Single-Phase Induction Machines
Single phase induction motors generally have a construction similar to that of a three phase motor: an ac windings is placed on the stator, short-circuited conductors are placed in a cylindrical rotor. The significant difference is, of-course, that there is only a single phase supply to the stator.
Consider the motor schematic shown below
If an ac supply is connected to the stator winding, a pulsating flux density will be produced, which will link the rotor circuits. The voltage induced in the rotor circuits will cause a current to flow, producing a flux density to oppose change in the stator flux linking the circuit. In the diagram above, both the stator and rotor flux densities will act in the y-direction. i.e. the cross product of flux densities will be zero, the motor produces no torque.
The qualitative analysis above indicates a problem, single phase supplies produce pulsating fields, not rotating fields and pulsating fields do not produce torque. However, single phase motors can be made, so there seems to be a contradiction in what we know works, vs. the analysis.
One method which can be used to explain the behavior of single phase motors is called "double revolving field theory". It is explained in the animation below.
Applying double revolving field theory, it is possible to sketch a torque-speed curve for a single phase machine as the product of two independent machines, one trying to rotate forwards, one backwards.
It can be seen that if there are two equal and opposing rotating fields, then the net torque at standstill will be zero. This is the case considered in the qualitative analysis shown earlier. Considering the torque curves, if a single-phase induction motor can be made to start, a torque is created and the machine will operate as a motor.
A number of different types of machine are used, which have slightly different approaches to creating a rotating field at starting. The majority work on the principle of applying two phase currents to the motor. In the illustration below, there are two phases supplying the motor. If the phase supplies are balanced, 90° apart electrically with a 90° electrical angle physically between the coils, then the result is a purely rotational field with no pulsations. If the supplies are unbalanced, or the phase angle is not exactly 90°, there will be a combination of a pulsating and rotating effect. We will consider how to achieve a two-phase machine from a single phase supply in the next section.