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Chapter 11 Chapter 11 Electrical GeneratorsElectrical Generators

• A generator is a device that converts mechanical A generator is a device that converts mechanical energy (motion) into electrical energy (current – energy (motion) into electrical energy (current – voltage).voltage).

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Basic Principle of Basic Principle of GeneratorsGenerators

• Faraday’s Law:Faraday’s Law:– ““When a piece of wire moves within a magnetic field, it When a piece of wire moves within a magnetic field, it

causes current to be induced in the conductor.”causes current to be induced in the conductor.”

See figures on page 101 and 102See figures on page 101 and 102

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Magnitude of Induced Magnitude of Induced CurrentCurrent

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

with respect to the magnetic field (B).with respect to the magnetic field (B).– I is max when v is perpendicular to B.I is max when v is perpendicular to B.– I is min (I=0) when v is parallel to B.I is min (I=0) when v is parallel to B.– I = K B V (sin of angle between v and B)I = K B V (sin of angle between v and B)

See figures on page 101 and 102See figures on page 101 and 102

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Basic GeneratorBasic Generator• The basic generator consists of a loop of wire wound on an The basic generator consists of a loop of wire wound on an

armature drum residing within a magnetic field (B) armature drum residing within a magnetic field (B) produced by a permanent magnet.produced by a permanent magnet.

• Each end of the loop is connected to a slip ring which Each end of the loop is connected to a slip ring which conducts electricity.conducts electricity.

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

See figures on page 103.See figures on page 103.

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Working of a GeneratorWorking of a GeneratorAs the loop (armature) turns, both ends of the loop start to As the loop (armature) turns, both ends of the loop start to

rotate a circular path.rotate a circular path.• At 0 degrees the loop v is parallel to B and I = 0At 0 degrees the loop v is parallel to B and I = 0• At 90 degrees, loop v is perpendicular to B and I reaches At 90 degrees, loop v is perpendicular to B and I reaches

its maximum value.its maximum value.• At 180 degrees, loop v is parallel to B and I = 0At 180 degrees, loop v is parallel to B and I = 0• At 270 degrees, loop v is perpendicular to B and I reaches At 270 degrees, loop v is perpendicular to B and I reaches

its minimum value.its minimum value.• At 360 degrees, loop is parallel to B and I = 0At 360 degrees, loop is parallel to B and I = 0• At any other angle I = K v B (sin angle v,B)At any other angle I = K v B (sin angle v,B)

See figures on pages 104, 105, and 106See figures on pages 104, 105, and 106

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Methods of Generating Methods of Generating ElectricityElectricity

1.1. Hydroelectric (Water) Plants.Hydroelectric (Water) Plants.

2.2. Nuclear power plants.Nuclear power plants.

3.3. Coal driven power plants.Coal driven power plants.

4.4. Wind power plants.Wind power plants.

5.5. Geothermal power plants.Geothermal power plants.

6.6. Solar power plants.Solar power plants.

• The first five operate with a generator moved by water, The first five operate with a generator moved by water, air or steam.air or steam.

• The last one (solar), the electricity is produced by the The last one (solar), the electricity is produced by the action of the sun given energy to electrons to move on action of the sun given energy to electrons to move on semiconductor materials.semiconductor materials.

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To Increase the currentTo Increase the current

• Remember that I = K v B (sin of angle v,B)Remember that I = K v B (sin of angle v,B)• Then we can increase I by:Then we can increase I by:

– Increasing v.Increasing v.– Increasing BIncreasing B– Using many loops in parallelUsing many loops in parallel

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To Increase the Frequency (f)To Increase the Frequency (f)

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

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

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Single-Phase GeneratorSingle-Phase Generator

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

See figure on bottom of page 107See figure on bottom of page 107

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Two-Phase GeneratorTwo-Phase Generator• Two-Phase Generator is a generator built with Two-Phase Generator is a generator built with

two loops at 90 degrees as shown in Figure on two loops at 90 degrees as shown in Figure on page 108.page 108.

See figure on page 108See figure on page 108

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Three-Phase GeneratorThree-Phase Generator• Three-Phase Generator is a generator built with Three-Phase Generator is a generator built with

three loops at 60 degrees as shown in figure on three loops at 60 degrees as shown in figure on page 109.page 109.

