← All revision guidesSEMESTER 4 · DI04000321
6FLASHCARDSElectrical Machines II
Induction motors, alternators and synchronous motors.
Induction machines
Q1Define slip of an induction motor.
s = (Ns − N)/Ns, usually expressed as a percentage.
Q2Why can an induction motor never reach synchronous speed?
At synchronous speed relative motion and rotor EMF become zero, so no torque is produced.
Q3What is the purpose of a star-delta starter?
It reduces starting phase voltage and therefore starting current and torque.
Synchronous machines
Q1Write the alternator frequency equation.
f = PN/120, where P is poles and N is speed in rpm.
Q2Why is a synchronous motor not self-starting?
At standstill the rotating field produces alternating torque with zero average value.
Q3How can a synchronous motor improve power factor?
With over-excitation it draws leading current and supplies reactive power to the system.