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
If a single-phase induction motor runs slower than normal, the most likely defect is
Worn bearing
Open circuit winding
Short circuit winding
faulty centrifugal switch
Worn bearing
A worn bearing increases the mechanical friction and air-gap irregularity in a single-phase induction motor, leading to an increased load torque on the rotor. As a result, the rotor slip increases to produce higher torque to overcome this friction, causing the motor to run significantly slower than its normal rated speed.
A worn bearing increases the mechanical friction and air-gap irregularity in a single-phase induction motor, leading to an increased load torque on the rotor. As a result, the rotor slip increases to produce higher torque to overcome this friction, causing the motor to run significantly slower than its normal rated speed.
NsтАЛ=P120fтАЛ тАФ Synchronous speed formula
N=NsтАЛ(1тИТs) тАФ Actual rotor speed as a function of slip s
In an induction motor, speed N=NsтАЛ(1тИТs). Under normal conditions, slip s is low. When a bearing becomes worn out, it causes mechanical dragging and increased load friction, requiring more electromagnetic torque. To deliver higher torque, the motor draws more current and operates at a higher slip (s), which directly reduces the shaft speed N.
Worn bearings cause mechanical friction, causing the rotor to experience resistance and run at a higher slip.
A severely worn bearing can also cause rotor rubbing against the stator core due to eccentricity.
An open circuit winding prevents the motor from starting or running at all.
A faulty centrifugal switch that remains open prevents the auxiliary winding from energizing, resulting in failure to start.
Option B (Open circuit winding): The motor will fail to start and will draw no current in the open branch.
Option C (Short circuit winding): Causes heavy current draw, localized overheating, humming, and potential tripping of protective devices rather than just running slightly slower.
Option D (Faulty centrifugal switch): If stuck open, the motor won't start; if stuck closed, the starting winding stays energized, causing overheating.
A is correct тАФ Worn bearings introduce extra mechanical friction, forcing the motor to operate at a higher slip and lower speed.
Always remember that electrical faults (open/short circuit) usually lead to catastrophic failure to start or excessive tripping, whereas mechanical issues like worn bearings lead to sub-normal operational speeds and noisy operation.