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RACM Period 3 Alberta > 140303A - Three Phase Motors > Flashcards

140303A - Three Phase Motors Flashcards

1
Q

What are the three most important terms related to 3P induction motors?

A
  • torque
  • mechanical power
  • efficiency
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2
Q

What is torque?

A

Also called moment force, torque is the force that moves a rotating body

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3
Q

Rotational motion is a result of the interaction between which two forces?

A

driving force and restraining force

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4
Q

What is a type of restraining force?

A

friction or some other conversion of mechanical energy

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5
Q

Give the formula for torque

A

Torque = Force (F) x Radius (R)

T = F x R

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6
Q

what are the units for torque?

A

Newton meters or N*m

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7
Q

What are the units of force?

A

Newtons or N

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8
Q

What are the units of Radius?

A

Meters or m

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9
Q

What is a result of imposing torque on a load?

A

counter torque

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10
Q

In what direction will the counter torque of a load be applied?

A

in the opposite of the direction of the motor torque

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11
Q

When are motor torque and counter torque balanced?

A

When the motor is run driving the load at a constant speed

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12
Q

What happens if the load on the motor increases? 5 points

A
  • Counter torque increases
  • motor slows down
  • motor draws more current
  • motor increases torque to match new counter torque
  • motor speed is now lower than it was before
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13
Q

What happens when load decreases? 3 points

A
  • less torque
  • less current
  • higher speed
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14
Q

What is the mechanical power of a rotating machine the product of?

A

Rotational speed
torque
a numerical constant

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15
Q

Formula for mech power in SI units

A

P = [2(pi)nT] / 60

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16
Q

What are the units of mechanical power?

A

Watts or W

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17
Q

what is 2(pi)?

A

the conversion between rotational and linear motion

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18
Q

what is n?

A

the rotational speed/ frequency in rpm

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19
Q

what is T?

A

Torque in N*m

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20
Q

How many Watts in 1 horsepower?

A

1 HP = 746 Watts

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21
Q

What is efficiency?

A

The ratio of output energy/ power over input energy / power

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22
Q

What is efficiency in relation to an electric motor?

A

Ratio of the mechanical power output to the electrical power input for any given load

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23
Q

Formula for elec motor efficiency?

A

P (in electrical) = P (out mechanical) + P(loss)

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24
Q

What are some electrical losses in an induction motor?

A
  • iron losses in the stator and rotor

- copper losses in the stator and rotor

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25
Q

What are some induction motor mechanical losses that ultimately convert to heat?

A
  • friction in the bearings and between brushes and slip rings
  • windage, the friction of air movement produced by the motor
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26
Q

Give formula for % motor efficiency

A

% Eff = (Pout / Pin) x 100

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27
Q

Give the 3 types of mechanical loads

A
  • variable torque
  • constant torque
  • constant horsepower
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28
Q

Describe a variable torque load’s characteristics. 2 points

A
  • low torque at low speed

- high torque at high speed

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29
Q

Name some types of variable torque loads

A
  • centrifugal pumps
  • centrifugal compressors
  • centrifugal fans and blowers
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30
Q

What is an advantage of variable torque loads?

A

they are easy to start due to low initial counter torque

31
Q

Describe a constant torque load and their other name

A

produces approx the same torque at all speeds

aka frictional loads

32
Q

Why are constant torque loads sometimes greater at start up?

A

due to viscous friction

33
Q

Give some examples of constant torque loads 7 total

A
  • printing press
  • conveyor belts
  • positive displacement pumps
  • reciprocating compressors
  • crushers
  • punch presses
  • low speed vehicles
34
Q

What is viscous friction and it’s other name

A

aka sticking friction is caused by greater viscosity of cold lubricant between stationary shaft and its bearing or metal on metal due to lack of lubricant initially

35
Q

Describe a constant horsepower load

A

produce counter torque inversely related to speed

36
Q

Give examples of constant horsepower loads

A
  • drill press
  • lathe
  • milling machines
37
Q

Why are constant HP loads often easy to start?

A

since the tool is disengaged initially

38
Q

why does the constant HP load have such characteristics?

A

Due to how machinists operate them. Small tools spin fast with low torque, large tools spin slow with high torque

39
Q

What are the main parts of an induction motor?

A
  • frame
  • stator
  • rotor
40
Q

What is a TEFC induction motor?

