GB2281417A

GB2281417A - Washing machine

Info

Publication number: GB2281417A
Application number: GB9417282A
Authority: GB
Inventors: Takashi Nonogaki; Nobuhide Horiuchi; Masahiro Imai
Original assignee: Toshiba Corp; Toshiba AVE Co Ltd
Current assignee: Toshiba Corp; Toshiba AVE Co Ltd
Priority date: 1993-08-26
Filing date: 1994-08-26
Publication date: 1995-03-01
Anticipated expiration: 2014-08-26
Also published as: JP3152819B2; CN1106481A; KR950006094A; CN1077172C; KR0149467B1; GB2281417B; GB9417282D0; TW245751B; JPH07112094A

Abstract

A washing machine includes a speed control circuit for a motor driving a dehydrating tub and a microcomputer. The microcomputer controls the motor so that it is decelerated when the motor speed has increased to a first reference value, for example, 600 rpm, after start of a dehydrating operation. The microcomputer then measures a time period t required for the motor speed to be decreased from the first reference value to the second reference value, for example, 550 rpm. The measured time period t is compared with a reference time period. Based on the result of comparison, the microcomputer determines the degree of unbalance of the dehydrating tub (3). The dehydrating operation is continued with a first target speed, for example, 1000 rpm corresponding to a normal condition set when the determined degree of unbalance is low. The dehydrating operation is continued with a second target speed, for example, 920 rpm lower than the first target speed set when the determined degree of unbalance is high. <IMAGE>

Description

WASHING MACHINE This invention relates to a washing machine having a dehydrating tub in which clothes are centrifugally dehydrated after a wash step, and more particularly to such a washing machine provided with means for coping with rotation of the dehydrating tub in an unbalanced state.

In washing machines, a dehydrating tub is caused to shake at large angles during rotation for dehydration when clothes are not evenly distributed in the dehydrating tub.

The shaking prevents the increase of a rotational speed of the dehydrating tub. In view of this problem, the conventional washing machines are usually provided with an unbalance detecting lever opposite to a circumferential wall of a water-receiving tub in which the dehydrating tub is mounted. An unbalance detecting switch is further provided to be turned on and off in response to movement of the lever. The dehydrating tub is caused to roll at large angles when rotated in the unbalanced state. The dehydrating tub comes into contact with the unbalance detecting lever, whereby the unbalance detecting switch is operated such that the motor is deenergized to stop the dehydration.

In the above-described arrangement, however, a rolling amplitude of the dehydrating tub is not sometimes increased to such an extent that it comes into contact with the unbalance detecting lever, depending upon the degree of unevenness of distribution of the clothes in the tub. In this case, the rolling of the dehydrating tub cannot be detected and its rotational speed is increased in a speed rise stage. When the dehydrating tub has reached a target rotational speed, vibration and noise are increased in one case. Alternatively, the unbalanced state of the clothes is spontaneously dissolved in the other case.

Therefore, a primary object of the present invention is to provide a washing machine wherein the unbalanced state of the dehydrating tub due to the uneven distribution of the clothes in the dehydrating tub can be detected on the basis of rotational characteristics of the dehydrating tub without using mechanical means such as the unbalance detecting lever.

A second object of the present invention is to provide a washing machine wherein a dehydrating operation can be continued even when the unbalanced state of the dehydrating tub has been detected.

A third object of the present invention is to provide a washing machine wherein the dehydrating operation is continued under a first target speed previously determined to correspond to a normal state of the dehydrating tub when the detected unbalanced state of the dehydrating tub is in a slight degree and the dehydrating operation is continued under a second target speed lower than the first target speed when the detected unbalanced state of the dehydrating tub is in a significant degree.

The present invention provides a washing machine comprising an electric motor for rotating a dehydrating tub, decelerating means for controlling a rotational speed of the motor so that the rotational speed of the motor is decreased after the rotational speed of the motor has been increased to a first reference speed upon start of a dehydrating operation, means for measuring a speed decreasing period of time necessitated for the motor speed to be decreased from the first reference speed to a second reference speed, means for changing a final dehydration speed of the motor to a second target speed when the measured speed decreasing period of time is equal to or below a reference period of time, the second target speed being lower than a first target speed corresponding to a normal condition, and means for increasing the motor speed to the second target speed after the motor speed has reached the second reference speed.

