Motor Control (Synchronous Motor Control) Flashcards
Describe the stator
Just like the SCIM
How are they started?
Use a pony motor or amortisseur windings up to approx. 99% sync speed
What is applied to the rotor? Why?
DC applied to the rotor to achieve pull-in torque
What 5 extra components are required for controlling synchronous motors?
- Field discharge resistor
- Field Contactor
- Polarized field frequency relay (PFR)
- Field rheostat
- Out-of-step relay
What is the purpose of the field discharge resistor?
Prevents large voltage build up across the field when accelerating and decelerating
What is the purpose of the field contactor?
- Applies DC to the field for pull-in and opens field discharge resistor
- Closes discharge resistor contacts and opens the DC field if breakdown occurs
What is the purpose of the polarized field frequency relay?
Senses voltage induced in the rotor when out of step
Describe how the PFR works?
When motor is energized, AC is induced in the rotor at a high level which energizes the PFR. When motor approaches sync speed, AC voltage in the rotor has decreased and the PFR de-energizes at the right moment.
What is the purpose of the field rheostat?
Adjusts Power Factor and torque angle
What is the purpose of the out-of-step relay? (2 things)
- Shuts down the circuit if motor does not accelerate within a certain amount of time or breakdown torque is exceeded
- protects amortisseur windings from overheating
What are synchronous motors rated with?
Rated with a starting duty cycle
What happens if the synchronous motor is started too often?
The amortisseur windings will burn out
What needs to happen if the out-of-step relay stops the motor?
a cooling period is required before restarting
What can the field rheostat control when adjusted?
Can control the power factor of the motor
what does decreasing resistance do?
Over excites the field giving a leading PF
