Introduction to 3-Phase Induction Motors

Question 1

What is an electrical motor?

Answer 1

A device which converts electrical power into mechanical power.

Question 2

How is rotational motion produced?

Answer 2

An imbalance between driving and restricting forces.

Question 3

When will a motor run at a constant speed?

Answer 3

When the motor torque and load counter torque are balanced.

Question 4

What happens when a motor’s load increases?

Answer 4

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.

Question 5

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

Answer 5

The moment of force.

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

Question 6

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

Answer 6

A prony brake.

A dynamometer.

Question 7

How is mechanical power output of a motor calculated?

Answer 7

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.

Question 8

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

Answer 8

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*.

Question 9

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

Answer 9

Maximum output mechanical power. (pre - service factor)

Question 10

What are the four main losses associated with motors?

Answer 10

Electrical:

Iron losses (Hysteresis, Eddy Currents)

Copper losses (I²R)

Mechanical:

Friction

Windage

Question 11

What are the three main parts of a motor?

Answer 11

Frame

Stator

Rotor

Question 12

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

Answer 12

End bells.

Question 13

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

Answer 13

In dry, indoor, non hazardous environments.

Question 14

Why are slot windings better than salient pole windings?

Answer 14

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

Question 15

How are concentric windings different from lap windings?

Answer 15

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.

Question 16

How are the stator windings of large motors always connected?

Answer 16

Delta.

Question 17

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

Answer 17

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

Question 18

What is synchronous speed?

Answer 18

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

It is constant.

Question 19

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

Answer 19

Squirrel cage.

Wound rotor motor.

Question 20

Why are SCIM rotor bars skewed?

Answer 20

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.

Question 21

Are rotor laminations usually the same size as stator laminations?

Answer 21

No they are usually thicker.

Question 22

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

Answer 22

Slip speed.

We need this speed difference for induced current to flow.

Question 23

What is a typical slip speed of a WRM?

Answer 23

2-5% of synchronous speed.

Question 24

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

Answer 24

One electrical cycle.

Question 25

What is magnetomotive force?

Answer 25

The movement of electric charges which creates magnetic flux.

Question 26

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

Answer 26

Direction of current flow.

Question 27

What is the basis of the left hand coil rule?

Answer 27

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

Question 28

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

• Thumb?
• Forefinger?
• Center finger?

Answer 28

• Thrust.
• Field direction (N to S).
• Current flow.

Question 29

Which force is responsible for motor rotation?

Answer 29

Lorentz Force.

Question 30

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

Answer 30

Secondary current increases, along with slip speed.

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

Primary current also increases.

Question 31

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

Answer 31

The stator current.

Question 32

What are the two categories of motor frame?

Answer 32

Fractional and integral horseppower.

Question 33

What are the standard fractional horsepower frames?

Answer 33

42, 48 and 56.

Question 34

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

Answer 34

Voltage may not exceed +/- 10%;

Frequency may not exceed +/- 5%; and

Combined variation may not exceed +/- 10%.

Question 35

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

Answer 35

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

Question 36

is rotor frequency different thatn rotational frequency?

Answer 36

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

Question 37

What is a typical slip speed?

Answer 37

2-8% of synchronous speed.

Question 38

How many poles do most motors have?

Answer 38

4 poles.

1800 rpm synchronous speed.

Question 39

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

High speed.

Low speed.

Answer 39

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

Question 40

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

Answer 40

When load speeds of less than 1150 rpm are required.

Question 41

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

Answer 41

Class H.

Class B and F.

Question 42

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

Answer 42

To estimate locked rotor current, or starting current.

Question 43

When is a motor automatically considered to be continuous duty?

Answer 43

When duty is not listed on the nameplate.

Question 44

What are the primary characteristics of the following rotor designs?

1. A
2. B
3. C
4. D

Answer 44

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.

Question 45

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.

Answer 45

125% of rated.

Question 46

Can a maximized service factor shorten motor life?

Answer 46

Yes, if excessively overloaded.

Question 47

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?

Answer 47

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

Clockwise.