Module IV: Electrical Machines - Solved Questions

Part	Function
Yoke	Mechanical support, carries magnetic flux, protective cover
Field Poles	Produce main magnetic field, made of laminated steel
Field Winding	Carries DC current to create magnetic field
Armature Core	Provides low reluctance path, houses conductors
Armature Winding	Carries current, produces torque
Commutator	Converts AC to DC (mechanical rectifier)
Brushes	Collect current from commutator (carbon/graphite)
Bearings	Support rotating shaft, reduce friction

Definition

When armature rotates in magnetic field, EMF is induced opposing applied voltage. Called Back EMF (E_b) or Counter EMF.

Formula

E_b = (φZNP)/(60A)

where φ=flux, Z=conductors, N=RPM, P=poles, A=parallel paths

Voltage Equation

V = E_b + I_aR_a

Significance

• Self-regulating: Limits armature current
• At start: E_b=0, so I_a=V/R_a (very high) - needs starter
• Running: E_b ≈ 90-95% of V
• Power: E_bI_a = Mechanical power

Principle

1. 3-phase supply to stator → Rotating Magnetic Field (RMF) at N_s = 120f/P
2. RMF cuts stationary rotor conductors → EMF induced (Faraday's law)
3. Rotor circuit closed → current flows
4. Current in magnetic field → force/torque (F=BIL)
5. Rotor rotates in direction of RMF, but never reaches N_s

Slip

s = (N_s - N_r)/N_s × 100% (typically 2-5%)

Types

Squirrel Cage	Slip Ring (Wound Rotor)
Rotor bars short-circuited	3-phase winding with slip rings
Simple, rugged	Complex construction
Low starting torque	High starting torque
No speed control	Speed control possible
Fans, pumps	Cranes, hoists, elevators

Not self-starting - single phase produces pulsating field, not rotating field.

Types

• Split-phase: Main + auxiliary winding, centrifugal switch (fans, blowers)
• Capacitor-start: Capacitor in series with aux winding, high starting torque (compressors)
• Capacitor-start-run: Two capacitors, best efficiency (AC, refrigerators)
• Shaded pole: Simplest, low torque, low efficiency (small fans, toys)

Key points: < 1HP, lower efficiency than 3-phase, used where 3-phase unavailable

Definition

Brushless DC motor uses electronic commutation instead of mechanical brushes.

Construction

• Stator: 3-phase armature winding
• Rotor: Permanent magnets
• Sensors: Hall effect (detect position)
• Controller: Electronic switching

Working

1. Hall sensors detect rotor position
2. Controller energizes stator windings in sequence
3. Magnetic field interacts with rotor magnets
4. Continuous rotation as sequence repeats

Advantages over Brushed DC

Higher efficiency (85-90%), longer life, low noise, no brush maintenance, wider speed range

Applications: Hard drives, drones, EVs, AC, medical equipment