According to the above-described arrangement, a decelerating period of time required for decreasing the rotational speed of the dehydrating tub from the first reference speed to the second reference speed under the decelerating control shortens as the degree of unbalance of the dehydrating tub becomes significant. Accordingly, the degree of unbalance of the dehydrating tub can be determined when the decelerating period of time is measured and then, compared with the reference period of time.

Upon determination of the degree of unbalance of the dehydrating tub, the first or second target speed according to the determined degree of unbalance is selected. The dehydrating tub is accelerated toward the selected target speed without stop. - In particular, the second target speed corresponding to the relatively significant degree of unbalance belongs to such a speed range that an abnormal vibration is not produced even under such a significant degree of unbalance. Consequently, the dehydrating operation can be continued at the target speed without hindrance.

In a first preferred form, the washing machine further comprises power-supply voltage detecting means for detecting a power-supply voltage applied to the motor and reference time period setting means for setting the reference period of time so that the reference period of time is lengthened as the voltage detected by the power-supply voltage detecting means becomes high.

In a second preferred form, the washing machine further comprises temperature sensing means for sensing a temperature of water supplied into the dehydrating tub and reference time period setting means for setting the reference period of time so that the reference period of time is shortened as the temperature sensed by the temperature sensing means becomes high.

In a third preferred form, the motor comprises a capacitor-start motor. Both of a main coil and an auxiliary coil of the motor are energized when the rotational speed of the motor is controlled to be increased. Either the main or the auxiliary coil of the motor is energized when the rotational speed of the motor is controlled to be decreased.

In a fourth preferred form, both of the main and auxiliary coils of the motor are deenergized when the rotational speed of the motor is controlled to be decreased.

In a fifth preferred form, the motor comprises a DC brushless motor and voltage changing means is provided for changing the voltage applied to the motor so that the rotational speed of the motor is controlled to be increased and decreased. The voltage changing means may comprise phase control means or pulse width modulation (PWM) control means.

The invention will be described, merely by way of example, with reference to the accompanying drawings, in which: FIG. 1 is a circuit diagram showing an electrical arrangement of a washing machine of a first embodiment in accordance with the present invention; FIG. 2 is a longitudinally sectional side view of the washing machine; FIG. 3 is a graph showing an energization pattern of a motor and the changes in the rotational speed of a dehydrating tub; FIG. 4 is a graph similar to FIG. 3, showing a washing machine of a second embodiment in accordance with the present invention; FIG. 5 is a graph similar to FIG. 3, showing a washing machine of a third embodiment in accordance with the present invention; FIG. 6 is a block diagram showing an electrical arrangement of a washing machine of a fourth embodiment in accordance with the present invention; FIG. 7 is a graph showing a voltage applied to the motor and the changes in the rotational speed of the dehydrating tub; FIG. 8 is a graph showing the relationship between the motor speed and the torque in the fourth embodiment; FIG. 9 is a circuit diagram showing an electrical arrangement of a washing machine of a fifth embodiment in accordance with the present invention; FIGS. 10A to lOD are graphs showing phase-controlled waveform patterns; FIG. 11 is a graph showing the relationship between the motor speed and the torque in the fifth embodiment; FIG. 12 is a circuit diagram showing an electrical arrangement of a washing machine of a sixth embodiment in accordance with the present invention; FIGS. 13A and 13B are graphs showing an on-off pattern of a switching transistor and a voltage waveform respectively; FIG. 14 is a circuit diagram showing an electrical arrangement of a washing machine of a seventh embodiment in accordance with the present invention; FIG. 15 is a circuit diagram 'showing a power-supply voltage detecting circuit employed in the washing machine of the seventh embodiment; FIG. 16 shows reference time periods set in accordance with the values of the DC power-supply voltage in the seventh embodiment; FIG. 17 is a circuit diagram showing an electrical arrangement of a washing machine of an eighth embodiment in accordance with the present invention; FIG. 18 is an electrical circuit diagram showing a sensor circuit; FIG. 19 is a longitudinally sectional side view of a lower portion of a water-receiving tub of the washing machine, on which portion a temperature sensor is mounted; and FIG. 20 shows the reference time periods set in accordance with the values of the power-supply voltage and the temperature of water supplied to the rotatable tub in a ninth embodiment.

