GB2188798A - Electrical apparatus with rush current controlling function - Google Patents

Abstract

In a microwave oven supply circuit, a thyristor 13 is connected between a commercial power supply 10 and a primary winding 14a of a high-voltage transformer 14, on-off operation of the thyristor 13 being controlled by a microcomputer 18. A synchronizing signal generator 26 generates a signal SA1 in synchronism with the supply 10 and applies it to the computer 18 which turns the thyristor 13 on in response to the signal SA1 for each half-wave period of the supply 10 and further controls the thyristor 13 so that an "on" period in a given cycle is longer than that in the preceding cycle. Before on-control of the thyristor 13 is started, a predetermined period for verifying stable operation is provided, so that accuracy of the synchronizing signal SA1 is ensured. In addition, an instantaneous disconnection detector 29 checks for disconnection of the supply 10, and computer 18 is then reset to the beginning of the starting sequence. <IMAGE>

Description

SPECIFICATION Electrical Machinery and Apparatus with Rush Current Controlling Function Background of the Invention Field of the invention The present invention relates to an electrical machinery and apparatus comprising a rush current controlling function and more particularly, to an improvement of the function for controlling occurrence of rush current in the electrical machinery and apparatus such as a microwave oven comprising a load supplied with electric power from a commercial power supply and capable of generating the rush current.

Description of the Prior Art A microwave oven has been known as an electrical machinery and apparatus comprising a load supplied with electric power from a commercial power supply and capable of generating a rush current. More specifically, a microwave oven generally comprises a high-voltage transformer having a primary winding supplied with commercial electric power and a secondary winding connected to a magnetron. At the beginning of starting the high-voltage transformer, large rush current occurs on the primary winding.

One of the techniques in the prior art for controlling occurrence of such rush current is to preexcite the high-voltage transformer at the begin flying of starting thereof, which is disclosed in Japanese Patent Publication No. 41496/1977. More specifically, at the beginning of starting the highvoltage transformer, electric power is supplied to the primary winding through a rush current preventing resistor, so that pre-exciting current flows. The rush current preventing resistor is shortcircuited by a delayed relay switch after a lapse of a time period when the rush current can occur, so that the high-voltage transformer is completely excited without any rush current.However, since coil impedance of a delayed relay is affected by impedance of the primary winding of the highvoltage transformer, variation in the delay time of the delayed relay is liable to occur due to variation in the latter impedance.

Another technique in the prior art for controlling occurrence of such rush current is to set the time when the power supply is initiated to the primary winding of the high-voltage transformer correctly, to the phase of supply voltage in which the rush current can not easily occur using a semiconductor switch, which is disclosed, for example, in Japanese Patent Publication No. 9177/1976. According to this method, even if there is variation in impedance of the primary winding of the high-voltage transformer, occurrence of the rush current can be always controlled effectively. However, the rush current of about 20A (commercial power supply voltage: 200V) remains.Therefore, when dynamic sensitivity is relatively high, as in a breaker for house wiring in Europe, and especially West Germany, the breakerfor house wiring is liable to operate by the above described remaining rush current.

An approach for solving the problems is disclosed in, for example, Japanese Patent Publication No.

27377/1977. According to the approach, a semiconductor controlled rectifying device is provided between an AC power supply and a power circuit of a magnetron. If the semiconductor controlled rectifying device is controlled such that the conduction angle may be gradually increased, the rush current may be alleviated when the power supply is initiated.

However, since a period for verifying stable operation at the beginning of control of the semiconductor controlled rectifying device is not provided in an apparatus comprising the above described rush current controlling function, control operation is liable to be started while a control signal for the semiconductor controlled rectifying device is unstable. In addition, since measure is not taken against instantaneous disconnection of the AC power supply, correct control can not be performed if such instantaneous disconnection occurs.

Summary of the Invention Briefly stated, the present invention provides an electrical machinery and apparatus comprising a load supplied with electric power from a commercial power supply and capable of generating rush current, a semiconductor switch interposed between the commercial power supply and the load for controlling conduction to the load, semiconductor switch controlling means for turning the semiconductor switch on for each of positive and negative half-wave periods of a waveform of the commercial power supply and making an "on" period in at least a certain half-wave period larger than that in the preceding half-wave period, and stable operation verifying period setting means for setting a stable operation verifying period before control of the semiconductor switch is started by the semiconductor switch controlling means and operating the semiconductor switch controlling means after a lapse of the period.

In accordance with an aspect of the present invention, the semiconductor switch controlling means comprises a synchronizing signal generator for generating a synchronizing signal which is in synchronization with the waveform of the commercial powersupplyto make a predetermined change, cycle setting means responsive to the synchronizing signal for setting a cycle which corresponds to one cycle of the waveform of the commercial power supply and turns the semiconductor switch on during a predetermined period in each of positive and negative half-wave periods, means for repeating the cycle a plurality of times, and "on" period controlling means for making the "on" period of the semiconductor switch in a certain cycle larger than that in the preceding cycle in at least a part of the cycle repetition period.

In accordance with another aspect of the present invention, the semiconductor switch controlling means comprises means for detecting a predetermined change in the synchronizing signal while a cycle is repeated and repeating again the stable operation verifying period and the repeated cycles if it is not within a predetermined time period.

In accordance with still another aspect of the present invention, the stable operation verifying period setting means defines the stable operation verifying period by repeating a predetermined change in the synchronizing signal predetermined times.

In accordance with still another aspect of the present invention, after a predetermined change in the synchronizing signal is repeated predetermined times, the stable operation verifying period setting means repeats the cycle if the subsequent change is within a predetermined time period and extends the stable operation verifying period if it is not within the time period.

In accordance with still another aspect of the present invention, the electrical machinery and apparatus further comprises an instantaneous disconnection detector for detecting instantaneous disconnection of the commercial power supply, the semiconductor switch controlling means comprising means responsive to output of the instantaneous disconnection detector for repeating again the stable operation verifying period and the repeated cycles.