A

A totally enclosed fan cooled motor

41
Q

Give parts of the frame 4 total

A
  • cast iron/forged steel shell
  • end bells or end brackets
  • bearings in end brackets
  • junction box
42
Q

Give parts of the stator 4 total

A
  • stator’s laminated and slotted iron
  • stator conductors
  • conductor insulation
  • protective sleeves and wedges for slots
43
Q

Give parts of the rotor 3 total

A
  • rotor’s laminated iron
  • rotor conductors
  • shaft
44
Q

What type of motor frame is best for clean and dry indoor environments and why?

A

Drip proof (DP) or open drip proof (ODP) motor frames are best. They allow air to flow through the motor to cool it

45
Q

What type of motor frame is best for wet, outdoor or dirty installations?

A

Totally enclosed frames (TE)

46
Q

Name the 3 types of TE motors

A
  • totally enclosed fan cooled TEFC
  • totally enclosed non- ventilated TENV
  • totally enclosed pipe ventilated TEPV
47
Q

Name 4 other motor enclosure types

A
  • open enclosures for very clean environments
  • explosion proof for hazardous locations
  • submersible motor/pump enclosures for deep wells
  • hermetically sealed refrigeration enclosures for motor compressor units
48
Q

Name the 4 advantages of slot windings over salient pole windings in stator construction

A
  • slot windings facilitate the use of standard parts meaning flexible and efficient manufacturing
  • slot windings draw lower harmonic content current than salient poles
  • more efficient motor and overall electrical system
  • reduced interference with other electrical systems
49
Q

Name the 2 slot winding methods

A
  • lap windings

- concentric windings

50
Q

What are lap windings

A

of slots = # of coils and all coils are identical in size and orientation to the others

51
Q

What are concentric windings?

A

-the coils for each phase for each pole become progressively larger while centered on the smallest coil for that phase and pole

52
Q

Name the connection type for stator windings of large motors

A

Delta type

53
Q

Name the stator winding connection type of smaller motors

A

wye connected

54
Q

Describe configuration of a wye connection

A

three lines of power all connecting at one common point, showing a Y shape. 3 or 4 wire, the 4th being a neutral in the center

55
Q

Describe a delta connection configuration

A

three lines connecting in the shape of a triangle. polarity must change from + to - around the delta connection for electrons to flow

56
Q

How are the stator windings insulated?

A

the stator is dipped in varnish or resin and baked in an oven

57
Q

what is synchronous speed

A

the speed of the rotating magnetic field of the stator

58
Q

formula for synchronous speed

A

n syn = 120f/ N

nsyn is synchronous RPM
120 is conversion between seconds and minutes and number of pairs of poles
f is elec frequency in Hz
N is number of poles

59
Q

What relationship do motor poles and synchronous speed have with each other?

A

More poles = slower synchronous speed

60
Q

What kind of current flows in the rotor?

A

Induced current, hence the term induction motor

61
Q

Define slip speed

A

Difference between rotor speed and the synchronous speed of the rotating magnetic field as a percentage

62
Q

Slip speed formula?

A

% Slip = syn speed - rotor speed / syn speed * 100%

63
Q

What are small and medium rotor bars made of?

A

Aluminum and copper

64
Q

What are large squirrel cage motor rotor bars made of?

A

Brass bars brazed to brass end rings

65
Q

Why are wound rotor motors used less than squirrel cage motors?

A

They cost more and squirrel cage motors already have good characteristics

66
Q

What are the 5 stages of operation of a 3 phase induction motor?

A
  1. 3 phase power is supplied, generating a rotating magnetic field in the stator (constant magnitude and speed)
  2. stators rotating field cuts rotor conductors and induces voltage and current, rotor develops torque
  3. Induction motor operates as a transformer with a rotating short circuited secondary winding
  4. the resistance and inductance of the rotor circuit affect rotor starting torque and starting current
  5. once started, slip speed is proportional to load counter torque
67
Q

How can a 3 phase motor have 6 different poles, but is classified as a 2 pole stator?

A

Due to the 3 electrical phases being tied to 2 poles each of opposite polarity

68
Q

What 2 terms describe movement of electrical charges?

A

Magnetomotive force and current

69
Q

What creates magnetic flux?

A

movement of electric charges

70
Q

At any instant, what is polarity and magnitude of flux proportional to?

A

magnetomotive force or current

71
Q

According to NEMA which is the front and back end of a motor?

A

shaft is back and front is without shaft

72
Q

According to IEC terminology what is the drive end and non-drive end?

A

shaft is drive end

73
Q

Which compressors are phase specific?

A

Scroll and screw

74
Q

Should you connect a 230V motor to a 208V system or vice versa?

A

No that is a misapplication unless the motor states it is rated for both

RACM Period 3 Alberta (3 decks)

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