The speed of a 4-pole induction motor is controlled by varying

The speed of a 4-pole induction motor is controlled by varying the supply frequency while maintaining the ratio of supply voltage to supply frequency (V/f) constant. At rated frequency of 50 Hz and rated voltage of 400 V its speed is 1440 rpm. What is the speed at 30 Hz, if the load torque is constant?

Since , N_S₁ =	120 f	=	120 × 50	= 1500
	P		4

∴ N = N_S₁ (1 – S)
⇒ 1440 = 1500 (1 – S)
⇒ (1 – S) = 0.96
⇒ S = 0.04

∴ N_S₂ =	120 × 30	= 900 rpm.
	4

∴ N = 900 (1 – 0.04) = 864 rpm.