ppt 3. dc_machines - large fonts
TRANSCRIPT
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DC Motors
EE 2802 Applied Electricity
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Contents
1. Introduction
2. Construction
3. Equivalent Circuit
4. Operation
5. Losses
6. Starting a DC Motor7. Types of DC Motors
8. Speed Control
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Why DC motors are so common, when DCpower systems are rare?
DC power systems are used in cars, trucks &aircrafts > use DC motors
Wide variation in speed is possible(today induction motors with solidstate drive packages are mostlyused)
1. Introduction
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2. Construction
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DC Motor Stator
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Permanent magnets
Can eliminate the power loss
Improve the efficiency Disadvantage - constant flux
Field windings around the poles
Can control flux(by regulating the current in the winding)
Stator
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DC Motor Rotor
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Rotating part is called the armature
Made of highly permeable electricallyinsulated thin laminations;
that are stacked together
Electrical insulation reduces eddy currentlosses
Slotsto house the armature windings
Rotor
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Commutator- made of copper segments- insulated from one another- converts alternating emf into a uni-
directional voltage
Electrical connection between external circuitand the armature coils
- established using the stationarybrushes
Rotor
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2 types Lap windingWave winding
Its the way the armature winding is connectedto the commutator
Armature winding
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Lap winding
No. of brushes and the parallel paths betweenbrushes = No. of poles
Used in low voltage, high current applications
Armature winding
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Wave winding
No. of brushes and the parallel paths betweenbrushes = 2
Used in high voltage, low current applications
Armature winding
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Armature circuit by Eaand Ra Field coils by Rfw and Nf Rfxexternal variable resistor to control the
amount of filed circuit current
3. Equivalent Circuit
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A uniform magnetic field is created by the poles
Armature conductors are connected to a DCpower source > carry current
Current carrying conductors are now placed in amagnetic field
Will experiences a force / torque
Armature starts rotating > an emf is induced
4. Operation
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Operation of a DC Generator
Driven by a source of mechanical power
(prime mover)
Prime mover
A steam turbineA diesel engine
An electric motor
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Induced emf
For a generator -> Generated emf
For a motor -> Back emf
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Developed Torque
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Magnetization Characteristic
If the armature is rotated at the rated speed;
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Current in each conductor under a same pole mustbe in the same direction
As the conductor moves from one pole to the next,
there must be a reversal of the current
This process of reversal is known as Commutation
Commutation
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Brush Positions
GNA (Geometrical Neutral Axis)
- axis of symmetry between 2 adjacent poles
MNA (Magnetic Neutral Axis)- axis drawn perpendicular to the mean
direction of the flux passing through the centre of
the armature
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For sparkles commutation, the brushes must lie
along MNA
With no current in the armature conductors, theMNA coincides with GNA.
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Armature Reaction
Flux by the field winding - main flux
Flux by the armature winding - armature flux
Armature flux distorts and weakens the main flux
This action is called armature reaction
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(Only one pole is shown for clarity)
Effects: Flux weakening MNA shift (brushes are not aligned with MNA)
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Solutions for Armature Reaction
1) Brush shifting
2) Commutating poles (Interpoles)
3) Compensating winding
Brush shifting
Automatic brush shifting mechanisms (in largemachines)
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Commutating poles (Interpoles)
Small poles placed between the main poles Interpole winding - in series with armature
winding Interpoles produce flux that opposes the
armature fluxCompensating winding
Slots cut in the pole faces Compensating winding on those slots Compensating winding produce flux that
opposes the armature flux
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1. Copper losses2. Rotational losses
1. Friction & windage losses
2. Magnetic loss (Core losses)
3. Stray load losses
Copper Losses
Armature winding loss Field (shunt / series) winding loss
Interpole winding loss (if any)
Compensating winding loss (if any)
5. Losses
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Rotational Losses
Friction and Windage Loss Bearing friction loss
- friction between bearings and shaft
Brush friction loss
- friction between brushes and commutatorWindage loss
- drag on the armature caused by air gap
Magnetic Loss
Hysteresis loss
Eddy current loss
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Stray Load Losses
Due to the distorted flux due to armaturereaction
About 1% of the output power in large
machines Can be neglected in small machines
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Power Flow Diagram For a Generator
For a Motor
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At starting Ea= 0
As Rais very small, Iawill be extremely high
This high current will permanently damage thearmature winding
6. Starting a DC Motor
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An external resistance is added in series with the
armature circuit
It is gradually decreased
Finally, cut out from the armature circuit
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7. Types of DC Motors
1. Separately Excited DC Motor
Field winding is supplied by a separate
power source
2. Series DC MotorField winding in series with armature
winding
3. Shunt DC Motor
Field winding in parallel with armaturewinding
4. Compound DC Motor
2 field windings (series & shunt)
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Separately Excited DC Motor
External power source for field winding
Mainly used in testing purposes (laboratories)
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Series DC Motor
Series filed winding carries rated Ia
Ia changes with load -> main flux changes
Flux is a function of Ia
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High torque at low speeds
Low torque at high speeds
Suitable for hoists, cranes and electric trains
Developed torque can be controlled bycontrolling the applied voltage
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Example 1
A series motor operates in the linear region inwhich the flux is proportional to the armaturecurrent. When the armature current is 12A, themotor speed is 600 rpm. The line voltage is 120 V,
the armature resistance is 0.7 and the seriesfield winding resistance is 0.5.
