Join 60,000+ competitive exam aspirants
or with email
Your Cart
SubtotalтВ╣0
TotalтВ╣0
Examoogle тАв User тАв info@examoogle.com тАв EE-2024-8821
Chapter 1 of 12 тАв Page 1 of 248ЁЯФТ Protected PDF тАв Watermarked
Join 60,000+ competitive exam aspirants
On __________ of the rated speed of the motor, the centrifugal switch disconnect the _________ winding from supply.
10тИТ20%, running
20 тАУ 30%, starting
30 тАУ 40%, running
70 тАУ 80%, starting
70 тАУ 80%, starting
In a split-phase single-phase induction motor, a centrifugal switch is connected in series with the starting (auxiliary) winding. When the motor accelerates and reaches approximately 70% to 80% of its synchronous/rated speed, the centrifugal force acts on the switch mechanism to open its contacts. This disconnects the starting winding from the supply, leaving only the main (running) winding in the circuit.
In a split-phase single-phase induction motor, a centrifugal switch is connected in series with the starting (auxiliary) winding. When the motor accelerates and reaches approximately 70% to 80% of its synchronous/rated speed, the centrifugal force acts on the switch mechanism to open its contacts. This disconnects the starting winding from the supply, leaving only the main (running) winding in the circuit.
NsтАЛ=P120fтАЛ тАФ Synchronous speed of the motor
NcutтИТoutтАЛ=(0.70┬аto┬а0.80)├ЧNsтАЛ тАФ Speed range at which centrifugal switch opens
During startup, both the main and auxiliary windings are energized to create a rotating magnetic field necessary to produce starting torque. As the rotor speeds up, centrifugal force on the governor weights increases against an opposing spring force. At around 70% to 80% of rated speed, the centrifugal force overcomes the spring tension, throwing the switch open and disconnecting the high-resistance starting winding to prevent overheating and improve operating efficiency.
The starting winding is designed with high resistance and low inductive reactance (RsтАЛ>XsтАЛ).
The running winding has low resistance and high inductive reactance (XmтАЛ>RmтАЛ).
If the centrifugal switch fails to open, the starting winding will burn out due to continuous high current flow.
If the centrifugal switch remains open at standstill, the motor will not develop starting torque and will hum without rotating.
Protects the auxiliary winding from burning out under continuous operation.
Improves running efficiency and power factor by leaving only the low-resistance main winding in operation.
Mechanical switch contacts are subject to wear, sparking, and failure over time.
Unsuitable for hermetically sealed units (e.g., refrigerators) where mechanical switches can cause arcing in refrigerant gas.
Split-phase induction motors used in fans, blowers, and centrifugal pumps.
Capacitor-start induction-run motors in washing machines and workshop tools.
Option A (10тИТ20%): At this low speed, the motor has not developed sufficient torque to continue running smoothly on the main winding alone.
Option B (20тАУ30%): Disconnecting at this speed would result in the motor stalling or drawing excessive current under load.
Option C (30тАУ40%): Still too early to cut off the starting winding; maximum torque point is typically reached near 75% of rated speed.
D is correct тАФ The centrifugal switch disconnects the starting winding from the AC supply when the motor reaches 70 тАУ 80% of its rated speed.
Always remember: in single-phase induction motors (split-phase or capacitor-start), the centrifugal switch is always in series with the STARTING (auxiliary) winding and opens at 70тАУ80% of rated speed. In capacitor-start capacitor-run motors, only the starting capacitor/auxiliary winding branch is disconnected or modified.