2. Chapter 11
DC Motors

3. 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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4. 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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5. Motor Operation Principles (Cont.)
Practical motors
Counterelectromotive force (counter emf, or cemf)
Overload protection
Commutation and interpoles
Speed regulation
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6. 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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7. 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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8. Counter emf Counter emf = Speed × Field strength × K
K equals a constant, which varies in different motors
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9. Counter emf (Cont.)
Actual effective voltage when applied to the windings in the armature
Esource – Ecounter = Earmature
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10. Counter emf (Cont.)
Current flowing in the armature windings at any given instant, when the resistance of the windings is known
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11. 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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12. Overload Protection Fuses and circuit breakers Thermo-overload devices
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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13. Overload Protection (Cont.)
Bimetallic overload devices
Contacts separate when current reaches a set level
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14. Commutation and Interpoles
Reducing sparks at commutator brushes
Interpole
Commutating poles
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15. Speed Regulation Percent of speed regulation
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16. Review What changes electrical power into mechanical power?
Electric motor
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17. Review What can replace permanent field magnets? Field windings
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18. Review What is torque? Turning power of the motor
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19. Review What is the formula for solving for counter emf?
Counter emf = Speed × Field strength × K
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20. Review
When rotation of the motor armature slows down, is there more or less counter emf generated?
Less
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21. Review
What percentage of full-load current should a fuse or breaker use?
No more than 150%
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22. Review What is a thermo-overload device?
A simple ratch wheel device that causes the circuit to open
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23. Review What is an interpole?
A smaller field pole placed midway between main field poles
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24. DC Motors Types Motor starting circuits Thyristor motor controls
Shunt
Series
Compound
Motor starting circuits
Thyristor motor controls
Universal motors
Permanent magnet motors
Servo motors
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25. Shunt DC Motors Constant speed motors Input power
Power = Applied voltage × Total current
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26. Series DC Motors Develop high torque under load Armature speed low
Low cemf
High armature current
Increased torque
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27. Compound DC Motors Series winding and shunt field winding
Cumulative compound motors
Develop high starting torque
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28. 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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29. Motor Starting Circuits
Variable starting resistance
Maximum resistance at start, then decreased
Manual starters
Automatic starters
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30. Manual Starters Starting resistance adjusted by hand
Resistance subject to human error
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31. 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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32. Thyristor Motor Controls
Semiconductors that control motor speed
Silicon controlled rectifier (SCR)
Controls half-wave
Triac
Controls full ac wave
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33. Universal Motors Series motor should be used AC used to run a dc motor
Not used for heavy-duty purposes
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34. Permanent Magnet Motors
1/50th–5 horsepower
Magnets used instead of electrical coils
(Baldor Electric Co.)
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35. DC Servo Motors
Feedback determines position of mechanism the motor is driving
Digital encoder as another feedback system
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36. 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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37. Review What are the three types of dc motors?
Shunt, series, and compound
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38. Review What is another name for a shunt motor? Constant speed motor
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39. Review What type of compound dc motor develops a high starting torque?
Cumulative compound motor
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40. 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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41. Review What are the two types of thyristor motor controls?
Silicon controlled rectifier (SCR) and triac
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42. Review What motors operate on either ac or dc power? Universal motors
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43. 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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44. 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?
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45. Glossary Bimetallic overload device Commutating pole
Overload protection device that contains two different metals that expand to different lengths, causing an open circuit.
Commutating pole
An auxiliary pole located midway between main poles of a generator to establish flux or satisfactory commutation.
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46. Glossary Constant speed motor Counterelectromotive force (cemf)
A motor in which the field windings shunt across, or parallel to, the armature.
Counterelectromotive force (cemf)
Voltage induced in a conductor moving through a magnetic field that opposes source voltage.
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47. Glossary Cumulative compound motor DC machine
A motor in which the magnetic field on the series winding reinforces the magnetic field of the shunt winding.
DC machine
A device that can be used as either a dc generator or a dc motor.
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48. Glossary Differential compound motor Digital encoder
A motor with two windings that oppose each other magnetically.
Digital encoder
A type of feedback system that sends pulses back to the control unit rather than varying the voltage.
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49. Glossary Electric motor Field winding
Converts electrical power into rotating mechanical power.
Field winding
An electromagnet that can replace a permanent field magnet in a generator motor to make it more powerful.
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50. Glossary Interpole Locked rotor
An auxiliary pole located midway between main poles of a generator to establish flux or satisfactory commutation.
Locked rotor
A rotor that is not turning while power is being applied to the motor because of a physical impedance.
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51. Glossary Percent of speed regulation Push-button starter
A ratio of the speed under no-load conditions to the speed under full-load, expressed as a percentage of the full-load speed.
Push-button starter
Heavy-duty relay operated by momentary contact push-button switches.
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52. Glossary
Servo motor
Any motor that is modified to give feedback concerning the motor’s speed, direction of rotation, and number of revolutions.
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53. Glossary Thermo-overload Torque
An overload protection device for motors that consists of a simple ratchet wheel held in place by a metal alloy such as solder. When the overload condition generates sufficient current flow to melt the solder, the wheel is free to rotate, causing the circuit to open.
Torque
Turning power of a motor.
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54. Glossary Universal motor
1. Series ac motor that also operates on dc. 2. Fractional horsepower ac/dc motor.
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