Electrical engineering - Basics Page 32

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4.4.3 Alternating current engines

Alternating current engines are relatively low power drive units. Above all, they are used in small electric devices and machines.
In the sanitary, heating and air-conditioning sector alternating current engines are e.g. used for circulation pumps, fans, etc. They are mostly being connected with single-phase alternating current (see diagram 47 below).
Very often a capacitor is necessary for starting assistance: Since a capacitor can store electric charges, it helps the engine to create a rotary field. The rotary field is bringing the engine rotor into a rotating movement (rotation).
Three-phase engines are above all used in big installations and for high performance engines. Three-phase engines do not need any capacitors for starting assistance since the three-phase current that is flowing through the windings of the engine produces a rotary field which is propelling the rotor.
Three-phase engines can be operated in y-connection or in delta connection. Up to a power of 3kW they are mostly connected in y-connection.
High-performance three-phase engines often start in y-connection (because of the smaller starting current) and then achieve their full engine performance in delta connection. The engine performance in y-connection reaches about only 1/3 of the engine performance in delta connection:

Diagram 47: Engine connections with protective grounding/earth PE

At the neutral point of the y-connection (see diagram 48) the voltages neutralise themselves (U 0V). Each string voltage is 230V. Between the string windings of the delta connection (see diagram 49) the voltage is 400V.

Diagram 48: Y-connection

Diagram 49: Delta connection


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