27-07-2012, 11:25 AM
AC Machine Stator
Machine Stator.ppt (Size: 981.5 KB / Downloads: 33)
Induction Motor
Most popular motor today in the low and medium horsepower range
Very robust in construction
Speed easily controllable using V/f or Field Oriented Controllers
Have replaced DC Motors in areas where traditional DC Motors
cannot be used such as mining or explosive environments
Of two types depending on motor construction: Squirrel Cage
or Slip Ring
Only Disadvantage: Most of them run with a lagging power factor
Slip Ring Rotor
The rotor contains windings similar to stator.
The connections from rotor are brought out using slip rings that
are rotating with the rotor and carbon brushes that are static.
Torque Production in an Induction Motor
In a conventional DC machine field is stationary and the current carrying conductors rotate.
We can obtain similar results if we make field structure rotating and current carrying conductor stationary.
In an induction motor the conventional 3-phase winding sets up the rotating magnetic field(RMF) and the rotor carries the current carrying conductors.
An EMF and hence current is induced in the rotor due to the speed difference between the RMF and the rotor, similar to that in a DC motor.
This current produces a torque such that the speed difference between the RMF and rotor is reduced.
Slip in Induction Motor
However, this speed difference cannot become zero because that would stop generation of the torque producing current itself.
The parameter slip ‘s’ is a measure of this relative speed difference
where ns,s are the speeds of the RMF in RPM and rad/sec respectively
nm,m are the speeds of the motor in RPM and rad/sec respectively
The angular slip frequency and the slip frequency at which voltage is induced in the rotor is given .
Induction Motor Example
A 100 hp, 8 pole, 60 Hz, 3 phase induction motor runs at 891 rpm
under full load. Determine the synchronous speed in rpm, slip, slip
frequency at full load. Also estimate speed if load torque becomes
half of full load torque, given the fact that torque is proportional to
slip in the region between breakdown torque or zero torque.
Synchronous Machine
The stator is similar in construction that of a induction motor
The rotor can be Salient or Non-Salient
Field excitation is provided on the rotor by either permanent or
electromagnets with number of poles equal to the poles of the
RMF caused by stator
Non-excited rotors are also possible as in case of reluctance motors
Synchronous Machine (2)
The rotor gets locked to the RMF and rotates unlike induction motor at synchronous speed under all load condition
All conventional power plants use synchronous generators for converting power to electrical form
They operate at a better power factor and higher efficiency thanequivalent induction machines