Special Machines – Question and Answer

1. What are the disadvantages of repulsion motor?
  1. Speed variations with the variations in load dangerously high at no load.
  2. Low power factor, except at high speeds.
  3. Requires frequent maintenance.
  4. Higher cost.
  5. Sparking at the brushes.
2. What are the applications of universal motor?
  1. Sewing machines
  2. High speed vacuum cleaners
  3. Electric typewriters
  4. Electric shavers
  5. Motion picture projectors
  6. Portable hand tools
  7. Hair dryers
  8. Cameras
  9. Food mixers
3. What are the application of hysteresis motor?

Due to their quiet operation and ability to drive high-inertia loads, hysteresis motors are used for driving i) electric clocks, ii) timing devices, (iii) tape-decks, (iv) turn tables and other precision equipment.

4. Write some advantages of Hysteresis motor over induction motor.
  • Here, hysteresis torque remains constant from locked rotor to synchronous speed. a hysteresis motor is able to synchronize any load.
  • Due to its smooth cylindrical rotor, the motor operates quietly and does not affected from magnetic pulsations cause by slots/salient poles that are present in the rotors of other motors.
5. What is Switched Reluctance Motor?

The switched reluctance motor is a double salient, singly-excited motor. This means that it has salient poler on both the rotor and the stator, but only one member carries windings. The rotor has no windings, magnets (or) cage winding. It works on variable reluctance principle.

6. Why rotor position sensor is essential for the operation of switched reluctance motor?

It is normally necessary to use a rotor position senser for commutation and speed feed back. The turning ON and OFF operation of the various devices of power semiconductor switching circuit are influenced by signals obtained from rotor position sensor.

7. List are the disadvantages of a switched reluctance motor?
  1. Stator phase winding should be capable of carrying magnetizing current.
  2. For high speed operation developed torque has undesirable ripples is a result develops undesirable noises (or) acoustic noises.
  3. For high speed current wave form has undesirable harmonics to suppress this effect large size capacitor is to be connected.
8. What are the types of permanent magnet DC motor?

PMBL square wave motor
PMBL sine wave motor

9. Why is the PMBLDC motor called electronically commutated motor?

The PMBL DC motor is also called electronically commutated motor because the phase winding of PMBL DC motor is energized by using power semiconductor switching circuits. Hence, the power semiconductor switching circuits act as a commutator.

10. What are the applications of PMSM?
  1. Used as a direct drive traction motor.
  2. Used as high speed and high power drives for compressors, blowers, conveyors, fans, pumps, conveyors, steel rolling mills, main line traction, ship propulsion, aircraft test facilities.

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Santhakumar Raja

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