A first embodiment of the present invention will now be described with reference to FIGS. 1 to 3. Referring to FIG. 2, an outer cabinet 1 of an automatic washing machine encloses a water-receiving tub 2 rockably mounted on a suspension mechanism (not shown). A rotatable tub 3 is rotatably mounted in the water-receiving tub 2. The rotatable tub 3 serves both as a wash tub and as a dehydrating tub. An agitator 4 is rotatably mounted on an inner bottom of the rotatable tub 3. A drive mechanism 6 is provided below the water-receiving tub 2. The drive mechanism 6 includes an electric motor 5 comprising a capacitor-start motor. The water-receiving tub 2 has in its bottom a drain hole connected through a drain valve 7a to a drain hose. The motor 5 serves both as a washing motor and as a dehydrating motor. A lever 9a and an unbalance detecting switch 9 are mounted on the upper interior of the outer cabinet 1. The water-receiving tub 2 collides against the lever 9a when it shakes upon rotation of the rotatable tub 3 in an unbalanced state. The unbalance detecting switch 9 is responsive to movement of the lever 9a.

A water supply valve 7 and a water level sensor 10 are provided in the rear interior of a top cover 8 covering the top of the outer cabinet 1. A control unit 11 is provided in the front interior of the top cover 8. The abovementioned drive mechanism 6 includes a reduction mechanism reducing the rotational speed of the motor 5 to a half and transmitting the reduced speed to the agitator 4 in a wash step.

Referring now to FIG. 1, a common connecting terminal 5a of the motor 5 is connected to one of terminals of an AC power source 12. The other terminals 5b and Sc are connected to the other terminal of the AC power source 12 through triacs 13 and 14, respectively. The terminals 5b, Sc are further connected to one ends of triacs 15 and 16 respectively. The other ends of the triacs 15, 16 are connected through a common starting capacitor 17 and coil 18 to the other terminal of the AC power source 12. Switching means 19 is composed of the triads 13-16. The motor 5 composes a capacitor-start motor with the starting capacitor 17.

The motor 5 is provided with speed detecting means 20 comprising Hall elements, for example. The speed detecting means 20 detects a rotational speed of the motor 5 and delivers a signal indicative of the detected motor speed to a control circuit 21. The control circuit 21 comprises a microcomputer, gate circuits and an analog-to-digital (A/D) converter. The control circuit 21 has a function of dehydration control means including the above-described switching means 19. The triacs 13 and 14 are controlled by the control circuit 21 to be turned on and off. The triacs 15 and 16 are controlled via respective photo couplers 22 and 23 by the control circuit 21 to be turned on and off.

The motor 5 is switched among an all-coil energizing mode, a main coil energizing mode and an auxiliary coil energizing mode, in accordance with on-off patterns of the triacs 1316. The following TABLE 1 shows the relation between the on-off patterns of the triacs 13-16 and the energizing modes.

TABLE 1 Relation between triacs and energizing mode Energizing mode Triac 13 14 15 16 All-coil energizing mode On Off Off On Main coil energizing mode On Off Off Off Auxiliary coil energizing mode Off Off Off On In the all-coil energizing mode, the triacs 13 and 16 are turned on while the triacs 14 and 15 are turned off.

Consequently, the coil 5e of the motor 5 is energized without through the starting capacitor 17 and the coil 18 and its coil 5d is energized through the starting capacitor 17 and the coil 18. Accordingly, the coil Se serves as a main coil and the coil 5d as an auxiliary coil in this case.

In the main coil energizing mode, only the triac 13 is turned on so that only the coil 5e is energized without through the starting capacitor 17 and the coil 18. In the auxiliary coil energizing mode, only the triac 16 is turned on so that only the coil 5d is energized through the starting capacitor 17 and the coil 18.

Switch signals from various switches 24 of the control unit 11 are supplied to the control circuit 21.

