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Energy Efficiency in Electrical Systems > Electric Motor > Flashcards

Electric Motor Flashcards

1
Q

What is Electric Motor?

A

Electric motors convert electrical power into mechanical power by the interaction between the magnetic fields set up in the stator and rotor windings within a motor.

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2
Q

What is Magnetic Field?

A

The magnetic field is the area around a magnet in which there is magnetic force. Moving electric charges can make magnetic fields.

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3
Q

What is Stator?

A

The stator is the stationary part of a rotary system, found in electric generators and electric motors. Energy flows through a stator to or from the rotating component of the system.

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4
Q

What is Rotor?

A

The rotor is a moving component of an electromagnetic system in the electric motor, electric generator, or alternator. Its rotation is due to the interaction between the windings and magnetic fields which produces a torque around the rotor’s axis.

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5
Q

What is the classification of industrial motor?

A

Induction Motors, Direct Current Motors or Synchronous Motors.

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6
Q

What are the operating components of motor?

A

Stator, Rotor, Bearing and Frame

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7
Q

What is Induction Motor?

A

An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding.

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8
Q

What are the types of Induction Motor rotors?

A

Cage Rotors Motor and Wound Rotors Motor.

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9
Q

In an induction machine, how rotor currents are induced?

A

Rotor currents are induced in the rotor windings by a combination of the time-variation of the stator currents and the motion of the rotor relative to the stator.

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10
Q

What is Magnetic Flux?

A

Magnetic flux is a measurement of the total magnetic field which passes through a given area.

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11
Q

What is E.M.F?

A

Electromotive Force, around a closed loop of conductor as the electromagnetic work that would be done on an electric charge if it travels once around the loop.

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12
Q

What is Faraday’s Law of Induction?

A

Whenever a conductor are placed in a varying magnetic field EMF are induced which is called induced EMF, if the conductor circuit are closed current are also induced which is called induced current.

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13
Q

What is Lenz’s Law?

A

The direction of an induced current is always such as to oppose the change in the circuit or the magnetic field that produces it.

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14
Q

What causes EMF in the rotor conductor?

A

Rotating Magnetic Flux that is produced in the stator windings because of the power supply.

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15
Q

What produces rotor current?

A

The induced EMF produces the rotor current.

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16
Q

What is the direction of rotor current?

A

Rotor current has the opposite direction of the cause produces it.

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17
Q

What is the direction of the rotation of rotor?

A

Rotor’s direction is same as of magnetic flux.

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18
Q

What is Synchronous Speed?

A

The speed at which the magnetic field rotates.

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19
Q

What is the other name of Wound Rotors Motor?

A

Slip-ring Motor.

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20
Q

What is the difference in the stator between Cage Motor and Slip-ring Motor?

A

No difference.

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21
Q

What is the difference in the stator between Cage Motor and Slip-ring Motor?

A

The rotor of a slip-ring motor is wounded with wire coils.

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22
Q

What is the relationship between slip and rotor resistance to generate maximum torque?

A

The slip necessary is directly proportional to rotor resistance.

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23
Q

What is the effect of high rotor resistance?

A

High rotor resistance can result in the pull-out torque occurring at almost zero speed, providing a very high pull-out torque at a low starting current.

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24
Q

What is Fleming’s left hand rule?

A

Whenever a current carrying conductor is placed inside a magnetic field, a force acts on the conductor, in a direction perpendicular to both the directions of the current and the magnetic field. [picture should be included]

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25
Q

How many types of DC motors are generally used?

A

Five.

  1. The separately excited dc motor
  2. The shunt dc motor
  3. The permanent-magnet dc motor
  4. The series dc motor
  5. The compounded dc motor
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26
Q

What is motor speed?

A

The speed of a motor is the number of revolutions of rotor in a given time frame. Unit- RMP [photo to be included]

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27
Q

What is Motor Slip?

A

Difference between synchronous and full load speed which is measured in percentage.

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28
Q

What is the relation between frequency and impedance in an induction motor?

A

Changes in frequency changes impedance as impedance is equal to 2πfl.

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29
Q

What does happen if the load of the motor comes down?

A

It reduces active current but magnetizing current stays same which results in a reduced power factor.

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30
Q

What parameters power loss depend mostly on?

A

Efficiency and Power Factor.

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31
Q

How efficiency can be increased?

A

By changing the motor design.

32
Q

What are the types of intrinsic losses in motor?

A

Fixed Loss (independent of motor load) and Variable loss (dependent on motor load).

33
Q

What are the types of Fixed Losses?

A
  1. Magnetic Core Loss or Iron Losses (Hysteresis and Eddy Current Loss)
  2. Friction and Wind Age Losses
34
Q

What are the types of Variable Losses?

A
  1. Resistance Losses in the stator and or rotor (I2R)

2. Miscellaneous Stray Losses

35
Q

What is the primary technical consideration for choosing a motor?

A

Torque requirement of the load.

36
Q

What are the important terms for choosing a motor?

A
  1. Maximum torque generated by the motor.
  2. Torque requirements for startup and during acceleration period
  3. Constant versus variable torque requirements
  4. Constant versus variable speed etc.
37
Q

Why no load test is required?

A

To measure efficiency & losses in a motor. Core loss, Friction and Wind age losses can be determined from no load test.

38
Q

What is the operating temperature of modern motors?

A

100°C to 120°C.

39
Q

What is the correction factor for motor temperature?

A

R2/R1=(235+t2)/(235+t1)

40
Q

How the rotor resistance can be determined?

A

From locked rotor test.

41
Q

How to measure rotor I2R loss?

