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close this bookElectrical Machines - Basic Vocational Knowledge
close this folder4. Synchronous machines
close this folder4.3. Operational behaviour
View the document4.3.1. Synchronous generator
View the document4.3.2. Synchronous motor

4.3.2. Synchronous motor

Starting

The rotating field immediately runs at low inertia once a stator winding of the synchronous motor is connected to the three-phase mains. Conversely, the field spider cannot follow at once because of its inertia mass. Constantly alternating north and south poles of the rotating field continuously cross the north pole of the field spider and give rise to pulsating force actions in the one or other direction. In this manner the synchronous motor does not develop a “starting torque”. For this reason a starting winding in the rotor is used.


Figure 46 - Starting winding of the synchronous rotor

1 Rods, 2 Rotating field, 3 Spur ring

This starting winding consists of aluminium or copper rods positioned in axial direction through the pole shoe and short-circuited at the ends through end rings. As the rotating field rotates a torque is attained (see short-circuit rotor principle) which causes the machine to accelerate. Switching on the exciting current draws the motor into the synchronous run of the rotational field.

Rated operation

The envisaged “rubber threads” (Cp 4.1.2.) stretch more and more. The synchronous speed is always retained. The synchronous motor does not alter its speed given an increased load.


Figure 47 - Speed-torque curve of the synchronous motor

1 Speed, 2 Torque, 3 Rated torque, 4 Breakdown torque, 5 · n = f (M)

Where the load is increased substantially over and beyond the rated torque, the magnetic coupling forces are no longer able to draw the rotor in to the synchronous run of the stator rotating field. One can imagine that the “rubber threads” could easily tear through such overload. The rotor “gets out of step”.

This takes place according to machine type overload between 1.5 and 1.9 times above the rated torque (breakdown torque MK).