Published on October 15, 2019
1. Shroff S.R. Rotary Institute of Chemical Technology Principal Supporter & Sponsor- UPL LTD./ Shroff family Managed By Ankleshwar Rotary Education Society Approved by AICTE, New Delhi, Govt. of Gujarat & GTU Affiliated Guided By: Mr. Krunal Shah Assistant Professor, Department of Electrical Engineering, Shroff S. R. Rotary Institute of Chemical Technology, Vataria, Ankleshwar Name of Student Chauhan Sachin V. Enrollment No. 170993109001 Semester 6TH Name of Topic Static Scherbius Drive Name of Course Control Of Electrical Drive Course Code 2160913 Academic Term January-June, 2019 Year of Admission 2017 Department of Electrical Engineering Active Learning Assignment (ALA)
2. Static Scherbius Drive: The main limitation of the Kraemer drive discussed is the operation at sub-synchronous range due to diode rectifier. Super synchronous speeds can be achieved if the power is fed to the rotor from the ac mains. This can be made possible by replacing the converter cascade by a static scherbius drive system using cycloconverter in rotor circuit.
3. A cycloconvereter allows power flow in either direction making the drive operate at both sub and super synchronous speeds. However, the converter becomes costly and has a complex control. The current in the rotor circuit is nearly sinusoidal. The torque pulsations and other reactions are minimal. The performance of the drive improves with respect to additional losses and torque pulsation. A smooth transition is possible from sub to super synchronous speed without any commutation problems. Speed reversal is not possible. A step up transformer may be interpossed between the lines and the converter to reduce the voltage reating of the converter.
4. Static Scherbius Drive, with power flow to and from the rotor, can also be realized by replacing the diode rectifier on the rotor side by means of a controlled bridge rectifier using thyristors, as shown in Fig. 4.18. This provides speeds below and above synchronous speed. The effects of non-sinusoidal rotor current are present. In addition the commutation of the bridge rectifier at very small slips (near synchronous speed) poses problems.
5. Either forced commutation or artificial commutation of the thyristors is employed at and about synchronous speed. These methods are shown in Fig. 4.18. A current source inverter can be advantageously employed to obtain a Static Scherbius drive.
6. Operating modes of static sherbius drive : The following are the operating modes of static scherbius dives : Sub-synchronous motoring : in this mode of operation the slip and torque both are positive and hence the injected voltage is in phase with rotor current. The power flows into the stator and feedback into the rotor circuit.
7. Super-synchronous motoring : when the speed of the motor is above the synchronous speed, then the slip is negative. Thus, the voltage and current are out of phase with each other. The power feeds into the rotor from the drive circuit along with input power flowing into the stator. Sub-synchronous generating : for sub-synchronous speed, the torque is required to be positive, although the slip is positive . The power is fed into the rotor through the slip-ring.
8. Super-synchronous generating : when the speed of the motor of the above the synchronous speed, then the slip and torque becomes negative. Thus, the injecting voltage is in phase with te rotor. The mechanical power is injected by the shaft and the output power is obtained from the stator and rotor circuit.