Module IV: Electrical Machines

This module covers theory only - numerical problems are not expected.

1. DC Motor

1.1 Principle of Operation

Working Principle

A DC motor works on the principle that when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force. This is based on Fleming's Left Hand Rule.

Force on Conductor

F = BIL

Where:

F = Force on conductor (Newton)
B = Magnetic flux density (Tesla)
I = Current through conductor (Ampere)
L = Length of conductor in magnetic field (meter)

Fleming's Left Hand Rule

Motor Action

In a DC motor:

DC supply is given to the armature winding through brushes and commutator
Current flows through the armature conductors
The field winding (or permanent magnets) creates a magnetic field
The current-carrying conductors in the magnetic field experience a force
This force creates a torque that rotates the armature
The commutator reverses current direction every half rotation to maintain continuous rotation

1.2 Constructional Details

DC Motor Construction

Component	Material	Function
Yoke (Frame)	Cast iron or steel	Provides mechanical support; carries magnetic flux
Pole Core	Laminated steel	Carries field winding; concentrates flux
Pole Shoe	Laminated steel	Spreads flux uniformly; supports field winding
Field Winding	Copper wire	Produces main magnetic field
Armature Core	Laminated silicon steel	Carries armature winding; provides low reluctance path
Armature Winding	Copper conductors	Carries current; produces torque
Commutator	Hard-drawn copper segments	Converts AC in armature to DC at brushes; reverses current
Brushes	Carbon or graphite	Conducts current between supply and commutator
Shaft	Mild steel	Transmits mechanical power

1.3 Classification of DC Motors

DC motors are classified based on the connection of field winding with respect to the armature:

DC Series Motor

Field winding is connected in series with the armature.

Field winding: Few turns, thick wire
High starting torque
Speed varies with load
Should never run without load (may overspeed)

DC Shunt Motor

Field winding is connected in parallel with the armature.

Field winding: Many turns, thin wire
Approximately constant speed
Medium starting torque
Can run without load safely

DC Compound Motor

Has both series and shunt field windings.

Cumulative Compound: Both fields aid each other - high starting torque, fairly constant speed
Differential Compound: Fields oppose each other - constant speed but poor torque

1.4 Applications of DC Motors

Type	Characteristics	Applications
Series Motor	High starting torque, variable speed	Electric trains, cranes, hoists, elevators, trolleys
Shunt Motor	Constant speed, medium torque	Lathes, drilling machines, fans, blowers, conveyors
Compound Motor	High torque, fairly constant speed	Rolling mills, punching machines, shears, elevators

2. Three-Phase Induction Motor

2.1 Principle of Operation

Working Principle

A three-phase induction motor works on the principle of electromagnetic induction and rotating magnetic field.

When three-phase supply is given to the stator, it produces a rotating magnetic field (RMF)
This RMF rotates at synchronous speed: N_s = 120f/P
The RMF cuts the rotor conductors (which are stationary initially)
By Faraday's law, EMF is induced in the rotor conductors
Since rotor circuit is closed, current flows in rotor conductors
These current-carrying conductors in magnetic field experience a force (motor action)
The rotor starts rotating in the same direction as RMF

The rotor can never reach synchronous speed! If it did, there would be no relative motion, no induced EMF, no current, and no torque. The rotor always runs at a speed slightly less than synchronous speed.

Important Formulas

N_s = 120f / P (Synchronous Speed)

Slip (s) = (N_s − N_r) / N_s

N_r = N_s(1 − s) (Rotor Speed)

Where: f = frequency, P = number of poles, N_r = rotor speed

2.2 Constructional Details

Main Parts

A three-phase induction motor consists of two main parts:

Stator: Stationary part that produces the rotating magnetic field
Rotor: Rotating part where torque is developed

Component	Description
Stator Frame	Cast iron or steel; provides mechanical support and protection
Stator Core	Laminated silicon steel; carries magnetic flux
Stator Winding	Three-phase winding placed 120° apart; produces RMF
Rotor Core	Laminated silicon steel; mounted on shaft
Rotor Winding	Squirrel cage or wound type
Air Gap	Small gap between stator and rotor (0.4mm to 4mm)

2.3 Types of Three-Phase Induction Motors

Squirrel Cage Induction Motor

The rotor consists of copper or aluminum bars short-circuited at both ends by end rings, resembling a squirrel cage.

Features:

Simple and rugged construction
Low cost and maintenance
High efficiency
Low starting torque
Speed control is difficult
Most widely used type (~90% of industrial motors)

Wound Rotor (Slip Ring) Motor

The rotor has a three-phase winding similar to stator, connected to slip rings and external resistances.

Features:

More complex construction
Higher cost and maintenance
High starting torque possible
Speed control possible via external resistance
Used where high starting torque needed
Slip rings require maintenance

2.4 Applications

Motor Type	Applications
Squirrel Cage Motor	Fans, blowers, pumps, compressors, conveyors, machine tools, textile mills
Wound Rotor Motor	Cranes, hoists, elevators, conveyors, crushers (where high starting torque needed)

3. Single-Phase Induction Motors

3.1 Principle of Operation

Why Single-Phase Motor is NOT Self-Starting?

