← All revision guidesSEMESTER 3 · DI03000081
6FLASHCARDSElectrical Machines I
Transformers, DC generators and DC motors.
Transformers
Q1State the transformer EMF equation.
E = 4.44 f N Φm volts for a sinusoidal flux.
Q2Why is a transformer rated in kVA?
Copper loss depends on current and core loss mainly on voltage, independent of load power factor.
Q3What does an open-circuit test determine?
Core loss and shunt-branch parameters at rated voltage.
DC machines
Q1What is back EMF in a DC motor?
The armature-generated EMF opposing the supply; Eb = V − IaRa.
Q2Why must a DC motor starter be used?
At starting Eb is zero, so a starter limits the dangerously high armature current.
Q3State the torque relation of a DC motor.
Developed torque is proportional to flux per pole multiplied by armature current: T ∝ ΦIa.