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INTRODUCTIONA 3-phase induction motor cannot be operated at
variable speed.1-phase induction motor is not self starting.Thus,
A.C. commutator motors have been developed to provide improved
starting torque, good power factor and better speed control.A.C.
Commutator Motor
1-Phase Motor 3-Phase Motor A.C. Series Motor 3- Series Comm.
Motor Universal Motor 3- Shunt Comm. Motor Repulsion Motor Schrage
Motor
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AC SERIES MOTORIf A.C. supply is given to a d.c. series motor it
can start and run because the torque produced is proportional to
the product of the flux and the armature current.
During positive half cycle, the flux and the armature current
are in positive direction thus torque is in positive direction.
During negative half cycle, both the flux and the armature
current reverse but its product is positive. So, torque remains
positive.
This kind of motor is known as A.C. Series motor.
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PROBLEMS IN A.C SERIES MOTORThe alternating flux would cause
excessive eddy current loss in the yoke and field cores which will
become extremely heated.
Vicious sparking will occur at brushes because of the huge
voltage and current induced in the short-circuited armature coils
during their commutation period.
Power factor is low because of high inductance of field and
armature circuits.
Torque developed is not of constant magnitude, but pulsates
between zero and maximum value each half-cycles.
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MODIFICATION IN DESIGN OF AC SERIES MOTORThe eddy current loss
has been reduced by laminating the entire iron structure of the
field cores and yoke by silicon steel.In order to improve power
factor ,reactance of the field winding and the armature winding
should be reduced.Field reactance is reduced by reducing the number
of turns on the field windings. For a given current, it will reduce
the field m.m.f. which will result in reduced air- gap flux.This
will increase the speed but reduce motor torque. For same torque,
the number of armature turns should be increase
proportionately.
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Increasing armature turns will increase inductive reactance of
the armature, so the overall reactance of the motor will not be
decreased.Increased armature m.m.f can be neutralized by
compensating winding .On the basis of the compensating winding
there are two types of connection :Conductively compensated
motorInductively compensated motor
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Conductively compensated motor:The compensated winding is
connected in series with the armature.Inductively compensated
motor:The compensated winding is short- circuited and has no
interconnectionwith the motor circuit.The compensating winding act
as a short-circuited secondary ,for which the armature winding acts
as a primary. The current in the compensating winding will be
proportional to the armature current and 180 out of phase with
it.
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In A.C Series motor,the field winding associated with the
armature coil undergoing commutation acts as primary and the
armature coil during its commutating period acts as a
short-circuited secondary. This transformer action produces heavy
current in the armature coil as it passes through its commutating
period and results in vicious sparking.Unless the transformer
voltage is neutralized
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Speed- Torque Characteristics
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UNIVERSAL MOTORThere are small capacity series motors which can
be operated on d.c. supply or single phase alternating supply of
same voltage with same characteristics, called universal
motors.
They are of two types.i) Projected pole - Non compensated
typeii) Distributed field-Compensated type
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Non-compensated Type: in this the pole and yoke are not separate
but there is single lamination for the two.Series field winding is
wound over the poles.Armature winding is placed in the armature
slots.Ends of armature coils are connected to the risers of the
commutator bars.
Non Compensated Universal MotorCross-section of non compensated
universal motor
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Compensated Type: There are no projected pole in this
type.Distributed type field winding similar to single phase
induction motor is placed in the stator.Armature winding is placed
in the armature slots.Ends of armature coils are connected to the
risers of the commutator bars.
Compensated type Universal Motor
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Working of Universal MotorSupply is given
Flux is produced due to field winding carrying current.
Torque is produced on the armature
When the current flows through armature winding
Armature rotatesDirection of torque remains same during the
negative half cycle of a.c supply as the direction of both the
field & armature current are reversed.
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Speed Torque Characteristic
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Speed Control of Universal MotorResistance Method Tap Field
MethodCentrifugal MechanismThe variable resistance connected in
series with the motor used in sewing machineThe field coil number
of turns are adjusted by tapping method using switch S.Switch S is
controlled by external lever. S open, R is introduced to reduce
speed.S close, R is shorted increasing speed.The C reduces the
sparking.Used in domestic machines such as mixers.
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Reversal of Direction of RotationBy reversing the flow of
current through the armature or field windings, the direction of
rotation can be reversed for salient pole non compensated type
universal motor. This is possible by interchanging the terminals on
brush holders .In case of compensated type, the armature or field
loads are interchanged and brushes are shifted against the
direction of rotation of motor, to achieve the reversal of
direction.
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Application of Universal MotorThe universal motors are used for
domestic applications likevacuum cleanersfood processorMixershair
drierscoffee grinderselectric shavers etc.
Their other applications are blowers, portable tools
likedrilling machines and small drivers.