Dynamic braking module (chopper) design

[LMAO]

Hello,
I am not too familiar with the detailed design of the choppers; is there a reason for them have DC capacitors (similar to inverters)? The chopper I am looking at has DC caps and pre-charge circuit just like an inverter but I just don't see why they need capacitors. The DC bus is already connected to inverters with their own capacitors which reduce the ripple voltage and store energy. Any idea why there is need for caps built in to chopper? This is a common DC system; aka multiple inverters running different motors on the same DC bus fed from common rectifiers.

 

[Jraef]

Hello,
I am not too familiar with the detailed design of the choppers; is there a reason for them have DC capacitors (similar to inverters)? The chopper I am looking at has DC caps and pre-charge circuit just like an inverter but I just don't see why they need capacitors. The DC bus is already connected to inverters with their own capacitors which reduce the ripple voltage and store energy. Any idea why there is need for caps built in to chopper? This is a common DC system; aka multiple inverters running different motors on the same DC bus fed from common rectifiers.

If it is ONLY a chopper, there would be no need for it to have it's own capacitors, no. But it sounds more like what you might have have is a DC bus supply module, maybe one that COMES WITH a chopper for Dynamic Braking. It would be redundant in a system wherein several drives are connected via a common DC bus, but each still has its own rectifier. There's no reason that you can't use it though, so long as the chopper has "the chops" for it... (sorry, couldn't resist.)

The other thing is, are you sure they are DC bus caps and not RC Snubber circuit caps?

Got a brand and model number?

 

[LMAO]

If it is ONLY a chopper, there would be no need for it to have it's own capacitors, no. But it sounds more like what you might have have is a DC bus supply module, maybe one that COMES WITH a chopper for Dynamic Braking. It would be redundant in a system wherein several drives are connected via a common DC bus, but each still has its own rectifier. There's no reason that you can't use it though, so long as the chopper has "the chops" for it... (sorry, couldn't resist.)

The other thing is, are you sure they are DC bus caps and not RC Snubber circuit caps?

Got a brand and model number?

I am sure these are DC bus caps. The choppers area external to the rectifiers and are placed on the same DC bus as multiple inverters inputs which all have their own DC capacitors. There is no reason for choppers caps.

The reason I am making a big deal out of it is that choppers caps require a lot of logic and circuitry to pre charge them prior to closing the DC disconnect switch. Adding a lot of cost.

 

[LMAO]

I talked to the gentleman who designs our choppers and he made it a little clear. The caps are there to protect chopper IGBTs because the inverter caps, even though on the same DC bus as chopper IGBTs, are too far from the chopper IGBTs to be able to smooth out the over voltage notches. The DC bus inductance is not negligible and it comes between inverters caps and chopper IGBTs.

I think I understand it.

 

[Jraef]

How big of a system are talking about? We call it "DC Bus Conditioning" in large drive systems, but I'm talking 250HP and up. I've never seen it as much of an issue on smaller drives.

 

[LMAO]

How big of a system are talking about? We call it "DC Bus Conditioning" in large drive systems, but I'm talking 250HP and up. I've never seen it as much of an issue on smaller drives.

about 5000HP at 690V

 

[Besoeker]

about 5000HP at 690V

We have designed and built systems up to about 9,000 HP where the recovery current goes back to the 11 kV supply.
What you are faced with is that brake chopper frequency doesn't match the supply frequency hence the need for intermediate energy storage. AKA capacitors.

On one we had around 400 2,200uF 450V electrolytics in a series/parallel arrangement.