25-09-2012, 12:17 PM
Split-phase induction motor
1Split-phase.pdf (Size: 298.56 KB / Downloads: 299)
Objective:
to examine the construction of the split-phase motor; to learn its wiring connections;
to observe its starting and running operations; to measure its starting and operating
characteristics under load and no-load conditions.
Theory:
Some means must be provided for getting two phases from the standard single-phase power supplied
to homes if we need to start and run an AC motor. The process of deriving two phases
from one is known as phase-splitting and is usually built into the stator circuit of the AC motor.
2-phase power creates the rotating magnetic field.
One method is a special auxiliary winding built into the stator called the start (auxiliary) winding
to differentiate it from the actual run (main) winding of the stator. In split-phase AC motors, the
start winding is used only for starting the motor and has a high resistance and low inductive reactance.
The run winding has low resistance and high reactance. When power is first applied, both
windings are energized. Because of their different inductive reactances, the run winding current
lags the start winding current, creating a phase difference between the two. Ideally, the phase
difference should be 90 degrees; but in practical motors, it is much less. Nevertheless, the
windings develop fields that are out of phase, which creates a rotating magnetic field in the
stator. This applies torque to the rotor, starting the motor.
When the motor gets up to operating speed, the rotor is able to follow the alternations of the
magnetic field created by the run winding without the field of the start winding. The start
winding is then switched out of the circuit by a mechanical device called a centrifugal switch,
because it is operated by the centrifugal force created by the rotor revolutions. The direction of a
split-phase rotating field can be reversed by reversing the connections to the start winding. This
changes the direction of the initial phase shift, creating a -magnetic field rotating in the opposite
direction.