JPS6181197A - Control circuit of induction motor - Google Patents

Abstract

PURPOSE:To effectively drive by varying an output voltage on the basis of the displacement of a current phase, thereby automatically regulating the drive voltage of a motor for the variation in the load of the motor or the alteration of the motor. CONSTITUTION:If a motor 7 is in a no load state, a signal (f) input to an output voltage controller 15 is preset to become substantially zero. If a load is applied to the motor 7, a current phase is displaced as compared with the no load case, a momentary current value is sampled by the displaced phase at the current sampling time signal c time, the output signal (e) of a current amount detector 13 which exhib its the current value is input though a limiter 14 to the controller 15 to alter the pulse width, thereby raising an output to output a signal to a transistor base driver 16 to supply the raised voltage to the motor 7.

Description

[Detailed Description of the Invention] "Field of Industrial Application" The present invention relates to a control circuit for an induction motor that drives the induction motor by supplying alternating current power of variable voltage and frequency, and in particular improves the output torque characteristics of the induction motor. This invention relates to a control circuit for an induction motor.

"Prior Art" In general, the output torque of an induction motor is explained using its equivalent circuit.As shown in Fig. 4, the equivalent circuit connects a secondary side impedance z2 and an excitation admittance YO in parallel, and connects both end points A of this parallel circuit. , B is replaced with a circuit in which an input voltage V is applied through a primary impedance Z1, and when the power supply frequency is f and the voltage applied between points A and B is E, the output torque T is: Therefore, when an induction motor is driven with variable voltage and variable frequency AC power and operated at variable speed, it is expressed as (E/f).
It is necessary to keep the value of f ) approximately constant, but this (
In order to supply the input voltage V while keeping the value of E/f) constant, it is necessary to take into account the voltage drop due to the negative side impedance Zl, and especially when driving at a low frequency, the primary side impedance Since the voltage drop due to the resistor R1 of Z1 occupies a larger proportion than other drops, the input voltage (2) was previously determined to be high when driving at a low frequency.

"Problem to be Solved by the Invention" However, if the voltage drop caused by the resistor R1 is fixedly corrected in advance, when the load of the motor changes or the motor is changed, the current flowing through the negative side impedance Z1 As a result, the voltage drop also changes, causing the motor to be in an overexcited state or in the completely opposite coarsely excited state, resulting in inconvenient phenomena such as an increase in the magnetic noise of the motor, overheating of the motor, or insufficient torque. Moreover, the relationship between the output torque T and the rotational speed Fr of the induction motor is as follows when the power supply frequency f is large, as shown by the dotted line in the characteristic diagram of FIG. In this case, the predetermined torque T1
In order to obtain the rotational speed Fr, the rotation speed Fr is obtained at a value Frl that is slightly smaller than the value F1 of the power supply synchronous speed F, whereas when the power supply frequency f is small, that is, when the power supply synchronous speed F of the induction motor is a small value F2. In order to obtain a predetermined torque TI, the rotational speed Fr is obtained at a value Frl that is much lower than the value F2 of the power supply synchronous speed F, and the slip S in both
were significantly different, and the rotational speed of the motor could not be controlled sufficiently. Therefore, depending on the rotation speed of the induction motor, the input voltage
It was necessary to compensate for the slip S by adjusting the Note that the slip S is generally expressed as ((F-Fr)/F).

"Means for Solving the Problem" The present invention provides a control circuit for an induction motor that supplies variable voltage, variable frequency alternating current power to drive the induction motor. and an output voltage control circuit that varies the output voltage of the inverter circuit in response to a phase shift in a loaded state using the shift as a reference.

"Function" With the above-mentioned configuration, the induction motor is efficiently driven by reducing the possibility that the induction motor becomes over-excited or becomes the completely opposite coarse excitation state in response to a change in the load of the induction motor or a change in the induction motor. It is possible to compensate for slippage with respect to the variable frequency and to sufficiently control the rotational speed of the motor.

