Operation of a single phase fully controlled bridge rectifier in DCM
Operation of a single phase fully controlled bridge rectifier in DCM:
- The voltage across the R and L component of the load is negative in the region π - θ ≤ ωt ≤ π α. Therefore i0 continues to decrease till a new pair of thyristor is fired at ωt = π α. Now if the value of R, L and E are such that i0 becomes zero before ωt = π α the conduction becomes discontinuous. at the boundary between continuous and discontinuous conduction the minimum value of i0 which occurs at ωt = α and ωt = π α will be zero. we obtain the condition for continuous conduction as
- On the boundary between continuous and discontinuous conduction modes the load current is still continuous. Therefore, all the analysis of continuous conduction mode applies to this case as well. However in the discontinuous conduction mode i0 remains zero for certain interval.
- During this interval none of the thyristors conduct. In this conduction mode i0 starts rising from zero as T1T2 are fired at ωt = α. The load current continues to increase till ωt = π – θ. After this, the output voltage v0 falls below the emf E and i0 decreases till ωt = β when it becomes zero.
- Since the thyristors cannot conduct current in the reverse direction i0 remains at zero till ωt = π α when T3 and T4 are fired. During the period β ≤ ωt ≤ π α none of the thyristors conduct. During this period v0 attains the value E. Performance of the rectifier such as VOAV, VORMS, IOAV, IORMS etc can be found in terms of α, β and θ.
- The performance of the converter is strongly affected by the value of β. The value of β in terms of the load parameters (i.e, θ, φ and Z) and α can be found as follows.
- The values φ, α and θ, the value of β can be obtained.
Questions of this topic
A 220 V, 20A, 1500 RPM separately excited dc motor has an armature resistance of 0.75 ohms and inductance of 50mH. The motor is supplied from a 230V, 50Hz, single phase supply through a fully controlled bridge converter. Find the no load speed of the motor and the speed of the motor at the boundary between continuous and discontinuous modes when firing angle is 25 degrees.