Electrical. Flashcards
(91 cards)
1
Q
Why is high grade silica steel used in 3 phase induction motors?
A
To reduce hysteresis.
2
Q
Why are thin sheets of steel used in 3 phase induction motors?
A
To reduce eddy currents.
3
Q
How many windings are in a 3 phase motor?
A
Three, while a single phase motor uses 2.
4
Q
How many degrees apart are the windings placed in the stator?
A
120 degrees.
5
Q
Do 3 phase motors have start windings or utilize centrifugal switches?
A
No, they have neither.
6
Q
What does NEMA stand for?
A
National Electrical Manufacturers Association.
7
Q
What are NEMA motors rated in?
A
Rated in H.P and operate at 60 hz.
8
Q
What does IEC stand for?
A
International Electrical Technical Commission.
9
Q
Where is the junction box on a NEMA motor?
A
It is on the side of the motor.
10
Q
Where is the junction box on a IEC motor?
A
It is located on the top of the motor.
11
Q
What are IEC motors rated in?
A
They are rated in KW.
12
Q
What are the terminals on a IEC motor?
A
U, V, and W.
13
Q
What frequency do IEC motors operate at?
A
They operate at 50 hz, or 60 hz at a reduced capacity.
14
Q
How do 3 phase motors produce torque?
A
The stator MMF and and rotor MMF.
15
Q
What are the three classifications of mechanical load on a motor?
A
- Variable torque
- Constant-torque
- Constant Horsepower
16
Q
What is variable-torque load?
A
This type of mechanical load produces low torque at low speed and hight torque at high speed. It is the opposite of constant load torque. Counter torque increases exponentially as speed increases.
ex. centrifugal pumps and compressors.
17
Q
What is constant-torque load?
A
This type of mechanical load produces approximately the same counter-torque at all speeds. It is sometimes greater at start up due to vicous friction.
ex. conveyor belt, recip compressor.
18
Q
What is constant-horsepower load?
A
This type of mechanical load produces counter-torque that is inversely related to speed.
ex. drill press, lathe.
19
Q
What is the purpose of an drip proof frame on a motor?
A
Allow outside air to enter one end of the motor and flow over the stator windings providing ventiliation.
20
Q
What are the three types of totally enclosed motor?
A
- Totally enclosed fan-cooled (TEFC)
- Totally enclosed non-ventiliated (TENV) (used in textile industry)
- Totally enclosed pipe ventilitated (TEPV)
21
Q
What are the two slot winding methods used on induction motor stators?
A
- Lap windings: There are many coils as there are slots with each coil identical in size and orientation to all others.
- Concentric windings, the coils for each phase for each pole become progressively larger while centred on the smallest coil for that phase and pole.
22
Q
The span from one side of a motor coil to the other is called?
A
Pitch.
Lap windings have identical pitch.
Concentric windings have varying pitch.
23
Q
What is the synchronous speed of a motor?
A
The speed of the rotating magnetic field produced by the stator.
Calculated by (120 * frequency) / number of poles.
Measured in RPM.
24
Q
What is slip speed?
A
The difference between the rotor speed and synchronous speed.
N slip = N synchronous - N rotor
25
What are the two types of motor frame size?
Fractional horsepower and integral horsepower.
26
What is the maximum voltage difference compared to nameplate acceptable for most motors?
Aroung 10%
27
What are the typical slip speeds seen on most motors?
2% - 5%
28
What is percent speed regulation?
A comparison between open-shafted speed and the synchronous speed.
29
Why would a motor require a belt drive or gearbox?
When load speeds are less than 1150 rpm as these motors require large amount of raw materials to produce the required torque at such low speeds.
30
What type of insulation would be used on high temperature motors?
Type H
31
If duty is not indiciated on the nameplate, what type of motor is it?
Continous duty motor. Suitable for continuous use and can run without interruption.
32
Which rule specifies the rules for selecting overload devices in the Canadian Electrical Code?
Rule 28-306.
33
What are the 4 NEMA rotor designs?
A: Highest possible efficiency. Suitable for most loads but not used often due to higher starting torques and current.
B: 500% of FLA at start-up. Suitable for most loads and are the most common type of rotor. Almost as efficient as type A.
