Thyristor speed controller for electric motors. Scheme, description

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Thyristor speed controller for electric motors. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Electric motors

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In the electric drive of electric transport, in automation, telemechanics and other fields of technology, DC electric motors are widely used. The main advantages of DC machines over asynchronous ones are a large starting torque and the possibility of smooth speed control.

The mechanical part of DC motors consists of an armature and a stator. The armature (rotor) is the rotating part of the electric motor. A collector is installed on the anchor. Magnetic poles with excitation windings are fixed in pairs on the stator.

The number of poles can be different (the maximum number has stepper motors of computer hard drives). In small electric motors, permanent magnets are used instead of parallel windings on the stator to create a magnetic field.

The magnetic field of the rotor windings interacts with the magnetic field of the stator windings or with permanent magnets and creates a torque. DC electrical machines are reversible, i.e. can work both in the mode of an electric motor and in the mode of a DC generator with mechanical rotation of the armature shaft.

The speed of rotation of electric motors is affected by the parameters of the excitation windings.

When you turn on the adjusting resistance (rheostat) in series with the armature and the excitation winding, you can achieve different performance characteristics of electric motors (from natural to steeply falling). Such characteristics are usually used in traction motors of urban electric vehicles.

With a decrease in the resistance of the rheostat, the speed of the electric motor increases, although at present rheostats are practically not used for speed control, since thyristor circuits for controlling the speed of electric motors are more effective.

In case of forced braking or stopping of electric motors, a mode of recovery (return) of electricity back to the power system is created, which is very economically beneficial. This is achieved by appropriately turning on the thyristors using an electronic device.

It is not recommended to turn on a DC electric motor with a series winding without load, as this can lead to an uncontrolled increase in the speed of the electric motor and its breakdown. To prevent this from happening, on the stator of the electric motor, in addition to the windings of series excitation, windings of parallel excitation are also placed.

Additional windings allow you to change the magnetic flux and adjust the speed of the electric motor. For example, in car generators, an electronic field current control circuit can be used to set the recommended battery charge current.

Adjusting the rotation speed by changing the armature current is used in low-power electric motors, and in powerful ones, the current of the parallel excitation winding changes.

Reversing (changing the direction of rotation) is achieved by reversing the armature supply or voltage on the parallel field winding.

You can regulate the speed of electric motors with electronic devices made on thyristors or transistors. The first option is more preferable, since the thyristors, due to the pulse current switching, heat up less during operation. For a reliable start of thyristors, frequency filling of the control signal is provided.

The proposed device for thyristor speed control of a DC motor (Fig. 1) consists of:

control signal generator;
amplifier;
switching element (thyristor);
negative feedback circuits for speed stabilization under variable load.

(click to enlarge)

The multivibrator on the DA1 analog timer chip operates as a rectangular pulse generator. The internal structure of the timer contains two comparators connected to inputs 2 and 6, an RS flip-flop, an output amplifier, and a switching transistor for discharging an external capacitor. The output 7 of the timer is connected to the collector of the internal reset transistor, the emitter of which is connected to the common wire. The state of this transistor is identical to the state of output 3 (open when the timer output is at zero potential). In this circuit, pin 7 of DA1 is used as an auxiliary output with increased load capacity to indicate the status of the timer. LED HL1 is lit when the internal transistor is turned off, indicating that output 3 of the timer is high.

Charging capacitor C1 occurs at a high level at output 3 DA1 through resistors R2 and R3. When the voltage on C1 is equal to 2/3 Upit, the internal trigger DA1 switches output 3 to zero, the capacitor is discharged through R2 and R3, then the output level changes again, i.e. at output 3, rectangular pulses are formed.

Pin 5 DA1 in the timer is used to control the circuit. A capacitor C2 and an adjustable zener diode DA2 with a load resistor R5 are connected to it. The signal to the control input 1 DA2 comes from the setting resistor R10, the capacitor C5 smoothes the voltage ripple generated by the motor armature during rotation. The VD2-C7-R9-C6 circuit reduces the effect of back-emf on the sparking of the collector and the operation of the thyristor. With an increase in engine speed, the voltage across capacitor C8 increases, the DA2 chip opens and shunts output 5 of DA1. the frequency of the generator on the timer drops and the speed of the electric motor M1 decreases. LED HL2 in the emitter circuit VT1 indicates the status of the device circuit.

Power to pin 8 DA1 is supplied from a stabilized source on the analog stabilizer DA3. which reduces the effect of powerful current surges when switching the electric motor on the operation of the timer. Diode VD1 protects DA3 from reversed power supply. The power amplifier for starting the thyristor is made on the transistor VT1.

The lithium source is made on a power transformer T1 with a powerful diode bridge VD3. To reduce interference from the thyristor regulator, capacitor C9 is installed on the power grid.

The adjustment of the circuit begins with a power check. The engine of the resistor R10 must also be in the lower (according to the diagram) position. When adjusting the speed with resistor R3, the stable rotation of the motor shaft is checked. When the feedback voltage is increased by the resistor R10, the feedback action on the electric motor inhibited by the mechanical load of the shaft is checked. The speed of an electric motor with feedback must be higher than without it. The differential voltage is supplied to the resistor R10 from the anode of the thyristor VS1. by changing the delay of the generator pulses relative to the beginning of each half-cycle of the mains voltage. Using R10, the optimal feedback voltage value is set.

The voltage at the anode of the thyristor VS1, while it is closed, is equal to the difference between the supply voltage and the voltage created by the rotating armature of the motor M1. Reducing the speed under load leads to an increase in the voltage applied to the motor and vice versa. Diode VD2 eliminates reverse current. generated by the rotation of the motor.

Resistors in the device are used S2-ZZN S1-4. Timer DA1 is replaced by an analogue of the 555 series. Transistor VT1 for a reliable start of the thyristor must be with a gain of more than 100. Thyristor VS1 is designed for a current of more than 10 A at a voltage of 100 V.

You can use thyristors of types KU202, T106 T112 T122 T137, VT138-152, MCR-25. The diode bridge consists of two avalanche diodes for a current of more than 10 A, but diode types will also work. D302.305 KD203 KD206 KD213B. Radiators should be installed on the thyristor and diodes, if necessary. The transformer is selected based on the power of the electric motor. Suitable - types of TN, CCI, TS and similar.

The device is assembled on a printed circuit board, the drawing of which is shown in Fig.2.

Thyristor motor speed controller

The board is designed for thyristors in different cases (plastic and metal). The GVS1 point is connected by a jumper to the control electrode of the thyristor, "katod" - to the cathode.

Author: V.Konovalov

See other articles Section Electric motors.

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Comments on the article:

Viktor Timchenko
Made this regulator. There were problems with the setup. Tell me how to set up the device correctly and is it possible to set it up without an electric motor for a 24 V incandescent lamp? And one more question: when setting up the device, the al2 LED burns out.

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