A single-phase induction motor is not self-starting because:

Single-phase supply produces a pulsating magnetic field, not a rotating field
This pulsating field can be resolved into two rotating fields of equal magnitude rotating in opposite directions
At standstill, torques due to both fields are equal and opposite → Net starting torque = 0
Once started (given an initial rotation), it will continue in that direction

Making Single-Phase Motor Self-Starting

To make a single-phase induction motor self-starting, we need to create a rotating magnetic field. This is done by using an auxiliary winding with a phase difference from the main winding.

3.2 Types of Single-Phase Induction Motors

1. Split-Phase Motor

Has main winding and auxiliary (starting) winding displaced 90° electrically
Starting winding has high resistance, low inductance
Creates phase difference to produce starting torque
Centrifugal switch disconnects starting winding after reaching 75% speed
Applications: Fans, blowers, washing machines, grinders

2. Capacitor-Start Motor

Similar to split-phase but with capacitor in series with starting winding
Capacitor creates 90° phase shift for better starting torque
Higher starting torque than split-phase (3-4.5 times full load torque)
Centrifugal switch disconnects capacitor after starting
Applications: Compressors, pumps, conveyors, refrigerators

3. Capacitor-Start Capacitor-Run Motor

Uses two capacitors: one for starting, one for running
Starting capacitor (large, electrolytic) gives high starting torque
Running capacitor (small, oil-filled) improves efficiency and power factor
Most efficient single-phase motor
Applications: Air conditioners, compressors, pumps

4. Shaded-Pole Motor

Simplest and cheapest single-phase motor
Copper ring (shading coil) on part of each pole
Shading coil creates phase shift in part of pole flux
Low starting torque and efficiency
Small sizes only (fractional horsepower)
Applications: Small fans, toys, hair dryers, record players

Type	Starting Torque	Efficiency	Cost	Applications
Split-Phase	Low-Medium	Medium	Low	Fans, blowers
Capacitor-Start	High	Medium	Medium	Compressors, pumps
Capacitor-Run	Very High	High	High	Air conditioners
Shaded-Pole	Very Low	Low	Very Low	Small fans, toys

4. BLDC Motor (Brushless DC Motor)

4.1 Principle of Operation

Working Principle

A BLDC motor is essentially a DC motor turned inside out. Instead of mechanical commutation using brushes and commutator, it uses electronic commutation.

Key Differences from Conventional DC Motor:

Permanent magnets are on the rotor (not stator)
Armature windings are on the stator (not rotor)
No brushes or commutator - uses electronic switching
Position sensors (Hall effect sensors) detect rotor position
Controller switches stator currents based on rotor position

BLDC Motor Operation

4.2 Construction

Component	Description
Stator	Laminated steel core with three-phase windings; produces rotating magnetic field
Rotor	Permanent magnets (usually rare earth - NdFeB) mounted on shaft
Position Sensors	Hall effect sensors or encoders to detect rotor position
Electronic Controller	Inverter with power transistors (MOSFETs/IGBTs) for switching

4.3 Advantages and Disadvantages

Advantages

No brushes - no sparking, no wear, no maintenance
High efficiency (85-95%)
High power-to-weight ratio
Excellent speed control
Long life and high reliability
Low noise and EMI
Better heat dissipation (windings on stator)
Can operate at higher speeds

Disadvantages

Higher initial cost
Requires electronic controller
More complex control circuitry
Controller failure stops the motor
Permanent magnets can demagnetize at high temperatures

4.4 Applications

BLDC motors are increasingly replacing traditional DC and AC motors in many applications due to their superior performance.

Sector	Applications
Automotive	Electric vehicles, power steering, window motors, seat adjusters, cooling fans
Consumer Electronics	Computer hard drives, DVD/Blu-ray drives, cooling fans, drones
Home Appliances	Washing machines, air conditioners, refrigerators, ceiling fans (energy-efficient)
Industrial	CNC machines, robotics, conveyor systems, pumps
Medical	Medical pumps, ventilators, surgical tools
Aerospace	Aircraft actuators, satellite systems

Summary: Electrical Machines

Motor Type	Working Principle	Key Feature	Main Application
DC Motor	Current-carrying conductor in magnetic field	Good speed control; uses commutator	Traction, cranes, machine tools
3-Phase Induction Motor	Rotating magnetic field; electromagnetic induction	Rugged, self-starting; most common	Industrial drives, pumps, fans
1-Phase Induction Motor	Pulsating field; needs starting mechanism	Not self-starting; domestic use	Fans, pumps, refrigerators
BLDC Motor	Electronic commutation; permanent magnet rotor	High efficiency; no brushes	EVs, drones, appliances