SCCR Ratings for 3-Phase Rectifier

[lec1982]

I am currently working on a Control Panel that needs to be UL508A Certified. The control has a 3-Phase Rectifier in the secondary of a 10.3kVA Power Transformer but I am unable to find a SCCR for this rectifier. I am still waiting to here back from the manufacturer whether or not they tested for a SCCR and I was also told from UL that Table SB4.1 which has maximum assumed values for different components does not cover rectifiers. Is there any way around this other then having UL test the rectifier?

 

[winnie]

Diodes have an Ifsm rating (see the data sheet). This is typically the surge current that the diode can survive for half a cycle. However I don't think that this specification means quite the same thing as SCCR.

Typically, when I am speaking with the inverter designers, they say that the diodes are there to protect the fuses in the event of a short circuit.

-Jon

 

[dbuckley]

...they say that the diodes are there to protect the fuses in the event of a short circuit.

With semiconductor power devices in general, they are a bit fragile, and the ways of trying to improve their lifespan include:

1) Massively overrate. Given your transformer is only 10KVA this may be a realistic and affordable proposition, depending on the voltage which you dont state.

2) Electronic protection, using current sensors to remove the source of power in an overload situation.

3) Fuses, but as noted, fuses are apt to survive and the device they are supposedly protecting is killed...

As noted, as a component, diodes don't have a SCCR rating in the normal sense of the word. If you put enough current through the device to exceed the junction temperatures as specified, you'll fry the device.

 

[rwilsond]

FYI: I was recently involved with several large Industrial Control Panels, via multiple suppliers/builders, that had large (1000A, 400A, etc) SCR's within them. Each panel was required to satisfy Article 409 of the 2005 NEC, and to be "Listed" (UL 508A) by the State where they would be installed. The Available Fault Currents to the panels exceeded 30 kA. Finding adequately rated internal components became very challenging for the SCR's. Ultimately, the suppliers ended up redesigning the panels, and replacing many components (including SCR's) with "Listed" components having higher kA ratings, and adding some current-limiting fuses.

I suggest speaking with application engineers at various manufacturers for advice.

 

[james_mcquade]

have you thought about a 3 phase mercury contactor?
regards,
james