Topic 1 three phase induction motors

Topic 1 Three phase induction motors contents: Revise the principles of electromagnetism • Phase sequence • Three phase induction motor action • Induction motor construction • stator construction • rotor construction • Speed and slip • Line and phase voltages • Terminal block markings • Motor nameplate details • Motor rating • Star and Delta connections • Electrical isolation • Electrical testing and fault finding • Rotation reversal • Maintenance requirements

Topic 1 Three phase induction motors

• Revise the principles of electromagnetism

• Phase sequence

• Three phase induction motor action

• Induction motor construction

• stator construction

• rotor construction

• Speed and slip

• Line and phase voltages

• Terminal block markings

• Motor nameplate details

• It was discovered that when a current

flows in a conductor, it creates a magnetic field around the conductor

• The strength of the magnetic field is

proportional to the current

• When current flows in a coil, the resultant magnetic fields around each conductor combine to

create a magnet.

• In this case the magnetic lines

of force are entering the bottom and leaving the top This would make the bottom a south and

the top a north.

Right hand grip rule

• Fingers follow the direction of the current through the coil, and the thumb points to the north pole

Core material

• By placing steel into the centre of the coil,

it increases the amount of magnetism

B/H curve

Iron

Air

Cast steel Silicon Steel

Three phase generation requires

three winding sets.

distributed around the

stator.

• These are referred to as

A, B and C or Red White

and Blue (they were

Red, Yellow and Blue)

Yellow has been used to represent white

phase

N

If the magnet spins clockwise

• The phase

sequence shall be

ABC

If the magnet spins anti clockwise

shall be CBA

N A

B

C

This can be represented as a

phasor

V A

V B

V C

3 phase line and phase voltages

Motor construction

Rotor

Stator

Frame (yoke)

Stator

The stator is made up of many

stacked thin sheets of silicon

steel called laminations This

provides the magnetic circuit

for the fields Laced into the

slots of the stator is the field

windings.

The frame or yoke supports the

stator, it does not provide any

magnetic circuitry for the fields

The frame provides heat

dissipation via conduction,

radiation or fan forced, bearing

supports, mounts, terminal

connections, name plate

details, etc

Copper rotor bars

Bars squired to improve torque and reduce cogging

The rotor is constructed from many

laminations mounted on a solid steel

shaft

The rotor bars (windings) are hand

wound or machine moulded into the

laminations.

Three windings 120° apart

A1

3 phase supply 120° apart

s

N

• On a 4 pole per phase machine would

require 2 cycles to complete on revolution,

on 50Hz, this would make 25 revolutions per second or 1500rpm

• From this we can use the formula

(120 is derived from 60 seconds in a minute and two poles per magnet)

120f

P

Direction of rotation

Reverse direction

Forward reverse switch

Forward reverse contactors

A cage is placed inside the rotating

magnetic field

As there is relative motion between the rotating magnetic field and the

bars of the rotor a voltage is

induced in the bars

Faraday’s

Law

As the rotor bar ends are shorted

by the end ring, a current flows in the bars, creating a magnetic field

The Iron core is made up of thin metal sheets called laminations Each

lamination is insulated from each other to reduce circulation ‘Eddy currents’

On start

• At standstill, also known as locked rotor, the

motor acts like a shorted transformer.

• A large current is drawn from the supply

operating current

• The current in the rotor creates a magnetic field

• Some text quote 6 -8 whilst others quote 8 – 10 so to simplify we say 6 -10 We shall use 6 times in most cases in this course.

This magnetic field interacts with

the RMF to create rotation

• As the speed of the rotor increases the relative motion is reduced

• The rotor has resistance and inductance.

• As frequency decreases so does XL

• When XL = R maximum interaction

between the magnetic fields occurs

• Known as break over or break down

torque

Rotor XL and R determine torque

curve

Rotor Frequency Rotor XL

Rotor R

The rotor cannot get to the same speed as the rotating magnetic field

• As the rotor approaches synchronous

speed, the speed of the RMF, the amount

of induced voltage is very low

• Therefore the current is also low

• Reducing torque

• Even with no load on the motor, bearing and windage loss prevents the motor from achieving Synchronous speed

The difference between RMF and

Different cages give different curves

Torque, speed and current

Motor inputs

Motor outputs

Motor connections

Motor windings are placed

1 2 3

5

Delta connection

Star connection

• Check continuity of windings, 1 – 4, 2 – 5, 3 – 6.

• Insulation test each winding to earth (500V)

5

6 terminal 3 phase motor

If each motor winding has a continuity of 10Ω calculate the resistance between

terminals

If each motor winding has a continuity of 10Ω calculate the resistance between

terminals

20Ω 6.66Ω

20Ω 6.66Ω

20Ω 6.66Ω

What would be the values if one

leg open circuits

What would be the values if one

leg open circuits

20Ω 10Ω

∞ Ω 20Ω

What would the values be if one

leg short circuits

What would the values be if one

leg short circuits

10Ω 10Ω

20Ω 0Ω

Nameplate details

1 Manufacturer name and manufacturer-specific ordering code This code will typically contain information about frame size and mounting

arrangement

2 Rated volts - The voltage at which the motor is

designed to operates and give optimum

performance

3 Full load amps –.used to facilitate cable sizing, starter selection and

motor protection

4 Rated frequency in Hertz, and rated full load speed in revs/minute at that frequency.

5 Rated temp rise or insulation class – Industry standard specification of the thermal tolerance of the motor insulation.

6 Rated power (kW).

7 Electrical connection and associated operating parameters – i.e three phase motors can be connected in a ‘star’ (Υ) or ‘delta’ (Δ) ) or ‘delta’ (Δ) ) configuration In a star configuration the current flowing from the supply

is reduced as is the torque.

8 Power factor for the motor.

9 Efficiency class – IEC nameplates quote EFF ratings at full load It is a measure of how well the motor converts

electrical energy to mechanical power.