14-12-2012, 06:06 PM
SLIP POWER RECOVERY SYSTEM OF SLIP RING INDUCTION MOTOR
SLIP POWER RECOVERY.pptx (Size: 2.07 MB / Downloads: 202)
INDUCTION MOTOR
Simple design, rugged, low-price, easy maintenance.
Wide range of power ratings: fractional horsepower to 10 MW.
Run essentially as constant speed from no-load to full load.
Its speed depends on the frequency of the power source.
Not easy to have variable speed control.
Requires a variable-frequency power-electronic drive for optimal speed control.
SLIP POWER RECOVERY SYSTEM OF SLIP RING INDUCTION MOTOR
In a wound-field induction motor the slip rings allow easy recovery of the slip power which can be electronically controlled to control the speed of the motor.
The oldest and simplest technique to invoke this slip-power recovery induction motor speed control is to mechanically vary the rotor resistance.
Instead of wasting the slip power in the rotor circuit resistance, a better approach is to convert it to ac line power and return it back to the line.
HOW SPEED CONTROL IS ACHIEVED?
The air gap flux is constant and the torque is controlled by the dc link current Id (controlled in the inner control loop).
The speed is controlled via the outer control loop (see performance curves below).
HARMONICS IN SLIP POWER RECOVERY SCHEME
The rectification of slip-power causes harmonic currents in the rotor which are reflected back into the stator.
This results in increased machine losses.
The harmonic torque is small compared to average torque and can generally be neglected in practice.