Electric Motors

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Fundamentals of DC Electric Machinery

• History• Introduction• Principle of Operation of Electric Machinery• Principle of Operation of AC Synchronous

Machines• Principle of Operation of DC Machines• Types of Electric Machines• Construction of DC Machines• DC Machine Torque Characteristics• DC Machine Speed Control• Exercise Questions

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History- Very advanced mechanisms are developed in the middle ages.- Development of the steam engine leads to the industrial

revolution in the mid 1700’s.- Michael Faraday runs the first electric motor experiment in

1821.

Images are in the public domain. Source: Wikimedia Commons, Wikimedia Foundation

A mechanical clock made in 1510

James Watt

Faraday and Daniell in the lab

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Introduction

• Electromechanical device that converts electrical energy to mechanical energy

• Mechanical energy used to:• Rotate pump impeller, fan, blower• Drive compressors• Lift materials

• Motors in industry: 70% of electrical load

What is an Electric Motor?

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• Electric Machinery:

- Electric Motors: convert electrical energy into mechanical energy.

- Electric Generators: convert mechanical energy to electric energy.

• The Electric machine is composed of:

- Stator

- Rotor

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The Stator is the stationary part of a motor.

The Rotor is a moving component of an electromagnetic system in the electric motor

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How Does an Electric Motor Work?

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Principle of Operation of AC Synchronous Machines

N S

N

S

N

S

NS

S

N

Alternators such as the one shown above are three phase synchronous machines. In synchronous motors, the magnetic field in the stator rotates, and the rotor tries to follow the stator’s magnetic field, i.e. it tries to synchronize itself with it, and thence the name synchronous motor.

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Principle of Operation of DC MachinesA Stationary magnetic field in the stator

An alternating magnetic field in the rotorBut how can we make the magnetic field alternate

in the rotor?Once the rotor rotates half a cycle, it switches

polarity, so that the magnets are again repelling

N

SBrushes

(To maintain electrical contact)

Commutators

A circuit that uses a commutation technique is used to supply an alternating current into the rotor.

Commutation Circuit

Every half cycle the current flowing in the rotor switches direction due to commutation, watch: When the rotor is energized, it rotates to the most

stable position:

N

S

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Types of Electric Motors

Electric Motors

DC MotorsSpecial Purpose

MotorsPermanent Magnet

DC (PMDC)

Series Motor

Shunt Motor

Compounded Motor

Induction Motor

Synchronous Motor

AC Motors

Separately Excited motor

Stepper motor

Brushless DC motor

Hysteresis motor

Reluctance motor

Universal motor

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Classification of Motors

Electric Motors

Alternating Current (AC) Motors

Direct Current (DC) Motors

Synchronous Induction

Three-PhaseSingle-Phase

Self ExcitedSeparately Excited

Series ShuntCompound

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Three types of Motor Load

Motor loads Description Examples

Constant torque loads

Output power varies but torque is constant

Conveyors, rotary kilns, constant-displacement pumps

Variable torque loads

Torque varies with square of operation speed

Centrifugal pumps, fans

Constant power loads

Torque changes inversely with speed

Machine tools

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DC Motors• Parts of a DC motor:

– Armature with windings– Brush assembly– Field windings– Commutator

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Construction of DC Machines

• Field Windings: In DC machines, this is the winding in the stator. Note: Permanent magnet dc motors do not have field winding but rather a permanent magnet.

• Armature Windings: The windings in the rotor .• Commutator segments: the method to convert the dc current

to an alternating current in the rotor of the dc motor is called commutation. They are connected to the rotor windings, and stay in contact with the brushes.

• Brushes: pieces made of carbon or graphite and pushed against a spring to maintain electrical contact with the commutator segments.

• Interpole windings: A third set of windings are mounted on the stator, and connected in series with the rotor to reduce the sparking between brushes and the commutator.

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• If the armature and field windings are connected in series, then the machine that results is called the DC series machine.

