Electrical Engineering ⇒ Topic : Split-phase resistance start induction motor
In a split-phase induction motor the stator is provided with two parallel windings displaced 90 electrical degrees in space and somewhat less than 900 in time. Fig.(a) shows the winding diagram of the two windings of a split-phase induction motor. The starting winding has fewer turns and is wound of smaller diameter copper than the running winding. The starting winding, therefore, has a high resistance and low reactance.The running or main, winding (heavier wire of more turns) has a low resistance and high reactance. Because of its lower impedance, the current in the running winding, Ir,. is higher than thecurrent in the starting winding, IS
The phase relations of the lock-motor currents at the instant of starting are shown in Fig.(b). The starting winding Is lags, the supply voltage by about 15°, while the greater running winding current lags the single-phase voltage by about 400. Despite the fact that the current in the two space quadrature windings are not equal, the quadrature components are practically equal.
(a) Connection diagram.
(b) Phase relations.
Fig. Splite phase resistance-start induction motor.
If the windings are displaced by 900 in space, and if their quadrature current components, which are displaced by 900 in time, are practically equal, an equivalent two-phase rotating field is produced at starting which develops sufficient starting torque to accelerate the rotor, in the direction of the rotating field produced by the currents.
If it is necessary to reverse the motor while it is rotating, then some means must be incorporated to slow the motor down to the speed where the starting-switch contacts close,placing the starting winding across the supply lines. This may be done by incorporating a timing 1 evice which first disconnects the motor entirely from the line and then reverses one field at the proper line. A mechanical braking device which can be electrically operated may also be used.
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