140303A - Three Phase Motors Flashcards
1
Q
What are the three most important terms related to 3P induction motors?
A
- torque
- mechanical power
- efficiency
2
Q
What is torque?
A
Also called moment force, torque is the force that moves a rotating body
3
Q
Rotational motion is a result of the interaction between which two forces?
A
driving force and restraining force
4
Q
What is a type of restraining force?
A
friction or some other conversion of mechanical energy
5
Q
Give the formula for torque
A
Torque = Force (F) x Radius (R)
T = F x R
6
Q
what are the units for torque?
A
Newton meters or N*m
7
Q
What are the units of force?
A
Newtons or N
8
Q
What are the units of Radius?
A
Meters or m
9
Q
What is a result of imposing torque on a load?
A
counter torque
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
11
Q
When are motor torque and counter torque balanced?
A
When the motor is run driving the load at a constant speed
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
13
Q
What happens when load decreases? 3 points
A
- less torque
- less current
- higher speed
14
Q
What is the mechanical power of a rotating machine the product of?
A
Rotational speed
torque
a numerical constant
15
Q
Formula for mech power in SI units
A
P = [2(pi)nT] / 60
16
Q
What are the units of mechanical power?
A
Watts or W
17
Q
what is 2(pi)?
A
the conversion between rotational and linear motion
18
Q
what is n?
A
the rotational speed/ frequency in rpm
19
Q
what is T?
A
Torque in N*m
20
Q
How many Watts in 1 horsepower?
A
1 HP = 746 Watts
21
Q
What is efficiency?
A
The ratio of output energy/ power over input energy / power
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
23
Q
Formula for elec motor efficiency?
A
P (in electrical) = P (out mechanical) + P(loss)
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
25
What are some induction motor mechanical losses that ultimately convert to heat?
- friction in the bearings and between brushes and slip rings
- windage, the friction of air movement produced by the motor
26
Give formula for % motor efficiency
% Eff = (Pout / Pin) x 100
27
Give the 3 types of mechanical loads
- variable torque
- constant torque
- constant horsepower
28
Describe a variable torque load's characteristics. 2 points
- low torque at low speed
| - high torque at high speed
29
Name some types of variable torque loads
- centrifugal pumps
- centrifugal compressors
- centrifugal fans and blowers
30
What is an advantage of variable torque loads?
they are easy to start due to low initial counter torque
31
Describe a constant torque load and their other name
produces approx the same torque at all speeds
| aka frictional loads
32
Why are constant torque loads sometimes greater at start up?
due to viscous friction
33
Give some examples of constant torque loads 7 total
- printing press
- conveyor belts
- positive displacement pumps
- reciprocating compressors
- crushers
- punch presses
- low speed vehicles
34
What is viscous friction and it's other name
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
Describe a constant horsepower load
produce counter torque inversely related to speed
36
Give examples of constant horsepower loads
- drill press
- lathe
- milling machines
37
Why are constant HP loads often easy to start?
since the tool is disengaged initially
38
why does the constant HP load have such characteristics?
Due to how machinists operate them. Small tools spin fast with low torque, large tools spin slow with high torque
39
What are the main parts of an induction motor?
- frame
- stator
- rotor
40
What is a TEFC induction motor?
A totally enclosed fan cooled motor
41
Give parts of the frame 4 total
- cast iron/forged steel shell
- end bells or end brackets
- bearings in end brackets
- junction box
42
Give parts of the stator 4 total
- stator's laminated and slotted iron
- stator conductors
- conductor insulation
- protective sleeves and wedges for slots
43
Give parts of the rotor 3 total
- rotor's laminated iron
- rotor conductors
- shaft
44
What type of motor frame is best for clean and dry indoor environments and why?
Drip proof (DP) or open drip proof (ODP) motor frames are best. They allow air to flow through the motor to cool it
45
What type of motor frame is best for wet, outdoor or dirty installations?
Totally enclosed frames (TE)
46
Name the 3 types of TE motors
- totally enclosed fan cooled TEFC
- totally enclosed non- ventilated TENV
- totally enclosed pipe ventilated TEPV
47
Name 4 other motor enclosure types
- 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
Name the 4 advantages of slot windings over salient pole windings in stator construction
- 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
Name the 2 slot winding methods
- lap windings
| - concentric windings
50
What are lap windings
of slots = # of coils and all coils are identical in size and orientation to the others
51
What are concentric windings?
-the coils for each phase for each pole become progressively larger while centered on the smallest coil for that phase and pole
52
Name the connection type for stator windings of large motors
Delta type
53
Name the stator winding connection type of smaller motors
wye connected
54
Describe configuration of a wye connection
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
Describe a delta connection configuration
three lines connecting in the shape of a triangle. polarity must change from + to - around the delta connection for electrons to flow
56
How are the stator windings insulated?
the stator is dipped in varnish or resin and baked in an oven
57
what is synchronous speed
the speed of the rotating magnetic field of the stator
58
formula for synchronous speed
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
What relationship do motor poles and synchronous speed have with each other?
More poles = slower synchronous speed
60
What kind of current flows in the rotor?
Induced current, hence the term induction motor
61
Define slip speed
Difference between rotor speed and the synchronous speed of the rotating magnetic field as a percentage
62
Slip speed formula?
% Slip = syn speed - rotor speed / syn speed * 100%
63
What are small and medium rotor bars made of?
Aluminum and copper
64
What are large squirrel cage motor rotor bars made of?
Brass bars brazed to brass end rings
65
Why are wound rotor motors used less than squirrel cage motors?
They cost more and squirrel cage motors already have good characteristics
66
What are the 5 stages of operation of a 3 phase induction motor?
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
How can a 3 phase motor have 6 different poles, but is classified as a 2 pole stator?
Due to the 3 electrical phases being tied to 2 poles each of opposite polarity
68
What 2 terms describe movement of electrical charges?
Magnetomotive force and current
69
What creates magnetic flux?
movement of electric charges
70
At any instant, what is polarity and magnitude of flux proportional to?
magnetomotive force or current
71
According to NEMA which is the front and back end of a motor?
shaft is back and front is without shaft
72
According to IEC terminology what is the drive end and non-drive end?
shaft is drive end
73
Which compressors are phase specific?
Scroll and screw
74
Should you connect a 230V motor to a 208V system or vice versa?
No that is a misapplication unless the motor states it is rated for both
