25-08-2012, 10:41 AM
AC Motors
EMC AC Motors.ppt (Size: 3.26 MB / Downloads: 121)
AC Motor Theory
Rotating flux field created by alternating field current.
Rotating field induces current within copper conductors in rotor.
Rotor rotates due to torque exerted on conductors.
Single-Phase AC Motors
Used for low-hp applications only (typically less than 5 hp).
Low operating efficiency (typ. 60-80%).
Same basic operating principles as 3-phase, but field alternates rather than rotating.
Direction of rotation not defined as startup. Must use supplemental method to create rotating field.
Single Phase Induction Motors
1.) Resistance Split Phase Motor
Secondary winding of fine high resistance wire used.
Change in inductive characteristics causes AC phase shift between primary and secondary windings, creating a rotating field.
I2R losses in secondary winding overheat quickly, suited for low loads only.
Centrifugal switch used to disconnect start winding once motor reaches about 75% of synchronous speed.
2.) Capacitor Start Motor
Motor wired with two identical windings 90 degrees apart.
Capacitor places in series with one winding, creating phase shift to generate rotating magnetic field
Lower I2R losses during start
Better suited to starting high inertial loads
Utilizes a centrifugal switch to drop start winding when motor speed reaches 75% of synchronous speed.
3.) Shaded Pole Motor
Part of pole surrounded by copper ring.
Ring delays formation of magnetic field in that portion of the pole, causing rotor to turn in that direction.
Fixed direction of rotation, cannot be changed after mfg.
Very inefficient, as start function cannot be disconnected once motor is as speed.
Used for fractional horsepower motors only (typ < .05hp)
Universal Motors
Operate on single phase AC or DC power
Essentially a series-wound DC motor, complete with armature, commutator, and brushes.
Operates on AC as both field and armature reverse, rotating in the same direction.
Applications: Vacuum cleaners, jig saws, drills …
High-speed at no-load, similar to the series DC motor.
Low efficiency
Synchronous Motors
Runs at synchronous speed, no slip.
Magnets on rotor ‘lock-on’ to rotating flux
Motor must used external start assist such as:
External start motor
Squirrel cage built into rotor, to start as an induction motor
Variable frequency supply – slowly increase speed.
Very high power (200 – 20,000 hp).
Rock crushers, rolling mills, paper mills.
Very low power
Timing devices (wall and oven clocks)
Sheet feed devices
Well suited to high-torque, low rpm applications – may eliminate the need for speed reducers