motors

[prp_senji]

Dear all:
Please clarify me why a three phase motor does not require a capacitor unlike a single phase motor?
With thanks & rgds,
Prasad, PR

 

[JimB]

The stator windings in a three phase motor generate a rotating magnetic field as they are energised by the the phase supply. This rotating field causes the rotor to turn.

In a single phase motor the magnetic field is stationary, we have to generate a second field out of phase with the main field. This is done by using a second winding and putting a capacitor in series with that winding so that it generates a magnetic field out of phase with the first winding and starts the rotor turning.

JimB

 

[eblc1388]

In a single phase motor the magnetic field is stationary, ....

To make matter more complicated, a single phase motor without a second field winding will run once it is assist/rotated by hand.

 

[fsahmed]

Induction motors r much like transformers. Infact their electrical models r almost identical, the turns ratio for transformer being replaced by slip only.

The stator voltage creates a 'rotating' magnetic field which is the result of the pjhase difference & the physical orientation of poles. This field, when cuts the rotor (which consists of copper bars shorted by end rings) creates circulating currents within the rotor bars, resulting in a secondary magnetic field. This field sort of follows the stator & the rotor moves.

IN single phase motors, the field doesnt rotate but oscillates perpendicularly. This kind of field cannot provide the starting torque to the rotor & the motor becomes useless unless someone gives it a starting turn. To create a rotating field another set of windings is added, with the CAPACITOR in series. The capacitor results in a voltage phase shift in the second winding, the physical orientation of the stator & the starter windings creates a rotating field enough to provide the starting torque. Once the motor starts running, a centrifugal switch disconnects the starter winding & capacitor, since inertia is enough to keep the motor running smoothly in the oscillating field.

The capacitor can be selected to improve the power factor of the motor, & can be left connected as well, only this reduces the starting torque.

 

[prp_senji]

Thanks a million one and all.