In accordance with still another aspect of the present invention, the semiconductor switch controlling means comprises a repetitive voltage generator for generating repetitive voltage having a repetition of non-rectangular wave with a predetermined period and plying at least one time between the maximum value and the minimum value of a predetermined range of voltage in each half-wave period of the waveform of the commercial power supply, a comparison voltage generator for generating a comparison voltage which changes in a single direction within the above described predetermined voltage range over a plurality of cycle periods of the waveform of the commercial power supply and an energization circuit for comparing the repetitive voltage with a comparison voltage and turning the semiconductor switch on in response to the comparison output.

In accordance with yet still another aspect of the present invention, the stable operation verifying period setting means defines, as the stable operation verifying period a period extending until the comparison voltage changing in a single direction is within a predetermined voltage range.

Therefore, a primary object of the present invention is to provide an electrical machinery and apparatus capable of controlling occurrence of the rush current almost completely.

Another object of the present invention is to provide an electrical machinery and apparatus in which control precision of the semiconductor switch for controlling the rush current is improved.

A principal advantage of the present invention is that since the stable operation verifying period is provided before control of the semiconductor switch is started, the time point when the control cycle of the semiconductor switch is started can be correctly determined in response to the stable synchronizing signal.

Another advantage of the present invention is that since the "on" period of the semiconductor switch is controlled in synchronization with the waveform of the commercial power supply while the control cycle of the semiconductor switch is performed, control precision in each cycle can be improved.

Still another advantage of the present invention is that if the commercial power supply is instantaneously disconnected, the effect can be eliminated and precise rush current controlling operation can be achieved.

These objects and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

Brief Description of the Drawings Fig. 1 is a circuit diagram showing an embodiment of the present invention; Fig. 2 is a waveform diagram for explaining operation of the circuit shown in Fig. 1; Figs. 3A, 3B and 3C are waveform diagrams for explaining operation of the circuit shown in Fig. 1; Figs. 4A, 4B, 4C and 4D are flow charts showing a control program according to an embodiment of the present invention; Fig. 5 is a circuit diagram showing a second embodiment of the present invention; and Figs. 6A, 6B and 6C are waveform diagrams for explaining operation of the circuit shown in Fig. 5.

Description of the Preferred Embodiments The present invention is used in an electrical machinery and apparatus having a load capable of generating rush current relative to a commercial power supply. For convenience, a microwave oven to which the present invention is applied is now described as an embodiment.

Fig. 1 is an electric circuit diagram showing an embodiment of the present invention.

Referring now to Fig. 1, description is made on a structure according to an embodiment of the present invention.

In Fig. 1, electric power is supplied to a primary winding 14a ofa high-voltagetransformer 14ofa leakage type from a commercial power supply 10 through a power switch 11, a door switch 12 and a bidirectional thyristor 13 serving as a semiconductor switch. The power switch 11 is provided on an operating panel (not shown) of a microwave oven and is turned on during a predetermined cooking period by users. The door switch 12 is adapted such that it is opened or closed ganged with opening or closing 6f a door (not shown) of the microwave oven. More particularly, the switch 12 is turned on when the door is closed.

An output of a secondary winding of the highvoltage transformer 14 is supplied to a magnetron 16 through a voltage multiplying rectifier 15, as is well-known. When a transistor 17 connected to a gate of the thyristor 13 is rendered conductive by a low level signal appearing on a C0 port of an output terminal of a microcomputer (referred to as uCOM hereinafter) 18 serving as control means, the thyristor 13 is turned on. In the present embodiment, a microcomputer LC-6527H manufactured by Sanyo Electric Co., Ltd. is employed as the pCOM 18.

A control power circuit 19 is supplied with electric power from the commercial power supply 10 through the power switch 11, the door switch 12 and a fuse 20 to produce a DC voltage (+7V, +5V) required for the I1COM 18 or the like. The power circuit 19 has a well-known structure comprising a step-down transformer 21, a full-wave rectifier portion 22 and a DC constant voltage controller portion 23.

A monitor switch 24 is connected in series with the commercial power supply 10, together with the power switch 11, the door switch 12, the fuse 20 and a current limiting resistor 25. The monitor switch 24 is ganged with opening or closing of a door of the microwave oven and operates in the opposite phase with the door switch 12. The monitor switch 24 is arranged for the following purpose. More specifically, if the door is opened during oscillation of microwave when the door switch 12 is welded, a short-circuit is almost formed in series as described above to immediately cut the fuse 20 and then, stop operation of the control power circuit 19, so that leakage of the microwave is prevented.

A synchronizing signal generator 26 is responsive to an AC voltage TB of the secondary winding of the step-down transformer 21 in the control power circuit 19to produce a synchronizing signal SA1 which is synchronized with the waveform of the commercial power supply 10 and to provide it to an A1 port of an inputterminal in the pCOM 18. More specifically, in the synchronizing signal generator 26, a phase-shifting portion 27 shifts, by 90 , a waveform of the AC voltage TB in phase with that of commercial supply voltage ACV to produce an AC waveform TB' centering +5V, as shown in Fig. 2.

Furthermore, a comparator 28 outputs a negative half-wave of the waveform TB' as a high level signal. Thus, a synchronizing signal SA1 changes from a high level to a low level at a 90 phase of the waveform of the commercial supply voltage ACV and becomes a pulse waveform changing from a low level to a high level at a 270 phase.

An instantaneous disconnection detector 29 detects decrease in voltage in the commercial power supply 10 and immediately feeds a detection signal SAo to an Ao port of the input terminal in the uCOM 18. More specifically, in the instantaneous disconnection detector 29, a voltage divider portion 30 divides a full-wave rectified voltage of the control power circuit 19 to produce a DC voltage Vb of about 6.5V. When the DC voltage Vb is less than 4V, a comparator 31 detects decrease in voltage to output the detection signal SAo at a low level. Decrease in voltage of the commercial power supply 10 immediately results in decrease in the above described DC voltage Vb.As a result, when the commercial power supply 10 is instantaneously disconnected, the detection signal SAo changes from a high level to a low level.

A reset circuit 32 feeds a reset signal at a low level to RES port of an inverted resetterminal in the uCOM 18 when an output voltage of the control power circuit 19 is less than a predetermined value.

The reset circuit 32 can be formed of IC, M51 951 manufactured by for example, Mitsubishi Electric Co., Ltd.

A clock generator portion 33 includes a resonator 33a. The uCOM 18 produces a system clock within the uCOM in cooperation with such a clock generator portion.

