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Problem 2

(Control of Synchronous Machines - Volts/Hz Control Refer to March 5th Class Material)

A 1.1 MVA, 4.6 kV, 60 Hz, three phase, 4-pole synchronous motor is to be driven from avariable-frequency, three-phase, constant V/Hz inverter rated at 1.25MVA. The synchronous motor has a synchronous reactance of 1.18 per unit and achieves rated opencircuit voltage at a field current of 85A
a) Calculate the rated speed of the motor in rpm and rated current of the motor.
b) When the motor is operating at rated voltage and speed and an input power of 1MW, calculate the field current required to achieve unity power factor operation.
c) If the motor speed is reduced to 1300 rpm and the motor load is adjusted to ratedtorque (note that the armature current would remain unchanged if

the machinekeeps running at rated torque), find the motor field current required to return themotor to unity power factor.
شكرا