Intro To Three-Phase Induction Motors

Question 1

The counter-torque produced by a dynamometer or prony brake is the product of what two (2) mechanical quantities?

Answer 1

1. Force

2. Radius (to the point where the torque is measured)

Question 2

Mechanical power is the product of what two (2) mechanical quantities?

Answer 2

1. Torque

2. Speed

Question 3

Two types of electrical losses

Answer 3

Iron losses (core losses) 
Copper losses

Question 4

Two types of mechanical losses

Answer 4

Friction

Windage

Question 5

The formula to calculate torque is

Answer 5

T = F x r

Question 6

The main parts of a rotor

Answer 6

Laminated iron
Conductors
Shaft

Question 7

What three (3) main parts are induction motors made of

Answer 7

Frame
Stator
Rotor

Question 8

The stator includes what 4 parts

Answer 8

Laminated and slotted iron
Conductors
Conductor insulation
Protective sleeve and wedges for the slots

Question 9

What two slot winding methods are used on induction motors?

Answer 9

Lap windings

Concentric windings

Question 10

Describe the frame and stator of an induction motor

Answer 10

The frame holds the stator and its end brackets with their bearings. The stator holds the stationary iron with its slots into which windings are inserted.

Question 11

State the advantage of windings in slots over windings in salient poles

Answer 11

Slot construction of the stator allows more flexible manufacturing so that motors of different speed, horse power and voltage can be constructed with the same standard parts. Slot construction also produces currents of lower harmonic content.

Question 12

What are the two major components of a stator?

Answer 12

The stator core

The stator winding

Question 13

Describe a squirrel cage rotor

Answer 13

Squirrel cage rotors have aluminum, brass or copper bars connected to two rings (One ring on each end of the bars). Squirrel cage rotors are rugged, and have a fixed resistance.

Question 14

Describe a wound rotor

Answer 14

Wound rotors have insulated copper conductors wound into coils that terminate at slip rings. Wound rotors permit rotor resistance to be varied so that a range of starting torques and currents are available. Wound rotors are more expensive than squirrel cage rotors and require more maintenance.

Question 15

State two types of rotors used in induction motors

Answer 15

Squirrel-cage

Wire wound

Question 16

What happens to a current carrying conductors are placed within a magnetic field?

Answer 16

A mechanical force is developed on the conductor

Question 17

Describe how three-phase alternating currents in the stator windings cause the rotor to turn

Answer 17

In each of the three single phase stator windings the current increases, decreases and changes polarity. At the same time, the flux developed by the magnetomotive forces changes in the samedirection. The 120 degree displacement between the currents causes the stator flux to rotate as current in one winding decreases and current in an adjacent winding increases. The rotation of stator flux induces rotor currents that interact with the stator flux to develop torque and rotate the rotor.

Question 18

According to NEMA Motor terminology, the front end of a motor has the motor shaft.

True or False?

Answer 18

False; the front is the non-drive end

Question 19

Looking down at a motor with the shop pointing away from you, the motors junction box is on the right side.

True or False?

Answer 19

True

Question 20

The front end of the motor in NEMA terminology is called the non-drive end in IEC terminology.

True or False?

Answer 20

True

Question 21

To designate output power, the rounded metric equivalent of 1 HP is 0.75 kW.

True or False?

Answer 21

True

Question 22

A motor where the name plate voltage is given as 230/460 V is suitable for connection to a 208 V system.

True or False?

Answer 22

False; rated voltage is indicated on the nameplate

Question 23

The NEMA standard for motors states that the connected voltage must not vary more than _______ from the nameplate voltage. The frequency cannot vary more than ________ from the nameplate frequency and the combined variation must not exceed __________.

Answer 23

The NEMA standard for motors states that the connected voltage must not vary more than +-10% from the nameplate voltage. The frequency cannot vary more than +-5% from the nameplate frequency and the combined variation must not exceed +-10%.

Question 24

A fully loaded motor connected to lower than rated voltage draws (more/less) then rated current and runs (faster/slower) than rated speed.

Answer 24

A fully loaded motor connected to lower than rated voltage draws MORE then rated current and runs SLOWER than rated speed.

Question 25

The speed of an open shaft motor connected to rated voltage is the name plate speed. True or False?

Answer 25

False; the speed on the nameplate is the fully loaded speed

Question 26

The speed of an open-shafted motor (the no-load speed) when the motor is connected to rated voltage, is 1-3 r/min less than the synchronous speed. True or False?

Answer 26

True

Question 27

The load of an induction motor is most proportional to: a) speed b) slip speed c) current d) power factor

Answer 27

b) slip speed

Question 28

Which motor should have the least weight? a) 10 hp, four-pole motor b) 10 hp, six-pole motor c) 25 hp, four-pole motor d) 25 hp, six-pole motor

Answer 28

a) 10 hp, four-pole motor

Question 29

The standard maximum ambient temperature for motors is 30°C True or False?

Answer 29

False; ambient temperature for motors is 40°C

Question 30

The NEMA starting code is a letter designating a decimal numeric range by which the nameplate current can be multiplied to give the possible blocked rotor current (initial starting current) True or False?

Answer 30

True

Question 31

The rotor design determines a motor starting characteristics. True or False?

Answer 31

True

Question 32

If provided, the nameplate efficiency and power factor Are the efficiency and power factor at 75% load. True or False?

Answer 32

False; The information on the nameplates are 100% load values

Question 33

The service factor of an electric motor is a multiplier applied to the nameplate horsepower to give the permissible output power that the motor can deliver under the conditions specified for the service factor. True or False?

Answer 33

True

Question 34

The full service factor of a motor is only available when: a) rated voltage and frequency are applied to the motor b) line voltages are balanced c) ambient temperature is less than or equal to 40°C d) all of the above

Answer 34

d) all of the above

Question 35

A motor has a service factor greater than 1.00. When the service factor is used, the motor has the same lifespan as it would if it delivered only the stated nameplate horsepower. True or False?

Answer 35

False; The motors lifespan will be reduced

Question 36

The service factor of a motor is the number by which the motors full-load current is multiplied to determine the maximum overload setting.

Answer 36

False; The overloads are sized in accordance with the Canadian electrical code

Question 37

The code term overheating protection refers to protection beyond normal overload protection. True or False?

Answer 37

True

Question 38

You are looking at the end of a conductor and the electrons are coming toward you. What direction are the magnetic lines of force situated in?

Answer 38

Circular and clockwise around the conductor.

Question 39

Listing of a motors performance describing the length of time that it can safely deliver its rated output is known as ________ cycle.

Answer 39

Duty

Question 40

The full-load speed of a 3-phase induction motor is the speed identified on the nameplate. a) True b) False

Answer 40

a) True