JPS6020782A - Speed controller for motor - Google Patents

Abstract

PURPOSE:To suppress the switching loss of a power transistor by lowering the carrier frequency of a pulse width modulation proportionally to the increase in the load current of a motor. CONSTITUTION:An error amplifier 21 outputs an error voltage between the speed voltage Vf outputted from a speed detector 17 and the speed command voltage Vs. A comparator 24 compares a triangular carrier signal from a variable frequency oscillator 22 with the error signal and supples a pulse width modulation signal to a power transistor 16. The frequency of the triangular carrier signal from the oscillator 22 decreases when the load current to a DC motor 2 increases. Thus, the switching loss of the power transistor can be suppressed.

Description

[Detailed description of the invention] Industrial applications The present invention relates to constant speed control of a DC motor.

Configuration of the conventional example and its problems Figure 1 is a configuration diagram of a speed control circuit of a DC motor in the conventional example. 1 is a DC power supply, 2 is a DC motor, and 3 is a power transistor. 4 is a speed detector directly connected to the DC motor 2, and its output signal, the speed voltage, is 7V.
Connected to ± terminal.

The vs terminal of 6 inputs the speed command voltage. 8 is an error amplifier that compares and amplifies the speed voltage and the speed command voltage. An oscillator 9 sends out a triangular wave carrier with a constant amplitude and a constant frequency, and the oscillator output signal and the output signal of the error amplifier 8 are operated by a comparator 10 to create a pulse width modulation (hereinafter abbreviated as PWM) signal. Said PW
The M signal drives the base of power transistor No. 3 to control the DC motor current of No. 2. 511-1: Flywheel diode. FIG. 2 shows the signal waveforms of each part in the control circuit of FIG. This is an output signal, and this signal drives the base of the power transistor 30.

Next, the operation of the conventional example will be explained. When the motor is stopped and a positive speed command voltage is applied to the vs terminal, a large negative error voltage is generated in the output signal of the error amplifier 8 because the speed voltage vf has not yet been generated.
The power transistor 3 is fully energized by the output signal 0. As a result, the motor speeds up rapidly.

Next, when the speed voltage vf (negative voltage) becomes larger than the absolute value of the speed command voltage v8, a positive error voltage is generated in the output signal of the error amplifier 8, and the ON time ratio of the power transistor 3 is determined by the output signal of the comparator 1o. In order to reduce the speed, the motor speed decreases. In other words, as a control system, the speed command voltage v
8 and the absolute value of the speed voltage vf are controlled to be always equal, so when the speed command voltage v8 is constant, the motor rotates at a constant speed. Further, even if the motor speed changes due to a change in the motor load, the motor rotates at a constant speed due to the control system.

However, in the configuration of the control system described above, the switching loss of the ζ power transistor increases rapidly as the motor load increases and the current of the power transistor increases. Therefore, it was necessary to take measures such as using an element that has a low allowable dissipation of the power transistor, or increasing the cooling effect by increasing the radiation fin by '=1. As a result, the power transistor,
There were drawbacks such as an increase in the cost of heat dissipation fins, etc., and a large installation space for each component.

OBJECT OF THE INVENTION The present invention is intended to solve the above-mentioned drawbacks, and provides a low-cost and highly reliable motor speed control device by suppressing the increase in switching loss of the power transistor when the motor load current increases. It is something to do.

Structure of the Invention The present invention provides an error amplifier circuit that compares and amplifies a speed voltage of a speed detector that generates a voltage proportional to the rotational speed of a DC motor and a speed command voltage, and an output signal of the error amplifier circuit. PWM that creates a PWM signal by comparing it with a triangular wave carrier signal of constant amplitude sent out from a variable frequency oscillator whose frequency changes in response to changes in the DC motor load current.
By driving an arithmetic circuit and the base of a power transistor connected in series with the DC motor to a DC power source using the PWM arithmetic circuit output signal, the speed of the DC motor is controlled and the output frequency of the variable frequency oscillator is controlled. is configured such that the oscillation frequency decreases in proportion to the motor load current, and has the unique effect of minimizing the increase in switching loss of the power transistor even if the motor load current increases.

Description of examples Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

FIG. 3 is a configuration diagram of a speed control circuit for a DC motor in one embodiment. 1411'i is a DC power supply, 15 is a DC motor, and 16 is a power transistor. 17 is a speed detector directly connected to the DC motor 15, and its output signal, speed voltage, is connected to the vf terminal 20. The vs terminal of 19 inputs the speed command voltage. 21 is an error amplifier that compares and amplifies the speed voltage and the speed command voltage. 22 is a variable frequency oscillator, and the oscillation frequency of 22 is changed by the output signal of the motor load current detection resistor 23. That is, when the motor load 15 increases and the current increases, the output frequency of the oscillator output triangular wave carrier signal 22 decreases with a constant amplitude due to the output signal of the current detection resistor 23.

