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Electrical Level - 3 > Motor Control (Braking) > Flashcards

Motor Control (Braking) Flashcards

1
Q

What are the 5 common methods for deceleration?

A
  1. Friction or electromechanical braking
  2. Plugging
  3. Dynamic Braking
  4. Regenerative Braking
  5. Eddy Current Braking
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2
Q

Describe Friction or Electromechanical braking. What are the 3 types?

A

Friction slows/stops the motor and load. Energy is dissipated as heat.

  1. Drum
  2. Band
  3. Disk
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3
Q

How do most motor friction or electromechanical brakes work?

A

Most motor friction brakes are spring applied and electromagnetically released.
FAIL SAFE

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4
Q

How do you vary the braking effect of friction/electro brakes?

A

Control current

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5
Q

Where is the drum attached?

A

Attached to the motor increasing surface area where the friction material makes contact

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6
Q

How are the shoes applied to the drum? How are the shoes released?

A
  • Applied to the drum by a spring

- Applying voltage to the solenoid releases the shoes from the brake

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7
Q

How are band brakes used?

A

Use a drum but employ a band instead of shoes

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8
Q

How are disk brakes mounted?

A

Normally mounted on the end bell

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9
Q

What is the disk lined with?

A

Friction material & sandwiched between 2 steel plates

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10
Q

how do disk brakes work?

A
  • Springs clamp the plates to the disk

- When the solenoid is energized the plates move apart allowing disk to turn

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11
Q

Advantages of friction brakes?

A
  • Hold the motor from rotating when stopped
  • Few control components
  • Automatically applies if the power fails
  • Brake cannot reverse the motor
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12
Q

Disadvantages of friction brakes?

A
  • Friction parts require maintenance
  • Brakes take up space
  • High initial cost
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13
Q

How does plugging work? What can it be used for?

A
  • Stops a motor by reversing the phase sequence

- Can be used for slowing/stopping or reversing a motor

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14
Q

What does the control circuit not sense when using time delays?

A

Does not sense the motor speed

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15
Q

With time delays plugging may happen _____________________________.

A

May happen for too long or not long enough

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16
Q

With a zero speed switch, plugging will happen ____________________.

A

Just long enough to stop the motor

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17
Q

What can a zero speed switch also be used for?

A

To plug a motor that has forward and reverse

18
Q

What does a lock out solenoid do in plugging?

A

Prevents the plugging from occurring before the motor is energized

19
Q

Advantages of plugging?

A
  • Fastest deceleration
  • No friction parts (low maintenance)
  • Suitable for large motors an severe duty
20
Q

Disadvantages of plugging?

A
  • Needs a reversing contactor
  • Requires a zero speed switch of time delay relay
  • Does not work if power fails
  • DRAWS LARGE PEAK CURRENTS
  • Stresses motor windings
  • Motor is free to rotate after stopping
  • Motor may reverse with time delay method
21
Q

How does Dynamic Braking work?

A

Slows or stops a motor by making it act like a generator

22
Q

How is the energy from the motor and load dissipated?

A

Dissipated as heat in the motor windings or external resistors

23
Q

Advantages of Dynamic Braking?

A
  • Wont reverse motor if applied too long
  • Smooth deceleration
  • Braking torque easily adjusted by varying applied DC
  • No friction parts (low maintenance)
24
Q

Disadvantages of Dynamic Braking?

A
  • DC excitation required
  • No braking effect is supply voltage is removed
  • Motor can rotate after stopping
  • Frequent stopping can overheat motor
25
Q

Describe Regenerative Braking.

A

When a motor acts as a generator and energy is recovered and fed back to supply

26
Q

What is regenerative braking used for?

A

Used to slow but not completely stop a motor

27
Q

When does regenerative braking also occur?

A

If a motor is being over-driven by a load making it turn faster than synchronous speed

28
Q

What happens at speeds higher than sync speed in regenerative braking?

A

Motor still draws reactive power from the source but still delivers active power (KW) to the supply

29
Q

What happens when supplying KW in regenerative braking?

A

Counter torque tends to slow the motor

30
Q

When does the motor supply rated power in regenerative braking?

A

at about 3-5% slip ABOVE sync speed

31
Q

Advantages of regenerative braking?

A
  • No extra equipment may be required

- No friction parts (low maintenance)

32
Q

Disadvantages of regenerative braking?

A
  • Only effective above sync speed
33
Q

How does modern regenerative braking work?

A

Employ complex solid state circuitry to reduce sync speed or rectify braking current, invert the energy and feed it back to the supply

34
Q

How does Eddy Current Braking work?

A

When drum rotates around the field, eddy currents are induced into the drum which create flux which react to the field poles creating counter torque which slows the motor

35
Q

Where are eddy current brakes attached?

A

to the motor shaft

36
Q

How are eddy brakes constructed?

A
  • Turns of wire wound around the field poles of an anchored iron core
  • Conductive drum is attached to the motor shaft that surrounds the field poles
  • DC supply connected to windings
37
Q

How do you get the braking effect in eddy current brakes?

A

Braking can be controlled by adjusting the DC current through the field poles

38
Q

Advantages of Eddy Current Braking?

A
  • Motor wont reverse if applied too long
  • Braking torque easily adjusted
  • No friction parts (low maintenance)
39
Q

Disadvantages of Eddy Current Braking?

A
  • More expensive than other braking methods
  • No braking can occur if DC field lost
  • Motor can rotate after stopping
  • Braking effect diminishes as motor slows
40
Q

Why are the rotor circuit resistors generally shorted-out during dynamic braking or Wound Rotor induction motors?

A

To allow max current to flow in the rotor

41
Q

For dynamic braking, what precaution is observed concerning the AC and DC supplies?

A

should never be connected at the same time

Electrical Level - 3 (9 decks)

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