Three-phase motors in single-phase operation

 

Three-phase motors can also be operated with single-phase alternating voltage. A time shift in the current in one winding strand is normally achieved using an operating capacitor. This in turn causes a magnetic field to be generated. However, due to the capacitor the currents flowing in the windings are not all of equal magnitude. The rotating field is no longer circular but has an elliptical shape. This leads to a considerable drop in power and starting torque.

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.

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.

 

Universal motor

Design of Universal motor

Universal motors have two windings connected in series – the exciter winding and the armature winding. The rotor is designed as a laminated steel core. Several coils form the armature winding, whose winding turns are arranged in grooves located on the rotor surface. The ends of each coil are connected to the neighbouring collector segments. The stationary carbon brushes feed the coils with current one after the other as the rotor rotates. The stator is equipped with two salient poles which accommodate the exciter windings and is also built of laminated steel sheeting. To reduce power losses the windings can also be inserted into grooves.

How the motor operates

 

As the exciter winding and the armature winding are connected in series, the same current flows through them both, i.e. the magnetic fields in the stator and the armature are in phase. The magnetic fields situated perpendicular to each other cause torque to be exerted on the rotor of the motor – the north pole of the armature field is drawn by the south pole of the stator field and repelled by the north pole. The important thing here is that both fields do not rotate. The alternating current only causes the simultaneous direction reversal of the two magnetic fields. The direction of the torque exerted on the rotor remains unchanged.

Torque, speed and back emf

 

As long as the rotor has not been put into motion, the armature current is solely limited by the winding resistance. The torque is at its maximum and the motor's speed rapidly increases. Back emf is induced in the armature winding, which opposes the externally applied voltage in conformity with Lenz's law. The higher the rotor speed, the greater the back emf induced in the armature. As a result the current and thus the torque developed by the motor is diminished. An equilibrium comes about – the motor turns with a constant speed at which the torque being developed is adjusted to the load.

Operating response

 

In its design and the way it operates the universal motor is analogous to the series-wound motor. It has a very high starting torque. Its rotation speed can also be high and is variable over a wide range. Phase control allows for continuously adjustable speed/power control. The pole reversal of the stator or armature winding causes reversal of the rotation direction. Short-term overloads are not dangerous for these motors. However, when the load is so high that the motor comes to a standstill, there is a danger that the winding insulation may become damaged. At no-load the universal motor can end up "racing" which can lead to the destruction of the armature winding.

Applications