Introduction to 3-Phase Induction Motors Flashcards

1
Q

What is an electrical motor?

A

A device which converts electrical power into mechanical power.

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2
Q

How is rotational motion produced?

A

An imbalance between driving and restricting forces.

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3
Q

When will a motor run at a constant speed?

A

When the motor torque and load counter torque are balanced.

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4
Q

What happens when a motor’s load increases?

A

When counter torque is increased, the motor will draw more current to increase torque and achieve balance.

The higher torque requirement represents a lower motor speed.

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5
Q

What is torque commonly referred to? How is it calculated?

A

The moment of force.

T (torque) = F (force) * r (radius)

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6
Q

What are two methods to measure a motor’s torque?

A

A prony brake.

A dynamometer.

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7
Q

How is mechanical power output of a motor calculated?

A

It is the product of torque and rotational frequency (speed) in radians per second;

or simply:

P (power) = [T (torque) * n (speed *rpm*)]/9.55

9.55 is the constant which converts rpm into rad/sec.

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8
Q

Will a motor always draw it’s full rated power + losses?

A

No, A motor only delivers the torque that a load requires. Therefore, if a load requires only half torque, then the motor only draws (*roughly*) half power.

*There are small speed changes which are neglected*.

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9
Q

What is signified by a motor’s nameplate power rating?

A

Maximum output mechanical power. (pre - service factor)

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10
Q

What are the four main losses associated with motors?

A

Electrical:

Iron losses (Hysteresis, Eddy Currents)

Copper losses (I2R)

Mechanical:

Friction

Windage

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11
Q

What are the three main parts of a motor?

A

Frame

Stator

Rotor

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12
Q

What is the front and back end of a motor frame called?

A

End bells.

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13
Q

When would a drip proof or ODP motor frame be used?

A

In dry, indoor, non hazardous environments.

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14
Q

Why are slot windings better than salient pole windings?

A

They facilitate the use of standard parts and cancel some harmonics produced by the salient poles.

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15
Q

How are concentric windings different from lap windings?

A

Concentric windings (varying pitch) have coils for each phase/pole, becoming progressively larger centered on the smallest coil.

Lap windings (identical pitch) are all similar and “lapped” over one another.

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16
Q

How are the stator windings of large motors always connected?

A

Delta.

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17
Q

How is the effect fo metal fatigue in the stator reduced?

A

The windings are wrapped, then dipped in liquid varnish or resin and baked to solidify the coils.

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18
Q

What is synchronous speed?

A

The speed at which the magnetic field rotates around the stator windings.

It is constant.

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19
Q

What are the two types of rotors used in induction motors?

A

Squirrel cage.

Wound rotor motor.

20
Q

Why are SCIM rotor bars skewed?

A

It prevents the stator and rotor teeth from magnetic locking (torque pulsations or cogging) and creates a uniform speed.

Makes motor operation smoother and quieter.

21
Q

Are rotor laminations usually the same size as stator laminations?

A

No they are usually thicker.

22
Q

What is the difference between synchronous speed and rotor speed defined as?

A

Slip speed.

We need this speed difference for induced current to flow.

23
Q

What is a typical slip speed of a WRM?

A

2-5% of synchronous speed.

24
Q

In a 2 pole motor, how many electrical cycles are needed for one mechanical rotation (of the stator field)?

A

One electrical cycle.

25
Q

What is magnetomotive force?

A

The movement of electric charges which creates magnetic flux.

26
Q

Using a left hand conductor rule, what does the thumb indicate?

A

Direction of current flow.

27
Q

What is the basis of the left hand coil rule?

A

With a hitchhiker hand, If current flows from wrist to fingertip, the thumb indicates flux (north to south).

28
Q

Regardless of left hand generation or right hand motor rules, what is indicated by your:

  • Thumb?
  • Forefinger?
  • Center finger?
A
  • Thrust.
  • Field direction (N to S).
  • Current flow.
29
Q

Which force is responsible for motor rotation?

A

Lorentz Force.

30
Q

When load counter torque is increased, how is a motor’s transformer action affected?

A

Secondary current increases, along with slip speed.

As slip speed increases, greater voltage and current are induced in the rotor.

Primary current also increases.

31
Q

Which value best reflects the power factor of the rotor circuit?

A

The stator current.

32
Q

What are the two categories of motor frame?

A

Fractional and integral horseppower.

33
Q

What are the standard fractional horsepower frames?

A

42, 48 and 56.

34
Q

What are the NEMA standard maximum variations in voltage and frequency of a motor?

A

Voltage may not exceed +/- 10%;

Frequency may not exceed +/- 5%; and

Combined variation may not exceed +/- 10%.

35
Q

What is a problem cuased by low voltage on a motor?

A

The low voltage requires higher current for the same electrical power input, possibly tripping overload devices.

36
Q

is rotor frequency different thatn rotational frequency?

A

Yes, the rotor frequency is the frequency of the current within the rotor, whereas rotational frequency is the speed which the rotor is spinning.

37
Q

What is a typical slip speed?

A

2-8% of synchronous speed.

38
Q

How many poles do most motors have?

A

4 poles.

1800 rpm synchronous speed.

39
Q

For the same horsepower size, which motor is physically larger?

High speed.

Low speed.

A

The low speed motor is larger as more iron is required to produce the needed torque.

40
Q

When are engineers likely to specify a belt or gear drive connected to a motor?

A

When load speeds of less than 1150 rpm are required.

41
Q

What insulation class allows the most heat? Which are the most common?

A

Class H.

Class B and F.

42
Q

Why is a NEMA starting code (kVA/hP) used?

A

To estimate locked rotor current, or starting current.

43
Q

When is a motor automatically considered to be continuous duty?

A

When duty is not listed on the nameplate.

44
Q

What are the primary characteristics of the following rotor designs?

  1. A
  2. B
  3. C
  4. D
A
  1. Normal starting torque, high starting current - can handle brief overloads
  2. Normal starting torque, low starting current
  3. High starting torque, low starting current - designed for starting heavy loads
  4. High starting torque, low starting current - High inertia starts; Poor speed reg.
45
Q

A motor with a service factor of 1.15 or greater allows a current that is _______, when:

  • Rated voltage and frequency are applied.
  • Line voltages are balanced.
  • Ambient temp is less than or equal to 40 degrees C.
A

125% of rated.

46
Q

Can a maximized service factor shorten motor life?

A

Yes, if excessively overloaded.

47
Q

If looking at a NEMA motor and the the junction box is on the left, which end faces you? Which direction will the shaft spin?

A

The back end (NEMA)/ drive end (IEC).

Clockwise.