Electrical Machines and Transformers (Introductory)
What the domain tests
Electrical Machines and Transformers (Introductory) tests the working principle, turns ratio and basic losses of transformers, and the difference between DC machines, induction motors and synchronous machines.
Core rules
- A transformer works on alternating current by mutual induction. It changes voltage and current, not frequency or power (apart from losses).
- Turns ratio: V₁/V₂ = N₁/N₂ = I₂/I₁. A step-down transformer has fewer secondary turns.
- Core loss (hysteresis and eddy current) is roughly constant at normal voltage; copper loss varies with load squared.
- A DC motor uses a commutator and brushes; an induction motor is AC, robust and self-starting; a synchronous machine runs at synchronous speed and can supply reactive power.
- Transformer rating is in kVA because heating depends on voltage and current, not directly on load power factor.
Worked example
A transformer has 200 primary turns and 50 secondary turns, with 220 V applied to the primary. V₂ = V₁ × N₂/N₁ = 220 × 50/200 = 55 V. If the secondary supplies 10 A, the primary current is approximately I₁ = I₂ × N₂/N₁ = 10 × 0.25 = 2.5 A, ignoring losses. The reasoning uses the turns ratio in the correct direction.
Traps
Do not connect a transformer to DC and expect induction-there is no changing flux, and the winding may overheat. Do not say a transformer changes frequency. Do not rate a transformer in kW without qualification; kVA is standard. Do not confuse an induction motor with a DC series motor.