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Electrical Level 3 and 4 > Motor Control > Flashcards

Motor Control Flashcards

(68 cards)

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1
Q

What is the difference btwn overload protection and overcurrent protection?

A 
Overload = within normal path, damage normally thermal
Overcurrent = outside normal path, damage thermal, magnetic and arcing
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2
Q

What is the difference btwn a schematic diagram and a wiring diagram?

A

Schematic shows only electrical relationships

Wiring shows electrical and physical relationship btwn components

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3
Q

What is electrical interlocking?

A

contacts placed in the control circuit to prevent energization of the forward and reverse contacts at the same time

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4
Q

List 2 types of thermal overload relays

A

Bi-metallic

Solder pot

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5
Q

Explain on delay

A

delay occurs upon energization

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6
Q

Explain off-delay

A

delay occurs upon de-energization

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7
Q

What is meant by the term jogging?

A

Energizing the motor for a very short interval

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8
Q

Stop buttons must always be connected in ____?

A

Series

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9
Q

Start buttons must be connected in _______ ?

A

Parallel

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10
Q

List three reasons why across the line starting is the most common method used for starting AC motors?

A

High starting T
Inexpensive
Simple

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11
Q

Why might across the line starting not always be suitable?

A

High starting current causing line disturbances

High starting torque damaging to some loads

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12
Q

List three types of reduced voltage starting methods?

A

Primary resistor or reactor
Autotransformer
Wye - Delta

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13
Q

Describe the term “open transition”

A

Motor momentarily disconnects from lines during transition

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14
Q

Describe the term “closed transition”

A

Motor continuously connected to lines during transition

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15
Q

What advantage is offered by closed transition versus open transition starting methods?

A

Less line disturbances with closed transition method

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16
Q

What advantage would reactors have over resistors for reduced voltage starting?
What disadvantage?

A

Reactor are more efficient

Poorer PF

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17
Q

What advantage is gained by the autotransformer starting method compared to the primary impedance method?

A

Less line current drawn for same torque

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18
Q

Can the wye-delta starting method be used for all 3 phase induction motors?

A

No, all leads must be brought out

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19
Q

What important consideration should be observed when selecting the OLR heaters for wye-delta starting method?

A

Must be sized with phase current

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20
Q

Sync Motor Control: Explain the purpose of the relay SCA

A

Shuts down circuit if motor does not accelerate within a certain amount of time or breakdown T is exceeded.
Protects the amortisseur windings from over heating

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21
Q

Sync Motor Control: Explain the purpose of the relay PFR

A

Senses voltage induced in rotor when out of step

Decides when to apply DC to the rotor for pull-in T to occur (92-97% of sync speed)

