Start relay for asynchronous electric motor. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Electric motors

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I want to share my experience in manufacturing a starting relay for asynchronous electric motors, including three-phase ones, powered by a single-phase network. I hope this is useful to someone.

To ensure the operation of such an engine, a phase-shifting capacitor is used. Moreover, its capacity when starting the engine should be four times greater than during operation. Therefore, for the start time (1 ... 3 s), a starting capacitor of the appropriate capacity is connected in parallel with the working capacitor.

The easiest way to connect a start capacitor is to use a push button switch with auxiliary contacts that are closed only while the Start button is pressed. The main contacts of the switch are also closed at the moment of pressing the "Start" button, and in order to open them, you need to press the "Stop" button.

Such a solution (it was used in old washing machines) is only possible with manual motor control. But sometimes it needs to be started remotely, only by supplying voltage. In such cases, one cannot do without a starting relay that connects an additional capacitor when the mains voltage is applied, and after a specified time, turns it off.

A possible scheme for switching on an engine with such a relay is shown in fig. one.

When it is connected to a 220 V network, a constant voltage appears at the output of the rectifier assembled on the diode bridge. Capacitor C4 starts charging. Its charging current is sufficient to operate the electromagnetic relay K1. With its closed contacts, it connects the starting capacitor Descent in parallel with the working phase-shifting capacitor Srab of the electric motor M1. Capacitor C3 is spark quenching.

As the capacitor C4 charges, the current through the relay winding K1 decreases and after a while reaches the release current. The relay contacts open and disconnect the starting capacitor from the motor. Thus, the time for which the starting capacitor is connected depends on the properties of the relay K1 and the more, the larger the capacitance of the capacitor C4. Restarting the engine is possible after disconnecting the device from the network for a time sufficient to discharge the capacitors C2 and C4 through the resistor R2.

The capacitance of the capacitor C1 is selected based on the current of the relay operation, with some margin. Approximately - 1 microfarad of capacitance for every 50 mA of current. The capacitor must be designed for continuous operation at an alternating voltage of 220 V, 50 Hz. For example, K73-17 is suitable for a constant voltage of 630 V. The required capacity can be obtained by connecting several capacitors in parallel.

Relay K1 must have a response voltage not exceeding the stabilization voltage of the zener diode VD2 (27 V for the D816B indicated on the diagram). Its contacts must be designed for switching a voltage of at least 350 V and a current twice the starting current of the motor. If there are several suitable relays, choose the one with the largest difference between the voltage (current) of operation and release.

If the contacts of the existing relay are not powerful enough, you can connect the starting capacitor to the motor using a triac assembly assembled according to the circuit shown in fig. 2.

It is connected to points A and B of the original circuit instead of the relay contacts and capacitor C3 shown there. Triac V51 is selected based on the switched voltage and current. Contacts K1.1 are now included in the triac control electrode circuit, where the current is very small.

In order to completely abandon the electromagnetic relay, it can be replaced by a triac optocoupler according to the circuit shown in Fig. 3.

The input circuit of the optocoupler is connected to points C and D (see Fig. 1) instead of the relay winding K1 with the obligatory observance of polarity, and the output circuit is connected to points D and E (see Fig. 2) instead of contacts K1.1. The UOZ diode protects the emitting diode of the optocoupler from the reverse voltage applied to it when the capacitor C4 is discharged.

You can do without the triac shown in the circuit diagram (see Fig. 2), if you use not a low-power optocoupler, but an opto-triac, or a special electronic relay of sufficient power for direct switching of capacitors. Unfortunately, these devices are quite expensive.

In series with capacitor C1, it is advisable to include a resistor with a resistance of 51 ... 82 Ohms with a power of 0,5 W. It will limit the current pulse through the rectifier diodes when the device is connected to the network.

Author: K.Subbotin, Kuznetsk, Penza region.

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