description - paulmarshall.id.au three phase motors.pdf · applications of a three phase induction...
TRANSCRIPT
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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
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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.
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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.
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• 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
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• 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.
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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.
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• 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.
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Right hand grip rule
• Fingers follow the direction of the current
through the coil, and the thumb points to
the north pole.
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Three windings 120° apart
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3 phase supply 120° apart
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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
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• 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
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A cage is placed inside the rotating
magnetic field
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As there is relative motion between
the rotating magnetic field and the
bars of the rotor a voltage is
induced in the bars
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As the rotor ends are shorted by
the end ring, a current flows in the
bars, creating a magnetic field
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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.
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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
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• 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
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Rotor XL and R determine torque
curve
Rotor
Frequency
Rotor XL
Rotor R
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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.
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The difference between RMF and
rotor
• This is known as slip
• It is expressed as a
percentage of RMF
%s =
nRMF - nROTOR
nRMF
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Different cages give different
curves
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Torque, speed and current
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Motor connections
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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
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1 2 3
56 4
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Delta connection
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Star connection
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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
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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
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Advantage
• By placing a magnet on the rotor the rotor
will rotate at the same speed as the
rotating magnetic field irrespective of load
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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