Furthermore, signals from the unbalance detecting switch 9, the water level sensor 10 and the speed detecting means 20 are also supplied to the control circuit 21. Based on these supplied signals, the control circuit 21 controls the motor 5, the drain valve 7a, the water-supply valve 7b and a display section 25 of the control unit 11 in accordance with an operation program stored therein, thereby controlling washing, rinsing and dehydrating operations.

In the dehydrating operation, the control circuit 21 functions as dehydration control means as follows. Upon start of the dehydrating operation, the triacs 13-16 of the switching means 19 are set for the all-coil energizing mode in which both main and auxiliary coils are energized, so that the motor 5 is energized under this mode to thereby rotate the rotatable tub 3. In this case, when the degree of unbalance of the rotatable tub 3 is excessively large, the water-receiving tub 2 collides against the lever 9a such that it is swung. The unbalance detecting switch 9 responds to the movement of the lever 9a, generating an unbalance detection signal. In response to the unbalance detection signal, the control circuit 21 immediately deenergizes the motor 5. However, the water-receiving tub 2 does not sometimes collide against the lever 9a depending upon the extent of uneven distribution of clothes in the rotatable tub 3 even while it is being rotated in an unbalanced state.

Thus, the above-described mechanical unbalance detecting means cannot detect the unbalanced state of the rotatable tub 3.

The rotational speed of the rotatable tub 3 is increased in the situation that its unbalanced state is not detected by the lever 9a and unbalance detecting switch 9, as shown in FIG. 3. The rotatable tub 3 reaches a first reference speed, for example, 600 rpm at time t1 or the motor 5 reaches 1,200 rpm. At time t1, the triacs 13-16 of the switching means 19 are switched only to the auxiliary coil energizing mode, so that the motor 5 is energized in this mode. Since an output torque developed by the motor 5 in the auxiliary coil energizing mode is reduced to or below a no-load torque in the dehydrating operation in the washing machine, the motor 5 positively decelerates. The rate of deceleration interrelates with the degree of unbalance of the rotatable tub 3. An inertia force due to rotation of the rotatable tub 3 is smaller and the rotatable tub 3 decelerates more rapidly when the degree of unbalance thereof is large than when it is small.

The control circuit 21 initiates a timing operation at the deceleration start time t1. The control circuit 21 stops the timing operation at time t2 or t3 when the rotational speed of- the rotatable tub 3 is decreased to a second reference speed, for example, 550 rpm. At time t2 or t3, the triacs 13-16 are switched to the all-coil energizing mode. When a measured time period t between t1 and t2 or t3 exceeds a reference period of time which is set for 7 seconds in an intended meaning in this case, as shown by time t3 and solid line K1 in FIG. 3, a final rotational speed of the rotatable tub 3 is increased to a first target speed of 1,000 rpm corresponding to a final speed in the normal condition and is maintained at this final speed.

Upon expiration of a set dehydrating period of time, the motor 5 is deenergized. Thus, when the measured time period t is relatively long, it is detected that the degree of unbalance is low or that the unbalanced state of the rotatable tub 3 does not occur. In such a case, the dehydrating operation is normally executed.

When the measured time period t is 7 seconds or below as shown by time t3 and broken line K2 in FIG. 3, the final rotational speed of the rotatable tub 3 is set for a second target speed, for example, 920 rpm, which speed is lower than the first target speed of 1,000 rpm. That is, when the measured time period t is relatively short, it is detected that the degree of unbalance is high. In such a case, the final speed of the rotatable tub 3 is rendered lower than that in the normal condition, so that the dehydrating operation is continued at such a rotational speed range that the vibration or the abnormal noise is not produced. The time chart of the energizing mode corresponding to broken line K2 is not shown in FIG. 3. Line K in FIG. 3 shows the characteristic of the rotatable tub in the mode in which all motor coils are energized.

According to the above-described embodiment, the rate of deceleration after arrival of the motor 5 at the first reference speed is detected to serve as the decelerating time period or the measured time period t, whereby the degree of unbalance of the rotatable tub 3 is detected.

Consequently, the unbalanced state of the rotatable tub 3 can be reliably detected even when it is not detected by the mechanical detecting means such as a lever. It is detected that the degree of unbalance is relatively high when the measured time period t is 7 seconds or below. In this case, the speed control is performed so that the final speed of the rotatable tub 3 is set for the second target speed lower than the first target speed. Consequently, the dehydrating operation can be continuously performed with prevention of occurrence of abnormal vibration or abnormal noise.

