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Chapter 11 DC Motors © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Objectives Explain the operating principles.

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Presentation on theme: "Chapter 11 DC Motors © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Objectives Explain the operating principles."— Presentation transcript:

1

2 Chapter 11 DC Motors

3 © 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.

4 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Motor Operation Principles Electric motors are used for hundreds of useful machines, including refrigerators and vacuum cleanersElectric motors Like poles repel each other Unlike poles attract each other

5 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Motor Operation Principles (Cont.) Practical motors Counterelectromotive force (counter emf, or cemf)Counterelectromotive force (counter emf, or cemf) Overload protection Commutation and interpolesinterpoles Speed regulation

6 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Motor Operation Principles (Cont.) The dc generator and motor are often interchangeable in use –dc machinesdc machines Field magnets can be replaced by field windingsfield windings

7 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Practical Motors Industry –Rotational force from different source –Interaction between two fields –Armature coils connected to commutator sections –Torque of motor increasedTorque

8 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Counter emf Counter emf = Speed × Field strength × K K equals a constant, which varies in different motors

9 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Counter emf (Cont.) Actual effective voltage when applied to the windings in the armature E source – E counter = E armature

10 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Counter emf (Cont.) Current flowing in the armature windings at any given instant, when the resistance of the windings is known

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

12 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Overload Protection Fuses and circuit breakers –No more than 150% of the full-load current –Protect against short circuits or locked rotorslocked rotors Thermo-overload devicesThermo-overload –Actual overload protection –115%–125% of full-load current

13 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Overload Protection (Cont.) Bimetallic overload devicesBimetallic overload devices –Contacts separate when current reaches a set level

14 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Commutation and Interpoles Reducing sparks at commutator brushes –Interpole –Commutating polesCommutating poles

15 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Speed Regulation Percent of speed regulation

16 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Review What changes electrical power into mechanical power? Electric motor

17 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Review What can replace permanent field magnets? Field windings

18 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Review What is torque? Turning power of the motor

19 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Review What is the formula for solving for counter emf? Counter emf = Speed × Field strength × K

20 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Review When rotation of the motor armature slows down, is there more or less counter emf generated? Less

21 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Review What percentage of full-load current should a fuse or breaker use? No more than 150%

22 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Review What is a thermo-overload device? A simple ratch wheel device that causes the circuit to open

23 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Review What is an interpole? A smaller field pole placed midway between main field poles

24 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. DC Motors Types –Shunt –Series –Compound Motor starting circuits Thyristor motor controls Universal motors Permanent magnet motors Servo motors

25 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Shunt DC Motors Constant speed motors Input power –Power = Applied voltage × Total current

26 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Series DC Motors Develop high torque under load Armature speed low Low cemf High armature current Increased torque

27 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Compound DC Motors Series winding and shunt field winding Cumulative compound motorsCumulative compound motors –Develop high starting torque

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

29 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Motor Starting Circuits Variable starting resistance Maximum resistance at start, then decreased Manual starters Automatic starters

30 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Manual Starters Starting resistance adjusted by hand Resistance subject to human error

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

32 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Thyristor Motor Controls Semiconductors that control motor speed –Silicon controlled rectifier (SCR) Controls half-wave –Triac Controls full ac wave

33 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Universal Motors Series motor should be used AC used to run a dc motor Not used for heavy-duty purposes

34 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Permanent Magnet Motors 1/50 th –5 horsepower Magnets used instead of electrical coils (Baldor Electric Co.)

35 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. DC Servo MotorsServo Motors Feedback determines position of mechanism the motor is driving Digital encoder as another feedback systemDigital encoder

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

37 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Review What are the three types of dc motors? Shunt, series, and compound

38 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Review What is another name for a shunt motor? Constant speed motor

39 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Review What type of compound dc motor develops a high starting torque? Cumulative compound motor

40 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Review How is a differential compound motor different from a cumulative compound motor? It has two windings connected to oppose each other magnetically

41 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Review What are the two types of thyristor motor controls? Silicon controlled rectifier (SCR) and triac

42 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Review What motors operate on either ac or dc power? Universal motors

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

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

45 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Glossary Bimetallic overload device –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.

46 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Glossary Constant speed motor –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.

47 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Glossary Cumulative compound motor –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.

48 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Glossary Differential compound motor –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.

49 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Glossary Electric motor –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.

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

51 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Glossary Percent of speed regulation –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.

52 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Glossary Servo motor –Any motor that is modified to give feedback concerning the motor’s speed, direction of rotation, and number of revolutions.

53 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Glossary Thermo-overload –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.

54 © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Glossary Universal motor –1. Series ac motor that also operates on dc. 2. Fractional horsepower ac/dc motor.


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