SPEED CONTROL OF SPLIT PHASE MOTORS

Speed control of split phase motors (without capacitors) is obtained by either pole changing method or by using special winding arrangements. These methods will, however, increase the size of such motors and make them expensive.

To obtain two speed operation of a split phase motor, the motor will have two main windings and two auxiliary windings, each set wound for different number of poles. However centrifugal switch is set to open at the lower of the two speeds. In case of fast speed operation, therefore, the rotor has to accelerate to full speed on the main winding only. No problem would arise on this account if the load on motor is moderate.

Figure shows the control circuit for a two speed motor with two main and two auxiliary windings. The control circuit permits starting of the motor at slow or fast speed depending upon the setting of the selector switch. During running condition, transfer can be made from one speed to the other speed.

Figure: Control circuit for a two speed split phase induction motor having two main and two auxiliary windings

For the selector switch setting for slow speed, as shown in Figure, the motor can be started by pressing the SLOW-push button. A relay CRS first gets energized through contact S₃ and then gets hold through its contact CRS₁. Contact CRS₂ also closes and makes the circuit ready for fast operation. Whenever a changeover is required for fast operation, the FAST-push button is to be pressed which would first release the slow contactor S and then energies the fast contactor F. A relay CRF would then get energized and get hold through its own contact. Contact CRF₂ of this relay closes allowing transfer from fast speed to slow speed operation when required. Thus, it follows from the above that the motor can be started for a particular operation, fast or slow, depending upon the setting of the selector switch and then change over to the other operation by pressing the corresponding push button.