JPS63302787A - Changeover of pwm carrier frequency - Google Patents

Abstract

PURPOSE:To keep the thermal loss by a combination of a power transistor and a heat sink below the rated value, by changing over the carrier frequency in several stages in accordance with torque command value. CONSTITUTION:A torque command TREF is inputted into an A/D converter 13 to be converted to a digital value and is taken into a microprocessor 10'. In the microprocessor 10' the torque command value is always monitored by software processing, where a carrier changeover signal 15 is set at a high level below a certain value while it is set at a low level above a certain value, turning OFF an analogue switch 14. The cycle of a triangular wave output signal is thereby changed over in accordance with the torque command value, so that the carrier frequency of a PWM signal is changed over.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a motor control circuit having a current control loop using PWM pulse distribution.

[Conventional technology]

FIG. 4 is a block diagram of a conventional example of this type of motor control circuit.

This motor control circuit generates U-phase, ■-phase, and W-phase reference current signals 12a and 12 in accordance with a magnetic pole detection signal 11 from a rotary encoder 2 attached to an AC servo motor 1.
b% 12c is generated by the microprocessor 10, and torque commands T are generated by the multiplication type D/A converters 9m, 9b, and 9c, respectively. , and converts it into a current command, and calculates the deviation between each current command and the current detection signal detected by the current detectors 3a, 3b, and 3c.
U phase, ■ phase, W phase current deviation signals AU, AV, AW
and convert these into comparators 6a, 6b, and 6c, respectively.
The U-phase, ■-phase, and W-phase PWM signals MU are compared with the triangular wave signal generated by the triangular wave generation circuit 8.

MV and MW are generated and applied to the power transistor of the three-phase transistor bridge 4 via the base drive circuit 5 to drive the AC servo motor 1.

(Problems to be Solved by the Invention) In the conventional PWM drive type motor control circuit described above, when the carrier frequency is set high in order to improve the high-speed response of the control system, the switching loss of the power transistor inevitably increases. Larger, higher rated power power transistors or larger heat sinks must be chosen. Increasing the capacity of power transistors or heat sinks has a significant impact on the cost and appearance of the device, so
Typically, heat losses calculated from the motor circuit rating are determined by the tolerances of the economically selected heat sink and power transistor.
The switching loss, that is, the carrier frequency, is designed to be kept low so that it is below the capacitance value.

An object of the present invention is to provide a PWM carrier frequency switching method that can suppress thermal loss in a combination of a power transistor and a heat sink below a rated value.

(Means for Solving the Problems) The PWM carrier frequency switching method of the present invention changes the carrier frequency according to the value of the torque command so that when the torque command is large, the carrier frequency is small and when the torque command is small, the carrier is large. The carrier frequency is switched in several stages or is changed continuously.

(Function) If the generated loss of the power transistor is PC, the generated loss PC is divided into switching loss and steady state loss, and if each of these is psw l PON, then pc=
It can be expressed as PiIw◆PON. The switching loss Pa1m is proportional to the product of the collector current IC and the carrier frequency, and is a steady state loss P. N is collector current■.

can be calculated approximately proportionally.

FIG. 3 shows the relationship between the loss PC generated in the power transistor and the collector current 1c when the carrier frequency is changed.

Here, ICO is the current value when the carrier frequency is switched from f to f/2, pswt is the switching loss when the carrier frequency is f (however, 0≦Ic<Ico), P8V
2 is the switching loss (I
C≧Ico), PON is the steady state loss, Pct is the sum of pswt and PON (O≦Ic<Ico) -PC
2 is the sum of P9W2 and PON (Ic≧Ico).

As can be seen in Figure 3, a certain point IC of the collector current IC
By lowering the carrier frequency to 172 at O, the collector current can flow up to ICI. This means that if the maximum current flowing through the motor is 1c2, and the carrier frequency remains f, the current can only flow up to ICI because it exceeds the allowable limit of the loss generated by the power transistor, and the allowable loss of - rise However, by switching the carrier frequency, the transistor can be used efficiently.

