Advantages and disavantages of motors Flashcards
1
Q
The speed at which the magnetic field of a motor is called what?
A
Synchronous speed
2
Q
What determines the output frequency of an alternator?
A
- Number of Stator poles.
2. Speed of the rotation of the rotor.
3
Q
What are the 3 factors that determine the amount of output voltage of an alternator?
A
- The Length of the armature or stator conductors ( number turns)
- The strength of the magnetic field of the rotor (which is controlled by controlling the DC current to the rotor.
- The speed of the Rotor.
4
Q
What is the cause of inrush current?
A
The cause of inrush current is cause by “SLIP” the difference between the speed of the magnetic field and the speed of the Rotor.bb
5
Q
What is Service Factor
A
The number by which the horsepower ratin is multiplied to determine the maximum safe load that a motor may be expected to carry continuously
EX. a 10 HP motor w/ a SF of 1.15 will deliver 11.5 without exceeding the allowable temperature rise of the insulation class.
6
Q
Advantages of Squirrel Cage Induction Motor
A
Cheaper cost wise
Less maintenance
High Efficiency
Operates at consistent speed high O/L
7
Q
Squirrel Cage Induction Motor Disadvantages
A
Poor starting torque, high in rush current
More sensitive to the supply voltage fluctuations
Speed control is not possible in squirrel cage induction motor
8
Q
Induction Motor Advantages
A
3 phase induction motors will have self starting torque unlike synchronous motors
Induction motors are cheaper in cost due to the absence of brushes, commutators, and slip rings
simple and rugged in construction
9
Q
What is the name given to the stationary part of the motor
A
the stator
10
Q
What name is given to the rotating a part of the motor
A
the rotor (squirrel cage)
11
Q
What is the purpose of the laminations on the cores of both the stator and rotor of the motor
A
To reduce Eddy current losses
12
Q
Why are the bars of the squirrel cage slightly skewed so that they are not parallel with the slots in the stator
A
they produce a smoother and quieter motor operation
13
Q
What is the main purpose of the end bells on a motor
A
To house the bearings that support the rotor shaft and keep it aligned with the stator to turn freely
14
Q
Name two ways motor enclosures are classified
A
- the way they protect the motor from the environment
- the way they are cooled
15
Q
What name is given to the machine with enclosure that does not prevent the free exchange of air between the inside and the outside of the machine
A
totally enclosed machine
16
Q
what is the name given to the machine with an enclosure that prevents the free extremes of air between the inside and the outside of the machine
A
open machine
17
Q
the winding of a three phase winding is made up of three Single phase windings placed _____electrical degrees apart from each other
A
120
18
Q
What are the two purposes of the stator core
A
- support the stator windings
- provide a low reluctance path for the magnetic current
19
Q
What does the frame does Frame destination tell about the motor
A
The physical dimensions of the motor distance from feet to centre from the shaft
20
Q
Does the horse power rating of the motor referred to the input power or to the output power
A
output power
21
Q
What does the service factor of the motor tell about the motor
A
maximum horsepower for a continuous supply without damage to the motor
22
Q
what would an electrician use the insulation system designation for when installing a motor
A
To select the right supply conductor to the motor
23
Q
What does the ambient temperature reading referred to
A
the highest temperature allowed for the cooling medium that contacts the heated parts of the motor
24
Q
And what units is the speed of the motor rated
A
revolutions per minute
25
for a motor to operate successfully under running conditions at full load by what maximum percentage me the frequency of the supply very
5%
26
For a mother to operate successfully under running conditions at full load what is the maximum variation from the rated voltage allowed
10%
27
what relationship is expressed by the design letter on a motor nameplate
between torque and the speed of the motor
28
define the term full load current
The current of the motor draws when operated at the rated voltage and supplies the rated horsepower
29
The flux created by any winding will be at a maximum when the current through that winding is at a_____
maximum
30
When the current through the winding reverses the flex direction created by that winding ___
