Three phase motors

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Author:
johnbowens
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107557
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Three phase motors
Updated:
2012-01-14 00:39:56
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Three phase motors
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NJATC chapter 4 three phase motors ISBN 978-0-8269-1982-3
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  1. The unmoving part of a 3 phase motor, consisting of a core and windings, or coil is called what?ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 74
    stator.
  2. An alternator provides what type of current? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 71
    AC.
  3. Which is cheaper? Three phase or single phase motors? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, WB PP 17

    Why?
    3 phase.

    Low maintenance requirements and economy of operation.
  4. Define synchronous speed. ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 79
    & what it is based upon.
    The speed of the field as it rotates around the stator and is based on the frequency and the number of poles.
  5. The formula to calculate synchronous speed is what? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 79
    • Synchronous speed = (120 x F)/P
    • P= number of poles
    • F= frequency
  6. There are how many stator windings for each pole in a 3 phase motor? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 76
    3
  7. In a 3 phase motor, there are 3 sine waves placed how far apart? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 79

    Is electrical degrees and mechanical degrees relatively the same thing? pp 79
    120 degrees.

    Not at all.
  8. Define rotor frequency. ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 80
    The rate at which the stator magnetic field passes the poles in the rotor.
  9. Do the rotor conductors have high resistance or reactance? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 80
    Low resistance but high reactance due to the high frequency.
  10. At synchronous speed the rotor continues to induce a current on the stator. T or F? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 80
    False. There is no longer any relative motion at synchronous speed. The stator current is controlled by CEMF determined by the rotor speed, therefore no slip.
  11. What is E coil for a Delta and a Wye configured motor? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 81
    • E coil (Delta) = E line
    • E coil (Wye) = .58 x E line
  12. What represents the true power and working force of a motor? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors
    The HP available at the end of the shaft or FLC.
  13. What must be done to reverse a 3 phase motor? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, WB 18

    What is the industry standard? Why?
    interchange any two of the 3 phase power lines to the motor.

    Interchange T1 and T3 only. Due to Hi legs, and allows T2 to be permanently connected to L2 to insulate T1 and T3 from each other.
  14. A motor operating at 60 HZ operates at what speed compared to one operating at 50 HZ? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 90
    20 percent faster.
  15. What is the difference between torque and work force? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 91
    • Torque is in lbs per ft and work is in ft lbs.
    • Torque = length x force
  16. The three types of loads are what? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 93
    constant horsepower, constant-torque, and variale-torque loads.
  17. Multispeed 3 phase motors are designed to operate at what number of speeds? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 93
    2, 3, or 4.
  18. During startup, when is maximum torque achieved? What happens to current and voltage? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 92
    When the rotor fequency decreases to the point at which the winding reactance is equal to its resistance. At this point, the current in the rotor lags exactly 45 electrical degrees behind the voltage.
  19. What happens in regards to Horsepower in a constant torque load motor? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 93
    HP changes proportionally to the speed.
  20. ____________ is also refered to as breakaway torque or starting torque. ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, WB 18
    Pull-up torque.
  21. When can true power only be produced? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 95
    When current and voltage are both positive or both negative.
  22. Power Factor is sometimes called what? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 96
    Displacement Power Factor.
  23. What is the Formula for Power Factor? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 96
  24. A low power factor indicates what about a motor? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 96
    That the motor is drawing more current than required by the load.
  25. The power factor can be improved how? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 96
    The power factor can be improved by adding capacitors in parallel with the inductance.
  26. A power factor correction capacitor should meet what requirements? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 96
    It should have the same amount of reactance as the inductor to canel out the reactive power of the unductor.
  27. Motor efficiency is calculated how? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 96
    By dividing the output power by the input power.
  28. The five major components of motor energy losses are what? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 96
    • The five major components of motor energy losses are
    • 1 resistance: stator and rotor
    • 2 bearing: friction loss
    • 3 core: hysterisis and eddy currents
    • 4 windage: cooling blades
    • 5 sound: it takes energy to make noise
  29. Resistance losses are found in what components of a motor? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 97
    Resistance losses in a motor are found in the stator and the rotor.
  30. Two types of diagrams used with motor cuircuits are? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 99
    Ladder and wiring.
  31. What type of drawing does not show the location of components in a motor circuit? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 99
    Ladder diagram versus wiring diagram.
  32. What can be used to measure the voltage and current unbalance? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 100
    A 3 phase power quality meter.
  33. Can a motor be rewired in the field from delta to wye or wye to delta? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 101 (summary)
    NO!
  34. Define eddy current. ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 98
    This type of current is an undesired current circulating in the stator and rotor core caused by magnetic induction.
  35. Define hysterisis. ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 98
    This is a loss due to the power consumed to realign the magnetic domains in the iron twice every electrical cycle.
  36. Define flux linkage loss. ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 98
    This is the loss of flux in the air gap because he air gap has increased reluctance compared to the cores.
  37. Power factor correction capacitors can be placed where on an electric motor drive in the AC supply lines? Not where? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 96
    These can be placed ahead of an electric motor drive in the AC supply lines but not between the drive and motor.
  38. The four most common types of torque related to motors are what? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 91
    Locked rotor torque, full-load torque, pull-up torque, and breakdown torque.
  39. At start up when a motor is in locked rotor, what are the only limits to current flow? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 80
    Windage resistance and reactants are the only limits to current flow at this point in a three-phase motor.
  40. At start up when a motor is in lock rotor, what are the only limits to current flow? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 80
    In a three-phase motor windage resistance and reactance are the only limits to current flow at this point.
  41. In a three-phase motor as a load is placed on the rotor, what happens? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 80
    The rotor slows and the frequency increases in a three-phase motor when this happens. Producing a stronger pole to provide the necessary torque to carry the load.
  42. Describe the operating principle that causes three-phase motor rotation. ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 79
    Electrical energy creates a field in the stater windings that rotates around the frame. These revolving field poles cut the conductors in the rotor, creating unlike poles that interact with each other to cause this.
  43. Defined a salient pole or projecting pole. ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 78
    This is a pole that extends away from the core toward the stator or extends away from the stator toward the rotor.
  44. In a three-phase motor, there are three stator windings for each pole. The windings are placed in groups of three with each group connected in series or parallel for each phase? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 76
    These are grouped and placed in series for each phase in a three-phase motor.
  45. The stator in a three-phase motor consists of what? What does the rotor consist of? ATP & NJATC, Motors 2nd edition, Chapter 4: Three phase motors, PP 74
    These both consist of a core and windings in each, respectively, in a three-phase motor.

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