ELECTRICAL MACHINE
(a) D.C. Machines
- Construction: Yoke, Pole core, Armature, Commutator, Brushes.
- Principle: Generator – Electromagnetic induction; Motor – Lorentz force.
- Motor Types: Series, Shunt, Compound.
- Speed Control:
- Armature control
- Field flux control
- Voltage control method
- Starting: 3-point and 4-point starters used for safety and controlled current.
- Braking Methods: Regenerative, Plugging, and Dynamic braking.
- Losses: Copper, Iron (hysteresis & eddy), Mechanical losses.
- Efficiency: η = Output / Input × 100
EMF Equation: E = (P × ϕ × Z × N) / (60 × A)
(b) Transformers (1-Phase & 3-Phase)
- Construction: Core (laminated), Windings (Primary & Secondary).
- Principle: Mutual Induction – E ∝ dϕ/dt
- Equivalent Circuit: Referred to primary or secondary side.
- Voltage Regulation: VR = (No-load V – Full-load V) / Full-load V × 100
- Tests:
- Open Circuit Test – to determine core losses
- Short Circuit Test – to determine copper losses
- Losses: Copper and Iron losses.
- Effect of Voltage/Frequency: High voltage or frequency increases core losses.
- Parallel Operation: Required for load sharing, must match voltage ratio, impedance, phase.
- Auto Transformer: Uses part of the winding for both primary and secondary.
EMF Equation: E = 4.44 × f × N × ϕ
(c) 3-Phase Induction Motors
- Working Principle: Rotating Magnetic Field (RMF) produces torque.
- Slip: s = (Ns - Nr) / Ns
- Starting Methods: Direct-on-line (DOL), Star-delta, Auto-transformer starter.
- Speed Control: Frequency variation, Pole changing, Rotor resistance.
- Torque-Speed Characteristics:
- High starting torque, stable operation near synchronous speed.
- Braking: Regenerative, Plugging, Dynamic.
- Effect of Voltage & Frequency: Affects torque and speed stability.
Torque ∝ sE² / R² + (sX)²
Previous Year Questions (SSC JE / RRB / HSSC)
- Q. In a DC motor, which part commutates current?
Ans: Commutator
- Q. Which test is performed to calculate no-load loss of transformer?
Ans: Open Circuit Test
- Q. In a 3-phase induction motor, what is the value of slip at synchronous speed?
Ans: 0%
- Q. What is the main advantage of auto transformer over two winding transformer?
Ans: Higher efficiency and lower copper usage
- Q. Which method is used for speed control of DC shunt motor?
Ans: Field flux control