The oscillator output signal 22 and the output signal of the error amplifier 21 are used to create a PWM signal by 24 comparators. Said P
The WM signal drives the bases of 16 power transistors to control the 15 DC motor currents. 18 is a flywheel diode.

The operation of the speed control circuit configured as described above will be explained below.

When the speed command voltage is applied, since the speed voltage vf has not yet been generated, a large negative error voltage is generated in the output signal of the error amplifier 21, and the comparator 24 fully energizes the 16 power transistors. As a result, the motor speeds up rapidly. Next, when the speed voltage vf (negative voltage) becomes larger than the absolute value of the speed command voltage v6, the error amplifier 21
Since a positive error voltage is generated in the output signal of the comparator 24, the ON time ratio of the 16 power transistors is reduced by the output signal of the comparator 24, so that the motor speed is reduced. In other words, the control system is controlled so that the absolute values of the speed command voltage vs and the speed voltage vf are always equal, so the speed command voltage v
If s is constant, the motor will rotate at a constant speed. Further, even if the motor speed changes due to a change in the motor load, the motor rotates at a constant speed due to the control system.

Next, when the motor load increases and the motor load current increases while the motor is rotating at a constant speed, the variable frequency oscillator 2
Focusing on the frequency of the triangular wave carrier signal No. 2, the output frequency of the oscillator 22 is configured to decrease due to the output signal of the motor load current detection resistor 23 when the load current increases. Figures 4 and 5 show the output waveforms of each part during operation of the control system. Figure 4 shows the state before the motor load increases, and 25 is the output signal of the variable frequency oscillator, referred to as T1. It oscillates periodically. 2
6 is the output signal of the 21 error amplifiers. 27 is the output signal of the comparator, and this signal drives the base of the power transistor 16 to control the motor load current of 16. Figure 5 shows the waveforms of various parts when the load current of the motor 15 increases, and 28 is the output signal of the variable frequency oscillator.As the motor load current increases, the oscillation period becomes T2 (T1
2), and it can be seen that the oscillation frequency has decreased. 29
21 is the output signal of the error amplifier, 30 is the output signal of the comparator, and the base of the power transistor 16 is driven by this signal.

As described above, when the motor load current increases, the carrier frequency of PWM decreases, so it becomes possible to suppress power transistor switching loss due to an increase in the load current. Generally, when the PWM carrier frequency decreases, the ripple of the motor load current increases, but in the embodiment of the present invention, the PWM carrier frequency decreases as the load current increases. Therefore, when the current is large, an increase in IJ sople due to intermittent current etc. does not occur due to the influence of the electric time constant of the motor, so this is not a big problem.

Effects of the Invention As described above, the present invention has the effect that when the motor load current increases, the PW
Since the frequency of the triangular wave carrier signal, which is the reference frequency of M, is configured to decrease, it is possible to suppress the switching loss of the switching power transistor for motor current control when the load current increases, and as a result, the capacity of the power transistor is reduced. This can be expected to reduce heat dissipation and reduce the size of heat dissipation fins, and the practical effects of this are significant.

[Brief explanation of drawings]

Figure 1 is a circuit diagram of motor speed control in a conventional example;
Figures a and b are signal waveform diagrams of each part in Figure 1, Figure 3 is a control circuit diagram in an embodiment of the present invention, Figures 4 and 5 a,
b is a signal waveform diagram of each part in FIG. 3; 14...DC power supply, 15...DC motor. 16... Power transistor, 17... Speed detector, 21... Error amplifier, 22...
Variable frequency oscillator, 23...Current detection resistor, 24
...Comparator. Name of agent: Patent attorney Toshio Nakao and 1 other person 1st
Figure 2 Figure 9 Figure 3 Figure 4

Claims (2)

[Claims] (1) An error amplification circuit that compares and amplifies the speed voltage of a speed detector that generates a voltage proportional to the rotational speed of the DC motor and a speed command voltage, and the output signal of the error amplification circuit and the DC motor load current. a pulse width modulation calculation circuit that creates a pulse width modulation signal by comparing the triangular wave carrier signal of constant amplitude sent out from a variable frequency oscillator whose frequency changes in response to changes in the frequency of the oscillator; A motor speed control device for controlling the speed of the DC motor by driving the base of a power transistor connected to the pulse width modulation calculation circuit output signal. (2) The motor speed control device according to claim 1, wherein the oscillation frequency of the variable frequency oscillator decreases in proportion to the load current of the DC motor.