Figs. 3A, 3B and 3C are waveform diagrams for explaining operation of the circuit shown in Fig. 1.

Referring now to Figs. 3A, 3B and 3C, description is made on operation according to an embodiment of the present invention shown in Fig. 1.

In a circuit structure of the microwave oven shown in Fig. 1, when a door of the microwave oven is closed and the power switch 11 is initiated, the control power circuit 19 begins to supply the ,uCOM 18 with control supply voltage and at the beginning of starting of supply, the reset circuit 32 generates a reset signal. As a result, the uCOM 18 starts controlling, so that a level of a C0 port of an output terminal is determined in response to the above described synchronizing signal Say, the detection signal SAo and the like and driving of the thyristor 13 is controlled. Such control operation of the uCOM 18 is completed by opening the power switch 11 or the door switch 12.

Fig. 3A illustrates the driving state of the thyristor 13 based on control operation of the uCOM 18 in association with the commercial supply voltage ACV. More specifically, when control operation of the I1COM 18 is started, the "on" period of the thyristor 13 gradually increases over a series of cycles comprising the first to fifteenth cycles, so that the thyristor 13 remains continuously conductive after the fifteenth cycle.

As shown in Fig. 3A, the "on" period of the thyristor 13 is very short in an initial cycle. Thus, the thyristor 13 is turned off before the rush current increases, so that no rush current substantially occurs. In addition, the cycle period in an arbitrary cycle corresponds to one cycle of the commercial supply voltage ACV and the thyristor 13 is turned on for each predetermined period in each of negative and positive half-wave periods of the ACV. Thus, in an arbitrary cycle, pseudo alternating magnetic flux occurs in the high-voltage transformer 14, whose magnitude gradually increases with the lapse from the first cycle to the fifteenth cycle. As a result, the thyristor 13 remains continuously conductive without any rush current, that is, transition to the steady state is achieved.

In each cycle, it is preferable to pass current in the primary winding 14a of the high-voltage transformer as much as possible during the entire "on" period of the thyristor 13 so that the magnetic flux may occur. From this viewpoint, in the initial cycle particularly having a short "on" period of the thyristor 13, the "on" period should be set so as to avoid such a phase that current becomes 0 within the period. The current lags by a phase difference of 90 , behind the supply voltage in an inductive load such as the high-voltage transformer. Therefore, if the "on" period of the initial cycle is set in the vicinity of a zero cross of the supply voltage ACV, as in the present embodiment, it is possible to pass current sufficiently in the "on" period of the thyristor in each cycle.

Fig. 3B is a waveform diagram showing an arbitrary ith cycle of the above described repeated control cycles. In the present embodiment, frequency of the commercial power supply is 50 Hz, so that the length of one cycle is 20 millisecond (ms). In the ith cycle, the length of each period xi (a first "off" period), yi (a first"on" period), Xi (a second "off" period), Yi (a second "on" period) of the cycle is represented bythefollowing expressions: xi=Xi=(16-i)x0.56 ms (1) yi=Yi=ix0.56+1 ms (2) xi+yi=Xi+Yi=10 ms (3) The first and second "on" periods yi and Yi are 1.56 ms in the first cycle, 2.12 ms in the second cycle and 9.4 ms in the fifteenth cycle.

In setting the length of the above described each period, the time length of xi is evaluated bythe above described expression (1) utilizing the cycle start time Tp as a reference in the actual control of the laCOM 18. After the lapse of xi, the time length of yi is evaluated by the above described expression (2) and 10 ms is counted, so that completion of Xi is determined. In addition, 5 ms is counted from the time Ti when the synchronizing signal SA1 is inverted to a low level after a lapse yi, so that a lapse of Yi is determined.

The above described cycle start time as a reference for counting the above described time length of xi is the time Tp when the second "on" period Yi-1 in the preceding i-1th cycle is elapsed.

Specially, the start time of the first cycle utilizes as a reference the time when the synchronizing signal SA, is inverted to a low level after a lapse of a certain time period since the power switch 11 is initiated.

Fig. 3C illustrates the state in the vicinity of the above described start time in the first cycle. A time origin 0 represents the time point when the power switch 11 is initiated. After the power switch 11 is initiated, an output voltage of the control power circuit 19 becomes the steady value with a certain time constant. When the output voltage becomes almost the steady state, the reset circuit 32 releases the reset signal. As a result, the laCOM 18 starts control operation from this time point It is expected that at the beginning of starting of control of the ilCOM 18, the synchronizing signal generator 26 and the clock generator portion 33 have not been in a stable operating state. Thus, a period for verifying the stable operation is provided at the beginning of control of the pCOM 18.In this period, after control is started, the synchronizing signal SA, is inverted from a high level to a low level five times and then, it is examined whether the subsequent sixth inversion occurs or not in a predetermined time period, that is, a time period between 19 ms and 21 ms as shown in Fig. 3C. In other words, the period of the synchronizing signal is evaluated and it is examined whether the period is within a predetermined time period or not. If the sixth inversion does not occur in such a time period, it is considered that the synchronizing signal SA, is not precise and it is examined in the same manner whether the subsequent synchronizing signal SA, is inverted to a low level in a predetermined time period or not.If the above described inversion of the synchronizing signal SA, occurs in a predetermined time period, the time point when 5 ms is elapsed from the inversion time To becomes the start time of the first cycle. Thus, the start time of the first cycle is correctly determined based on a correct synchronizing signal.

Referring now to Fig. 3B again, the time point when the second "on" period Yi is completed in each cycle becomes the start time in the subsequent cycle, as described above. Since a reference for setting the start time is the time Ti immediately before the period Yi, the time Ti must be correct. For this test, it is examined whether the synchronizing signal SA, is inverted to a low level or not within a predetermined time period, that is, a time period between 4 ms and 6 ms as shown in Fig. 3B after the first "on" period yi. In other words, it is examined whether the period of the synchronizing signal is within a predetermined time period or not.If such inversion does not occur within a predetermined time period, it is considered that there is a slight variation in the phase relation between the commercial supply voltage and the synchronizing signal SA1, so that control operation of the uCOM 18 is returned to the state where the power switch 11 is initiated and the stable operation verifying is performed again.