FIG. 4 shows a second embodiment of the invention. In the second embodiment, only the main coil of the motor 5 is energized when the motor 5 is decelerated. The same effect can be achieved in the second embodiment as in the first embodiment.

FIG. 5 shows a third embodiment of the invention. In the third embodiment, both main and auxiliary coils of the motor 5 are deenergized when the motor 5 is decelerated.

The same effect can be achieved in the third embodiment as in the first embodiment.

FIGS. 6 to 8 show a fourth embodiment of the invention.

An electric motor 31 comprises a DC brushless motor. A motor drive control circuit 32 controlling drive of the motor 31 includes an inverter circuit or drive circuit 33 having switching elements on-off controlling motor coils and a control section 34 controlling an "on" duty ratio of each switching element to change a voltage applied to the motor 31 and controlling a commutation timing. The control section 34 serves as applied voltage changing means. The DC power from a DC power-supply circuit 35 is supplied to the drive circuit 33. FIG. 8 illustrates the characteristics of the brushless motor 31. Reference symbol L1 denotes a load line in the state of normal dehydrating rotation. On line L1, the motor 31 develops torque To at applied voltage V0 where V0 > V1 > V2 > V3. In this case, the motor speed is 2,000 rpm. The motor 31 develops torque T1, T2 and T3 when the motor speeds are 1 ,600 rpm, 1 ,200 rpm and 600 rpm respectively.

VoLtage V2 is applied to the motor 31 so that the dehydrating operation starts. Thereafter, the control circuit 36 serving as dehydration control means changes the applied voltage to voltage V3 lower than voltage V2 at time t1 when the rotational speed of the rotatable tub has reached 600 rpm, as shown in FIG. 7. In this case, the rotatable tub is decelerated from 600 rpm finally to 300 rpm as understood from the characteristics shown in FIG. 8.

The control circuit 36 then measures the time period t required for the speed of the rotatable tub to be decreased to 550 rpm. When the measured time period t exceeds the reference time period, for example, 7 seconds, the applied voltage is changed from V3 to V0 at time t3 and the final speed of the rotatable tub is set for 1,000 rpm, as shown by reference symbol K1 in FIG. 7. When the measured time period t is the reference time period or below, the applied voltage is changed from V3 to V1 at time t2 and the final speed is set for 920 rpm. The same effect can be achieved in the fourth embodiment as in the above-described first embodiment.

FIGS. 9 to 11 show a fifth embodiment of the invention.

The power-supply circuit for the motor 5 includes triacs 41 and 42 and a zero crossing detecting circuit 43 detecting zero crossing of the AC voltage. Each of the triacs 41, 42 is turned on by the control circuit 44 in any one of patterns shown in FIGS. 1OA-1OD, thereby controlling the phase of AC voltage. An energization angle a1 is the minimum in FIG. 10A and is sequentially increased in the order of FIGS. lOB, 10C and 10D. The voltage applied to the motor is changed from V3 to V2, V1 and V0 sequentially by the phase control in which the energizing angle is changed from coils to a2, a3 and aq sequentially. The motor 5 has the characteristic that its speed is 600 rpm, 1,200 rpm, 1,840 rpm and 2,000 rpm at the applied voltages V3, V2, V1 and V0 on the normal load line L1 of the dehydrating operation.

The control circuit 44 serving as the dehydration control means performs the phase control by the energizing angle 2 when the dehydrating operation is initiated. The energizing angle is switched from a2 to a1 when the motor speed has reached the first reference speed of 1,200 rpm.

Consequently, the voltage applied to the motor 5 is changed from V2 to V3 to be thereby decreased. When the motor speed has reached 1,100 rpm, the time period t between the first and second reference speeds is measured. Based on the measured time period t, the control circuit 44 controls the motor speed so that it is set for 2,000 rpm or 1,840 rpm in the same manner as in the first embodiment.

