SLIP POWER RECOVERY SYSTEM

Prepared By:-• Kaushal Boghani(130760109002).• Kishan Patel(130760109013).Guided By:-• Prof. Keyur Denpiya.

Slip power recovery

• 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. Two types of converter provide this approach:

1) Static Kramer Drive - only allows operation at sub-synchronous speed. 2) Static Scherbius Drive - allows operation above and below synchronous speed

Conventional kramer system

Conventional kramer system• In this method, the speed of an induction motor is controlled

by injecting a voltage in the rotor circuit • The injected voltage to have same frequency as the slip

frequency• When we insert a voltage which is in phase opposition to

induced rotor emf,it amounts to increasing the rotor resistance

• When we insert a voltage which is in phase to induced rotor emf, its equivalent to decreasing the rotor resistance

• Hence by changing of injected emf and the rotor resistance, the speed can be controlled

ADVANTAGES 1.Any speed within the working range can be obtained,

instead of only two or three as with other methods2.If the rotor converter is over excited, it will take a lead

current which compensates the lagging current drawn by the main motor hence improves the pf of the system

Static kramer system

• At zero speed (s=1) the motor acts as a transformer and all the real power is transferred back to the line (neglecting losses). The motor and inverter only consume reactive power.

• At synchronous speed (s=0) the power factor is the lowest and increases as slip increases. The PF can be improved close to synchronous speed by using a step-down transformer. The inverter line current is reduced by the transformer turns ratio -> reduced PF.

AC Equivalent Circuit of Static Kramer Drive

Modified kramer system

Static Scherbius Drive

Static Scherbius Drive Equation

• Static scherbius drives are capable of bi-directional power flow, with both positive and negative injected voltages possible, in phase with or opposing the rotor current. As a result, a wider set of operating conditions is possible. Considering the torque equation for slip energy recovery: when motoring, torque is positive, when generating torque is negative.

Closed loop control of scherbius system

References:-

•www.nptel.ac.in/courses/108108078/pdf/chap6/student_slides06.pdf•www.electrical4u.com/speed-control-of-three-phase-induction-motor/•Power electronics by muhammad

rashid 4th edition pearson publication.

THANK YOU