• If the armature and field windings are connected in parallel, then the machine that results is called the DC shunt machine.

• If the armature is connected with a field winding in series and another field winding in parallel, then the resulting machine is called the compounded machine.

• Note that the interpole windings are always connected in series with the rotor.

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• Name the type of each of the motors A, B, and C shown in the figure below:

A: Shunt motor

B: Series motor

C: Compound motor

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DC Motor ConstructionMECH1200

Interpole Windings• Due to shifting of the

magnetic flux lines, arcing and short-circuiting can result

• Interpole windings counteract this shift and reduces the arcing

• Are in series with field coils

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Recall from 1100: an induced voltage in a wire (or any conductor) results from relative motion of either the wire or the magnetic field.

Likewise, as the armature cuts through the magnetic field of the field coils (stator), a voltage is induced. (Think how a generator works.)

NS

NS

Counter (or Back) EMFMECH1200

Combined Armature and Field Lines of Force

Counter (or Back) EMFMECH1200

This induced voltage causes current to flow in the opposite direction of the applied DC current, thus “countering” it, reducing the overall current and voltage.

Counter (or Back) EMF

Counter EMF current is “out of page” (+z direction)

Armature current is “into page” (- z direction)

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Permanent Magnet DC Motors

• Excellent starting torque• Good speed regulation• Limited to low HP loads• Torque is limited to 150%

of rated torque.• Reverse direction of

rotation by interchanging armature connections.

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Series Wound - DC MotorsMECH1200

Series Wound - DC Motors

• Highest starting torque• Not used with loads that are belt or chain

coupled• Loads should always be directly coupled.

• Reverse direction of rotation by interchanging armature connections.

• Cannot be used without a load attached

Examples: Locomotives, Cranes, Hoists..

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Series Wound – DC Motors

High starting torque

Runaway motor at low/no loads

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Shunt Wound DC Motors• Best speed regulation• Open shunt field winding

will cause motor speed to increase to dangerous levels.

• Field loss relay should always be used.

• Allows simplified control for reversing the motor.

• http://www.youtube.com/watch?v=VGmoKK5Iqrs

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Shunt Wound DC Motors• Field winding are connected in parallel to the

armature windings.• Field and armature windings can be hooked to

same or different sources.• “Separately excited” – independent control of

armature and field.

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Shunt Wound DC MotorsSelf excited –

When constant speed is desired.

Field is independent of armature current.

Doesn’t provide high starting torque

Ex: Elevator, Centrifugal Pumps

Separately excited –

Can operate above its rated speed.

Speed can be increased by weakening the field in the field windings. Results in lower torque

Requires two power supplies

Ex: Woodworking Machinery, Printing Press

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• Field winding parallel with armature winding

• Current = field current + armature current

Speed constant independent of load up to certain torque

Speed control: insert resistance in armature or field current

Shunt Wound DC Motors• Properties:

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• Reverse direction by switching either armature or field winding polarities

Shunt Wound DC MotorsMECH1200

Shunt DC Motor Terminal Characteristics

Question: At which speed should we operate the motor to achieve maximum power?

Hint: The power corresponds to the area of the rectangle under the operating point on the speed-torque line. Which of the three rectangles has the largest area?

Torque (ft-lb)

Speed (RPM)

Stall point

No load speed

5 10

3000

2400

18001200

600

Maximum power is at 50% of no load speed

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Shunt DC Motor Speed Control

• There are three methods to control the speed of a DC motor:

1. Changing the armature voltage.

2. Changing armature resistance (by adding a resistor in series with armature)

3. Change the field (stator) magnetic field

Torque

Speed (ω)

As Va increases

Speed (ω)

As Ra increases

Torque

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Compound Wound DC Motors• Starting torque :

– Better than shunt-wound DC motor.

– Not as good as series-wound DC motor.

• Speed regulation – Better than series-wound DC

motor – Not as good as with shunt-wound

DC motor.• Reverse direction of rotation

by interchanging armature connections.

