Operation and start of a 3-phase electric motor in the event of a phase failure. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Electric motors

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Phase failure in villages is a common thing: either a line break or theft, an electrician or the villagers themselves are repairing something, or a powerful switch at a transformer substation has blown, broken or stolen ...

To protect the electric motor (EM) in the event of a phase failure, protection systems of an increased level of complexity are being developed, although a pair of relays can be completely dispensed with. There is a well-known law: "system reliability is inversely proportional to the number of elements", so a complex electronic system can fail at the most inopportune moment. And then the metal-cutting machine will stop, and the scarce carbide tool will fail; the water pump will stop and the expensive steam heating boiler will glow white; ventilation will stop, and the air of the "harmful" shop will become hazardous to health, etc.

I think that when a phase disappears, it is not the ED that should be turned off, but the "sick" phase.

The circuit (Fig. 1) consists of 3 relays of the type RP21-003-UHL4A or similar. Such relays are very economical, the electricity meter simply does not feel them.

The protection is suitable for several EDs of medium power (small workshop); to protect a more powerful load, magnetic starters should be used instead of relays. All pairs of contacts and relays, and magnetic starters should be paralleled.

If the EM is switched by mechanical switches, then in case of phase loss, these motors continue their operation. When using magnetic starters, shutdowns are possible due to a short-term (at the time of protection operation) lack of current in the phase to be switched off. This can only happen if the working coils of the magnetic starters are powered from the switched off phase (1/3-2/3 cases).

All single-phase loads (lighting lamps, single-phase motors, heaters, etc.), as well as non-inductive 3-phase loads, should be connected before protection, although I have seen several light bulbs work satisfactorily from a 2 kW ED operating in two phases.

With an increase in the load on the engine, the brightness of the light bulbs decreased. Why do EDs burn out when a phase is lost? Yes, because at the time of the accident they become dynamic transformers, pumping electricity to the "sick" phase from two "healthy" ones. Such generators feed with their single winding (when connected by a star) a third of the consumers of the entire energy branch! Of course, ordinary EDs are not ready for such a role, since their power is too low - so they warm up, smoke and quickly burn out. Quite a different thing happens if you quickly turn off the "sick" phase, along with all its consumers: the engine continues to work, having lost a third of its power. This is still better than working in one phase, and this is how most EDs operated in villages work.

If the missing phase appears again, then the protection will immediately connect it. If an ED operating on two phases is turned off, then one of the circuits (the simplest - Fig. 2, the most complex Fig. 3) will be required to re-enable, which have long been used to reverse and start EMs included in one phase of 220 V.

Since operation on two phases (380 V) requires capacitors of a smaller capacity and designed for a higher operating voltage, you can use the circuit in Fig. 4, in which the start button switches (when released) the capacitors from parallel to series.

You can do without the diagrams of Fig. 2-4 if, in the event of a phase failure, you simply do not turn off the ED, but start others from it. The role of a "starter" can be played by a low-speed (less than 1000 rpm) EM, which is easily started manually at idle, or a powerful three-phase transformer.

Author: Y. Bearded

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