Chapter 11 Generator

8/8/2019 Chapter 11 Generator

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Chapter 11

Electrical Generators• A generator is a device that converts

mechanical energy (motion) into electricalenergy (current – voltage).

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Basic Principle of Generators

• Faraday’s Law:

– “When a piece of wire moves within amagnetic field, it causes current to be inducedin the conductor.”

See figures on page 101 and 102

2



Magnitude of Induced Current

• I is proportional to the speed of the conductor (v)

• I is proportional to the angle of travel of theconductor with respect to the magnetic field (B).

– I is max when v is perpendicular to B.

– I is min (I=0) when v is parallel to B.

– I = K B V (sin of angle between v and B)

3



Basic Generator

• The basic generator consists of a loop of wirewound on an armature drum residing within amagnetic field (B) produced by a permanentmagnet.

• Each end of the loop is connected to a slip ring

which conducts electricity.

• Attached to each slip ring are electrical contactorscalled “brushes”.

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See figures on page 103.



Working of a Generator

As the loop (armature) turns, both ends of the loopstart to rotate a circular path.

• At 0 degrees the loop v is parallel to B and I = 0

• At 90 degrees, loop v is perpendicular to B and Ireaches its maximum value.

• At 180 degrees, loop v is parallel to B and I = 0

• At 270 degrees, loop v is perpendicular to B and Ireaches its minimum value.

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• egrees, oop s para e o an =

• At any other angle I = K v B (sin angle v,B)

See figures on pages 104, 105, and 106



Methods of Generating Electricity

1. Hydroelectric (Water) Plants.

2. Nuclear power plants.

3. Coal driven power plants.

4. Wind power plants.

5. Geothermal power plants.

6. Solar power plants.

• The first five operate with a generator moved by

6

, .

• The last one (solar), the electricity is produced

by the action of the sun given energy toelectrons to move on semiconductor materials.



To Increase the current

• Remember that I = K v B (sin of angle v,B)

• Then we can increase I by:

– Increasing v.

– Increasing B – Using many loops in parallel

7



To Increase the Frequency (f)

• Increase the angular velocity of the loop (rotationof the loop)

• However, by doing this, the amplitude of currentand voltage also increase.

8



Single-Phase Generator

• A generators with a single source or ACvoltage is called a Single-Phase generator.

See figure on bottom of page 107

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Two-Phase Generator

• Two-Phase Generator is a generator builtwith two loops at 90 degrees as shown inFigure on page 108.

See figure on page 108

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Three-Phase Generator

• Three-Phase Generator is a generatorbuilt with three loops at 60 degrees asshown in figure on page 109.


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To Increase the Generator Output

1. Increase the strength of the electromagneticfield.

2. Increase the number of wires which make upthe loop.

3. Increase the rotation speed of the loop,however, remember that by doing this, theoutput frequency is also changed.


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Website



Chapter 12: DC Motors

• Motor operation is dependent on theinteraction of magnetic fields

• To understand how a motor operates, weneed to review:

– The rules of magnetism. – The relationship between I and B.

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Magnetism

• A permanent magnet has two poles, Nand S, and has an electromagnetic field(B), with flux lines traveling from N to Sexternal to the magnet.

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Effect of Like / Unlike Poles

• Like poles of a magnet repel.

• Unlike poles attract each other.

15



Current Flow and Magnetic Field

• A current flow in a conductor produces amagnetic field.

– Perpendicular to the current flow.

– Left-hand rule gives direction of magnetic field

• Thumb = direction of conductor motion

• Index finger = magnetic lines of force

• Middle finger = induced current in conductor.

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See figure on top of page 105.



Electromagnets

• Electromagnets are built with a conductorformed into a coil around an iron core.

See figure on top of page 114

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Direct Current Motors

• A simple DC motor looks just like the DCgenerator. . .

• . . . but, in the generator the input is themotion and the output is the current . . .

• . . . and, in the DC motor the input is thecurrent and the output is the motion.

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See figure on page 112.



Operation of a DC Motor• When voltage is applied to the loop of wire a

current flows, and a magnetic field is created thatwill interact with the field of the magnet.

• Repulsion and attraction of the fields will causethe loop to turn.

• The loop moves away from the strong field towardthe weak field.

• The direction of the rotation can be determined by“the right-hand rule”.

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See figure on page 113.



Motor / Generator notation

• Field Windings / Armature

• Stator / Rotor


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Types of DC Motors

• DC motors have two types of windings:

1. Field windings (Electromagnets)

2. Armature winding (Loop)

• Depending on how these windings areconnected to the voltage supply,motors are classified in to three types

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1. Series DC Motor

2. Shunt DC Motor

3. Compound DC Motor



Series DC Motor

• Field and armature windings in series.

• Use left-hand rule to find “N” and “S”

of magnets.

• Use right-hand rule to find if motor isturning CW or CCW.

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Use of DC Series Motors

• To operate small electrical appliances

• Portable electric tools

• cranes, winches, hoists

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Load Concerns of DC Motors

• Some load must ALWAYS be connectedto a Series DC Motor.

• Otherwise its speed will increase andmay damage the bearings or windings.

• Small motors, such as the ones used inelectric hand drills, have enough internalresistance to load themselves

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Shunt DC Motor

• Field windings and armature windings areconnected in parallel.

• Use the left-hand rule to draw theelectromagnetic fields, and right-hand

rule to show that the motor turns CW.

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See figures on bottom of page 114, and top of page 115



Speed of DC Shunt Motors

• Shunt DC motors provide constantspeed, even if the load requirementschange during operation.

• Therefore, the shunt DC Motors show

excellent speed regulation.

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Reversing a DC Motor

• Will the rotation of the motor change if we switchthe connections at the voltage source?

• NO! Because if reverse the polarity, the currentwill flow in opposite directions in both armatureand field windings.

• Verify this by inspection in the figure of page 115(shown below). Remember to use the left-handrule for the fields and the right-hand rule for the

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See figure on middle of page 115



Reversing a DC Motor (2)

• To change the rotation of a DC motor we need toensure that the current flowing in only one of thewindings (Field or loop) changes its flow.

• Using left-hand rule for the field and right-handrule for the rotation, verify that the motor below

turns CCW.


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Generator Action in DC Motors

• In the figure below, the battery voltage (emf )makes the current flow from its negative to itspositive terminal.

• Since the armature is a loop turning in a magneticfield, it induces a current in opposite direction tothe battery current producing a voltage in opposite

direction ( cemf ).

• Therefore, the total voltage (EMFT) is

emf Total = emf – cemf

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Effect of cemf on DC Motor Speed

emf Total = emf - cemf

• cemf strength depends of loop rotation speed

• Therefore, a motor at rest has no cemf .

• As motor starts to turn, cemf increases.

• Thus emf decreases and…

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• cemf is a self speed regulation in a DC motor



Speed Regulation in DC Motor

• REVIEW

• Speed of a DC motor depends on ILOOP

• ILOOP is proportional to emf Total

• cemf is directly proportional to motor speed

• SPEED REGULATION

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• oa ncreases, en mo or spee ecreases,

• cemf decreases, emf Total increases , & ILOOP

increases

• Since ILOOP increases, motor generates moreForce

• Loop turns faster compensating for anyreduction in speed due to mechanical load



Measure of Motor Output

• Output is measure in horsepower (hp).

• 1 hp = power to lift 550 pounds one foot

in one second.

1 hp = 746 Watts

- For residential uses motors < 1 hp

- Motors < 1hp are called

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* For large industrial applications motorswith multiple horsepower ratings are used

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Chapter 11 Generator