Furthermore, the pCOM 18 performs a processing for protecting from instantaneous disconnection of the commercial power supply 10. When the commercial power supply 10 is instantaneously disconnected, the output voltage of the control power circuit 19 is almost the same as the steady value. However, since the input voltage TB to the synchronizing signal generator 26 disappears, the synchronizing signal SA, outputted from the synchronizing signal generator 26 largely varies, so that precise synchronous control can not be performed to the thyristor 13. To prevent this, during the above described stable operation verifying period and the subsequent process for counting time, the uCOM 18 tests periodically the input level of the Ao port of the input terminal. When it is determined that the detection signal SAo occurs (a low level), control operation of the COM 18 immediately returns to the state where the power switch 11 is initiated.

Figs. 4A, 4B, 4C and 4D are flow charts showing a control program contained in the uCOM 18 to perform the above described series of control operations.

Referring now to Figs. to 4D, description is specifically made on a control program according to an embodiment of the present invention.

Control of the program is started when reset is released at the time of initiating the power switch 11 and is completed when the power switch 11 is opened or a door of the microwave oven is opened, as described above.

Control is started in the step represented by S1. In the step S1, the port CO of the output terminal becomes a high level and the thyristor 13 is turned off, so that the microwave is turned off.

Steps S2 to S18 are processings for the period for verifying stable operation. The steps S2 to S6 correspond to a processing of a lapse of five cycles, the steps S7 to S18 correspond to a processing for determining the time To when the synchronizing signal is inverted, and the step S8 or S15 corresponds to a processing at the time of instantaneous disconnection of the power supply.

More specifically, "5" is set to a counter CNT1 (step S2), and the uCOM 18 waits until the input port A1 becomes a high level (HI) (step S3). When A1 becomes HI, the program proceeds to the step S4 and the uCOM 18waits until the input port A, becomes a low level (LO). When A, becomes LO, "1" is subtracted from the CNT1 (step S5). If the result is not "0", the program returns to the step S3 (step S6). If the result is "0" (step S6), it is verified that the synchronizing signal SA1 is inverted from a high level to a low level predetermined times (five times), so that the program proceeds to the steps S7 to S18 for a processing for determining the time To when the synchronizing signal is inverted.

More specifically, data N1 for counting 19 ms is set to the CNT1 and the I1CO M 18 waits for a lapse of 19 ms in the steps S9 and S10. While the uCOM 18 waits for the lapse, instantaneous disconnection of the power supply is detected in the step S8. When the input level SAo of the Ao port becomes LO (step S8), the program immediately returns to the state where control is to be started. When 19 ms is elapsed (step S10), the state of the A, port is detected (step S11).In the step S11, if it is detected that the input level of the A, port is LO, it is determined that the synchronizing signal SA1 falls before a lapse of 19 ms, so that "1" is set to the CNT1 (step S12) and then, the program returns to the step S3. Thereafter, the same steps S3 to S12 are repeated with respect to fall of the subsequent synchronizing signal. On the other hand, in the step S11, if it is detected that the input level of the A1 port is HI, data N2 for counting 2 ms is set to the CNT1 (step S13), and 2 ms is counted in the steps S16 and S17 while fall of the synchronizing signal SA, is detected in the step S14. During this period, in the step S15, instantaneous disconnection of the power supply is detected and processed in the same manner as the above described step S8.When 2 ms is elapsed, it is determined that the subsequent fall does not occur if 21 ms is elapsed from the previous fall of the synchronizing signal, so that "1" is set to the CNT1 (step S18) and then, the program returns to the step S3. Thereafter, the same steps S3 to S18 are repeated with respect to the subsequent fall of the synchronizing signal. On the other hand, before 2 ms is elapsed, if the input level of the A, port is inverted to LO (step S14), it is determined that the fall of the synchronizing signal SA, is within a range of 20 ms+ 1 ms from the previous fall. Thus, it is determined that the synchronizing signal is precise, so that the program proceeds to the step S19.

Then, "15" corresponding to the number of cycles from the first cycle to the fifteenth cycle shown in Fig. 3A is set to a second counter CNT2 (step S19).

Subsequent steps S20, S22 and S23 correspond to a processing for determining a lapse of 5 ms from the time To when the synchronizing signal is inverted, a step S21 corresponds to detection and processing of instantaneous disconnection of the power supply.

Data N3 for counting 5 ms is set to the CNT1 (step S20). The laCOM 18 waits for a lapse of 5 ms in the step S21 to S23 while instantaneous disconnection of the power supply is detected in the step S21.

When 5 ms is elapsed (step S23), it is determined whether the data of the CNT2 is "0" or not, that is, the fifteenth cycle is completed or not (step S24). If the data is "0", the program proceeds to a step S25 (Fig. 4D). In this case, since "15" is previously set in the step S19, the program proceeds to the step S35 (Fig. 4B).

Steps S35 to S41 include a processing for determining the first "off" period xi in an arbitrary ith cycle described referring to Fig. 3B and a processing for instantaneous disconnection of the power supply. More specifically, in the first cycle, the data of the CNT2, that is, "15" which is the remaining number of cycles is set to the CNT1 (step S35) and "1" is subtracted from the data of the CNT2 (step S36). The uCOM 18 has a timer function. If certain data is set to the timer register, a flag TMF is set after a lapse of a time period depending on the data. The data corresponding to 0.56 ms is set to the timer register utilizing this function (step S37).The pCOM 18 waits till it is detected that the TMF is set to "1", that is, a lapse of 0.56 ms is detected (step S39) while instantaneous disconnection of the power supply is detected (step S38). When it is detected that the TMF is set, that is, 0.56 ms is elapsed (step S39), "1" is subtracted from the CNT1. When the result is "0", the program proceeds to a step S42. In this case, since the value of the CNT1 is "15-1=14", the program returns to the step S37. Afterthe steps S37 to S41 are repeated fifteen times, the program proceeds to the step S42. As can be seen from the above described expression (1), the first "off" period x1 in the first cycle is (16-1)x0.56 ms=1 5x 0.56 ms. Thus, when the above described steps S37 to S41 are repeated fifteen times, the first "off" period x, is defined.

