Induction Motors

Lecture 69 September 2003

MMME2104Design & Selection of Mining Equipment

Electrical Component

3-Phase Induction MotorsIntroduction

• 3-phase induction motors are simple, rugged, low-cost, and easy to maintain.

• They run at essentially constant speed from zero-to-full load.

• Therefore, they are the motors most frequently encountered in industry.

Lecture Outline• Induction Motor Components• Operating Principle• Synchronous Speed and Slip• Active Power Flow• Torque/Speed Curves• Starting & Braking Induction Motors• Abnormal Operating Conditions• Standard Classifications of Induction Motors

Induction Motor ComponentsA 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.

Induction Motor Components

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 ComponentsSquirrel 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 die-cast in aluminium to form an integral block.

Induction Motor ComponentsA 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.

Induction Motor: Operating principleOperation 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.

Induction Motor: Operating principleThe 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 –you have an induction motor!

Induction Motor: Rotating FieldConsider 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.

Induction Motor: Rotating Field

Induction Motor – Rotating Field:Direction of rotation

The phase current waveforms follow each other in the sequence A-B-C.

This produces a clockwise-rotating magnetic field.

If we interchange any two of the lines connected to the stator, the new phase sequence will be A-C-B.

This will produce a counter-clockwise rotating field, reversing the motor direction.

Induction Motor: Stator WindingIn 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.

Induction Motor:Number of Poles – Synchronous SpeedThe 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 / p

Ns = synchronous speed (rpm)f = frequency of the supply (Hz)p = number of poles

In Australia (50Hz), synchronous speeds include 3000rpm, 1500rpm, 1000 rpm, 750rpm…

Induction Motors: OperationLocked rotor: When the rotor is stationary, the field rotates at a

frequency (relative to the rotor) equal to the supply frequency. This induces a large voltage – hence large currents flow within the rotor, producing a strong torque.

Acceleration: When released, the rotor accelerates rapidly. As speed increases, the relative frequency of the magnetic field decreases. Therefore, the induced voltages and currents fall rapidly as themotor accelerates.

Synchronous speed: The relative frequency of the rotating field is zero, so the induced currents and voltages are also zero. Therefore, the torque is zero too. It follows, that induction motors are unable to reach synchronous speed due to losses such as friction.

Motor under load: The motor speed decreases until the relative frequency is large enough to generate sufficient torque to balance the load torque.

Induction Motors: SlipThe difference between the synchronous speed and rotor speed can be expressed as a percentage of synchronous speed, known as the slip.

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.

NsNNss −=

Induction Motors:Frequency induced in the rotor

sffR =

The frequency induced in the rotor depends on the slip:

fR = frequency of voltage and current in the rotor

f = frequency of the supply and stator field

s = slip

Induction Motors: Active Power FlowEfficiency – by definition, is the ratio

of output / input power:

Rotor copper losses:

Mechanical power:

Motor torque:

s

rm N

PTπ30=

rjr sPP =eL PP /=η

( ) rm PsP −= 1

Induction Motors: Torque/Speed Curve

Induction Motors: Torque/Speed Curve

Induction Motors: Torque/Speed Curve

Induction Motors: Torque/Speed Curve

The torque (Ts) produced at a slip (s) by an induction motor is given (approximately) by:

where sm is the slip for maximum torque and Tm is the maximum torque

( )ssssTT

mmm

s

+= 2

Complete Torque-Speed Curve for and Induction Motor

Induction Motors:Effect of Rotor Resistance

Induction Motors:Effect of Rotor Resistance

Induction Motors:Effect of Rotor Resistance

Wound rotor machine provides option of variable rotor resistance

Starting an Induction MotorHigh-inertia loads put a strain on induction motors

because they prolong the starting period. The current is high during this interval such that overheating is a major concern.

Rule of Thumb 1:The heat dissipated in the rotor during start-up

(from zero to rated speed) is equal to the final kinetic energy stored in all the revolving parts.

Assumes motor is not loaded mechanically (apart from inertia)

Braking an Induction MotorSometimes an induction motor (and its load) needs to be stopped

suddenly. This can be achieved by interchanging the phase sequence, so that the field is rotating backwards relative to the rotor. This is known as plugging.

During plugging, the motor absorbs kinetic energy from the still-rotating load and dissipates it as heat in the rotor. However, the motor also continues to receive electrical power from the supply, which is also dissipated as heat in the rotor.

Rule of Thumb 2:The heat dissipated in the rotor during plugging (from rated to zero

speed) is equal to three times the kinetic energy stored in all the revolving parts.

Assumes motor is not loaded mechanically (apart from inertia)

Abnormal Operating Conditions

1. Mechanical overload2. Supply voltage changes3. Frequency variation

Abnormal Operating Conditions

4. Single phasing

Standardisation of Induction MotorsThe frames of all industrial motors under 500hp

have standardised dimensions.Therefore, motors (of the same frame size) can

be interchanged without changing the mounting holes, the shaft height or the shaft coupling.

The standards also establish limiting values for electrical, mechanical and thermal characteristics (such as starting torque, locked-rotor current, overload capacity and temperature rise).

Classifications According to Operating Environment

1. Drip-proof motors2. Splash proof motors3. Totally enclosed,

non-ventilated motors

4. Totally enclosed, fan-cooled motors

5. Explosion-proof motors

Explosion proof motor

Classifications According to Electrical/Mechanical Properties

1. Motors with standard locked-rotor torque (NEMA B)• Good for fans, centrifugal pumps, machine tools…

2. High-starting torque motors (NEMA C)• Good for starting under load – hydraulic pumps and

piston-type compressors3. High-slip motors (NEMA D)

• Good for starting high-inertia loads

Classifications According to Electrical/Mechanical Properties