Running Conditions

Author: E.E. Kimberly

In Fig. 19-3 a synchronous motor with a four-pole field is shown at a definite instant while rotating counter-clockwise. At the instant chosen in Fig. 19-3, the stator fields are at the axis x-x and the rotor and its fields are at the axis y-y. The angle a between the two axes, or the angle by which the rotor lags the stator fields, is called the torque angle. If the axes x-x and y-y were coincident, the flux in the air-gap would be radial and would produce no turning effort on the rotor. As the load on the motor increases, the rotor lags behind the rotating field by a greater angle, to produce the necessary torque. Thus, for a definite constant load the rotor runs in perfect synchronism with, and at a definite torque angle behind, the rotating field. With an increase in load, the rotor drops back to a new position, to meet the new demand for torque. The action is similar to that of dragging the rotor around by springs in tension.

Fig. 19-3. Elements of a Synchronous Motor

The amortisseur winding, which is a rudimentary squirrel-cage winding, was first used on alternating-current generators driven by reciprocating engines to damp out fluctuations in speed caused by the torque impulses of the engine. "Amortisseur" means "damper."

If the load be too greatly increased, the magnetic coupling between the rotor and the stator fields will be over-stressed and broken. Having slipped a pole, the rotor will come to rest very quickly unless supported by a strong amortisseur-winding torque. At synchronous speed the amortisseur winding is electrically inert, because it has no motion relative to the rotating field. Momentary changes in torque angle caused by changes in load or slipping of poles are opposed by the amortisseur damping action.

A salient-pole synchronous motor, after having been synchronized, can carry a small fraction of its rated load even without field excitation. Induction motors are available with slots cut longitudinally in the rotors to produce rudimentary salient poles, in order to simulate unexcited synchronous motors. Their low ratio of rated power to weight seriously limits the use of such motors. They are called "reluctance" motors because their torque derives from the attempt of the flux to decrease the reluctance of the path through which it must pass.