- "Example" The present invention will be described below based on the drawings as an example. In general, induction motor control methods are divided into current type and voltage type, and among voltage type, pulse amplitude control (PAM control) and pulse duration control (PW
M control), and within pulse control (PWM control), it is divided into unequal pulse width control and equal pulse width control. Width control (PWM control) - Unequal pulse width control. FIG. 1 shows a circuit block diagram, in which 1 is a three-phase AC power supply and 2 is a converter circuit composed of a diode bridge 3-, which rectifies the AC voltage of the AC power supply 1 and outputs a pulsating current. do. 4 is a smoothing capacitor connected to the output of the converter circuit 2, which smoothes the ripple current that is the output voltage of the converter circuit 2, and converts it into a DC voltage. Reference numeral 5 denotes an inverter circuit consisting of transistors 6 and 6. The transistors 6 and 6 are connected in series to form an arm, and three sets of these arms are connected in parallel to form an inverter circuit, and the output voltage of the smoothing capacitor 3 is input to each arm. Transistor 6 of each arm
A three-phase AC power source consisting of voltages with unequal pulse widths is generated by performing switching operations with a phase shift of 120 degrees. Reference numeral 7 denotes an induction motor driven by a three-phase AC voltage output from the inverter circuit 5. The switching operation of the inverter circuit 5 is controlled by the control circuit 8, and the output three-phase AC voltage is made into a variable voltage and variable frequency to drive the induction motor 7 at variable speed. A current detector 9 detects the one-phase line current supplied to the induction motor 7, and outputs a waveform corresponding to the current flowing in the negative phase.

Next, the above-mentioned control circuit 6 will be explained.
1 is a setting circuit 10 that variably sets the frequency, and outputs a continuously variable DC voltage as a setting signal (a). Reference numeral 11 denotes a sine wave pulse train generation circuit, which inputs the setting signal (a) of the setting circuit 10 and generates a signal in an unequal pulse whose output signal frequency changes in proportion to the magnitude of the voltage of this setting signal (a). Output (b). 12 is a current sampling time generation circuit, which inputs the output signal (b) of the sine wave pulse train generation circuit 11 and generates this signal (
A current sampling time signal (C) is generated in synchronization with b). In the embodiment, when the induction motor 7 is at the zero position in the current waveform under no load, that is, when the current is at zero cross, an extremely short pulse signal is outputted to the sampling signal (c) of the time generating circuit 12 and the current detector. 9 detection signal (
d), and when the sampling signal (C) is input, the current detector 9 detects the value of the detection signal (d), holds that value until the next detection, and outputs the signal (e). A limiter circuit 14 inputs the output signal (e) of the current amount detection circuit 13, limits the signal (e) so that it does not exceed a predetermined value, and outputs a signal (f). 15 is an output voltage control circuit, which outputs the output signal (b) of the sine wave pulse train generation circuit 11;
and the output signal (, f) of the limiter circuit 14 are input, and the pulse width is varied by adding the voltage bone corresponding to the output signal (f) of the limiter circuit 14 to the output signal (b) of the sine wave pulse train generation circuit 11. Then, a signal (g) of unequal pulse width with the output voltage adjusted is output. 16 is a transistor base drive circuit which inputs the output signal (g) of the output voltage control circuit 15 and drives the transistors 6 and 6 of the inverter circuit 5.
- and are controlled respectively.

Incidentally, FIG. 2 shows the signal states of each part in the above-mentioned circuit block diagram, and (A) shows the output signal (a) of the setting circuit 10,
(B) is the output signal (b
), (C) is the output signal (C) of the current sampling time generation circuit 12, (D) is the output signal (d) of the current detector 9
, (E) are the output signals (6), (F
) shows the output signal (f) of the limiter circuit 14, and (G) shows the output signal (g) of the output voltage control circuit 15. This figure 2 shows the induction motor in a normal loaded state, and (
In D) to (G), in addition to the solid line when the induction motor is in the normal loaded state, there is a solid line when the induction motor is in the no-load state (
(dashed line) and when the induction motor is overloaded (dotted line).

To explain this operating state, when the induction motor 7 is in a no-load state, the setting signal of the setting circuit 10 ( Corresponding to a), the sine wave pulse train generation circuit 11 outputs a signal (b) in the unequal pulse having a predetermined frequency, and further synchronizes with the output signal (b) of the sine wave pulse train generation circuit 11 to output a predetermined signal (b). Current sampling time signal with phase shift (
c), and also generates a current sampling time signal (C)
is input to the current amount detection circuit 13, and the instantaneous value of the phase current of the induction motor 7 is outputted using the detection signal (d) output from the current detector 9 when the current sampling time signal (C) is output. In this case, since there is no load, the instantaneous value of the current is approximately zero, as set in advance. Furthermore, the output signal (e) of the current amount detection circuit 13 is input to the limiter circuit 14, and the output signal (f) of the limiter circuit 14 is input to the output voltage control circuit 15. Furthermore, the output voltage control circuit 1
Since the signal input to the transistor base drive circuit 5 is approximately zero, the voltage is determined based on the proportional relationship between the frequency and the voltage set in advance, and the signal (g) is output to the transistor base drive circuit 16. This output signal (g) causes the transistor base drive circuit 16 to be connected to the inverter circuit 5 and the induction motor 7.
When is under load, the current sampling time signal (
c) is input to the current amount detection circuit 13, and when the current sampling time signal (C) is output, the instantaneous value of the phase current of the induction motor 7 is output using the detection signal (d) outputted from the current detector 9. However, at this time, due to the load being applied to the induction motor 7, the electrical resistance increases, so the phase shift is smaller than when there is no load, and the shift occurs at the current sampling time signal (C). An output signal (e) of the current amount detection circuit 13 indicating an instantaneous current value that starts at a phase corresponding to that phase and indicates this instantaneous current value.
is input to the output voltage control circuit 15 via the limiter circuit 14, and the output voltage control circuit 15 outputs the output signal (e).
That is, the output voltage is increased by varying the unequal pulse according to the current value, and a signal (g) is output to the transistor base drive circuit 16, and the increased AC voltage is transmitted to the induction motor 7 via the inverter circuit 5. supply

