Starting asynchronous motors. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electric motors
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If there are three phases, starting asynchronous motors does not cause problems (Fig. 1), but if there is only one phase, difficulties arise. To overcome them, there are many different ways that can be divided into electrical and mechanical.
Usually a combination of these techniques is used, but before talking about them, it is necessary to agree on concepts. I will try to offer such a classification of asynchronous motors according to their use at different powers and maximum speeds (see table).
According to this table, high-speed engines will be considered "three-thousanders", and powerful - with a power above 1,5 kW. Operation in everyday life of engines more powerful than 7,5 kW is impractical. Since there is no consensus on the names of engine groups, the table in brackets indicates the speed used by the older generation of electricians. All this does not apply to brushed or commutator motors, which have very high speeds and high power density.
Electrical methods for facilitating the starting of induction motors can be divided into capacitor (or RLC methods) and switching methods, which go back to mechanical ones.
If you are offered formulas for calculating the capacitance of capacitors for starting an electric motor, then they are most likely incorrect, since they do not take into account the operating speed of the engine, the load on it, line congestion, etc. What to do? And what and always try. Often used to start electric motors, polar (electrolytic) capacitors from tube TVs and radios. But to include them in an alternating current circuit means to break through. It will be correct to divide all the electrolytic capacitors used for starting into two identical groups and turn them on according to the diagram in Fig. 2.
The worst results are given by the scheme of Fig. 3, but it is still safer than direct inclusion.
The switching of the capacitor start of low-power electric motors can be automated by a starter from a washing machine or refrigerator (Fig. 4). There is already a positive experience in using a contactless starter to start low-power electric motors from the demagnetization loop of a color TV kinescope. Such starters are widely used abroad, as well as in new refrigerators in Minsk.
For starting "difficult" motors, the best method is considered to be star-delta switching [1,2], which helps to start any three-phase motor. This method belongs to electromechanical. The serial connection of the motor windings allows you to stretch the start-up process in time and thereby normalize the starting currents, which is essential for a three-phase motor operating on a single phase. In two-speed motors (Fig. 5), starting can be assisted by switching them to a lower speed.
In the villages, the electric lines are heavily overloaded. Such lines constantly "warm the sky" and starting a high-speed engine from them turns into sheer torment. Usually, they use a "starter rope" wound around the engine pulley. After the engine has started, a V-belt drive belt is thrown over it. This method of starting is purely mechanical. To safely start the engine in this way, it is best to use the belt tensioner (Fig. 6). To do this, it is necessary to loosen the belt tension for the start-up time (Fig. 6, a) and smoothly restore it to the end (Fig. 6, b).
References:
- Bearded Yu. Inclusion of a three-phase motor in a single-phase network, facilitating the start//Electrician. - 2000. - No. 8. - C.15.
- Bearded Yu. On the inclusion of a three-phase motor in a single-phase network, facilitating the start//Electrician. -2002. - No. 4. - C.13.
Author: Y. Bearded
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