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22
Q

Sync Motor Control: Explain the purpose of the contactor FC

A

Controls contacts which connect DC and field resistor

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23
Q

Sync Motor Control: Explain the purpose of the reactor

A

Provides voltage drops for PFR to measure

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24
Q

Sync Motor Control: Explain the purpose of the discharge resistor

A

Protects against high induced voltages in field winding when DC is applied

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25
Across the line voltage starters can result in extreme _______ and cause ______ ?
In rush currents | Brown outs
26
Reduced voltage starters are commonly employed for 3 phase SCIMS in the ____ to ___ range?
5hp to 200hp | Larger motors >200hp always use RVS
27
Primary resistance starter advantages and disadvantages?
Advantages: Simple, reduces voltage and current, smooth accel., OK for 3, 6, 9 & 12 lead motors Disadvantages: Power consumption, inefficient, limited capacity
28
Primary reactance advantages and disadvantages?
Advantages similar to resistance starter but more efficient | Disadvantages - size of inductor(s) needed.
29
Another name for both resistance and reactance voltage reducing starters?
Primary impedance starters
30
Another name for an autotransformer starter?
Compensator starter
31
Advantages of autotransformer starters?
High T to motor, good for large motors Multiple taps permit on-site selection Efficient
32
Most common taps for an autotransformer?
50, 65 and 80
33
A wye-delta starter reduced the starting T and starting I to what?
1/3 or 33%
34
A wye delta starter can not be used with what types of motors?
3 or 9 lead
35
List 2 ways to removed the R from a WRIM motor controller?
Manually with a drum controller | Automatically with contactors
36
What are the two common methods to do auto acceleration of WRIMs?
Definate - time acceleration using time delay relays | Speed sensing relays which sense the speed of the motor
37
If some R is left in a WRIM rotor circuit, the motor will run with a higher _______ ?
Slip speed (reduced T)
38
Below ____ % speed regulation is very unstable for WRIM speed regulator method?
50%
39
If the out-of-step relay stops the motor, a _______ is require before restarting.
Cooling period
40
Why is it essential that 2 of the contactor coils in a wye-delta starter be mechanically interlocked?
to prevent a short is both contactors closed at the same time
41
Give 5 methods of braking an AC motor?
``` Friction Plugging Dynamic Regenerative Eddy Current ```
42
Describe plugging
Energizing a motor in reverse to stop it quickly
43
Describe Dynamic braking
Slows/stops a motor by turning it into a generator. | Energy from motor and load dissipated as heat in motor windings or external resisitors
44
What are the advantages of plugging?
Fastest deceleration No friction parts Suitable for large motors and severe duty
45
Disadvantages of plugging?
``` Needs a reversing contactor Requires a zero speed switch or time delay relay Does not work if power fails Draws large peak currents Stresses motor windings Motor is free to rotate after stopping With time delay method motor may reverse ```
46
What prevents plugging from occuring before the motor is energized?
A lock-out solenoid
47
Describe friction/electromechanical braking
Friction slows/stops the motor and load using a drum, band or disc brake. Energy is dissipated as heat
48
List the three types of friction brakes?
Drum Disc Band
49
Most motor friction brakes are ______ applied and ______ released.
Spring | Electromagnetically
50
How can the braking effect be varied on a friction brake system?
Controlling the current
51
Advantages of friction brakes
Hold the motor from rotating when stopped Few control components Automatically applies if the power fails Brake can not reverse the motor
52
Disadvantages of friction brakes
Friction parts require maintenance Brakes take up space High initial cost
53
Dynamic braking is also referred to as ?
DC injection braking
54
Advantages of dynamic braking
Wont reverse the motor if applied to long Gives a smooth deceleration Braking torque is easily adjusted by varying the applied DC No friction parts = minimal maintenance
55
Disadvantages of dynamic braking
DC excitation required for braking No braking effect if supply voltage removed (loss of power) Motor can rotate after stopping Frequent stopping can over heat the motor
56
Describe regenerative braking
When a motor acts as a generator and energy is recovered and fed back to the supply
57
Advantages of regenerative braking
No extra equipment may be required | No friction parts
58
Disadvantages of regenerative braking
Only effective above sync speed
59
Advantages of eddy current braking
The motor wont reverse if applied to long Braking torque is easily adjusted No friction parts
60
Disadvantages of regenerative braking
More expensive than plugging, dynamic or regen braking No braking can occur if the DC field is lost Motor can rotates after stopping Braking effect diminishes as motor slows
61
Motor starters are rated in HP because?
They must safely interrupt the locked rotor current of the motor
62
A 3 phase SCIM can be stopped using dynamic braking which?
Applied DC to the stator after disconnecting the AC supply
63
Plugging on a 3 phase induction motor is accomplished by?
Reversing the phase sequence to the stator
64
Starter size required for a motor load depends on?
Motor duty cycle
65
A wye/delta starter is commonly open transition but could be closed transition by incorporating _______ into the power circuit.
Resistors
66
Contractors and disconnect switches must be _________ rated
Horsepower
67
A shading coil is
A copper ring wrapped around the core face of an AC armature, used to reduce contact chatter
68
What type of motor requires 2 sets of overloads?
Multi speed motors

Electrical Level 3 and 4 (10 decks)

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