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Speed of 4 pole 60 Hz alternator is
1500
1800
2000
2500
1800
Given: Number of poles P = 4, Frequency f = 60 Hz.
Number of poles P = 4, Frequency f = 60 Hz.
Ns=P120×f
Identify given parameters
The number of poles P is 4 and the supply frequency f is 60 Hz.
P=4,f=60
Apply synchronous speed formula
The synchronous speed Ns in RPM is determined by the formula relating frequency, poles, and speed.
Ns=4120×60
Calculate the result
Perform the multiplication and division: 120×60=7200, then 7200/4=1800.
Ns=1800 RPM
B is correct because the synchronous speed calculated for a 4-pole 60 Hz alternator using the standard formula is 1800 RPM.
This relationship Ns=P120f is fundamental to induction motors as well, as they operate at a speed slightly less than the synchronous speed (the slip speed).