What is the torque developed by the motor?
For the motor to operate at a speed of 2400 rpm,determine the;
(a) armature current and
(b) driving torque
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For a constant Vs
-> main flux is constant
Shunt DC Motor
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When the load (Ia) increases-> Eadecreases
When armature reaction is negligibleFlux is constant speed decreases (curve a)
When armature reaction is considered
Flux decreases speed increases (curves b, c & d)
Curve d is not desirable
add stabilizing winding in series with armature
winding
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Compound DC MotorLongshunt compound DC motor:
Shunt filed winding is connected across thepower source
Flux created by the shunt filed winding is
constant
Shortshunt compound DC motor:
Shunt field winding connected across the
armature terminals
Flux created is decreases with an increase in
the load
(due to voltage drop across series filed
winding)
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Long shunt compound DC motor
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Short shunt compound DC motor
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Cumulative compound motor:Flux produced by the series field windingaids the flux produced by the shunt fieldwinding
Differential compound motor:Flux produced by the series field windingopposes the flux produced by the shunt
filed winding
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As we increase the load on a long-shuntcompound motor;
1) Series winding current increases
-> total flux increases/decreases
(+) for cumulative and () for differential
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2) Torque increases/decreases at a faster rate thana shunt motor
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3) Increases the voltage drop across Ra and Rs
- > decrease/increase the speed rapidly thanin a shunt motor
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1. Armature resistance control method
2. Field control method
3. Armature voltage control method(Ward Leonard system)
8. Speed Control
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Add external resistance in the armature circuit
Starting resistor can also be used
Increase external resistance -> decrease speed
Suitable to operate at a speed < rated speed
(while delivering the same torque)
Power loss in the external resistor
Efficiency decreases
Armature Resistance Control Method
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Example 2
A 220V DC shunt motor drives a pump whosetorque varies as the square of the speed. When themotor runs at 900 rpm, it takes 47A from the
supply. The shunt filed current is 2A, and thearmature resistance is 0.5.
What resistance must be inserted in the armature
circuit in order to reduce its speed to 600 rpm?
Calculate the power loss at the external resistance.
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Control field current -> control flux
Insert an external resistor
in series with the shunt field winding
in parallel with the series field winding
Field current is very small -> power loss at theexternal resistor is small
Flux decrease -> speed increase
Suitable to operate at a speed > rated speed
Filed Control Method
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Inserting an external resistor in series with thearmature circuit
= Applying a voltage < rated value across thearmature terminals
Apply a reduced voltage across the armature
terminals Voltage across the shunt field winding is held
constant
Armature Voltage Control Method(Ward Leonard System)
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Requires 2 generators and an AC motor
3 phase AC motor acts as a prime mover thatdrives both generators
One generator (exciter) -> field winding
Other generator -> provides a variable voltageacross armature terminals
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Advantages:
Eliminate the power loss exists in armatureresistance control method
Wide and very sensitive speed control
Disadvantages:
Requires 2 power sources
Expensive
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References
Electrical Machinery & Transformers
Guru & Hizirogly
Electrical Machines Theory & Practice
M N Bandyopadhyay
Electrical Machinery Fundamentals
Stephan J Chapman