Two winding motor

Two winding motor Design

Most single-phase induction motors are designed as dual-winding machines. In contrast to single-phase motors there are two separate winding phases built into the stator of the two-phase motor – the power winding and the auxiliary winding. The auxiliary winding is normally disconnected after the motor has successfully started. The auxiliary winding coils are laid between the coils of the power winding. A squirrel-cage rotor is used as a rotor.

Direction of magnetic fields

 

If you connect the main winding to an AC voltage, the motor responds like a transformer with a short-circuited secondary winding. If the rotor had been purely inductive, the magnetic fields in the stator and rotor would have been phase-shifted by 180°. However, due to the fact that the rotor impedance also contains a resistive component, the north pole of the rotor field is always 15° behind the south pole of the stator field. This means that a torque cannot be generated when the motor is switched on.

Types of induction motors with squirrel-cage rotors

 

The starting torque can be produced with the aid of a rotating magnetic field. The prerequisite for this is a phase-shift between the currents flowing in the working and auxiliary windings which are situated at 90° to one another. The phase-shifted voltage feed of the auxiliary winding can be achieved either through an equivalent resistance, a capacitor or a choke. Depending on the circuitry used for the phase shift, a distinction can be drawn between five different types of two-phased asynchronous motors.

How the motor works

The phase shift between the currents in the two stator windings is achieved in this motor by implementing a considerable amount of active resistance in the auxiliary winding. The auxiliary winding comprises either a larger number of winding turns, lower wire cross-section or is made of resistive material. If the current in the auxiliary winding leads the current in the power winding, the rotation direction is from the pole of the auxiliary winding to the next pole of the power winding. After start-up the auxiliary winding is normally disconnected using a centrifugal switch or a current relay.

Operating characteristics

 

It is the asymmetrical rotating field which is responsible for the motor's relatively low torque during starting. The starting current amounts to approximately six times the rated current, while the starting torque is about equal to the rated torque. After the auxiliary winding is disconnected, the motor demonstrates the characteristics of single-winding single-phase motors. For that reason such motors are nowadays manufactured exclusively with power levels of under 1 kW and used where they do not need frequent starting.

Capacitro motor with starting capacitor

A capacitor is connected in series with the auxiliary winding. It is selected so that, in phase terms, the current in this winding is ahead of the applied voltage by 40°. The inductance of the power winding is the reason for the fact that current in this winding lags the voltage by 50°. The result of this is a 90° phase shift between the winding currents. When 75% of the nominal speed is reached the auxiliary winding is disconnected from the capacitor. Because the small electrolytic capacitor is only in operation for a few seconds, its value can be chosen to be adequately large – around 200 µF.

Operating response

 

Capacitor motors with starting capacitors are the most popular single-phase AC motors in use. Their starting characteristics are better than those for motors with resistive auxiliary phase windings. The starting torque is higher, the starting current consumed is lower and the power factor is better. After the auxiliary winding is disconnected, the motor demonstrates the operational characteristics of single-winding single-phase motors.