FIGS. 12 and 13 show a sixth embodiment of .the invention. The difference between the fifth and sixth embodiments will be described. Pulse width modulation (PWM) control means 54 is connected to one of the power-supply lines of the AC power source 51. The PWM control means 54 comprises a diode bridge circuit 52 and a switching transistor 53. A flywheel circuit 55 is provided between the power-supply lines of the AC power source 51 for discharging to the power-supply side energy stored in an inductance of the motor 5.

The control circuit 56 turns the switching transistor 53 on and off by the PWM control signal with an optional duty ratio as shown in FIG. 13A, whereby the PWM control signal is converted to a voltage having a waveform as shown in FIG. 13B so that the voltage Va applied to the motor is changed. The electrical arrangement in the sixth embodiment except for the motor speed changing means is the same as in the first embodiment.

FIGS. 14 to 16 show a seventh embodiment of the invention. Differing from the first embodiment, a powersupply voltage detecting circuit serving as power-supply voltage detecting means is provided for detecting the voltage of the power supply 12 and the control circuit 62 serves as reference time period setting means. The powersupply voltage detecting circuit comprises the control circuit 62 which is a part of the microcomputer and a voltage change output circuit 61 comprising resistors 63 to 67, a capacitor 68 and a diode 69 as shown in FIG. 15. The output circuit 61 supplies the control circuit 62 with the DC voltage whose value changes in accordance with the voltage fluctuation of the AC power supply 12.

The control circuit 62 detects the value of the powersupply voltage on the basis of the magnitude of the supplied DC voltage. The control circuit 62 then sets the reference time period in accordance with the detected voltage value, as shown in FIG. 16. The set reference time period is compared with the above-described measured time period t.

Based on the result of comparison, the control circuit 62 controls the final speed of the rotatable tub 3 in the same manner as in the first embodiment.

The motor torque is reduced in the decelerating period of the motor 5 under the auxiliary cbil energizing mode when the power-supply voltage is low. In this case, the rate of deceleration is large and accordingly, the second reference speed is rapidly reached. On the other hand, when the power-supply voltage is high, the motor torque is increased in the decelerating period. The rate of deceleration is small and accordingly, the second reference speed is reached late. Consequently, when the reference time period is set at a fixed value regardless of the power-supply voltage, the degree of unbalance cannot be accurately detected.

In the seventh embodiment, however, the reference time period is rendered short when the power-supply voltage is low while it is rendered long when the power-supply voltage is high, as understood from FIG. 16. Consequently, the degree of unbalance can be detected without influence of the magnitude of the power-supply voltage.

FIGS. 17 to 20 show an eighth embodiment of the invention. A temperature sensor serving as temperature sensing means is provided for sensing a temperature of water supplied to the rotatable tub 3. The control circuit 72 is provided with a function of reference time period setting means.

The temperature sensor 71 is mounted on a mounting plate 73 further mounted on the outer bottom of the waterreceiving tub 2. The mounting plate 73 is covered by a cover plate 74, as shown in FIG. 19. The temperature sensor 71 comprises a sensor circuit 77 including a resistor 75 and an analog-to-digital (A/D) converter 76, as shown in FIG. 18.- A temperature detection signal indicative of the sensed temperature is processed by the sensor circuit 77 and then supplied to the control circuit 72.

Upon receipt of the temperature detection signal, the control circuit 72 sets the reference time period in accordance with the temperature detection signal as shown in the following TABLE 2: TABLE 2 Temperature of water (OC) 10 or below 11 to 30 31 to 50 51 or above Reference time period (sec.) 7.5 7.0 6.5 6.0 As understood from TABLE 2, the set reference time period is rendered longer as the sensed water temperature becomes high. The set reference time period is compared with the time period t measured as described above by the control circuit 72. Based on the result of comparison, the control circuit 72 controls the final speed of the rotatable tub 3 in the same manner as in the first embodiment.

The above-described setting of the reference time period in accordance with the water temperature has the following purpose. The rigidity of the rotatable tub 3 usually formed of a synthetic resin is slightly reduced when the temperature supplied thereto is higher. Accordingly, the rotatable tub 3 is sometimes deformed temporally while it is being rotated in the dehydrating operation. In the eighth embodiment, since the reference time period is rendered short when the temperature of the water supplied to the rotatable tub 3 is high, the occurrence of rotation of the rotatable tub in the unbalanced state or abnormal vibration can be prevented and furthermore, the plastic deformation of the rotatable tub 3 due to its rotation under the unbalanced state can be prevented.