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DC compound motor

DC Motors

Field winding in series and parallel with armature winding

Good torque and stable speed

Higher % compound in series = high starting torque

Suited for high starting torque if high % compounding: cranes, hoists

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Series DC Motor Terminal Characteristics

Torque (ft-lb)

Speed (RPM)

Very high starting torque

If the series motor is unloaded, speed increases significantly (this condition is called Runaway,

and it must be avoided)

Shunt motor

Series motor

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DC Motor Rating PlatesMECH1200

DC STEPPER MOTORS

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• Voltage Rating – provides desired torque

• Resistance-per-winding determines– the current draw of the motor– Maximum operating speed

• Degrees per Step– Sets the number of degrees the

shaft will rotate for each full step

DC STEPPER MOTORSMECH1200

STEPPER MOTOR CONTROL

• 4 signal wires fired in the correct sequence will turn the motor

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Stepper motor locationMECH1200

How does the stepper motor fit inMECH1200

Stepper motorSTEPS

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• Winding(s) which produce the magnetic field(s) necessary to cause the rotor to turn.

3 Phase; 2 Pole Motor

What are Poles in a Motor?MECH1200

Brushless DC Motors• Rotor is permanent

magnet• Stator is wired to

provide rotating magnetic field

• HTTP://WWW.YOUTUBE.COM/WATCH?V=ZAY5JINYHXY

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Types of Motor Enclosures

• ODP – Open Drip Proof– Air flows through motor (fan blades help flow)– Used in environments free from contaminants

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Types of Motor Enclosures

• TENV – Totally Enclosed Non-Ventilating– Protect motor from corrosive and harmful

elements– Frame fins help to dissipate heat

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Types of Motor Enclosures

• TEFC – Totally enclosed Fan Cooled– Similar to TENV except has external fan for

cooling

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Types of Motor Enclosures

• XP – Explosion Proof – Similar to TEFC but enclosures are cast iron

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Hazardous Locations

Division I – Hazardous material present in the air as a norm

Division II - Hazardous material present in the air as an abnormal event

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Hazardous Locations/ ClassesMECH1200

Summary

• DC motors are:– permanent magnet– series-wound– shunt-wound– compound-wound

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Exercise Questions

1. How does the rotor of a dc motor maintain electrical contact with its commutation circuit?

By using brushes that are pushed against a spring.

2. Name two motors that do not need brushes for their rotor windings.

Squirrel cage induction motor and the brushless DC motor, and some types of stepper motors

3. Which motor supplies the highest output torque to weight ratio?

The series DC motor.

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Exercise Questions4. Why is it not recommended to use brushed dc motors in

spacecraft and some aircraft applications?

Because in the space environment there is no air to carry away the heat generated by the brushes of the dc motor, causing them to wear very quickly. The same thing applies in the low pressure environment of aircraft. Thus a brushless dc motor is used.

5. In an environment that contains explosive gases, such as in mines, which motor do you recommend using:

a) series dc motor

b) shunt dc motor

c) induction motor

d) universal motor

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6. Which motor is recommended for artificial heart applications

a) Series dc motor

b) Shunt dc motor

c) Induction motor

e) Brushless dc motor

7. What type of machines is used as an electric generator in vehicles to recharge the battery?

The synchronous generator

8. What is added to the dc motor to reduce sparking in dc motor brushes?

Interpole windings are added.

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9. Which machines have a more complex construction, DC or AC machines?

DC machines are much more complex, due to existence of interpoles and commutation circuits.

10. Which motor is potentially dangerous to operate with no load on it?

The series dc motor

11. Why series dc motors should not be used in belt drives?

Because if the belt breaks, the motor will be unloaded. Series dc motors are dangerous to operate if unloaded, as their speed will increase significantly until the motor is damaged.

12. Which type of motors can operate on both ac and dc power supplies?

The universal motor

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