DC motor control. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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DC motor control. 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 many machine tools, DC electric motors (EM) are used. They easily allow you to smoothly control the rotational speed by changing the constant component of the voltage on the armature winding, with a constant voltage of the excitation winding (0V).

The electrical circuit (Fig. 1) will be useful to those who assemble the necessary machine or device with an electric drive for themselves. The scheme allows you to control an electric motor with a power of up to 5 kW.

Fig.1 (click to enlarge)

Powerful DC EMs have several features that must be considered:

a) it is impossible to apply voltage to the EM armature without applying the rated voltage (usually 180 ... 220 V) to the field winding;

b) in order not to damage the motor, it is unacceptable to immediately apply the rated voltage to the armature winding when turned on, due to the large starting current, which is ten times higher than the rated operating current.

The above scheme allows you to provide the necessary mode of operation - smooth start and manual setting of the desired speed of the EM.

The direction of rotation will change if the polarity of the wires on the field winding or armature is reversed (this is only done when the EM is turned off).

Two relays are used in the circuit, which allows automatic protection of circuit elements against overload. Relay K1 is a powerful starter, it eliminates the possibility of turning on the EM when the initial speed set by resistor R1 is not zero. To do this, a lever is attached to the axis of the variable resistor R1, connected to the button SB2, which closes (with a lever) only at the maximum resistance value (R1) - this corresponds to zero speed.

When contacts SB2 are closed, relay K1, when the START button (SB1) is pressed, will turn on and its contacts K1.1 will self-lock, and contacts K1.2 will turn on the drive.

Relay K2 provides overload protection in the absence of current in the EM excitation winding circuit. In this case, contacts K2.1 will turn off the power to the circuit.

The control circuit is powered without a transformer, directly from the network through the resistor R3.

The value of the effective value of the voltage on the armature winding is set by changing the opening angle of the thyristors VS1 and VS1 by the resistor R2. Thyristors are included in the bridge arms, which reduces the number of power elements in the circuit.

A pulse generator is assembled on a unijunction transistor VT2, synchronized with the period of mains voltage pulsation. Transistor VT1 amplifies the current pulses, and through the isolating transformer T1 they are fed to the control outputs of the thyristors.

When performing the design, thyristors VS1, VS2 and diodes VD5, VD6 must be installed on a heat sink plate (radiator).

Part of the control circuit, highlighted in the figure by a dotted line, is placed on the printed circuit board (Fig. 2).

DC motor control
Ris.2

Fixed resistors are used of type C2-23, variable R1 - type PPB-15T, R7 - SP-196, R3 - type PEV-25. Capacitors C1 and C2 of any type, for an operating voltage of at least 100 V. Rectifier diodes VD1 ... VD4 for a current of 10 A and a reverse voltage of 300 V, for example D231 D231A D232, D232A, D245, D246.

The pulse transformer T1 is made on an M2000NM ferrite ring of size K20x12x6 mm and wound with a PELSHO wire with a diameter of 0,18 mm. Winding 1 and 2 contain 50 turns each, and 3 - 80 turns.

Before winding, the sharp edges of the core must be rounded off with a needle file to prevent punching and shorting of the turns.

When the circuit is initially turned on, we measure the current in the excitation winding circuit (0V) and, according to Ohm's law, calculate the value of the resistor R2 so that relay K2 operates. Relay K2 can be any low-voltage (6 ... 9 V) - the lower the response voltage, the better. When choosing a resistor R2, it is also necessary to take into account the power dissipated on it. Knowing the current in the 0V circuit and the voltage across the resistor, it is easy to calculate it using the formula P = UI. Instead of K2 and R2, it is better to use special current relays manufactured by the industry, but they are not available to everyone due to their narrow scope. It is easy to make a current relay on your own by winding about 20 turns on a larger reed switch with a PEL wire with a diameter of 0.7 ... 1 mm.

To set up the control circuit, instead of the anchor circuit of the motor, we connect a lamp with a power of 300 ... 500 W and a voltmeter. It is necessary to make sure that the voltage on the lamp changes smoothly with resistor R1 from zero to maximum.

Sometimes, due to the spread of the parameters of the unijunction transistor, it may be necessary to select the value of the capacitor C2 (from 0,1 to 0,68 μF) and the resistor R7 (R7 sets the maximum voltage at the load at the minimum value of the resistance R1).

If, with proper installation, the thyristors do not open, then it is necessary to swap the conclusions in the secondary windings of T1. Incorrect phasing of the control voltage coming to thyristors VS1 and VS2 cannot damage them. For the convenience of controlling the operation of thyristors, it is permissible to apply the control voltage first to one thyristor, and then to the other - if the voltage on the load (lamp) is regulated by the resistor R1, the phase of connecting the control pulses is correct. When both thyristors are working and the circuit is tuned, the voltage at the load should vary from 0 to 190 V.

It is also possible to exclude the possibility of applying maximum voltage to the armature winding at the moment of switching on electronically, using a circuit similar to that shown in Figure 6.17. (Capacitor C2 provides a smooth increase in the output voltage at the moment of switching on, and subsequently does not affect the operation of the circuit.) In this case, switch SB2 is not needed.

See other articles Section Electric motors.

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