See figure on page 109See figure on page 109

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To Increase the Generator To Increase the Generator OutputOutput

1.1. Increase the strength of the electromagnetic field.Increase the strength of the electromagnetic field.2.2. Increase the number of wires which make up the loop.Increase the number of wires which make up the loop.3.3. Increase the rotation speed of the loop, however, remember Increase the rotation speed of the loop, however, remember

that by doing this, the output frequency is also changed.that by doing this, the output frequency is also changed.

See figure on page 110See figure on page 110

WebsiteWebsite

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Chapter 12: DC MotorsChapter 12: DC Motors• Motor operation is dependent on the interaction Motor operation is dependent on the interaction

of magnetic fieldsof magnetic fields• To understand how a motor operates, we need to To understand how a motor operates, we need to

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

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MagnetismMagnetism• A permanent magnet has two poles, N and S, and A permanent magnet has two poles, N and S, and

has an electromagnetic field (B), with flux lines has an electromagnetic field (B), with flux lines traveling from N to S external to the magnet.traveling from N to S external to the magnet.

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Effect of Like / Unlike PolesEffect of Like / Unlike Poles• Like poles of a magnet repel.Like poles of a magnet repel.• Unlike poles attract each other.Unlike poles attract each other.

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Current Flow and Magnetic Current Flow and Magnetic FieldField

• A current flow in a conductor produces a A current flow in a conductor produces a magnetic field. magnetic field.

– Perpendicular to the current flow.Perpendicular to the current flow.– Left-hand rule gives direction of magnetic fieldLeft-hand rule gives direction of magnetic field

• Thumb = direction of conductor motionThumb = direction of conductor motion• Index finger = magnetic lines of forceIndex finger = magnetic lines of force• Middle finger = induced current in conductor.Middle finger = induced current in conductor.

See figure on top of page 105.See figure on top of page 105.

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ElectromagnetsElectromagnets• Electromagnets are built with a conductor formed into a coil Electromagnets are built with a conductor formed into a coil

around an iron core.around an iron core.

See figure on top of page 114See figure on top of page 114

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Direct Current MotorsDirect Current Motors• A simple DC motor looks just like the DC generator. . .A simple DC motor looks just like the DC generator. . .• . . . but, in the generator the input is the motion and the output is the . . . but, in the generator the input is the motion and the output is the

current . . .current . . .• . . . and, in the DC motor the input is the current and the output is the . . . and, in the DC motor the input is the current and the output is the

motion.motion.

See figure on page 112.See figure on page 112.

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Operation of a DC MotorOperation of a DC Motor• When voltage is applied to the loop of wire a current flows, and a When voltage is applied to the loop of wire a current flows, and a

magnetic field is created that will interact with the field of the magnetic field is created that will interact with the field of the magnet.magnet.

• Repulsion and attraction of the fields will cause the loop to turn.Repulsion and attraction of the fields will cause the loop to turn.• The loop moves away from the strong field toward the weak field.The loop moves away from the strong field toward the weak field.• The direction of the rotation can be determined by “the right-hand The direction of the rotation can be determined by “the right-hand

rule”.rule”.

See figure on page 113.See figure on page 113.

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Motor / Generator Motor / Generator notationnotation• Field Windings / ArmatureField Windings / Armature

• Stator / RotorStator / Rotor

See figure on top of page 107See figure on top of page 107

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Types of DC MotorsTypes of DC Motors• DC motors have two types of windings:DC motors have two types of windings:

1. Field windings (Electromagnets)1. Field windings (Electromagnets)

2. Armature winding (Loop)2. Armature winding (Loop)

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

1.1. Series DC MotorSeries DC Motor

2.2. Shunt DC Motor Shunt DC Motor

3.3. Compound DC MotorCompound DC Motor

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Series DC MotorSeries DC Motor

• Field and armature windings in series.Field and armature windings in series.

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

of magnets.of magnets.

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

See figure on top of page 114See figure on top of page 114

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Use of DC Series MotorsUse of DC Series Motors• To operate small electrical appliancesTo operate small electrical appliances• Portable electric toolsPortable electric tools• cranes, winches, hoistscranes, winches, hoists

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Load Concerns of DC Load Concerns of DC Motors Motors

• Some load must ALWAYS be connected to a Some load must ALWAYS be connected to a Series DC Motor.Series DC Motor.