reverses
31
What effect does increasing the frequency of the supply have on the speed of the routine magnetic field
it increases the frequency of the supply
32
What effect does increasing the number of poles have on the speed of the rotating magnetic field
increasing the number of the pools decreases the speed of the rotating magnetic field
33
Define the term synchronous speed
synchronous speed is the speed of the rotating Magnetic field
34
Since there is no electrical connection between the stator and the rotor how is the current need to flow in the rotor of a squirrel cage induction motor
through electromagnetic induction
35
Define the term the slip speed
Synchronous speed minus rotor speed the relative speed between the rotating stator field and the road or decreases
36
Define the term percent slip
percentage of the synchronous speed percent slip = synchronous speed - rotor speed divide by synchronous speed
37
Define the term speed regulation
the difference between the speed of the motor at no-load compared to the speed of the motor at full load Speed regulation equals no load speed minus full load speed
38
How is the direction of the rotation reversed in a squirrel cage induction motor
the rotor tries to match the rotating ring attic field thereby into changing any to stator leads at the source will do so
39
Define the term full load torque
The full load torque of motor is the torque necessary to produce rated horsepower at rated full load speed
40
Define the term break down torque
maximum torque that the motor will produce a.k.a. stalling torque
41
Define the term locked rotor torque
at the moment power is applied to the stator of a squirrel cage induction motor the router is not yet moving.
42
Define the term starting torque
the maximum torque for the motor will develop at rest for all angler positions of the rotor with rated voltage applied at rated frequency at what motor speed will the Stator current be the highest in the rotor is at a standstill 100% slip
43
what type of loads is a NEMA design class C motor suitable for
higher interia starts Crusher drums loaded compressors/conveyors
44
What part of the linear-induction motor actually moves
The stator
45
In what units is his Internet speed of a linear- induction motor measured.
Meters per second
46
Relative to the direction of the moving magnetic field, in what direction does the linear-induction motor move
the Stator travels in the direction opposite to the magnetic field
47
Many motors fail early because of simple maintenance program is not implemented
true
48
Why is it important to ensure unrestricted airflow around the motor
Air flow equals cooling of motor. A restricted motor overheats and damages the insulation
49
is too much grease better than to the grease on the bearings on motor
no to little grease is better too much can damage insulation values
50
what minimum resistance is adaptable for a low voltage motor
One megaohm
51
is excessive vibration likely caused by misaligned motor and load
yes
52
when taking insulation resistance test with a Megger the planer first dips down to low distance and then slowly rises is this cause for concern
No, the initial dip is likely caused by capacitive charging between the winding being tested and ground
53
To clean dust out of some motors it is better to use a vacuum cleaner than compressed air
true
54
List for senses that should be used when inspecting motors
Sight
Sound
Touch
Smell
55
How does the rotor of a round Road or induction motor differ from that of a squirrel cage induction motor
the rotor contains an insulated three phase core not a squirrel cage external resistance maybe place in rotor circuit
56
what is the purpose of a three slip rings and the corresponding brushes on the wound rotor motor
slip rings and bushes allow the rotor windings it to be connected to an electrical resistance grid
57
How are the three rotor terminals designated
M1 M2 M3
58
What is the purpose of the spring in the bush holders
the spring is used to force the push against the slip ring continuous contact
59
What is the purpose of the pink tail or shunt
attach each brush the pigtail her shunt connects the brushes to the external rotor (secondary) terminals.
60
What two additional pieces of information are normally supplied on the name plate of a wound rotor induction motor they would not find on a squirrel cage induction motor
max rotor voltage and current ratings
61
For what would an electrician use the two pieces of information of rotor voltage and current readings for?
