Sawtooth Wave after Rectification

[roro36]

We have an induction furnace that I try and maintain at work. I use a digital scope to take readings from around the machine when there is a fault. I have taken readings from the recifier circuit and the waveform is what comes out. With a lage capacitor bank at the output I would have thought that the voltage would be almost purely DC? Any explanation?

 

[Mike_In_Plano]

Looks like a phase controlled rectifier. The 6 SCRs serve as rectifiers, but they don't carry current until activated. The DC voltage should be present after the inductors and across the three capacitors on the right.

A control circuit typically sends pulses to turn on the SCRs at the correcyt time in the AC line cycle. By varying when the SCRs are turned on, the output voltage is controlled.

A much simplier version of this circuit is used for motor speed controls and light dimmers.

 

[jim hardy]

how much load is on that circuit?

what i think you ought to see is voltage rising to peak* six times per line cycle
then decaying at rate : volts per second = (load in amps) / (filter capacitance in farads)

do i read 'scope right?
is it cycling between about 200 and 530 volts? every 3.3 milisec?

looks like maybe a 600 volt 50hz supply with load of about 1/10 amp per microfarad?

and your 388 Vrms reading includes the DC component (true RMS) ?

* (value it rises to will depend on where in line cycle the SCR fires. )

are these traces from one of those modern battery powered handneld 'scopes?
Your circuit looks like it could be difficult to measure with an old fashioned line powered 'scope because it's not obvious whether it's grounded..

old jim