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3rd Year ELectrical - Motors Theory > Intro To Three-Phase Induction Motors > Flashcards

Intro To Three-Phase Induction Motors Flashcards

1
Q

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

A
  1. Force

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

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2
Q

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

A
  1. Torque

2. Speed

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3
Q

Two types of electrical losses

A 
Iron losses (core losses) 
Copper losses
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4
Q

Two types of mechanical losses

A

Friction

Windage

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5
Q

The formula to calculate torque is

A

T = F x r

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6
Q

The main parts of a rotor

A

Laminated iron
Conductors
Shaft

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7
Q

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

A

Frame
Stator
Rotor

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8
Q

The stator includes what 4 parts

A

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

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9
Q

What two slot winding methods are used on induction motors?

A

Lap windings

Concentric windings

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10
Q

Describe the frame and stator of an induction motor

A

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.

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11
Q

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

A

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.

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12
Q

What are the two major components of a stator?

A

The stator core

The stator winding

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13
Q

Describe a squirrel cage rotor

A

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.

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14
Q

Describe a wound rotor

A

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.

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15
Q

State two types of rotors used in induction motors

A

Squirrel-cage

Wire wound

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16
Q

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

A

A mechanical force is developed on the conductor

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17
Q

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

A

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.

18
Q

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

True or False?

A

False; the front is the non-drive end

19
Q

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

True or False?

A

True

20
Q

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

True or False?

A

True

21
Q

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

True or False?

A

True

22
Q

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?

A

False; rated voltage is indicated on the nameplate

23
Q

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 __________.

A

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%.

24
Q

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

A

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

25
Q

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

True or False?

A

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

26
Q

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?

A

True

27
Q

The load of an induction motor is most proportional to:

a) speed
b) slip speed
c) current
d) power factor

A

b) slip speed

28
Q

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

A

a) 10 hp, four-pole motor

29
Q

The standard maximum ambient temperature for motors is 30°C

True or False?

A

False; ambient temperature for motors is 40°C

30
Q

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?

A

True

31
Q

The rotor design determines a motor starting characteristics.

True or False?

A

True

32
Q

If provided, the nameplate efficiency and power factor Are the efficiency and power factor at 75% load.

True or False?

A

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

33
Q

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?

A

True

34
Q

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

A

d) all of the above

35
Q

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?

A

False; The motors lifespan will be reduced

36
Q

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

A

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

37
Q

The code term overheating protection refers to protection beyond normal overload protection.

True or False?

A

True

38
Q

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?

A

Circular and clockwise around the conductor.

39
Q

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

A

Duty

40
Q

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

a) True
b) False

A

a) True

3rd Year ELectrical - Motors Theory (7 decks)

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