Lecture 6Three Phase Induction Motors

Dr. Mostafa Elshahed

Objectives

• Learn a brief history of the electrical power systems construction.

• Differentiate between the various types of generating electric power stations.

• List and describe the more commonly used equipment in an electric substation.

• Understand and define the different elements of a protection system.

• Learn the principle of power factor improvement and evaluating the cost of

electricity.

• Define the equivalent circuit of the transformer and calculate its efficiency and

voltage regulation.

• Derive the induced voltage and torque in DC & A.C. machines and determine their

equivalent circuit.

• Determine the power flow diagram and the efficiency DC & A.C. machines.

• Differentiate between the various types of Power Electronics Rectifiers

Housing

Motor

•Large three-phase induction motor•Stator of a large induction motor

Different Types of Electrical Motors

INTRODUCTION

• The three-phase induction motor is the extensively used for various

kinds of industrial drives.

• AC induction motors are also the most common motors used in

main powered home appliances.

Advantages of 3 phase induction motor

• Generally easy to build and cheaper than corresponding dc or

synchronous motors

• Induction motor is robust

• The motor is driven by the rotational magnetic field produced by 3

phase currents, hence no commutator or blush is required

• Maintenance is relatively easy and at low cost

• Satisfactory efficiency and reasonable power factor

• A manageable torque-speed curve

• Stable operation under load

• Range in size from few Watts to several MW

Disadvantages of 3 phase induction motor

• Induction motor has low inherent starting torque

• Draw large starting currents, typically 6-8 x their full load values

• Speeds not easily controlled as DC motors

• Operate with a poor lagging power factor when lightly loaded

Induction Motor Components

An 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.

•The rotor is separated from the stator by a small air-gap which ranges

from 0.4 mm to 4 mm, depending on the power and the size of the

motor.

•Induction Motor: Stator Winding

• Spreading the coil in this manner creates a sinusoidal flux distribution per pole,

which improves performance and makes the motor less noisy (sound and

electrically).

Types of ac induction motor rotors

There are two-types of rotor windings:

Squirrel-cage windings, which produce a

squirrel-cage induction motor (most common)

Almost 90% of the three-phase AC

Induction motors are of this type.

Conventional 3-phase windings made of

insulated wire, which produce a wound-rotor

induction motor (special characteristics)

Winding/Rotor Arrangements for3 Phase Induction Machine

Three Phase Slip Ring Induction Motor

• A wound rotor or slip ring motor 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.

ThreePhaseSupply

Brush

Rotor Windings Slip RingsStator Windings

Running Position

Starting PositionExternal Resistors

•Expensive to manufacture and it is vulnerable to overheat

•but we can control the starting torque and running characteristics

•Squirrel Cage Rotor

Induced Voltage Generation

Faraday’s law

• Voltage is induced in a

conductor that moves

perpendicular to a

magnetic field,

• The induced voltage is:

E= Blv

Magnetic field B into page

Conductormoving

upward withspeed v

Induced voltage V

Conductor length L

v v

•Voltage induced in a

conductor moving through a

magnetic field.

Motor Force Generation

• The interaction between the magnetic field B and the current generates a force

F = B L I+

B B B B

F

B

•Force direction on a current-carrying conductor placed in a magnetic field (B) (current into the page).

Rotating Field Concept

• Before discussing the theory of operation for the induction motor a

very basic concept, that of a rotating field, must be understood.

• A rotating and constant resultant magnetic field rotating at a

constant speed may be produced by any three-phase group of

windings displaced in space if the currents flowing through the

windings are also displaced in time.

Instantaneous Values of three phase Voltages

VoltageField

Operation Principle

• The three fluxes generated by the phase windings are separated by 120° in space and in time for a two-pole motor

• The total flux in the machine is the sum of the three fluxes.

• The summation of the three ac fluxes results in a rotating flux, which turns with constant speed and has constant amplitude.

• The rotating flux induces a voltage in the short-circuited bars of the rotor. This voltage drives current through the bars.

• The interaction of the rotating flux and the rotor current generates a force that drives the motor.

• The force is proportional with the flux density and the rotor bar current

• The voltage and current generation in the rotor bar require a speed difference between the rotating field and the rotor.

• Consequently, the rotor speed is always less than the magnetic field speed.

• The speed of the rotating field:

• Rotor Speed:

• Slip Speed:

• Slip:

• The frequency in the rotor:

fR = frequency of rotor voltage and

current

P

fns

120

mn

ms nn

s

ms

n

nns

sff R

For an induction motor with P poles and f frequency,

•Slip

• For induction motors a very important parameter is the slip of the

motor The slip, s, defines the relative speed difference between

synchronous speed and rotor speed and is given by:

• where ω is expressed in rads/s and n is expressed in 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 (Starting).

s

ms

s

ms

n

nns

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 counterclockwise rotating

field, reversing the motor direction.

Stator Rotor

Xrot_m

= rot

Lrot

Rrot

Irot

Vrot = s Vrot_s

Rsta

Irot_t

VstaVsup

IstaXmRc

Xsta = sy Lsta

Phase equivalent circuit of a three-phase induction motor.

Torque versus speed

Fan

Conveyer

Torque versus slip

s

ms

n

nns

Power Flow Diagram

Power and Losses Chart of an induction motor

AIR

GAP

Stator Copperloss Pcu1

Statorinputpower

Pin

Powertransferred

to rotor

Rotorinputpower

Core loss(Hysteresis andeddy current)

Pc1

Rotor Copper LossPcu2

MechanicalLosses (Fricton& Windage)

Pmech

Net (usable)Mechanical

Power outputPout

GrossMechanical

PoweroutputPgross

• The supply power is:

cos3nP LL IVi • The motor efficiency:

• Motor torque:

• Rotor cu losses, gap power and developed power:

in

out

P

P

dev

devout

out

PT

PT

GapCuGapDevGapCu PsPPPsPP 1 22

Example

Example

Selected Problems

3 A three phase, four- pole, 30-hp, 220-V, 60-Hz, Y-connected

induction motor draws a current of 77 A from the line source at a

power factor of 0.88. At this operating condition, the motor losses

are known to be the following:

Stator copper losses = 1033 W Stator core losses = 485 W

Rotor copper losses = 1299 W Rotational losses = 540 W

Determine the power transferred across the air gap, the internally

developed torque, the slip, the horsepower output, the motor speed

in rpm, the torque corresponding to the rotational losses, and the

efficiency of operation at the stated condition.

Typical Name Plate Of An Ac Induction Motor

P

fns

120

P

fns

120

Gives step speeds Washing Machines