C: Suitable for hard-to-start loads that are started infrequently because they do not dissipate heat well. Double SCIM
D: Designed for maximum torque, however, at running speeds the higher rotor resistance means the motor has greater slip, lower full-load speed, and lower efficiency. Not meant for general purpose motors.
34
What is the purpose of a wound-rotor motor?
Adds resistance to the rotor windings giving the ability to change the torque.
35
How many slip rings are in a wound-rotor motor?
3
36
How many slip rings are in a synchronous motor?
2
37
What is torque measured in?
Newton Meters.
38
How is a 3-phase wye dual voltage motor wired for high voltage?
In series.
39
How is a 3-phase wye dual voltage motor wired for low voltage?
In parallel.
40
How is a 3-phase delta dual voltage motor wired for high voltage?
In series.
41
How is a 3-phase delta dual voltage motor wired for low voltage?
In parallel.
42
NEMA motors designate the front of the motor as _______________?
The end without the shaft. The back of the motor is the shaft end.
43
What is the purpose on an a 3 phase electric motor and why are they so common in our industry?
Convert electrical energy into mechanical energy. Because most commercial buildings are three phase supplied.
44
What are the two types of winding connections that can be seen in the stator of a three phase motor?
Wye and Delta.
45
The stator produces a rotating magnetic field of constant magnitude and that rotates at a constant speed. This is also known as ___________________.
Synchronous speed. Calculated by [120*frequency]/number of poles.
46
What purpose does varnish and surface oxidization have on the rotor conductors?
These provide sufficient resistance to keep the current induced in the rotor conductors.
47
Why are rotor bars typically skewed?
To reduce noise and provide more uniform torque.
48
Apparent power is rated in ____________ and is the product of the _____ and _______ of the motor.
Voltamps (VA). Volts. Amps.
49
What is apparent power?
Apparent power (S) is the total amount of power consumed by an inductive load and takes into account both real and reactive power.
50
What is true/real power?
The amount of power consumed to do actual work. Measured in watts.
51
What is reactive power?
Reactive power (measured in vars, QxL) is the amount of power lost to counter emf forces on any inductive load. This resistance in a circuit is called inductive reactance and causes the amps to lag to the volts.
52
How do you calculate horsepower?
(Torque in n.m * speed in rpm) / 7121
53
What is "blocked rotor"?
Same as locked rotor, this is a situation in which a motors rotor is prevented from moving when the stator is energized.
54
What is the slip % when a motor is experiencing locked rotor?
100%
55
How is rotor torque calculated?
K (constant) * rotor amps * rotor P.F.
56
How is rotor current calculated?
I (rotor current) = E (rotor voltage) / Z (rotor impedance)
57
What is motor service factor?
A motor service factor (SF) is a multiplier that indicates the permissible power loading a motor can handle beyond its rated power, allowing for temporary overloads or variations in load conditions without causing overheating or premature failure
58
How is service factor used to calculate permissible overload sizing?
1.15 or over = 125% max allowable overload
less than 1.15 = 115% max allowable overload
If not labelled, assume SF is 1.
59
What is a motors rated speed?
The RPM that a motor is designed to operate at under full load amps. This is also the number specified on the nameplate.
60
What is % slip?
The ratio of the rotor speed to the synchronous speed.
% slip is calculated by [ (Synchronous speed - Rotor speed) / Synchronous speed) ]*100
61
The horsepower of a motor is determined by?
The torque and the speed.
62
% speed regulation is an indication of how much the motor will slow down when it's _________________?
Overloaded.
63
% speed regulation is calculated how?
[(No load speed - Full load speed)/Full load speed]*100
This gives us an idea of how well a motor maintains speed under varying loads. A lower % speed regulation is more favorable as it indicates that the motor slows down a smaller percentage as the load on it increases.
64
What is No load speed?
The speed a motor will run without an external load attached to it. Normally 1-3 rpm less than sychronous speed.
65
Increasing the resistance of a motor ______________ the % speed regulation of a motor.
Increases. This is because as resistance goes up, there is a higher voltage drop across the armature and therefore less voltage is avaliable to drive the motor, thus causing a higher % speed regulation.
66
What is breakdown torque?
Breakdown torque is the amount of torque a motor can produce before experiencing a significant drop in speed. This indicates the point at which the motor can no longer maintain speed under a given load.
67
What are disadvantges of a wound-rotor motor?
These motors are expensive, require many parts, and can be quite inefficient due to adding resistors to the motor windings.