Steps S42 to S56 are processings for determining the first "on" period yi in an arbitrary ith cycle, as described referring to Fig. 3B. More specifically, in the first cycle, the output of the CO port is forced to LO, so that the thyristor 13 is turned on (step S42).

Data corresponding to 10 ms is set to the above described timer register (step S43). In addition, data of the CNT2, i.e., "14" is set to a third counterCNT3 (of 4 bits) (step S44) and then, a complement of the data of the CNT3 is evaluated, which value is set as the data of the CNT3 (step S45). Furthermore, data N5 for counting 0.56 ms is set to the CNT1 (step S46), the uCOM 18 waits for a lapse of 0.56 ms (steps S48 and S49) while instantaneous disconnection of the power supply is detected (step S47). If 0.56 ms is elapsed, "1" is subtracted from the CNT3 (step S50). Since the data of the CNT2 is "14" ("1110" in the binary presentation) in the step S44, the complement thereof, i.e., "1" ("0001" in the binary representation) is set to the CNT3.When "1" is subtracted in the step S50, the CNT3 becomes "0" (step S51) and then, the program proceeds to the step S52. Data N6 for counting 1 ms is set to the CNT1 (step S52). The llCOM 18 waits for a lapse of 1 ms (steps S54 and S55) while instantaneous disconnection of the power supply is detected (step S53). If 1 ms is elapsed, the output of the CO port is forced to HI (step S56), so that the thyristor 13 is turned off. As can be seen from the above described expression (2), y, in the first cycle is 1 x0.56+1 ms.

When the times of 0.56 ms and 1 ms are counted, the first "on" period y, is defined.

Steps S57 to S69 (Fig. 4C) include a processing for determining the reference time point Ti shown in Fig. 3B and a processing for instantaneous disconnection of power supply, and steps S70 to S76 include a processing for determining the time when the second "on" period Yi is completed and a processing for instantaneous disconnection of power supply.

Data N7 for counting 4 ms is set to the CNT1 and the uCOM 18 waits a lapse of 4 ms in the steps S61 and S62. During the period, it is checked whether 10 ms set to the timer register in the above described step S43 is elapsed or not (step S58) and whether the power supply is instantaneously disconnected or not (step S60). When 4 ms is elapsed (step S62), the program proceeds to the step S63, where the data N2 for counting 2 ms is set to the CNT1. The uCOM 18 waits for a lapse of 2 ms in the steps S68 and S69. During the period, while it is checked whether 10 ms set to the timer register in the step S43 is elapsed or not (step S64), the fall of the synchronizing signal SA, is detected (step S66) and instantaneous disconnection of power supply is checked (step S67).If the fall of the synchronizing signal is detected before a lapse of 2 ms, the program proceeds to the step S70. If the fall is not detected, it is determined that normal synchronous detection is not performed, so that the program returns to START (step S69). In addition, the data N3 for counting 5 ms is set to the CNT1 (step S70) and the uCOM 18 waits for a lapse of 5 ms (steps S74 and S75). During the period, it is checked whether 10 ms set to the timer register in the step S43 is elapsed or not (step S71) and then, instantaneous disconnection of the power supply is checked (step S73). If 5 ms is elapsed, it is determined that the second "on" period Y, is terminated.Thus, after the output of the CO port is forced to HI (step S76)so that thethyristor 13 is turned off, the program is jumped to the step S24.

During the above described steps S58 to S76, if TMF=1, i.e., a lapse of 10 ms is detected in the steps S58,S64 and 871,the output of the CO port becomes LO at the time point (steps S59, S65 and S72) so that thethyristor 13 is turned on. As a result, the second "on" period Y, is started. When termination of Y1 is determined (steps S75 and S76) and the program is jumped to the step S24, as described above, it is determined whether the data of the CNT2 is "0" or not, that is, the fifteenth cycle is terminated or not.

The data of the CNT2 is "14" (the second cycle), the program proceeds to the step S35. Thereafter, the above described steps are repeated. In this case, since the value set to the CNT1 in the step S35 is "14", timing for proceeding to the step S42 is faster, by 0.56 ms, than that in the first cycle. More specifically, the first "off" period x2 becomes shorter, by 0.56 ms, than that of the first cycle, so that the thyristor 13 is turned on at a phase faster, by 0.56 ms, than that in the first cycle. On the other hand, since the data of the CNT3 is "13" ("1101" in the binary representation) in the step S44, the time period when the program proceeds to the step S52 through the steps S46 to S51 becomes 0.56 msx2=1 .12 ms. Thereafter, 1 ms is elapsed in the steps S52 to S55 and then, the program proceeds to the step S56.Timing when the first "on" period y2 is terminated is in the vicinity of 0V phase of the commercial power supply ACV, which timing is the same as that in the first cycle. When the cycle in the steps S35 to S76 is repeated fourteen times and then, the program is jumped to the step S24 for the fifteenth time, the program proceeds to the step S25 (Fig. 4D) because the value of the CNT2 is "0".

The steps S25 to S34 correspond to the period when the thyristor 13 remains continuously conductive after termination of the fifteenth cycle.

The output of the CO port is forced to LO (step S25), so that the thyristor 13 is turned on. The data N4 for counting 14 ms is set to the CNT1 (step S26), and the ,uCOM 18 waitsfor a lapse of 14 ms (steps S28 and S29) while instantaneous disconnection ofthe power supply is checked (step S27). The data N2 for counting 2 ms is set to the CNT1 (step S30), instantaneous disconnection of power supply is checked (step S31) and the fall of the synchronizing signal is detected (step S32). When the fall of the synchronizing signal is detected (step S32), the program returns to the step S20. If the content of the CNT2 has been "0", it remains "0", so that the steps S20 to S34 continue to circulate, so that the output of the CO port is fixed to LO.

In the above described period when the thyristor remains continuously conductive, either one or both of cycles in the steps S26 to S29 or the steps S30 to S34 can be omitted.

Additionally, the above described embodiment can be modified as follows: (1) The second "on" period Yi is longer than the first "on" period yi in an arbitrary cycle.

(2) The "on" periods of the thyristor are the same in, for example, the first to third cycles of the first to fifteenth cycles, so that the "on" period is gradually increased. More specifically, according to the present invention, it is important that the "on" period of the thyristor in the subsequent cycle is larger than that in the previous cycle in at least a part of a period when the cycles are repeated.

