chapter 11 dc motors © goodheart-willcox co., inc.permission granted to reproduce for educational...

Chapter 11

DC Motors

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Objectives

• Explain the operating principles of dc motors.

• Explain counterelectromotive force.• Identify various dc motors.• Discuss the purpose for, and operation of,

motor starting circuits.• Identify and explain the operation of various

dc motors.

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Motor Operation Principles

• Electric motors are used for hundreds of useful machines, including refrigerators and vacuum cleaners

• Like poles repel each other• Unlike poles attract each other

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Motor Operation Principles (Cont.)

• Practical motors

• Counterelectromotive force (counter emf, or cemf)

• Overload protection

• Commutation and interpoles

• Speed regulation

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Motor Operation Principles (Cont.)

• The dc generator and motor are often interchangeable in use– dc machines

• Field magnets can be replaced by field windings

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Practical Motors

• Industry– Rotational force from different source– Interaction between two fields– Armature coils connected to commutator

sections– Torque of motor increased

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Counter emf

• Counter emf = Speed × Field strength × K

• K equals a constant, which varies in different motors

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Counter emf (Cont.)

• Actual effective voltage when applied to the windings in the armature

• Esource – Ecounter = Earmature

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Counter emf (Cont.)

• Current flowing in the armature windings at any given instant, when the resistance of the windings is known

armature

armaturearmature R

EI

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Counter emf (Cont.)

• As rotation of the motor armature slows down, less counter emf is generated

• There will be an increase in current through the armature

• Current increases until the motor stops rotating

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Overload Protection

• Fuses and circuit breakers– No more than 150% of the full-load current– Protect against short circuits or locked rotors

• Thermo-overload devices– Actual overload protection– 115%–125% of full-load current

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Overload Protection (Cont.)

• Bimetallic overload devices– Contacts separate

when current reaches a set level

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Commutation and Interpoles

• Reducing sparks at commutator brushes– Interpole– Commutating poles

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Speed Regulation

• Percent of speed regulation

%100 load-full Speed

load-full Speed– load-no Speed regulation speed ofPercent

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Review

What changes electrical power into mechanical power?

Electric motor

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Review

What can replace permanent field magnets?

Field windings

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Review

What is torque?

Turning power of the motor

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Review

What is the formula for solving for counter emf?

Counter emf = Speed × Field strength × K

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Review

When rotation of the motor armature slows down, is there more or less counter emf generated?

Less

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Review

What percentage of full-load current should a fuse or breaker use?

No more than 150%

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Review

What is a thermo-overload device?

A simple ratch wheel device that causes the circuit to open

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Review

What is an interpole?

A smaller field pole placed midway between main field poles

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DC Motors

• Types– Shunt– Series– Compound

• Motor starting circuits• Thyristor motor

controls

• Universal motors• Permanent magnet

motors• Servo motors

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Shunt DC Motors

• Constant speed motors• Input power

– Power = Applied voltage × Total current

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Series DC Motors

• Develop high torque under load• Armature speed low• Low cemf• High armature current• Increased torque

K strength Field

emfCounter Speed

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Compound DC Motors

• Series winding and shunt field winding

• Cumulative compound motors– Develop high starting

torque

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Compound DC Motors (Cont.)

• Differential compound motors– Starting torque is low– Used where heavy loads are applied with

varying speeds– Good speed regulation if loads do not vary

greatly

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Motor Starting Circuits

• Variable starting resistance

• Maximum resistance at start, then decreased

• Manual starters• Automatic starters

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Manual Starters

• Starting resistance adjusted by hand

• Resistance subject to human error

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Automatic Starters

• Switches• Armature contactors• Push-button starters

– The start button is pushed closed to start

– When the stop button is pushed, the circuit is opened

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Thyristor Motor Controls

• Semiconductors that control motor speed– Silicon controlled

rectifier (SCR)• Controls half-wave

– Triac• Controls full ac wave

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Universal Motors

• Series motor should be used

• AC used to run a dc motor

• Not used for heavy-duty purposes

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Permanent Magnet Motors

• 1/50th–5 horsepower• Magnets used instead of electrical coils

(Baldor Electric Co.)

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DC Servo Motors

• Feedback determines position of mechanism the motor is driving

• Digital encoder as another feedback system

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Submarines

• DC motors are propulsion units• Battery bank used to power ships when

primary source is not available• Conventional diesel engine submarines

and surface ships use electric motors for ship’s propellers

• High rpm from dc electric motor cannot be matched by addition of gear box

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Review

What are the three types of dc motors?

Shunt, series, and compound

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Review

What is another name for a shunt motor?

Constant speed motor

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Review

What type of compound dc motor develops a high starting torque?

Cumulative compound motor

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Review

How is a differential compound motor different from a cumulative compound motor?

It has two windings connected to oppose each other magnetically

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Review

What are the two types of thyristor motor controls?

Silicon controlled rectifier (SCR) and triac

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Review

What motors operate on either ac or dc power?

Universal motors

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Review

What is a motor that is modified to give feedback information concerning the motor’s speed, direction of rotation, and number of revolutions?

DC servo motor

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For Discussion

• Discuss the reasons a motor gets hot when overloaded.

• Why are starters necessary on heavy-duty motors?

• Why does a dc motor increase in speed when its field strength is decreased?