If the centrifugal switch does not open at 70 to 80 percent of a synchronous speed of the motor, it would result in

• A. None of these
• B. Overloading of running winding
• C. Damage to the starting winding
• D. Damage to the centrifugal switch

Answer: (C)

All split-phase motors have a start and run winding. The start windings must be disconnected from the circuit within a very short period of time or they will overheat. Several methods are used to disconnect the start windings. The centrifugal switch is the most common method in open motors; however, electronic start switches are sometimes used.
The centrifugal switch is used to disconnect the start winding from the circuit when the motor reaches approximately 75% of the rated speed. 
A centrifugal switch is a mechanical device attached to the end of the shaft with weights that will sling outward when the motor reaches approximately 75% speed. For example, if the motor has a rated speed of 1725 rpm, the centrifugal weights will change position at 1294 r.p.m (1725 x 0.75) and open a switch to remove the start winding from the circuit. This switch is under a fairly large current load, so a spark will occur.
If the switch fails to open its contacts and remove the start winding, the motor will draw too much current, and the overload device will cause it to stop. When the motor is de-energized, it will slow down and the centrifugal switch will close its contacts in preparation for the next motor starting attempt. The more the switch is used, the more its contacts will burn from the arc. If this type of motor is started many times, the first thing that will likely fail is the centrifugal switch. This switch makes an audible sound when the motor starts and stops.
Hence if the starting switch fails to open when needed, the starting winding mill almost always overheats and burns out.