(3) In the steady state after the fifteenth cycle, the thyristor 13 is not continuously turned on but in a constant duty cycle.

According to the above described embodiment, although control of the semiconductor switch for controlling the rush current is performed in response to the synchronizing signal which is in synchronization with the waveform of the commercial power supply, even if there is variation of the synchronizing signal or instantaneous disconnection of the commercial power supply, the effect is eliminated, so that precise operation for controlling the rush current can be achieved.

Fig. 5 is an electric circuit diagram showing a second embodiment of the present invention and Figs. 6A, 6B, and 6C are waveform diagrams for explaining operation according to the embodiment shown in Fig. 5.

In Fig. 5, electric power is supplied to the primary winding 14a of the high-voltage transformer 14 of a leakage type from the commercial power supply through the power switch 11, and the bidirectional thyristor 13 serving as a semiconductor switch. The power switch 11 is provided on an operating panel (not shown) of the microwave oven and is initiated during a desired cooking period. The output of the secondary winding of the high-voltage transformer 14 is supplied to the magnetron 16 through the voltage multiplying rectifier 15, as is well-known.

On the other hand, electric power of the commercial power supply 10 is applied to a control power circuit 40 through the power switch 11. The power circuit 40 comprises a step-down transformer 41, a full-wave rectifier portion 42 and a constantvoltage controller portion 43 to produce a driving voltage Vcc for control (7V in the present embodiment) for the other circuit portions.

The output voltage ACR of the full-wave rectifier portion 42 is compared with a first reference voltage Vsi in a first comparator 51 of a repetitive voltage generator 50. In the present embodiment, the ACR is full-wave rectified waveform having a base voltage 0V and a peak voltage 13V, while Vs1 is set to 1V.

Thus, a synchronizing signal voltage Vsy outputted from the first comparator 51 becomes a square wave which becomes a low level in the vicinity of the zero cross with respect to the waveform of the commercial supply voltage ACV, as shown in Fig.

6A. In a charging portion comprising a resistor 52 and a capacitor 53, the charged charge is rapidly discharged through the first comparator 51 when the synchronizing signal voltage Vsy is a low level.

Thus, a repetitive voltage RPV which is an output of the capacitor 53 becomes a triangular wave having a period of a half-cycle ofthewaveform of the commercial supply voltage ACV, as shown in Fig.

6A. In the present embodiment, the base voltage Vmin of such a triangular wave is 0.7V and the peak voltage Vmax thereof is 4.0V.

A capacitor 61 within a comparison voltage generator 60 is completely discharged before the power switch 11 is initiated, so that a transistor 62 is turned on immediately after the power switch 11 is initiated. However, the terminal voltage of the capacitor 61 approaches Vcc after the power supply is initiated, so that the transistor 62 is turned off.

More specifically, when the transistor 62 is turned on, a capacitor 63 is almost discharged, so that the terminal voltage, i.e., a comparison voltage COV is an initial value V0 (6.2V in the present embodiment).

Thereafter, when the transistor 62 is turned off, the charged current flows through a charging path comprising the capacitor 63 and a resistor 64, so that the comparison voltage COV is gradually decreased.

Change in the comparison voltage COV is shown in Fig. 6B. Referring to Fig. 6B, the comparison voltage COV is gradually decreased from the initial value V0 ( > Vmax) after the power switch 11 is initiated and passes in the single direction within a voltage range (VmaxVmjn) of the repetitive voltage RPV and then, becomes a final value VE (=or).

According to the present embodiment, capacitance of the capacitor 63 is 1 uF and the resistance value of the resistor 64 is 100 kQ, so that the comparison voltage changes from the initial value V0 to the final value VE in 250 millisecond. If frequency of the commercial power supply 10 is 50 Hz, the time period for such change corresponds to 12.5 cycles of the waveform of the commercial power supply.

A second comparator 71 of the energization circuit 70 compares the repetitive voltage RPV applied from the repetitive voltage generator 50 with the comparison voltage COV applied from the comparison voltage generator 60, so that a high level signal is outputted in the case of RPV > COV. A transistor 72 is turned on by such a high level signal, so that the thyristor 13 is turned on. Therefore, as shown in Fig. 6B, the "on" period TON of the thyristor 13 is0 immediatelyafterthe power supply is initiated. However, the period ToN is gradually increased with the lapse of time and finally extends over each cycle of the repetitive voltage RPV.

As shown in Fig. 6C, the thyristor 13 is turned on only in the period TON for each half-cycle of the waveform of the commercial supply voltage ACV.

Thereafter, the period TON is gradually increased and finaliy the thyristor 13 remains continuously conductive.

More specifically, in the initial cycle, the "on" period TON of the thyristor 13 is very short. Thus, the thyristor 13 is turned off before the rush current is increased, so that no rush current occurs substantially. In addition, the thyristor 13 is turned on in each of negative and positive half-cycles of the waveform of the commercial supply voltage ACV, pseudo alternating magnetic flux occurs in the highvoltage transformer 14 at the beginning, which magnitude is gradually increased with the lapse of time. As a result, the thyristor 13 remains continuously conductive, that is, transition to the steady state is achieved with no rush current.

It is preferable that the output of the second comparator 71 is held to a low level until the repetitive voltage RPV occurs stably immediately after the power switch 11 is initiated. Therefore, in the present embodiment, the time period extending until the comparison voltage COV is in the voltage range of the repetitive voltage RPV since the power supply is initiated is defined as the stable operation verifying period and is set to the length more than the time required to stabilize the repetitive voltage RPV, i.e., 120 millisecond.

An instantaneous disconnection detector 80 makes a full-wave rectified voltage ACR smooth in a smoothing portion 81 to produce an inspection voltage VINE. Athird comparator 82 compares such an inspection voltage VINS with a second reference voltage Vs2. When the commercial power supply 10 is instantaneously disconnected, the driving voltage Vcc produced by the control power circuit 40 is almost the steady value and the inspection voltage VINE is immediately decreased. Therefore, the output of the third comparator 82 is a low level, so that the charge charged in the capacitor 61 of the comparison voltage generator 60 is rapidly discharged through the third comparator 62. As a result, the transistor 62 is turned on and charge stored in the capacitor 63 is rapidly discharged through a transistor 62.Thus, the comparison voltage COV returns to the initial value VO, so that the thyristor 13 is turned off. Thereafter, when the commercial power supply 10 is recovered, there occurs the above described state where the power supply is initiated, so that the high-voltage transformer 14 is driven without any rush current.

