Starting a three-phase motor from a single-phase network without a capacitor

The article is devoted to the possibility of starting a three-phase asynchronous motor with a power of 250 W from a 220 V network not using a starting capacitor, but using a home-made starting electronic device. Its circuit is very simple: on two thyristors, with thyristor switches and transistor control.

Device diagram

This engine control is little known and practically not used. The advantage of the proposed starting device is that the loss of engine power is significantly reduced. When starting a three-phase 220 V motor using a capacitor, the power loss is at least 30%, and can reach 50%. Using this starting device reduces power loss to 3%, with a maximum of 5%.


Single-phase network is connected:

The starting device is connected to the engine instead of a capacitor.

A resistor connected to the device allows you to regulate engine speed. The device can also be turned on in reverse.

An old Soviet-made engine was used for the experiment.

With this starter, the engine starts instantly and runs without any problems. This scheme can be used on almost any engine with a power of up to 3 kW.

Note: in a 220 V network, it simply does not make sense to turn on motors with a power of more than 3 kW - household electrical wiring will not withstand the load.
The circuit can use any thyristors with a current of at least 10 A. Diodes 231, also 10 A.
Note: the author has 233 diodes installed in the circuit, which does not matter (only they run at a voltage of 500 V) - you can install any diodes that have a current of 10 A and hold more than 250 V.
The device is compact. The author of the circuit assembled the resistors simply in sets, so as not to waste time selecting resistors at their nominal value. No heat sink required. A capacitor, a zener diode, and two 105 diodes were installed. The circuit turned out to be very simple and effective in operation.

Recommended for use - assembling the starting device will not create problems. As a result, when connected, the engine starts at its maximum power and with virtually no power loss, in contrast to the standard circuit using a capacitor.

Watch a video about how the device works