The reference time period may be set in accordance with the power-supply voltage and the temperature of the water supplied to the rotatable tub 3. In this case, the reference time period may be set in accordance with the graph of FIG. 20 showing a ninth embodiment of the invention.

The reference time period is rendered longer as the power-supply voltage becomes high in the condition that the temperature of the water supplied to the rotatable tub 3 is fixed. The reference time period is rendered shorter as the water temperature rises in the condition that the powersupply voltage is fixed. Accordingly, the reference time period is rendered considerably short when the power-supply voltage is low and the water temperature is high while it is rendered considerably long when the power-supply voltage is high and the water temperature is low.

A tenth embodiment will be described though it is not shown in the drawings. In the tenth embodiment, dehydrating t illustrative of the principles of the present invention and are not to be interpreted in a limiting sense. Various changes and modifications will become apparent to those of ordinary skill in the art. All such changes and modifications are seen to fall within the true spirit and scope of the invention as defined by the appended claims.

Claims

WE CLAIM:

1. A washing machine comprising: a) an electric motor for rotating a dehydrating tub; b) decelerating means for controlling a rotational speed of the motor so that the rotational speed of the motor is decreased after the rotational speed of the motor has been increased to a first reference speed upon start of a dehydrating operation; c) means for measuring a speed decreasing period of time necessitated for the motor speed to be decreased from the first reference speed to a second reference speed; d) means for changing a final dehydration speed of the motor to a second target speed when the measured speed decreasing period of time is equal to or below a reference period of time, the second target speed being lower than a first target speed corresponding to a normal condition; and e) means for increasing the motor speed to the second target speed after the motor speed has reached the second reference speed.

2. A washing machine according to claim 1, further comprising: power-supply voltage detecting means for detecting a power-supply voltage applied to the motor; and reference time period setting means for setting the reference period of time so that the reference period of time is lengthened as the voltage detected by the power supply voltage detecting means becomes high.

3. A washing machine according to claim 1 , further comprising: temperature sensing means for sensing a temperature of water supplied into the dehydrating tub; and reference time period setting means for setting the reference period of time so that the reference period of time is shortened as the temperature sensed by the temperature sensing means becomes high.

4. A washing machine according to claim 1, further comprising: power-supply voltage detecting means for detecting a power-supply voltage applied to the motor; temperature sensing means for sensing a temperature of water supplied into the dehydrating tub; and reference time period setting means for setting the reference period of time on the basis of results of detection of the power-supply voltage detecting means and the temperature sensing means.

5. A washing machine according to any one of claims 1 to 4, wherein the motor comprises a capacitor-start motor; both of a main coil and an auxiliary coil of the motor are energized when the rotational speed of the motor is controlled to be increased; and the auxiliary coil of the motor is energized when the rotational speed of the motor is controlled to be decreased.

6. A washing machine according to any one of claims 1 to 4, wherein the motor comprises a capacitor-start motor; both of a main coil and an auxiliary coil of the motor are energized when the rotational speed of the motor is controlled to be increased; and the main coil of the motor is energized when the rotational speed of the motor is controlled to be decreased.

7. A washing machine according to any one of claims 1 to 4, wherein the motor comprises a capacitor-start motor; both of a main coil and an auxiliary coil of the motor are energized when the rotational speed of the motor is controlled to be increased; and both of the main and auxiliary coils of the motor are deenergized when the rotational speed of the motor is controlled to be decreased.

8. A washing machine according to any one of claims 1 to 4, wherein the motor comprises a DC brushless motor and voltage changing means is provided for changing the voltage applied to the motor so that the rotational speed of the motor is controlled to be increased and decreased.

9. A washing machine according to any one of claims 1 to 4, further comprising phase control means for controlling the voltage applied to the motor so that the rotational speed of the motor is controlled to be increased and decreased.

10. A washing machine according to any one of claims 1 to 4, further comprising pulse width modulation (PWM) control means for controlling the voltage applied to the motor so that the rotational speed of the motor is controlled to be increased and decreased.

11. A washing machine substantially as herein described with reference to the accompanying drawings.