Collector motor control. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Electric motors

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The article describes a scheme for automatically maintaining the rotation speed of a DC collector electric motor (EM) (Fig. 1), which does not require a special speed sensor. Its peculiarity lies in the fact that the speed of rotation of the EM is determined by the magnitude of the voltage on the armature rotating by inertia (with a load on the shaft) at the moments of short-term regularly repeated disconnections from the power source.

Speed ​​control is performed in a pulse-width manner by using the voltage mentioned above as a negative feedback for the control circuit. The EM rotation speed is automatically maintained within the zone, which has a set maximum and minimum, and is set by an external voltage from a manual or software speed controller. The width of the control zone is equal to:

where: U⁺ - positive saturation voltage of the OS output, V; U^- - negative saturation voltage of the OS output, V; R1' - resistance of the grounded part of the resistor R1, Ohm.

The width of the control zone in the form of a deviation in the number of revolutions ΔN EM can be represented by the expression

where: N is the number of revolutions of the EM shaft per unit time at the rated armature supply voltage U2.

When the supply voltage U2 changes, as well as the load value, the EM shaft speed is automatically kept within the set control zone.

Structurally, the control circuit can be represented in the form of two blocks: a regulator and a key amplifier A1 (Fig. 2). Figure 1 shows the electric motor M1 with excitation from a permanent magnet. If an excitation winding is used in the ED, then its supply voltage must be stable.

If this voltage is unstable, then the speed control with a change in load still occurs, but each voltage of the excitation winding corresponds to its own EM speed automatically maintained when the load changes.

The change in load and the voltage supplying the armature corresponds to the inversely proportional to the disturbing effect of the EM speed inside the established control zone. According to the classification in automation, this is proportional automatic control.

The width of the control zone decreases with a decrease in the set rotation speed and vice versa, since it depends on the position of the variable resistor R1 slider (i.e., on the resistance R1'), and hence the EM speed. Thus, the ratio of the width of the control zone to the speed of rotation of the EM remains constant when setting any speed.

This useful property will not be observed when operating from an external control voltage source with a constant output impedance. the power supply for the selected op-amp (±U1) can also be used to power the EM speed controller (R1), but then it must be stable.

The EM armature is powered by a separate U2 source. If the voltage U2=U1, the resistor R6 is not installed, and a jumper is soldered instead of R5. The voltage U2 should always be slightly higher than the rated supply voltage of the selected EM to be able to maintain a constant speed when setting its maximum value. However, it does not require stabilization.

Consider the operation of the regulator. We consider that the variable resistor R1 slider is set approximately to the middle position. Operational amplifier DA1, chains C1-R3 and R4-R1' form a rectangular pulse generator. With positive pulses from the output DA1, the armature M1, through the key amplifier A1, receives power from the source U2 and rotates, the VD1 diode is locked by reverse voltage at this time, the capacitor C1 is charged through the resistor R3.

When the voltage at C1 exceeds the voltage at the non-inverting input DA1, its output switches to negative polarity, the switch A1 disconnects M1 from the source U2, but its armature, along with the load, continues to rotate by inertia (short-term shutdown of the ED only slightly reduces its speed).

If at the same time the voltage at the armature of the ED turns out to be lower than the voltage at C1, then this capacitor is connected to the armature of the ED through the opened diode VD1, and the voltage across them (taking into account the voltage drop across VD1) equalizes. The generator output goes into the positive phase of the generation cycle starting from this voltage.

At the same time, ED is gaining momentum.

Resistors R1, R4 form a voltage divider taken from the DA1 output and create positive feedback, which provides the conditions for generation and hysteresis of the output voltage of the op-amp when it is switched. The EM speed control zone is equivalent to this voltage. The presence of an established control zone does not mean that the EM speed will "scour" from maximum to minimum within this zone.

It will be kept constant until the load or supply voltage changes its values. Then the ED speed will be set at a new stable level, but will not leave the zone. Regulation within the zone is smooth.

The frequency of operation of the control circuit depends on the time constant of the C1-R3 circuit, the supply voltage U2, the EM load, the set speed and hysteresis of the OS, the moment of inertia of the EM armature together with the load, and is a complex function. However, by choosing the time constant C1-R3 and resistance R4, it is easy to set the mode in which the EM speed will be in the control zone with the desired switching frequency with real changes in the EM load and deviations in its supply voltage U2. This determines the general setting of the regulator for the selected EM.

Circuit elements and their parameters are not critical. DA1 can also be 140UD12. In the case of using a unipolar power supply U1, you can take a dual op-amp AS339N (LM339N, K140CA1, KR110CA2).

It has an open collector output, which must be loaded with a resistor (one op-amp is used in the circuit). In addition to the key, the circuit of which is shown in Fig. 2, optoelectronic relays and MIS transistors can be used.

To suppress interference in the network during the operation of the regulator, additional measures may be required, for example, shunting the armature of the ED with an RC circuit.

It will have virtually no effect on the operation of the control circuit.

The rotation speed of the ED can be measured without the use of a tachometer, by measuring the voltage at the armature with a pointer voltmeter (the inertia of its moving system will smooth out voltage ripples).

Author: V. Gusarov, Minsk

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