A

Rotor I2R loss can be measured for rated speed, output and slip.
Rotor I2R losses = Slip × (Stator Input – Stator I2R Losses – Core Loss)

42
Q

What is the IS and IEC standard for stray loss?

A

0.5% of output but actual value of stray loss likely to be more.

43
Q

What is the IEE standard for stray loss?

A

0.9%-1.8%

Motor Rating Stray Losses
1 – 125HP 1.8 %
125 – 500HP 1.5 %
501 – 2499HP 1.2 %
2500 and above 0.9 %

44
Q

How superior the energy-efficient motors are over standard motor?

A

3 to 4 percentage point higher than standard motor.

45
Q

Within which limitation of load energy-efficient motor can operate without loss in efficiency?

A

75-100% of the rated capacity.

46
Q

What is the percentage of Stator and Rotor I2R Losses?

A

55-60% of total losses.

47
Q

What is the percentage of Core Losses?

A

20-25% of total losses.

48
Q

What is the percentage of Friction and Wind age Losses?

A

8-12% of total losses.

49
Q

What is the percentage of Stray Load Losses?

A

4-5% of total losses.

50
Q

How to reduce Iron losses?

A

Use of thinner gauge, lower loss core steel reduces eddy current losses. Longer core adds more steel to the design, which reduces losses due to lower operating flux densities.

51
Q

How to reduce stator I2R losses?

A

Use of more copper and larger conductors increases cross sectional area of stator windings. This lowers resistance (R) of the windings and reduces losses due to current flow (I).

52
Q

How to reduce rotor I2R losses?

A

Use of larger rotor conductor bars increases size of cross section, lowering conductor resistance (R) and losses due to current flow (I).

53
Q

How to reduce Friction and Wind age losses?

A

Use of low loss fan design reduces losses due to air movement.

54
Q

How to reduce Stray load losses?

A

Use of optimized design and strict quality control procedures minimizes stray load losses.

55
Q

What is voltage unbalance?

A

The condition where the voltages in the three phases are not equal.

56
Q

Why does voltage unbalance happen?

A

Supplying single-phase loads disproportionately from one of the phases and use of different sizes of cables in the distribution system.

57
Q

What is the percent unbalance?

A

Percent Unbalance = ((Vmax-Vavg)/Vavg)×100

58
Q

What is recommended maximum voltage unbalance at motor terminal?

A

Not more than 1%.

59
Q

What are the common causes of voltage unbalance?

A
  • Unbalanced incoming utility supply
  • Unequal transformer tap settings
  • Large single phase distribution transformer on the system
  • Open phase on the primary of a 3 phase transformer on the distribution system
  • Faults or grounds in the power transformer
  • Open delta connected transformer banks
  • A blown fuse on a 3 phase bank of power factor improvement capacitors
  • Unequal impedance in conductors of power supply wiring
  • Unbalanced distribution of single phase loads such as lighting
  • Heavy reactive single phase loads such as welders
60
Q

What does voltage unbalance cause?

A
  • Overheating
  • Premature motor failure
  • Extremely high current imbalance (6-10 times of voltage unbalance)
61
Q

How to calculate additional temperature rising due to voltage unbalance?

A

Additional temperature rise = 2 x (% Voltage unbalance)^2

62
Q

What can an Energy Manager do to minimize voltage unbalance?

A
  • Balancing any single phase loads equally among all the three phases
  • Segregating any single phase loads which disturb the load balance and feed them from a separate line / transformer.
63
Q

What should be done to the motors which consistently operates below 50% of rated capacity?

A

Operate in star mode; a change from the standard delta operation to star operation involves re-configuring the wiring of the three phases of power input at the terminal box.

64
Q

How does star connection help to increase efficiency?

A

Voltage reduction in star mode occurs by a factor of √3 where motor output falls by one third that of in delta mode. Thus, full-load operation in star mode gives higher efficiency and power factor than partial load operation in the delta mode.

65
Q

What are the prerequisite consideration of speed control system?

A

A detailed review of the load characteristics, historical data on process flows, the features required of the speed control system, the electricity tariffs and the investment costs.

66
Q

What does Load refer to for motor?

A

Torque output and corresponding speed required of a motor.

67
Q

What is the classification of Load?

A

Loads can be broadly classified as constant torque, variable torque and constant power.

68
Q

What is constant torque load?

A

Constant torque loads are those for which the output power requirement may vary with the speed of operation but the torque does not vary. Conveyors, rotary kilns, and constant-displacement pumps are typical examples of constant torque loads.

69
Q

What is variable torque load?

A

Variable torque loads are those for which the torque enquired varies with the speed of operation. Centrifugal pump sand fans are typical examples of variable torque loads (torque varies as the square of the speed).

70
Q

What is constant power load?

A

Constant power loads are those for which the torque requirements typically change inversely with speed. Machine tools are a typical example of a constant power load.

71
Q

In which cases multi-speed motors are economical?

A

For applications which requires limited speed control.

72
Q

What are the characteristics of Fluid Coupling?

A
  • No-load start of the prime mover
  • Overload protection
  • Wide range of choice of power transmission
  • Variable torque transmitting capacity
  • Same characteristics in both directions of rotation
73
Q

Which losses does occur in Fluid Coupling?

A

Circulation and Slip losses. Circulation losses are a constant percentage of the rated capacity of the unit—typically 1.5%.

74
Q

What is the power range of motor for Variable Pitch Drive?

A

5 HP to 50 HP.

75
Q

What will happen if only frequency is changed without changing voltage?

A

When the supply frequency is reduced, the equivalent impedance of electric circuit reduces. This results in higher current drawn by the motor and a higher flux. If the supply voltage is not reduced, the magnetic field may reach the saturation level.

Energy Efficiency in Electrical Systems (2 decks)

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