68
As the rotor speed increases, the % slip ________________?
Decreases, this is because as the rotor speeds up it gets closer and closer to synchronous speed and the % slip becomes smaller.
69
In a synchronous motor, rotor speed _________ synchronous speed at all times, unless the load exceeds the motor capabilities.
Equals.
70
How does a synchronous motor match rotor speed with synchronous speed during operation?
Instead of the stator inducing a MMF in the rotor, the rotor has an opposing MMF
71
Why is slip speed necessary on an induction motor?
Without slip speed we would have no relative motion between the magnetic field produced by the stator and the conductors (rotor bars in this case) cutting the lines of flux to induce current and produce rotation.
72
What factors affect torque in a squirrel cage induction motor?
Rotor resistance, slip, stator flux, and rotor current.
73
What affect does load have on the synchronous speed of a SCIM?
Load has no affect on the synchronous speed of an induction motor. The synchronus speed of a motor is the speed of the rotating magnetic field produced in the stator.
74
When is maximum torque produced in a motor?
When R = XL
75
What are two ways to change the speed of a motor?
Changing the number of poles and changing the frequency.
76
What are two types of motors that can change the number of poles to change the speed of the motor?
Dual wound motor and consequent pole motor.
77
How does a dual wound motor change the speed of the motor?
By switching between two different three pole contactors, this will add/remove the different windings (with differing numbers of poles) to the system. Each starter requires its own overloads and each winding must be checked to ensure proper rotation.
78
What speed ratios is a dual wound rotor designed for?
Anything other than 2:1.
79
How does a dual wound rotor ensure that both windings are not energized at the same time?
The control circuit is either set up for a push button interlock with the two sets of windings or using auxillary contacts. Can also use mechanical interlock to physically prevent both windings from being energized at the same time.
80
How does a consequent pole motor function to change the speed of the motor?
By changing polaritys of the existing poles, we are able to generate a stator core with twice the number of poles. This is accomplished with one 5 pole contactor for low speed and a 3 pole contactor for high speed. Each contactor needs it's own set of overloads. These are only in 2:1 speed ratios.
81
What are the three types of consequent pole motors?
Constant horsepower, constant torque, variable torque.
82
What is the horsepower ratio of a constant horsepower consequent pole motor?
1:1. As the speed decreases, the amount of torque required increases. Therefore in the equation horsepower = (rpm * torque / 7121), the top number stays the same regardless of if the motor is running in high or low speed. These are best for drills that require more torque at low speed.
83
What is the horsepower ratio of a constant torque consequent pole motor?
2:1. The horsepower in this case is directly proportional to the speed, which is always a 2:1 ratio for these types of motors. In the equation horsepower = (rpm * torque / 7121), Torque remains constant and therefore as speed (rpm) doubles, the horsepower is also doubled. These are best for consistent loads like conveyor belts.
84
What is the horsepower ratio of a variable torque consequent pole motor?
4:1. Torque and speed are directly proportional, therefore a doubling in speed equals a doubling in torque, which equals 4x horsepower. In the equation horsepower = (rpm * torque / 7121), when RPM is doubled, torque is also doubled.
85
How does a VFD reduce starting current?
By manipulating the voltage and frequency applied to the motor.
86
What are the 4 basic components of a VFD?
Rectifier: converts AC into DC.
DC link: Filters and smooths DC voltage
Control logic: Able to manipulate frequncy and voltage to motor.
Inverter: Converts DC to AC.
87
What are the two different types of VFDs?
Pulse Width Modulation and Voltage source inverter.
88
Why are pulse width modulation VFD's more common than voltage source inverter VFD's?
Reliable, affordable, least amount of harmonics, and can work well on motors from 1/2 HP all the way to 500HP.
89
Why would a consequent pole motor be preferred over a dual wound motor?
Cheaper.
90
What are the three most common volts/hertz ratios used for VFD's? Why do we use these ratios?
10:1, 8:1, 4:1.
The volts/hertz ratio is a ratio that determines the amount of voltage that must be decreased for every 1hz of frequency. This is because as we decrease frequency, we also decrease impedance which leads to an increase in current. To counteract this, we also decrease the voltage. For instance, in a 10:1 ratio, if we decrease the frequency from 60hz to 50hz, we would also decrease the voltage from 600V to 500V.
91
