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Level 3 Electrical, BCIT > Motor Control > Flashcards

Motor Control Flashcards

1
Q

A coil is designed to energize at what percent of its rated voltage?

A

85%

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2
Q

A zero speed switch used for plugging is NO or NC?

A

NO

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3
Q

A zero speed switch used for anti plugging is NO or NC?

A

NC

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4
Q

What are the four types of reduced voltage starters?

A

Auto TX, primary impedance/resistance, solid state, part winding

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5
Q

What is the reduced current starter?

A

Wye Delta

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6
Q

Which starting method provides max torque with minimum acceleration time?

A

Across the line / full voltage

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7
Q

What is the disadvantage of an open transition start?

A

Because the motor is disconnected from the line, a torque dip can occur causing line disturbances.

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8
Q

What is the main consideration for whether a motor can be started across the line?

A

Permissible voltage fluctuations to the power distribution system (usually determined by Hydro)

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9
Q

What value of starting resistance is used in a primary resistance starter?

A

Between 65-80% of full load voltage

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10
Q

What is the disadvantage of a primary resistance starter?

A

Large power losses in the resistor during start up

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11
Q

How many resistors are used in a three point primary resistor starter?

A

Two in each phase

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12
Q

What is the usual voltage limitation for primary resistance starter?

A

600V

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13
Q

What is the disadvantage of the primary reactance starter?

A

Lowers the power factor even more than a primary resistance start. Poor voltage regulation because of poor power factor

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14
Q

Why does the auto transformer provide more starting torque per ampere than any other starter?

A

The motor starting current is greater than the line current because of the transformation in the auto tx

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15
Q

What are the disadvantages of the open delta auto tx

A

Unbalanced phase voltages for larger induction motors

The moment that the circuit is open during transition can cause current line surges

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16
Q

What is the usual auto tx duty?

A

Intermittent

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17
Q

To protect an auto tx from overheating, what type of switch is wired into the control circuit?

A

A manually-reset temp switch

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18
Q

What is the simplest and most economic control of the reduced voltage starters?

A

The open delta auto transformer

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19
Q

What are some components used to shunt the resistors from the resistance starter?

A

Permanent magnets with a Hall effect sensor, current transformers, speed sensor, timing relays

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20
Q

Auto TX’s are used for which type of motor?

A

SCIM. Auto TX starters are inductive type loads and affect the power factor during start up.

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21
Q

Open transition auto tx starters are generally used on what size motor?

A

Size 5 and smaller

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22
Q

Closed transition auto tx starters are generally used on what size motor?

A

Size 6 or larger

23
Q

The overload heaters of a wye delta starter are rated by what current value?

A

The phase current.
If two current values are listed on a nameplate, the lower current value is the phase current

24
Q

How should a good SCR read on an ohmmeter?

A

Near infinity resistance.

Zero resistance means bad SCR

25
Q

What are the two main requirements that must be met for a wye delta starter to be used?

A

The motor must be designed for the stator windings to be connected in delta for the run period

All stator winding leads must be accessible.

26
Q

What type of starting is generally used for synchronous motors?

A

Across the line, wye delta, reactor, and sometimes Auto TX

27
Q

What are the components necessary for a synchronous motor start?

A

Out of step relay, field discharge resistor, field contactor, PFR, and field rheostat

28
Q

What is the function of the out of step relay?

A

A timer that contains a current operated coil. Disconnects the motor from power if the rotor takes too long to excite. Prevents excessive current from burning up the motor.

29
Q

What is the function of the field contactor?

A

Contains a DC coil that is energized by the excitation current of the rotor. Controls the application and disconnection of the DC excitation field and the discharge resistor. Provides a positive overlap to ensure the field circuit is never open.

30
Q

What is the function of the PFR?

A

Sensing rotor speed and controls the operation of the field contactor.

31
Q

How do you adjust the speed at which the PFR drops out and the sync motor becomes synchronized?

A

Add or remove shims or spacers to the face of the relay core.

Adding shims: lowers speed
Removing shims: increases speed

32
Q

What are the advantages of a wound rotor motor?

A

Higher starting torque, smoother acceleration

33
Q

What are the two types of automatic starting for a wound rotor motor?

A

Timed acceleration, and speed sensing acceleration

34
Q

What is the disadvantage of the time delay wound rotor motor start?

A

Timers are not load sensitive and can’t always match the requirements of each load.

35
Q

Which wound rotor motor starter is considered to be the best option?

A

Frequency control

36
Q

What are the four necessary components of a VFD?

A

Rectifier, Inverter, DC Bus, CPU

37
Q

In the control section of the VFD, what is the purpose of the inverter?

A

Varying the frequency and magnitude of the output wave

38
Q

What are the three basic inverter schemes?

A

VVI: variable voltage inversion
CSI: current source inversion
PWM: pulse width modulation.

39
Q

Which motor ratings determine which size VFD is required?

A

HP

40
Q

How does an inverter function?

A

Changes DC to AC using a mechanical switch inverter, or electronic switching with an SCR, transistor, or an IGBT

41
Q

What are the components used in a PWM VFD?

A

Diode bridge rectifier, filter section, and IGBTs

42
Q

What are the two most common types of braking?

A

Dynamic and regenerative

43
Q

What is an IGBT?

A

Insulated gate bipolar transistor

44
Q

What is an advantage of IGBT drives?

A

Reduces problems caused by harmonics

45
Q

What is a disadvantage of IGBT drives?

A

Can cause voltage spikes which will destroy the motor

46
Q

What type of motor should be used with an IGBT drive?

A

Inverter rated motor.

47
Q

What is ramping?

A

The ability (parameter) to control speed increase and decrease time

48
Q

What is switching frequency?

A

The rate that the IGBTs switch per second

49
Q

What is the benefit of Sensorless Vector Control (SVC)?

A

Allows AC motors to be controlled with the same precision as DC motors

50
Q

What is phase control in relation to an AC soft start?

A

Delaying each SCR gate signal to the appropriate point of each half cycle

51
Q

What are the electrical characteristics used to control the speed and direction of a 3 phase induction motor?

A

Speed: frequency
Direction: phase sequence

52
Q

What is constant torque operation?

A

The torque required for the load is the same during the entire speed range, constant torque is the motors ability to maintain constant flux.

Ex) conveyor

53
Q

What is constant HP operation?

A

Load requires a constant force as radius changes.

Ex) winder

54
Q

What is reflected wave voltage?

A

Occurs in PWM VFD’s. When voltage pulses reach the motor they encounter a different impedance than the cable provided, pulses reflect back into the VFD doubling the voltage level. The longer the cable run the higher the voltage.

Level 3 Electrical, BCIT (7 decks)

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