Single Phase AC Motors pot

Single Phase AC
Motors

Operation

A 3 phase motor self starts due to a rotating
magnetic field in the windings

If a single phase was applied to one winding,
it would simply pulse and produce no torque

If we could simply ‘push’ the rotor, it would
begin to spin for the same reasons in a 3
phase motor

Operation

To make the spin happen, we add a start
winding to the motor

A start winding usually has the same number
of turns as the run winding but a smaller
diameter wire

This creates an inductive reactance causing
the current to lag

Split Phase Motor

A split phase motor has 2 windings:


Run winding

Start winding

Split Phase Motor

The two windings cause the current to lag
depending on the inductive reactance of each

This mimics the effect of a changing magnetic field

Once started, the switch cuts out to stop burning out


Split Phase Motor

Can be reversed by changing connections to
any one winding

Has a high starting current

Used in simple domestic machines

Capacitor Start Motor

Similar in effect to the split phase motor

Capacitor causes current in start winding to
lead voltage


Start and Run windings are approximately
90° out of phase

Capacitor Start Motor

High torque

Used in compressors, refrigerators and when high
torque starting is needed

Capacitor Start – Capacitor
Run Motor

2 capacitors are
connected in parallel
with the start winding

Once started, the first
capacitor cuts out and
leaves the second to
continue to maintain an
out of balance magnetic
field

Capacitor Start – Capacitor
Run Motor

This type is similar to
the last but has:


Better torque

Improved power factor