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

• Small motors, such as the ones used in electric Small motors, such as the ones used in electric hand drills, have enough internal resistance to hand drills, have enough internal resistance to load themselvesload themselves

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Shunt DC MotorShunt DC Motor• Field windings and armature windings are Field windings and armature windings are

connected in parallel.connected in parallel.• Use the left-hand rule to draw the Use the left-hand rule to draw the

electromagnetic fields, and right-hand rule to electromagnetic fields, and right-hand rule to show that the motor turns CW.show that the motor turns CW.

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

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Speed of DC Shunt MotorsSpeed of DC Shunt Motors• Shunt DC motors provide constant speed, even if Shunt DC motors provide constant speed, even if

the load requirements change during operation.the load requirements change during operation.• Therefore, the shunt DC Motors show excellent Therefore, the shunt DC Motors show excellent

speed regulation.speed regulation.

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Reversing a DC MotorReversing a DC Motor• Will the rotation of the motor change if we switch the Will the rotation of the motor change if we switch the

connections at the voltage source?connections at the voltage source?

• NO! Because if reverse the polarity, the current will flow in NO! Because if reverse the polarity, the current will flow in opposite directions in both armature and field windings.opposite directions in both armature and field windings.

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

See figure on middle of page 115See figure on middle of page 115

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Reversing a DC Motor (2)Reversing a DC Motor (2)• To change the rotation of a DC motor we need to ensure that the To change the rotation of a DC motor we need to ensure that the

current flowing in only one of the windings (Field or loop) changes its current flowing in only one of the windings (Field or loop) changes its flow. flow.

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

See figure on page 116See figure on page 116

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Generator Action in DC Generator Action in DC MotorsMotors• In the figure below, the battery voltage (In the figure below, the battery voltage (emf emf ) makes the ) makes the

current flow from its negative to its positive terminal.current flow from its negative to its positive terminal.

• Since the armature is a loop turning in a magnetic field, it Since the armature is a loop turning in a magnetic field, it induces a current in opposite direction to the battery induces a current in opposite direction to the battery current producing a voltage in opposite direction ( current producing a voltage in opposite direction ( cemfcemf ). ).

• Therefore, the total voltage (EMFT) isTherefore, the total voltage (EMFT) is

emfemfTotal Total = emf – cemf= emf – cemf

See figure on page 117See figure on page 117

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Effect of Effect of cemfcemf on DC Motor Speed on DC Motor Speed emfemfTotal Total = emf - cemf= emf - cemf

• cemfcemf strength depends of loop rotation speed strength depends of loop rotation speed

• Therefore, a motor at rest has no Therefore, a motor at rest has no cemfcemf..

• As motor starts to turn, As motor starts to turn, cemfcemf increases. increases.

• Thus, Thus, emfemfTotalTotal decreases, and… decreases, and…

• cemfcemf is a self speed regulation in a DC motor is a self speed regulation in a DC motor

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Speed Regulation in DC MotorSpeed Regulation in DC Motor• REVIEWREVIEW

• Speed of a DC motor depends on ISpeed of a DC motor depends on ILOOPLOOP

• IILOOP LOOP is proportional to is proportional to emfemfTotalTotal

• cemf cemf is directly proportional to motor speed is directly proportional to motor speed

• SPEED REGULATION SPEED REGULATION

• If load increases, then motor speed decreases,If load increases, then motor speed decreases,

• cemfcemf decreases, decreases, emfemfTotalTotal increases , & I increases , & ILOOP LOOP increasesincreases

• Since ISince ILOOP LOOP increases, motor generates more Forceincreases, motor generates more Force

• Loop turns faster compensating for any reduction in Loop turns faster compensating for any reduction in speed due to mechanical loadspeed due to mechanical load

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Measure of Motor OutputMeasure of Motor Output• Output is measure in horsepower (hp).Output is measure in horsepower (hp).• 1 hp = power to lift 550 pounds one foot 1 hp = power to lift 550 pounds one foot

in one second.in one second.

1 hp = 746 Watts1 hp = 746 Watts

- For residential uses motors < 1 hpFor residential uses motors < 1 hp- Motors < 1hp are called Motors < 1hp are called

Fractional Horsepower MotorsFractional Horsepower Motors

* * For large industrial applications motors with For large industrial applications motors with multiple horsepower ratings are usedmultiple horsepower ratings are used