Three Phase Fully Controlled Converter With Source Inductance
Three phase fully controlled converter with source inductance:
- The load is assumed to be highly inductive such that the load can be replaced by a current source. Commutations are not instantaneous due to the presence of source inductances. It takes place over an overlap period of “μ1” instead.
- During the overlap period three thyristors instead of two conducts. Current in the outgoing thyristor gradually decreases to zero while the incoming thyristor current increases and equals the total load current at the end of the overlap period.
- If the duration of the overlap period is greater than 60º four thyristors may also conduct clamping the output voltage to zero for sometime.
- Due to the conduction of two devices during commutation either from the top group or the bottom group the instantaneous output voltage during the overlap period resulting in reduced average voltage. The exact amount of this reduction can be calculated.
- In the time interval α < ωt ≤ α μ, T6 and T2 from the bottom group and T1 from the top group conducts.
The above equation holds for μ ≤ 60º , for which,
Fig: Equivalent circuit during commutation from T6 to T2
Rearranging, we have,
- It should be noted that RC is a “loss less” resistance, since the overlap process does not involve any active power loss.
Questions of this topic
A 220V, 1450 RPM, 100A separately excited dc motor has an armature resistance to 0.1 ohms. It is supplied from a 3 phase fully controlled converter connected to a 3 phase 50 Hz ac source. The ac source has an inductive reactance of 0.5 ohms at 50 Hz. The line voltage is adjusted such that at firing angle = 0; the motor operates at rated speed and torque. The motor is to be braked regeneratively in the reverse direction at rated speed using the converter. What is the maximum braking torque the motor will be able to produce under this condition without causing commutation failure?