ppt 3. dc_machines - large fonts

7/26/2019 Ppt 3. DC_Machines - Large Fonts

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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

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