This document discusses slip power recovery in induction motors using static Kramer and Scherbius drives. It provides details on: 1. The conventional Kramer system controls motor speed by injecting a voltage into the rotor circuit at the slip frequency. This allows increasing or decreasing the rotor resistance to control speed. 2. Static Kramer drives only allow sub-synchronous operation while Static Scherbius drives allow both above and below synchronous speeds. 3. Scherbius drives enable bi-directional power flow using positive and negative injected voltages phase with or opposing the rotor current, allowing a wider range of operating conditions than Kramer drives.

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

2. 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

3. Conventional kramer system

4. 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

5. ADVANTAGES
1.Any speed within the working range can be obtained,
instead of only two or three as with other methods
2.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

6. Static kramer system

7. • 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.

8. AC Equivalent Circuit of Static Kramer
Drive

9. Modified kramer system

10. Static Scherbius Drive

11. Static Scherbius Drive Equation

12. • 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.

13. Closed loop control of scherbius
system

14. 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.