Universal control unit for polyphase motors. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Electric motors

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There is a huge variety of asynchronous, stepper, collector and all kinds of high-frequency multi-phase motors operating at a frequency of 400 ... 1000 Hz, which cannot be made to work efficiently from a single-phase network. However, modern electronics make this quite easy to do. In order to make the rotor of a polyphase motor rotate, it is necessary to apply a strictly defined sequence of pulses to its windings, i.e. create a rotating magnetic field. But how to do this if, apart from a single-phase network, there is nothing. A three-phase motor designed for 380 V / 50 Hz, of course, can also be started from a single-phase network using phase-shifting capacitors, but its efficiency will be very low, and there is nothing to dream of changing the speed of an asynchronous motor. Stepper and high-frequency motors cannot be started at all.

To solve all these problems, a universal control unit was created. By simple reprogramming of the ROM, it is possible to change the algorithm of the output keys, and hence, adaptation to any engine. Consider the operation of the main unit, the scheme of which is shown in Fig.1.

(click to enlarge)

A master oscillator at a frequency of 1 kHz is assembled on the D1: 1, D2: 2 chip. Its frequency is predetermined mainly by the engine speed and the amount of ROM used. To form steep fronts, the pulses from the generator pass through two Schmitt triggers.

On the front of the pulse from the output D2: 1 switches counters D3-D5. On the decline of the same pulse, inverted by the D2: 2 chip, data is overwritten from the ROM into the register on the D7 chip. When the device is turned on, the counters are set to zero by the C2R3 chain. In the process of operation, the counting coefficient depends on in which memory cell of the D7 discharge of the D6 chip the log. "1" will be written, which will predetermine the reset time of the counters. The D7 register is necessary so that the pulses that occur at the time of switching ROM addresses do not affect the algorithm of the keys. The number of counters depends on the number of addresses used by the D6 chip, and can vary from one to ten. Loads up to 7...20 mA can be connected directly to the outputs of register D30. In the case of using a larger load, it is necessary to use buffer elements, for example, a D8 chip.

Now let's talk about the output keys and the algorithm for the operation of different engines. To begin with, consider a collector motor operating from a constant voltage of 27 V. Its switching circuit is shown in Fig. 2.

This is the simplest transistor key assembled on VT1. This transistor has a fairly large gain and a diode connected between the emitter and collector. Therefore, its base through a current-limiting diode can be connected directly to the output of the D7 microcircuit (Fig. 1).

Fig. 3 shows a graph explaining the operation of the motor in pulse-width modulation (PWM) mode.

If the transistor for a period of time T will be more in the closed state, then the engine speed will be minimal, and vice versa. At the end of the period, it is necessary to write the log "8" in the D1 discharge in order for the cycle to repeat. If you need to create a complex speed mode, for example: for 1 s, the speed should be maximum, for the next 10 s - at the level of 20%, for the next 5 s - at the level of 60%, etc., then reset the counter must be written to end of the cycle of the entire adjustment process, and select the accuracy of the timing by changing the frequency of the master oscillator. You can install your own key with an engine or load on each data bus if their common cycles match.

To control a stepper motor, it is necessary to use three or six keys, depending on the motor, draw a motor control algorithm, calculate the required number of pulses per motor cycle and program the microcircuit. The motor speed can be controlled by changing the frequency of the master oscillator. Here is a diagram (Fig. 4), an algorithm (Fig. 5) and a program (Table 1) for a motor with three windings.

Table 1

(click to enlarge)

Consider the operation of a three-phase motor. The block diagram of connecting the motor with a star is shown in Fig.6.

Various key schemes will be given later. The first key is controlled from the data bus D0, the second - D1, and so on. If the engine is designed for a frequency of 400 ... 1000 Hz, then a simple algorithm is suitable for it, shown in Fig. 7.

In the algorithm, the moment of turning on the keys must be shifted by time t. For different keys, this delay is different and ranges from several microseconds to several milliseconds. It is necessary so that through currents do not occur through the transistors of the keys. To control asynchronous motors designed for a frequency of 50 Hz, it is necessary to introduce PWM modulation with a frequency of 10 ... 20 kHz.

Figure 8 shows a positive half-wave of a sine wave and its approximate filling with PWM pulses.

To keep the motor power unchanged at different frequencies, it is necessary to calculate the total half-wave area and bring the PWM modulation area into line. For low engine speeds, this is fraught with the installation of ROM chips with a very large volume of cells and, accordingly, painstaking calculation of their contents. The general picture of the PWM control algorithm for a three-phase motor is shown in Fig. 9, and the ROM firmware with PWM modulation at a frequency of 2 kHz is shown in Table 2. The engine speed is 60 rpm.

(click to enlarge)

To control the engine, I have tried various types of power switches. All have their own merits and demerits.

Figure 10 shows the simplest circuit without decoupling from the mains voltage and a small supply voltage. On transistors VT1-VT2, resistors R1-R3 and diode VD1, a key for a positive half-wave is assembled. On the transistor VT3 - the key of the negative half-wave.

Figure 11 shows a bipolar transistor circuit. Its disadvantage is that each key requires an additional unstabilized 24 V power supply.

(click to enlarge)

Figure 12 shows a field-effect transistor circuit with optocoupler isolation. To open field-effect transistors, a large current is not needed, so the keys are powered from the same circuit as the engine.

The power supply circuit with optocoupler isolation for this switch is shown in Fig.13.

All switches, in which optocouplers are used, have one significant drawback: with an increase in the modulation frequency, the pulse fronts are tightened.

Perhaps the most optimal at the moment is the use of a specialized three-phase driver chip IR2130, IR2131 from International Rectifier. It provides current protection, which disables all keys and generates an error signal. The microcircuit is a driver of six keys - IGBT or MOS.ET transistors. When using IR.740 transistors, motor power up to 5 kW can be controlled.

Details about the microcircuit and the principles of motor control can be found in [1]. The driver inputs are consistent with TTL logic. It is possible to coordinate it with the above control unit.

References:

1. Obukhov D., Stenin S., Strunin D., Fradkin A. Electric drive control module based on PIC16C62 microcontroller and IR2131 driver//Chip News. - 1999. - No. 6.

Author: S.M. Abramov

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