Electrical Machines (Electrical)

Transformers:

• Working principle: mutual induction.
• Turns ratio: V₁/V₂ = N₁/N₂ = I₂/I₁.
• Step-up: higher secondary voltage.
• Step-down: lower secondary voltage.
• Auto transformer: single winding tapped.
• Three-phase: star/delta connections.

Losses:

• Iron / Core loss: hysteresis + eddy. Constant.
• Copper / Joule loss: I²R. Variable with load.

Efficiency: η = P_out / P_in. Typical 95-99% for large transformers.

Cooling: ONAN (oil natural, air natural), ONAF, OFAF, etc.

DC Machines:

• Generator: mechanical → electrical (EMF generation).
• Motor: electrical → mechanical (rotation).
• EMF equation: E = (φZNP)/(60A).
• Back EMF in motor: E_b = V − I_a R_a.
• Speed: N ∝ E_b/φ.

Types:

• Separately excited.
• Self-excited: series, shunt, compound.

Induction Motor (Squirrel Cage / Slip Ring):

• Most common motor.
• Slip: s = (N_s − N)/N_s, where N_s = synchronous speed = 120f/P.
• Synchronous speed: for 50 Hz: 4-pole 1500 rpm; 6-pole 1000 rpm.
• Starting torque: low for squirrel cage; high for slip-ring.

Synchronous Motor:

• Constant speed (N_s).
• Used for power factor correction.

Stepper Motor:

• Rotates in discrete steps.
• Used in CNC, robotics.

RRB JE focus: transformer ratios, motor types, synchronous speed for given frequency/poles.