Insulation ratings and current readings for equipment on the secondary of the motor
62
Name the advantages of wound rotor motor
- high starting to work with low current -smooth acceleration under heavy load no abnormal heating during starting
- good running characteristics after the string resistance is removed -adjustable speed
63
three Disadvantages of a wound rotor motor
- -physical size -higher initial and maintenance costs
- more complicated to control
- poor speed regulation when the secondary resistance is left in the circuit
64
Applications for a wound rotor motor
driving overhead cranes, starting ball mills, driving loaded conveyors
65
a wound rotor motor is designed to be
a variable torque motor
66
With respect to the lock rotor reactance what value of rotor resistance will give Maxim starting torque
Rotor resistance should be approximately equal to locked rotor reactance
67
Torque is directly proportional to go to current why is it then the increasing the resistance of the rotor circuit increases the torque output of the motor
increasing the resistance of the rotor circuit improves the Power factor and he brings rotor current more in phase with the stator flux
68
With the secondary terminals M1 M2 and M3 short-circuited will the starting torque of the wound rotor motor be high or low
low
69
Why is the efficiency of the round rotor motor poor when resistance is left in the rotor circuit
resistors dissipate energy in the form of heat
70
To decrease the speed of awound rotor motor the resistance is
added to the rotor circuit
71
Why is not practical to control the speed of around what are motor below 50% slip
the speed of the motor becomes too unstable beyond 50% slip
72
What does the term duty cycle mean when applied to the secondary resistors
The amount of time the resistors are expected to carry current versus the amount of time they are not
73
how is the direction of the rotation reversed on a wound rotor motor
interchange in the two line leads to stator
74
state three advantages of a wound rotor motor has over a squirrel cage induction motor
```
high starting torque, low current
Smooth acceleration under heavy load
Adjustable speed
No abnormal healing during starting
good running characteristics after starting resistance is removed
```
75
State three at the disadvantages of a wound rotor motor has over a squirrel cage induction motor
Physical size
more complicated to control
higher initial and maintenance costs
poor speed regulation when secondary resistance is left in the circuit
76
What sort of symptoms what you expect if an open occurs in the secondary circuit of a wound rotor Motor
Failure of the motor to start or motor turns slowly
77
Where might the open in the secondary circuit of a wound rotor motor occur?
In the rotor, at the brushes and slip rings, in the controller, in the resistance bank or in the connections of the above
78
If the manufacture's specifications for bush pressure are not available what is a safe rule to use?
a pressure of 2 PSI or 1.38 KPA
79
Is it better to have the brush pressure too high or too low?
too high
80
When should A brush be replaced?
When they are worn to about one quarter of their useful length or original
81
If a wound rotor motor is connected up and runs in the with the wrong direction how do you remedy the situation?
switch any two of the leaves to the stator
82
What is the name of the squirrel cage winding placed near the surface of the field poles in a synchronous motor?
The amortisseur winding or damper winding
83
What are the two functions of the squirrel cage winding?
Dampens and speed fluctuations during the load changes
| Also used for starting the motor
84
Is the rotor field winding fed with alternating current or direct current?
Direct current
85
How is the field excitation supplied to the rotor?
Through slip rings and bushings
86
How are the field terminals of the synchronous motors identified?
F1 & F2
87
What is the purpose of the end bells and bearings?
End bells support bearings, bearings support the rotor shaft that's keeping rotor in alignment with the stator
88
When are external converters normally used to supply the field expectation to a synchronous motor?
When the supply is already available in the plan due to supplying other machines
89
How is the level of DC excitation controlled in external converter?
By placing a rheostat in series with the DC supply
90
Where is the DC exciter for a synchronous motor normally mounted?
On the same shaft as a synchronous motor
91
how is the level of DC excitation controlled in a built in exciter
By varying the exciter's voltage output by using a rheostat in series with the shunt field of the exciter
92
What provides the DC excitation in a synchronous motor with brushless exciter?
Three phase alternator and rectifier assembly mounted on the rotor of the synchronous motor
93
How is the level of DC excitation controlled on a synchronous motor equipped with a brushless exciter?
