Motor Control (Braking) Flashcards
What are the 5 common methods for deceleration?
- Friction or electromechanical braking
- Plugging
- Dynamic Braking
- Regenerative Braking
- Eddy Current Braking
Describe Friction or Electromechanical braking. What are the 3 types?
Friction slows/stops the motor and load. Energy is dissipated as heat.
- Drum
- Band
- Disk
How do most motor friction or electromechanical brakes work?
Most motor friction brakes are spring applied and electromagnetically released.
FAIL SAFE
How do you vary the braking effect of friction/electro brakes?
Control current
Where is the drum attached?
Attached to the motor increasing surface area where the friction material makes contact
How are the shoes applied to the drum? How are the shoes released?
- Applied to the drum by a spring
- Applying voltage to the solenoid releases the shoes from the brake
How are band brakes used?
Use a drum but employ a band instead of shoes
How are disk brakes mounted?
Normally mounted on the end bell
What is the disk lined with?
Friction material & sandwiched between 2 steel plates
how do disk brakes work?
- Springs clamp the plates to the disk
- When the solenoid is energized the plates move apart allowing disk to turn
Advantages of friction brakes?
- Hold the motor from rotating when stopped
- Few control components
- Automatically applies if the power fails
- Brake cannot reverse the motor
Disadvantages of friction brakes?
- Friction parts require maintenance
- Brakes take up space
- High initial cost
How does plugging work? What can it be used for?
- Stops a motor by reversing the phase sequence
- Can be used for slowing/stopping or reversing a motor
What does the control circuit not sense when using time delays?
Does not sense the motor speed
With time delays plugging may happen _____________________________.
May happen for too long or not long enough
With a zero speed switch, plugging will happen ____________________.
Just long enough to stop the motor
What can a zero speed switch also be used for?
To plug a motor that has forward and reverse
What does a lock out solenoid do in plugging?
Prevents the plugging from occurring before the motor is energized
Advantages of plugging?
- Fastest deceleration
- No friction parts (low maintenance)
- Suitable for large motors an severe duty
Disadvantages of plugging?
- 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
How does Dynamic Braking work?
Slows or stops a motor by making it act like a generator
How is the energy from the motor and load dissipated?
Dissipated as heat in the motor windings or external resistors
Advantages of Dynamic Braking?
- Wont reverse motor if applied too long
- Smooth deceleration
- Braking torque easily adjusted by varying applied DC
- No friction parts (low maintenance)
Disadvantages of Dynamic Braking?
- DC excitation required
- No braking effect is supply voltage is removed
- Motor can rotate after stopping
- Frequent stopping can overheat motor
Describe Regenerative Braking.
When a motor acts as a generator and energy is recovered and fed back to supply
What is regenerative braking used for?
Used to slow but not completely stop a motor
When does regenerative braking also occur?
If a motor is being over-driven by a load making it turn faster than synchronous speed
What happens at speeds higher than sync speed in regenerative braking?
Motor still draws reactive power from the source but still delivers active power (KW) to the supply
What happens when supplying KW in regenerative braking?
Counter torque tends to slow the motor
When does the motor supply rated power in regenerative braking?
at about 3-5% slip ABOVE sync speed
Advantages of regenerative braking?
- No extra equipment may be required
- No friction parts (low maintenance)
Disadvantages of regenerative braking?
- Only effective above sync speed
How does modern regenerative braking work?
Employ complex solid state circuitry to reduce sync speed or rectify braking current, invert the energy and feed it back to the supply
How does Eddy Current Braking work?
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
Where are eddy current brakes attached?
to the motor shaft
How are eddy brakes constructed?
- 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
How do you get the braking effect in eddy current brakes?
Braking can be controlled by adjusting the DC current through the field poles
Advantages of Eddy Current Braking?
- Motor wont reverse if applied too long
- Braking torque easily adjusted
- No friction parts (low maintenance)
Disadvantages of Eddy Current Braking?
- 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
Why are the rotor circuit resistors generally shorted-out during dynamic braking or Wound Rotor induction motors?
To allow max current to flow in the rotor
For dynamic braking, what precaution is observed concerning the AC and DC supplies?
should never be connected at the same time
