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INDUCTION MOTORS

Group members:1) Haji2) Ali3) Ahmad4) Adeel5) Jawad6) Babar7) Muneeb8) GM9) Umer

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ELECTRIC MOTOR

An 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.

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CLASSIFICATION OF MOTORS

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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

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AC MOTOR: INDUCTION MOTOR

Most common motors in industry

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

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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.

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CUT AWAY IMAGE

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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)

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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.

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Induction Motor: Wound Rotor

A wound rotor has a 3-phase winding, similar to the stator winding.

The rotor winding terminals are connected tothree slip rings which turn with the rotor. Theslip rings/brushes allow external resistors to beconnected in series with the winding.

The external resistors are mainly used duringstart-up –under normal running conditions thewindings short circuited externally.

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Wound Rotor & its connections

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EQUIVALENT CIRCUIT

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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) whoseextremities are shorted by bars A and B. A permanent magnet moves at a speed v, so that its magnetic field sweeps across the conductors.

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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 inductionmotor is formed!

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CONSTRUCTIONInduction 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.

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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.

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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.

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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 field

s = slip

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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

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Energy Efficiency OpportunitiesEnergy Efficiency Opportunities

Reduce intrinsic motor losses Efficiency 3-7% higher Wide range of ratings More expensive but

rapid payback Best to replace when

existing motors fail

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Use Energy Efficient Motors

Power Loss Area Efficiency Improvement

1. Fixed loss (iron) Use of thinner gauge, lower loss core steel reduces eddy current losses. Longer core adds more steel to the design, which reduces losses due to lower operating flux densities.

2. Stator I2R Use of more copper & larger conductors increases cross sectional area of stator windings. This lower resistance (R) of the windings & reduces losses due to current flow (I)

3 Rotor I2R Use of larger rotor conductor bars increases size of cross section, lowering conductor resistance (R) & losses due to current flow (I)

4 Friction & Winding Use of low loss fan design reduces losses due to air movement

5. Stray Load Loss Use of optimized design & strict quality control procedures minimizes stray load losses

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Electric Motors (INDUCTION)Electric Motors (INDUCTION)

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