Motor Control (Wound Rotor Motor Controllers) Flashcards
How are Wound Rotor Motors normally started?
With all resistance in the circuit
What does the resistance do?
Limits rotor starting current and improves power factor
What does the improved PF do?
Creates higher starting torque
As rotor speed increases, the accelerating _____ decreases. How can this be held fairly constant and what does it provide?
Torque. Lowering rotor resistance will allow accelerating torque to be held fairly constant providing smooth acceleration
How is resistance removed?
in stages. may be done manually with a drum controller or automatically with contactors.
What are the two common methods to do automatic acceleration of WRIMS?
- Definate - time acceleration using time delay relays
2. Speed sensing relays which sense the speed of the motor
Disadvantages of time-delay WRIM starters?
- Not sensitive to amount of load
- Takes longer to accelerate larger loads but the timers don’t adjust the delay
- Acceleration rate may not be suitable for the load
What is automatic speed sensing acceleration?
Frequency sensing relays used to sense speed of the rotor & remove secondary resistance according to applied load
What are secondary resistors used for? What do they need?
Speed regulators, need a higher power rating than those used solely for starting
Where are secondary speed regulators rarely used?
Below 50% speed regulation (very unstable and rarely used below 50%)
How is energy lost using secondary resistors? How can it be replaced?
Lost due to heat. Electronic circuitry can be used to replace the resistors.
When does regeneration occur?
When energy is transferred from the secondary circuit through rectifiers and DC is fed to the inverters
Explain regeneration
Inversion coverts DC to 3-phase AC. Circuitry synchronizes the AC to the supply and controls the voltage of the inverter to vary the load on secondary and level of regeneration back to the supply
What does regeneration allow?
Allows speed control with greater power efficiency