It should be noted that the method of varying the unequal pulse width works to improve the ratio and reduce the slip within the same cycle.

In addition, in the case of overload, as shown in Figure 2 (dotted lines in (D) to (G)), the increase in output torque or slippage cannot yet follow the increase in supply voltage, and current sampling is not performed. If the instantaneous current value from the current detector 9 continues to be high at the time of the time signal, the limiter circuit 14 stops the voltage from rising, thereby reducing the possibility that the induction motor will become abnormally overexcited and burn out.

Furthermore, when the setting signal (a) of the setting circuit 10 is changed, the setting circuit 1
In response to the setting signal (a) of 0, the sine wave pulse train generation circuit 11 generates a signal (b) of unequal pulse width with a predetermined frequency.
The induction motor 7 is driven by outputting power of variable voltage and frequency corresponding to each output. It should be noted that when the frequency is high, the voltage has already increased, so the increase in voltage is not as great as when the frequency is low, depending on the size of the load.

In this way, by automatically changing the drive voltage depending on the presence or absence of a load or the size of the load, the 31st! As shown by 1 (indicated by the solid line), it is possible to reduce the change in slip due to the height of the frequency, and it is now possible to compensate for the slip for the variable frequency and to sufficiently control the rotational speed of the motor.

As described above, in the embodiment, voltage type-pulse control (PWM
Control) - Compare the output voltage of the current detector that detects the line current supplied to the induction motor with the unequal pulse control for the phase shift with the drive voltage waveform, and use the phase shift near the no-load state as a reference. The above proposed system efficiently drives the induction motor by varying the output voltage of the inverter circuit in response to the phase shift under load, thereby increasing the output torque and reducing slippage.
Of course, any voltage type can be applied.

"Effects of the Invention" As described above, the present invention includes a current detector that detects the line current supplied to the induction motor, and the output voltage of this current detector is inputted to compare the phase shift with the drive voltage waveform. Since an output voltage control circuit is provided that varies the output voltage of the inverter circuit in response to the phase shift based on the phase shift in the vicinity of the state, the induction motor Since the driving voltage of the motor is automatically adjusted, the induction motor is less likely to be in an over-excited state or the completely opposite coarse-excited state, and the induction motor can be driven efficiently. This has the effect of sufficiently controlling the rotation speed of the electric motor.

[Brief explanation of drawings]

Figure 1 is a circuit block diagram, Figure 2 is a signal diagram of each part of the circuit block diagram in Figure 1, Figure 3 is a characteristic diagram of rotational speed and torque, and Figure 4 is an equivalent circuit diagram of an induction motor. be. 1 - AC power supply, 2 - converter circuit, 3 - diode bridge, 4 - smoothing capacitor, 5 - inverter circuit, 6 - transistor, 7 - induction motor, 8 - control circuit, 9 - current detector , 10--setting circuit, 11-sine wave pulse train generation circuit, 12-current sampling time starting circuit, 13-current amount detection circuit, 14-
-Limiter circuit, 15-/Output voltage control circuit, 16-
Transistor-based drive circuit.

Claims (1)

[Claims] (1) A DC voltage is applied to an arm configured by connecting a pair of switching elements in series, and the AC voltage output between the switching elements is applied to an inverter circuit that supplies the induction motor and the switching element of this inverter circuit. An induction motor control circuit comprising a control circuit that supplies a signal and varies the driving frequency and driving voltage of the induction motor substantially proportionally, the control circuit comprising: a current detector that detects a line current supplied to the induction motor; , input the output voltage of this current detector, compare the phase shift with the drive voltage waveform, and calculate the output voltage of the inverter circuit corresponding to the phase shift in the loaded state, using the phase shift near the no-load state as a reference. 1. A control circuit for an induction motor, comprising: an output voltage control circuit that varies the output voltage.