AC Machines (WRIM) Flashcards
What are the main constructional differences between a SCIM & WRIM?
Rotor construction: 3 separate windings 120 electrical degrees displaced from eachother with ends brought out to slip rings for external brush connection
What 2 major advantages does a WRIM have over a SCIM?
- High starting torque/low current
- Variable speed
- Smooth acceleration under heavy loads
How would you reverse the direction of rotation for a WRIM?
Interchange any two stator leads
2 disadvantages of a WRIM?
- High Initial cost
2. More maintenance
Why is a WRIM still classified as an induction motor?
Relies on currents induced in the rotor (by the stator flux) to develop torque
If a fully-loaded WRIM is being operated with some resistance left in the rotor circuit, will speed regulation be good or bad?
Higher torque but slower speed. Will be poorer with no resistance
List 3 examples where a WRIM could be used?
- Elevators
- Cranes
- Compressors
How many slips rings will you find on a WRIM?
3
How is the starting current in a large WRIM limited to a safe value?
Connecting external resistors into the rotor circuit during starting
What are the slip rings usually made from?
Brass or Copper
What are the brushes commonly made from?
Graphite Carbon
What is attached to each brush and what is it used for?
A piece of flexible conductor (pigtail or shunt) used to connect the brushes to the external rotor terminals
Why are the max voltage and current ratings on the nameplate important?
For selecting the proper insulation and current ratings for the equipment attached to the rotor circuit
What are the resistors used for?
Starting and speed control, must dissipate a large amount of energy in the form of heat
how do you avoid damaging resistors?
Determine the wattage rating