According to the above described embodiment, output circuits of the first, second and third comparators 51,71 and 82 are of open drain type, so that the output circuits become a high output impedance state when the high level is outputted.

Thus, when for example, the third comparator 82 is a high level, no current flows from the comparator 82 to the capacitor 61. Zener diodes included in the repetitive voltage generator 50 and the instantaneous disconnection detector 80 prevent excessive input to the first comparator 51 and the third comparator 82.

In the above described embodiment, the waveform of the repetitive voltage RPV may be replaced by anothertriangularwave or a sine wave and a period of the repetitive voltage RPV can be a quarter the period ofthewaveform of the commercial power supply. Briefly stated, it is sufficient for repetitive voltage RPV to have a repetition of non-rectangular wave with a predetermined period and ply at least one time between the maximum value and the minimum value of a predetermined voltage range within the time period of a half-cycle length of the commercial power supply.

In the above described embodiment, the initial value of the comparison voltage COV may correspond to Vmax of the repetitive voltage RPV.

Furthermore, the above described embodiment may be achieved in a negative logic form, and the comparison voltage COV may be changed from Vmin to Vmax of the repetitive voltage RPV.

Additionally, in the above described embodiment, the final value Vs of the comparison voltage COV may be set within the voltage range (VminVmax) of the repetitive voltage RPV. When the comparison voltage COV reaches the final value V5, the "on" period TON of the thyristor at that time point may be maintained to be a steady state. In this case, the output of the magnetron 16 is less than 100%.

Therefore, the output of the magnetron 16 in the steady state can be adjusted by using the variable final value VE.

Therefore, in the second embodiment, occurrence of the rush current can be surely controlled and the same control effect can be obtained if the commercial power supply is instantaneously disconnected.

Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.

Claims (42)

1. An electrical apparatus comprising: an electrical load coupled to an input for receiving electric power from a commercial power supply and capable of generating rush current, a semiconductor switch interposed between said input and said load for controlling conduction to said load, semiconductor switch controlling means for turning said semiconductor switch on for each of positive and negative half-wave periods of a waveform of said commercial power supply, an "on" period in at least a certain half-wave period being larger than that in a preceding half-wave period, and stable operation verifying period setting means for setting a period for verifying stable operation before control of said semiconductor switch is started by said semiconductor switch controlling means, said semiconductor switch controlling means being operated after a lapse of said period.

2. An electrical apparatus in accordance with claim 1,wherein said semiconductor switch controlling means comprises: a synchronizing signal generator for generating a synchronizing signal which is in synchronization with the waveform of said commercial power supply to make a predetermined change, cycle setting means for setting a cycle corresponding to one cycle of the waveform of said commercial power supply and turning said semiconductor switch on in the predetermined period in each of positive and negative half-wave periods in response to said synchronizing signal, means for repeating said cycle a plurality of times, and "on" period controlling means for making an "on" period of said semiconductor switch in a certain cycle larger than that in the preceding cycle in at least a part of said cycle repetition period.

3. An electrical apparatus in accordance with claim 2, wherein said semiconductor switch controlling means further comprises "on" period terminating means for terminating the "on" period of said semiconductor switch in the vicinity of zero volt of the waveform of said commercial power supply.

4. An electrical apparatus in accordance with claim 2, wherein said "on" period controlling means makes the "on" period of said semiconductor switch shortest in the first cycle of said repeated cycles.

5. An electrical apparatus in accordance with claim 4, wherein said "on" period controlling means gradually increases the "on" period of said semiconductor switch as said cycle is repeated.

6. An electrical apparatus in accordance with any of claims 2 to 5, wherein said semiconductor switch controlling means further comprises means for continuously turning said semiconductor switch on after said cycle is repeated a plurality of times.

7. An electrical apparatus in accordance with any of claims 2 to 5, wherein said semiconductor switch controlling means further comprises means for making the "on" period of said semiconductor switch constant after said cycle is repeated a plurality of times.

8. An electrical apparatus in accordance with any of claims 2 to 5, wherein said semiconductor switch controlling means comprises means for detecting a predetermined change in said synchronizing signal while said cycle is repeated and repeating again said stable operation verifying period and said repeated cycles when the predetermined change is not within a predetermined time period.

9. An electrical apparatus in accordance with claim 2, wherein said synchronizing signal makes a predetermined change in the vicinity of the maximum value of the absolute value of the waveform of said commercial power supply, and said cycle setting means counts from the reference time of each cycle a first "off" period during which said semiconductor switch is turned off, counts after a lapse of said first period a first "on" period during which said semiconductor switch is turned on, and starts counting a first constant period, defines a second "off" period which is a period from termination of the first "on" period to termination of counting said first constant period, said semiconductor switch being turned off during the period, starts counting a second constant period when a predetermined change in said synchronizing signal is detected during counting said first constant period, and defines a second "on" period which is a period from termination of counting said first constant period to termination of said second constant period, said semiconductor switch being turned on during the period, said semiconductor switch being turned off when counting of said second constant period is completed and the time when said second constant period is terminated being the reference time of the subsequent cycle.

10. An electrical apparatus in accordance with claim 9, wherein in each of said cycles, said first "on" period and said second "on" period are equal, and said first "off" period and said second "off" period are equal.

11. An electrical apparatus in accordance with claim 9, wherein in at least one cycle of said plurality of cycles, said second "on" period is longer than said first "on" period.

12. An electrical apparatus in accordance with claim 9, wherein said "on" period controlling means makes said first "on" period and said second "on" period shortest in the first cycle of said repeated cycles.

13. An electrical apparatus in accordance with claim 12, wherein said "on" period controlling means gradually increases said first "on" period and said second "on" period as said cycle is repeated.

14. An electrical apparatus in accordance with any of claims 9 to 13, wherein said cycle setting means makes said "on" period continuous after said cycle is repeated a plurality of times.

