Description - paulmarshall.id.au three phase motors.pdf · applications of a three phase induction...

Description

• Methods for determining polarity magnetic flux in relation to current flow in straight conductors and solenoids

• circuit operating characteristics

• characteristics of the magnetic field produced by a three phase winding

• calculated speed of rotation of the rotating magnetic field

• basic principle of operation, construction and applications of a three phase induction motor

Description

• three phase induction motor connections

• reversing the direction of rotation of a

three phase induction motor

• equipment and methods for testing the

motor winding resistance and insulation

properties

• effects of incorrect wiring a three phase

motor.

Electromagnets

• 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.

• The direction of the magnetic field is set by the direction of the current.

• The direction can be found by using the right hand thumb rule.

• The thumb is placed in the direction of the current and the fingers follow the magnet field

• This is can also be shown by looking at

the ends of the conductor.

• Cross represents current flowing into the

screen, dot represents current flowing out

of the screen.

Made into a coil

• Many have found on the job,

that by placing a conductor

through the jaws of a clamp

meter several times the

reading is increase by a

multiplying the current by the

number of turns.

• This would read twice 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.

Three windings 120° apart

3 phase supply 120° apart

Rate of rotation

• On a 2 pole per phase machine as shown,

one revolution will occur for every cycle,

on 50Hz, this would make 50 revolutions

per second or 3000rpm.

• 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

• n = speed in rpm

• f = frequency in Hertz

• P = number of poles per phase

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

n =

120f

P

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

As the rotor ends are shorted by

the end ring, a current flows in the

bars, creating a magnetic field

On start

• At standstill, also known as locked rotor, the

motor acts like a shorted transformer.

• A large current is drawn from the supply

• This can be between 6 – 10 times the normal

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

• Therefore the amount of induced voltage

is reduced

• Therefore the current in the bars would be

reduced

• At the same time the frequency of the

induced voltage is reducing

• 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

rotor

• This is known as slip

• It is expressed as a

percentage of RMF

%s =

nRMF - nROTOR

nRMF

Different cages give different

curves

Torque, speed and current

Motor connections

Motor windings are placed

diagonally

• 1 - 4

• 2 - 5

• 3 - 6

• 1 - 5

• 2 - 6

• 3 - 4

1 2 3

56 4

1 2 3

56 4

1 2 3

56 4

Delta connection

Star connection

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

• Each reading should be identical

• Insulation test each winding to earth (500V)

• Insulation test between windings (1000V)

• Not less than 1MΩ

1 2 3

56 4

Synchronous motor

• A synchronous motor is very simular to an

Induction motor

• The stator is identical

• The rotor is the only change

• Instead of a cage a magnet or an

electromagnet is placed on the rotor

Advantage

• By placing a magnet on the rotor the rotor

will rotate at the same speed as the

rotating magnetic field irrespective of load

By increasing or decreasing the

current in the field winding, the

motor input power factor can be

changed, allowing a BMS to control

power factor correction