By varying the field current to the brushless exciter
94
Why is the synchronous motor not self starting?
The rotating stator field is traveling too fast for the rotor to accelerate and lock in step
95
At what point in the starting process is the DC excitation applied to the field
When the rotor is close to synchronous speed. the percent of slip varies with the mechanical load on the motor.
96
During acceleration and deceleration of the rotor, why is the field short-circuited or connected across a discharge resistor?
To prevent a high-voltage being induced in the windings
97
Can the direction of rotation of the synchronous motor be reversed perverts in the player the of the DC field supply?
No only by reversing any of the two leads on the stator
98
What type of induction motor is sometimes used to start synchronous motors that have a large starting torque requirement?
A pony motor otherwise known as a wound rotor motor
99
A pony motor otherwise known as a wound rotor motor
A pony motor otherwise known as a wound rotor motor
100
Define the term Pull -in Torque
helps the rotor poles to catch the imaginary stator poles and run in synchronism.
101
What is the term used to describe the angle between the centre of the stator polls and the centre of the rotor poles?
The Torque angle
102
Does this angle increase or decrease as load is added to the motor?
Increases as load is added
103
Define the term pull out torque
The maximum sustained torque that the motor can handle at synchronous speed
104
What happens if the pull out torque is exceeded ?
the motor slows down and stalls
| pulls out of synchronism
105
As load is added to the synchronous motor what happens to the power factor of the motor?
It becomes less leading and more lagging
106
What controls the magnitude of the current flowing in the stator windings?
Controlled mainly by the amount of counter EMF or see EMF that is created
107
How does the torque angle of the rotor affect the stator currents?
When the torque angle changes the angle of the voltage induced in the stator by the rotor also changes new resultant enough in the Stator. new voltage, new current
108
With the load fixed which of the following cases will cause the lowest stator current?
unity power factor operation
109
What happens to the power factor of a synchronous motor when the field current is increased under a fixed load
increasing the field current with a fixed load will cause the power factor to become more leading or less lagging
110
When a synchronous motor is operated over or under excited does this change the in phase component of the stator current?
No
111
List four problems with the field circuit that could cause the motor to fail to synchronize?
Open in the field coils
No exciter voltage
Open in the field rheostat
Rheostat resistance set too high
112
Explain the reason for skewing the rotor bars relative to the slots in the stator core.
So the motor will run more smoothly and quietly.
113
At approximately what percentage of full-load speed do the centrifugal switch contacts open?
75%
114
Describe the constructional difference between the main and auxiliary windings.
Main winding is made of many turns of larger wire and is placed lower in the stator. Lower resistance, higher inductance.
Auxiliary windings are made with fewer turns of smaller wire and placed near the surface of the stator. High resistance, low inductance.
115
What is the physical displacement of the main and auxiliary windings of a two-pole spilt-phase motor?
2 pole - 90 mechanical and electrical degrees
| More then 2- 90 electrical, less than 90 mechanical
116
How does a magnetic pole move?
From an auxiliary winding to a main winding in the stator.
117
Which type of split -phase motor combines good starting torque with good overload capacity?
Capacitor - start, capacitor-run
118
Which type of split-phase motor operates at synchronous speed?
The reluctance motor
119
In a dual-capacitor type of split-phase motor which capacitor is larger
The starting capacitor
120
The output voltage of an alternator is maintained at a steady level through the use of an...
Automatic voltage regulator
121
The voltage regulation of an alternator will be poorest with a _____ power factor load.
Low (0%)
122
What are the three main reasons for connecting a three-phase alternator in wye?
To obtain two different voltages
Insulation requirements are reduced to phase voltage values.
Provides a neutral point for system ground.
123
Given a six-coil, three phase alternator, which one of the following connections yields the highest out put current?
Parallel wye, parallel delta, series wye, series delta.
Parallel delta
