Electrical Machines: Transformers, DC and AC Machines
What this domain tests
This domain covers transformer action and the EMF equation, DC generator and motor behaviour, and the rotating-field induction motor. Expect calculations of turns ratio, current ratio, speed and slip.
Core rules
- Transformer EMF equation: E = 4.44 f N Φm. Voltage ratio equals turns ratio: V1/V2 = N1/N2; for an ideal transformer, V1I1 = V2I2, so current ratio is inverse.
- A transformer changes voltage and current but not frequency or power; losses are copper (I²R) and core (hysteresis and eddy current), and efficiency = output/input.
- DC generator EMF: E = PΦNZ/60A. DC motor back EMF Eb = V − IaRa; torque depends on flux and armature current.
- Synchronous speed Ns = 120f/P. Induction motor slip s = (Ns − N)/Ns, usually a few percent at full load.
Worked example
A transformer has 1000 primary turns on 2000 V and a 200 V secondary. N2 = N1 × V2/V1 = 1000 × 200/2000 = 100 turns. If the secondary supplies 100 A, the ideal primary current is 100 × 200/2000 = 10 A, confirming that the volt-ampere rating is unchanged apart from losses.
Traps that cost marks
Stating that a transformer changes frequency; using the current ratio the same way round as the voltage ratio; calculating slip as a fraction but reporting it without the percentage conversion; and ignoring that a DC motor's back EMF depends on speed.