Asynchronous motor

Induction motors with squirrel-cage rotors

 

In the repulsion motor the collector and brushes are only used in the starting phase. After a certain speed has been attained the rotor is short-circuited. The next logical step in the simplification of its construction is the short-circuit rotor, usually referred to as a squirrel-cage. The single-phase asynchronous machines have the same simple mechanical design as three-phase motors. A distinction is drawn between single-phase and two-phase motors depending on the number of phase windings.

Design

This motor has a particularly simple and straight-forward design. The laminated stator is equipped with a power winding taking up 2/3 of the grooves. The squirrel-cage consists of two short-circuit rings, which are interconnected by aluminium or copper bars running close beneath the surface of the rotor core. In order to keep power losses low, a deep-bar rotor is not used. The grooves can take a variety of forms.

Direction of the magnetic fields

 

 If the power winding is connected to an alternating voltage, the motor responds like a transformer with short-circuited secondary winding. There is a 180° phase shift between the magnetic fields in the stator and rotor – the fields oppose each other and there is no starting torque generated when the motor is switched on.

How it works

 

The pulsating alternating field in the stator can be perceived as two magnetic fields rotating in opposite directions, which at standsill build up two torques of opposite and equal magnitude. The resulting torque is equal to zero. The hand-operated pony motor does not start by itself – it must be cranked in one direction or the other. After start-up, one of the partial moments will predominate so that the resulting torque becomes greater than zero – the motor accelerates until it reaches its rated speed. Here the same regularities and functional principles apply as elaborated on in the section on the repulsion motor.

 

Operating attributes

 

Compared to other motor types the hand-operated single-phase motor (pony) demonstrates a number of negative characteristics.
Pony motors are only used for small concrete mixers or grinding machines.