ELECTRIC MOTORAn electric motor is an electromechanical device that converts electrical energy to

mechanical energy.

The mechanical energy can be used to perform work such as rotating a pump

impeller, fan, blower, driving a compressor, lifting materials etc.

BASIC WORKING PRINCIPLE

TYPES OF MOTOR LOADSMotor loads

Description Examples

Constant torque loads

Output power varies but torque is constant

Conveyors, rotary kilns, constant-displacement pumps

Variable torque loads

Torque varies with square of operation speed

Centrifugal pumps, fans

Constant power loads

Torque changes inversely with speed

Machine tools

CLASSIFICATION OF MOTORS

Electric Motors

Alternating Current (AC) Motors

Direct Current (DC) Motors

Synchronous Induction

Three-PhaseSingle-Phase

Self ExcitedSeparately Excited

Series ShuntCompound

TYPES OF AC MOTORS

* Electrical current reverses direction* Two parts: stator and rotor

Stator: stationary electrical component Rotor: rotates the motor shaft

* Speed difficult to control* Two types

Synchronous motor Induction motor

AC MOTOR: INDUCTION MOTOR Most common motors in industry

Advantages: Simple design Inexpensive High power to weight ratio Easy to maintain Direct connection to AC power source

COMPONENTS OF INDUCTION MOTOR

A 3-phase induction motor has two main parts:

• A stator – consisting of a steel frame that supports a hollow, cylindrical core of stacked laminations. Slots on

the internal circumference of the stator house the stator winding.

• A rotor – also composed of punched laminations, with rotor slots for the rotor winding.

COMPONENTS OF INDUCTION MOTOR contd…

There are two-types of rotor windings:

• Squirrel-cage windings, which produce a squirrel-cage induction motor (most common)

• Conventional 3-phase windings made of insulated wire, which produce a wound-rotor induction motor (special characteristics)

Induction Motor: Squirrel cage rotor Squirrel cage rotor consists of copper bars,

slightly longer than the rotor, which are pushed into the slots.

The ends are welded to copper end rings, so that all the bars are short circuited.

In small motors, the bars and end-rings are diecast in aluminium to form an integral block.

Induction Motor: Wound Rotor A wound rotor has a 3-phase winding, similar to

the stator winding. The rotor winding terminals are connected to

three slip rings which turn with the rotor. The slip rings/brushes allow external resistors to be connected in series with the winding.

The external resistors are mainly used during start-up –under normal running conditions the windings short circuited externally.

* Construction is on next slide.

Wound Rotor & its connections

Induction Motor: Operating Principle Operation of 3-phase induction motors is based upon the

application of Faraday’s Law and the Lorentz Force on a conductor.

Consider a series of conductors (length L) whose extremities are shorted by bars A and B. A permanent magnet moves at a speed v, so that its magnetic field sweeps across the conductors.

Operating Principle Contd… The following sequence of events takes place:1. A voltage E = BLv is induced in each conductor while it

is being cut by the flux (Faraday’s Law)2. The induced voltage produces currents which circulate

in a loop around the conductors (through the bars).3. Since the current-carrying conductors lie in a magnetic

field, they experience a mechanical force (Lorentz force).

4. The force always acts in a direction to drag the conductor along with the magnetic field.

Now close the ladder upon itself to form a squirrel cage, and place it in a rotating magnetic field – an induction motor is formed!

Induction Motor: Rotating Field Consider a simple stator with 6 salient poles -

windings AN, BN, CN. The windings are mechanically spaced at 120° from

each other. The windings are connected to a 3-phase source. AC currents Ia, Ib and Ic will flow in the windings, but

will be displaced in time by 120°. Each winding produces its own MMF,which creates a

flux across the hollow interior of the stator. The 3 fluxes combine to produce a magnetic field that

rotates at the same frequency as the supply.

Rotating Field Contd…

Induction Motor: Stator Winding

In practice, induction motors have internal diameters that are smooth, instead of having salient poles.

In this case, each pole covers 180° of the inner circumference of the rotor (pole pitch = 180°).

Also, instead of a single coil per pole, many coils are lodged in adjacent slots.

The staggered coils are connected in series to form a phase group.

Spreading the coil in this manner creates a sinusoidal flux distribution per pole, which improves performance and makes the motor less noisy.

Stator Winding Contd…

Number of Poles – Synchronous Speed The rotating speed of the revolving flux can be reduced by

increasing the number of poles (in multiples of two). In a four-pole stator, the phase groups span an angle of 90°.

In a six-pole stator, the phase groups span an angle of 60°. This leads to the definition of synchronous speed: Ns = 120 f / pWhere Ns = synchronous speed (rpm)f = frequency of the supply (Hz)p = number of poles

For 50Hz ,synchronousSpeeds (Ns) include 3000rpm,1500rpm, 1000 rpm, 750rpm…

INDUCTION MOTOR : SLIP The difference between the synchronous speed and

rotor speed can be expressed as a percentage of synchronous speed, known as the slip.

s = (Ns – N) Ns

Where s = slip, Ns = synchronous speed (rpm), N = rotor speed (rpm)

• At no-load, the slip is nearly zero (<0.1%).• At full load, the slip for large motors rarely exceeds

0.5%. For small motors at full load, it rarely exceeds 5%.

• The slip is 100% for locked rotor.

Induction Motor: Frequency induced in the rotor

The frequency induced in the rotor depends on the slip:

fR = s f

fR = frequency of voltage and current in the rotor

f = frequency of the supply and stator fields = slip

Induction Motor: Active Power Flow Efficiency – by definition, is the ratio of output /

input power: η = PL / Pe Rotor copper losses: PJr = s Pr

Mechanical power: Pm = ( 1-s)Pr

Motor torque: Tm = 30Pr

πNs

Where: Pe = active power to stator

Pr = active power supplied to rotor

PL = Shaft Power

Power Losses

Induction Motor: Relationship between Load, Speed and Torque

At full speed: torque and stator current are zero

At start: high current and low “pull-up” torque

At start: high current and low “pull-up” torque

At 80% of full speed: highest “pull-out” torque and current drops