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Chapter 11
DC Motors
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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?