15. An electrical apparatus in accordance with any of claims 9 to 13, wherein said cycle setting means makes said first "on" period and said second "on" period constant after said cycle is repeated a plurality of times.

16. An electrical apparatus in accordance with any of claims 9 to 13, wherein said semiconductor switch controlling means comprises: means for counting a third predetermined period and a fourth predetermined period which is longer than said third predetermined period from the time when said first "on" period is elapsed and repeating again said stable operation period and said repeated cycles when a predetermined change in said synchronizing signal is not swithin a time period from termination of said third predetermined period to termination of said fourth predetermined period.

17. An electrical apparatus in accordance with claim 2, wherein said stable operation verifying period setting means defines said stable operation verifying period by repeating a predetermined change in said synchronizing signal predetermined times.

18. An electrical apparatus in accordance with claim 17, wherein after a predetermined change in said synchronizing signal is repeated predetermined times, said stable operation verifying period setting means repeats said cycle if the subsequent change is within a predetermined time period and extends said stable operation verifying period if the subsequent change is not within said time period.

19. An electrical apparatus in accordance with claim 18, wherein said stable operation verifying period setting means extends said stable operation verifying period until it is detected that a predetermined change in said synchronizing signal is within a predetermined time period.

20. An electrical apparatus in accordance with claim 2, further comprising an instantaneous disconnection detector for detecting instantaneous disconnection of said commercial power supply, wherein said semiconductor switch controlling means comprises means for repeating again said stable operation verifying period and said repeated cycles in response to the output of said instantaneous disconnection detector.

21. An electrical apparatus in accordance with claim 20, wherein said instantaneous disconnection is detected during said stable operation verifying period.

22. An electrical apparatus in accordance with claim 20 or 21, wherein said instantaneous disconnection is detected during said cycle repetition period.

23. An electrical apparatus in accordance with any of claims 20,21 and 22, wherein said instantaneous disconnection is also detected after repetition of said cycle is terminated.

24. An electrical apparatus in accordance with any of claims 20 to 23, wherein said instantaneous disconnection detector comprises: comparator means for comparing a voltage corresponding to said commercial power supply with a predetermined reference voltage to output a predetermined detection signal when the voltage corresponding to said commercial power supply is lowerthan said reference voltage and apply it to said semiconductor switch controlling means.

25. An electrical apparatus in accordance with claim 2, further comprising a control power circuit supplied with electric power from said commercial power supply for supplying a DC power supply to said semiconductor switch controlling means.

26. An electrical apparatus in accordance with claim 2, wherein said synchronizing signal generator comprises: means for shifting, by +90 , a predetermined voltage corresponding to said commercial power supply voltage, and means for generating a pulse signal which is inverted between a high level and a low level in the vicinity of zero volt of said shifted predetermined voltage and outputting it as said synchronizing signal.

27. An electrical apparatus in accordance with claim 1,wherein said semiconductor switch controlling means comprises: a repetitive voltage generator for generating a repetitive voltage having a repetition of nonrectangular waveform with a predetermined period and plying at least one time between the maximum value and the minimum value of a predetermined voltage range in each half-wave period of the waveform of said commercial power supply, a comparison voltage generator for generating a comparison voltage which changes in a single direction within said predetermined voltage range over a plurality of cycle periods ofthewaveform of said commercial power supply, and an energization circuit for comparing said repetitive voltage with said comparison voltage and turning said semiconductor switch on in response to the comparison output.

28. An electrical apparatus in accordance with claim 27, wherein said stable operation verifying period setting means is included in said comparison voltage generator and defines as said stable operation verifying period a period extending until said comparison voltage changes in a single direction within said predetermined voltage range.

29. An electrical apparatus in accordance with claim 27 or 28, further comprising: a control power circuit for receiving said commercial power supply and supplying the DC power supply to said semiconductor switch controlling means.

30. An electrical apparatus in accordance with claim 27, wherein said comparison voltage generator comprises: means responsive to the DC power supply supplied from said control power circuit when said commercial power supply is initiated for generating an initial voltage and supplying it as said comparison voltage, and means for gradually increasing or decreasing said initial voltage after said commercial power supply is initiated.

31. An electrical apparatus in accordance with any of claims 27 to 30, wherein said repetitive voltage generator comprises: means for generating a synchronizing signal of a rectangular wave corresponding to the waveform of said commercial power supply, and means for generating said repetitive voltage in response to said synchronizing signal of a rectangular wave.

32. An electrical apparatus in accordance with claim 31, wherein said means for generating said repetitive voltage comprises a charging portion including a resistor and a capacitor.

33. An electrical apparatus in accordance with claim 32, wherein said repetitive voltage is of a triangular wave having a period of half-cycle of the waveform of the commercial power supply.

34. An electrical apparatus in accordance with claim 30, wherein said initial voltage is equal to either the maximum value or the minimum value of said predetermined voltage range.

35. An electrical apparatus in accordance with claim 27, wherein change in said comparison voltage is terminated within a predetermined voltage range.

36. An electrical apparatus in accordance with claim 35, wherein the final value of said comparison voltage is variable.

37. An electrical apparatus in accordance with claim 27, wherein change in said comparison voltage is terminated without said predetermined voltage range.

38. An electrical apparatus in accordance with claim 27, further comprising an instantaneous disconnection detector for detecting instantaneous disconnection of said commercial power supply, wherein said semiconductor switch controlling means returns said comparison voltage generator to the state where said commercial power supply is initiated in response to the output of said instantaneous disconnection detector.

39. An electrical apparatus in accordance with claim 1,wherein said load is a magnetron generator including a high-voltage transformer.

40. An electrical apparatus in accordance with claim 1,wherein said semiconductor switch is a bidirectional thyristor.

41. A power supply for an electrical load, the power supply comprising an input for coupling to an AC power source, an output for coupling to the load, and a controllable switching means coupling the input to the output for controlling the electrical power supplied to the output, and control means for producing a control signal for controlling the conduction of the switching means, application of the control signal to the switching means being delayed until production thereof is stable.

42. An electrical apparatus substantially as hereinbefore described with reference to Figures 1 to 4A to 4C, or to Figures 5 and 6A to C.