PWM speed controllers for low-power collector motors. Scheme, description

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PWM speed controllers for low-power collector 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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It is possible to regulate the rotational speed of a low-power collector electric motor (EM) by including a resistor in series with it. However, this option gives low efficiency, does not make it possible to make smooth adjustment (variable resistors of several tens of ohms are not common). The main thing is that this measure sometimes leads to a stop of the shaft rotation: the EM "freezes" at a low supply voltage in a certain position of the rotor. The regulators given in this article are free from these shortcomings. The device can also be used to adjust the brightness of incandescent lamps.

PWM speed controllers for low-power collector motors
Fig. 1

Figure 1 shows a diagram of one of the regulators. On the unijunction transistor VT1, a sawtooth voltage generator (repetition frequency 150 Hz) is made, and the operational amplifier (op-amp) DA1 acts as a comparator that forms a PWM sequence based on the key transistor VT2. Regulate the rotational speed with the potentiometer R5, which changes the width of the pulses. Due to the fact that their amplitude is always equal to the supply voltage, the ED never "freezes", moreover, it can rotate much more slowly than in the nominal mode.

PWM speed controllers for low-power collector motors
Fig. 2

The second circuit (Fig. 2) is similar to the previous one, but op-amp DA1 is used as a master oscillator. This op amp operates as a triangular voltage generator with a repetition rate of 500 Hz. Potentiometer R7 sets the speed.

PWM speed controllers for low-power collector motors
Fig. 3

An interesting circuit of the regulator, made on the integrated timer NE555, is shown in Fig. 3. The master oscillator operates at a frequency of 500 Hz. The duration of the pulses, and hence the frequency of rotation of the ED can be adjusted in the range from 2 to 98% of the repetition period. The output of the generator through the current amplifier on the transistor VT1 controls the ED M1. The disadvantage of the above schemes is the lack of circuits for stabilizing the rotational speed when the load on the EM shaft changes. The scheme shown in Fig. 4 will help to solve this problem.

PWM speed controllers for low-power collector motors
Fig. 4

Like most of these devices, this regulator contains a triangular-shaped voltage generator with a repetition rate of 2 kHz on DA1.1.DA1.2, a DA1.3 comparator, an VT1 electronic key and a pulse duty cycle regulator (EM speed) R6. A feature of the circuit is the presence of positive feedback through the elements R12, R11, VD1, C2, DA1.4, which stabilizes the speed of the shaft M1 when the load changes. When adjusting with a specific ED resistor R12, such a depth of the POS is selected at which self-oscillations of the rotational speed do not yet occur when the load on the motor shaft changes.

Details

In the above circuits, the following replacement parts can be used: the KT117A transistor can be replaced by KT117B-G or 2N2646; KT817B - KT815, KT805; chip K140UD7 - K140UD6, KR544UD1, TL071, TL081; timer NE555 - S555, KR1006VI1; chip TL074 - TL064, TL084, LM324. To connect a more powerful load, the KT817 key transistor can be replaced with a powerful field-effect transistor, for example, IRF3905 or the like. Such a transistor can switch currents up to 50 A.

Author: A.V. Tymoshenko, Chernihiv region; Publication: cxem.net

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