Mumbai University > Electronics Engineering > Sem7 > Power Electronics 2

Marks: 10M

Year: Dec14, Dec11, May11

1) Compared to static Kramer system, the diode bridge is replaced by a SCR bridge

2) Therefore both the converters are controlled converter. Because of this the power flow in the rotor circuit becomes bidirectional and the induction motor can be operated in sub synchronous as well as super synchronous region of operation.

3) Subsynchronous mode

• Power must be extracted from the rotor
• Therefore Bridge 1 act as rectifier ( α1 < 90° )
• Bridge 2 act as inverter ( α2 > 90° ) to feed the slip power ( extracted from the rotor) back to A.C. supply.
• The slip power flows from t rotor circuit to bridge1, bridge2 and transformer and to the supply.
• At subsynchronous speeds the slip power sPm is supplied to the rotor by the exciter and so the remaining output power (1-s)Pm is supplied to the shaft.

4) Super- synchronous mode

• Power must be supplied to the rotor
• Bridge ‘1’ act as the inverter ( $α_1$ > 90° )
• Bridge ‘2’ operates as the rectifier ( $α_2$ < 90° )
• The power flow is now from the supply to transformer, bridge2, bridge1 and to the rotor circuit.

• At supersynchronous speeds, the rotor output power flows in the opposite direction so that the total shaft power increases to (1+s)$P_m$.

  Table 5.1 shows the summary of operation

5) Rotor voltage and frequency vary linearly with deviation from synchronous speed. For example, if the shaft speed varies in the range of 800-1600 rpm with 1200 rpm as the synchronous speed (s=±0.33) the range of slip frequency will be 0->20Hz for a 60Hz supply frequency.

6) Near synchronous speed, slip frequency emf’s are insufficient for natural commutation of thyristors. This difficulty can, however, be overcome by using forced commutation.

7) Thus, the provision of both subsynchronous and supersynchronous speed operation complicates the static converter system and nullifies the advantages of simplicity and economy which are inherent in a purely subsynchronous drive.

8) In addition, static scherbius drive is expensive than static Kramer drive because six diodes are replaced by six thyristors and their controlled circuitory.

9) Advantages

• The machine can be controlled continuously about 50% above and below the synchronous speed with a converter rating of about 50% of the machine capacity
• The static Scherbius drive overcomes the forward motoring only limitation of the static Kramer drive