The speed of a 3– phase, 440 V, 50 Hz induction motor

The speed of a 3– phase, 440 V, 50 Hz induction motor is to be controlled over a wide range from zero speed to 1.5 time the rated speed using a 3-phase voltage source inverter. It is desired to keep the flux in the machine constant in the constant torque region by controlling the terminal voltage as the frequency changes. The inverter out put voltage vs frequency characteristic should be

For speed control by varying frequency method, ratio	V	must be kept constant,
	f

so that maximum flux density may remain constant. i.e.,
⇒ V = 4.44 Nf B_m A

⇒	V	= 4.44 N B_m A = constant
	f

But for frequency above 50 Hz, voltage can’t be increased alone rated voltage. So, above 50 Hz voltage kept constant.