Question

A 6-pole and three-phase induction motor has synchronous speed of 1125 RPM.

a) Find the no-load and full-load operating speed of motor for the cases given

1. 2.5Hz electrical frequency of rotor for no-load condition. (15p)

2. 6.3Hz electrical frequency of rotor for full-load condition. (15p)

b) Find the speed regulation of motor by using parameters found above. (10p)

c) Determine the electrical frequency of rotor under full-load condition so that speed regulation would be equal to 5%? (Electrical frequency of rotor for no-load condition is still 2.5Hz.) (10p)

Answer 1

question solution - ( a ) - 3 phase induction motor has synchronous speed ( N_s ) 1125 RPM

operating speed of 3 phase induction motor is given as -

N = N_{​​​​​​s} ( 1 - S ) where S = slip = ( N_{​​​​​​s} - N )/N_{​​​​​​s}

at no load condition operating speed of 3 phase induction motor equals to synchronous speed

it means S = 0 , N = N_{​​​​​​s} = 1125 RPM

at full load slip ( s ) = 5% = 0.05

hence at full load N = 1125 × ( 1 - 0.05 ) = 1068.75 RPM

( b ) - speed regulation = ( N_{​​​​​​no load} - N_{​​​​​​full load} ) / N_{​​​​​​full load}

speed reguation = ( 1125 - 1068.75 ) / 1068.75 = 5.26%

( c ) - electrical frequency of rotor ( f_{​​​​​​r} ) is given as -

f_{​​​​​​r} = s.f , where f = frequency of rotor at no load = 2.5 Hz , s = slip

f_{​​​​​​r} = 0.05 × 2.5 = 0.125 Hz