Load commutated chopper

By Kaviyarasan.s

commutation

Commutation is the process of turning of a circuit.

Types Forced commutationLoad commutation

commutation

Forced commutation

Load commutation Voltage commutation

Current commutation

constructionThyristor t1Thyristor t2Thyristor t3Thyristor t4CapacitorFree wheeling

diodeLoad

load commutationAchieved by a) The IL = 0 due to the nature of

load circuit parameters b) IL is transferred to another device

from conducting SCR

Principle Working of this chopper starts

only if the Capacitor “C” is charged

Thyristor working:

If T1,T2 – Main Thyristors, then T3,T4 & C forms commutating elements

If T3,T4 – Main Thyristors , then T1,T2 & C forms commutating elements

Assumptions:

Load current is constantThyristor and Diodes are Ideal

Mode 1 (t=o)T1,T2 are triggeredT3,T4 are reverse biased

Mode 2(T=T1)Capacitor gets chargedFree wheeling diode is forward biased

Mode 3(t=t2) T3,T4 are triggered

T1,T2 are reversed biased and turned-off

wave form

merits Capable of commutating any

amount of load current

No commutating inductor is required

As it can work in high frequencies (KHz),Filtering requirements are minimal

demerits Peak load voltage is twice the supply

voltageFor high power applications – higher

switching losses at high operating frequencies –efficiency may low

FD is subjected to twice the supply voltage

Capacitor current which is alternating to be sensed to turn on one pair of SCRs when the other pair is commutated