Electrical Machines and Transformers
Core Concepts
Understand transformer principles: turns ratio (V1/V2 = N1/N2 = I2/I1), types (core, shell, auto), losses (copper, iron), and efficiency. Know DC machines: shunt, series, compound motors and generators, and their characteristics. Understand AC machines: induction motors (squirrel cage, slip ring), synchronous machines, and their starting methods. Know motor ratings, duty cycles, and cooling. Understand the concept of slip in induction motors: slip = (Ns - N)/Ns × 100%. Know generator principles and EMF equation.
Traps in Questions
- Transformer does not change frequency; it changes voltage and current.
- Auto-transformer does not provide isolation.
- DC series motor should never be started unloaded (it can overspeed).
- Induction motor is not synchronous; it runs at slip speed.
- Slip is positive for motoring, negative for generation.
Worked Example / Rule of Thumb
Example: A transformer has 500 primary turns and 100 secondary turns, with 230 V primary. Secondary voltage = 230 × (100/500) = 46 V. If secondary current is 10 A, primary current = 10 × (100/500) = 2 A. For a 4-pole induction motor at 50 Hz, synchronous speed Ns = 120f/P = 120×50/4 = 1500 rpm. If actual speed is 1440 rpm, slip = (1500-1440)/1500 × 100 = 4%. Rule of thumb: Transformer turns ratio directly proportional to voltage ratio; induction motor slip typically 2-5% at full load.