Incidentally, an invention similar to the present application (Japanese Unexamined Patent Publication No. 60-66674) has been filed by the present applicant. This prior application performs smooth switching of the carrier frequency with respect to the frequency command signal Vrar. On the other hand, in the present application, the frequency conversion itself uses a classical analog circuit, and is not an innovative circuit.

By switching the frequency, the switching loss of the inverter is kept low during overload output. As a result, the power conversion efficiency of the inverter can be increased,
Therefore, it becomes possible to use an inverter with low thermal overload resistance. In other words, running costs can be lowered, and the inverter can be made smaller, lighter, cheaper, and more efficient. These effects can be achieved by detecting the overload state of the inverter and switching the PWM carrier frequency. In the present application, the torque command signal Ttaf is used to detect an overload state. Furthermore, since it is constructed with an analog circuit, there is no shock when switching frequencies.

(Example) Next, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of a motor control circuit to which the PWM carrier frequency switching method of the present invention is applied, and FIG. 2 is a circuit diagram of a triangular wave generation circuit 8.

This embodiment has the structure of the conventional example shown in FIG.
The microprocessor lO converts r into A/D.

An A/D converter 13 and an analog switch 14 are added, and the configurations of the microprocessor 10° and the triangular wave generating circuit 8' are different from the conventional example shown in FIG. This triangular wave generating circuit 8° is a well-known circuit, with comparators I,
The main components are an integral circuit consisting of an operational amplifier IC, resistors R1 and R2, and a capacitor C, and a resistor R2.
An analog switch 14 is connected in series with the resistor R1 and in parallel with the resistor R1. Here, assuming that the resistance values of the resistors R, , R, are equal to R, the period of the triangular wave output signal is proportional to R,xC=RxC when the analog switch 14 is on, and (R , xR, /(R, +R2)) xC
=RxC/2, and the carrier frequency is 1:2 when the analog switch 14 is on and when it is off.

Next, the operation of this embodiment will be explained.

Torque command T. f is input to the A/D converter 13, converted into a digital value, and taken into the microprocessor lO°. The microprocessor 10° constantly monitors the torque command value Tref through software processing, and when it is below a certain value, the carrier switching signal 15 is set to high level and the analog switch 14 is turned on, and when it is above a certain value, the carrier switching signal 15 is set to low level. to turn off the analog switch 14. Therefore, since the cycle of the triangular wave output signal changes according to the torque command value Tree, the carrier frequency of the PWM signal changes.

(Effects of the Invention) As explained above, the present invention allows the carrier frequency to be high in the torque range that is most frequently used in normal use by switching the carrier frequency according to the torque command. In addition to reducing noise, it is possible to select an element that is efficient in terms of the rating of the power transistor or a heat sink that is economical and extremely small in size, which has the effect of making the control device inexpensive and compact.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an embodiment of a motor control circuit to which the PWM carrier frequency switching method of the present invention is applied, Fig. 2 is a circuit diagram of a triangular wave generation circuit 8゛, and Fig. 3 is a diagram showing when the carrier frequency is switched. FIG. 4 is a block diagram of a conventional example, which shows the relationship between the loss PC generated in the power transistor and the collector current IC. 1-AC servo motor, 2... rotary encoder, 3a, 3b, 3c = current detector, 4-3-phase transistor bridge, 5... base drive circuit, 6a, 6b, 6c - comparator, 7a, 7b, 7cm--Current amplifier, 8°...triangular wave generation circuit, 9a, 9b, 9c,...multiplying D/A converter, to'
- Microprocessor, it - Magnetic pole detection signal, 12a... U-phase reference current signal, 12b-V-phase reference current signal, 12cm-W-phase reference current signal, 13-A/D converter, 14- ...Analog switch, +5-...Carrier switching signal.

Claims (1)

[Claims] In a motor control circuit that has a current control loop using PWM pulse distribution, the carrier frequency is set in several stages according to the value of the torque command so that when the torque command is large, the carrier frequency is small and when the torque command is small, the carrier becomes large. PWM carrier frequency switching method that switches or continuously changes.