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sparkyMV (Electrical)
23 Jul 07 11:37
I was told that the Robicon Gen 3 power cells do not use a pre-charge circuit for the DC Bus and they actually use the high impedance of their special pahse shifting, multi tap tansformer. Can anyone confirm or deny this? if so it will be the first VSD that I have seen to not use a pre-charge circit (resistor/swicth) to prevent high inrush currents upon powering up the dc bus capacitors.
DickDV (Electrical)
23 Jul 07 13:04
I don't know about your Robicon but I've been told that AB model 1336 drives don't have a precharge circuit either, instead using SCR's instead of diodes in the rectifier section and charging up the bus slowly to limit the current.

Maybe someone else could confirm/deny this.

jraef (Electrical)
23 Jul 07 13:42
That's actually become somewhat common (SCR front end) on larger HP drives LV drives now. The drawback is the extra circuitry (i.e. gate firing card) for the SCR front end and  line phase loss sensitivity, but the payoff is eliminating the precharge resistor as a catastrophic failure point. if the gate circuit fails, the drive just doesn't power up.

As to Robicon's design, that has been around for quite a while now. As was explained to me, the drawback (if you want to call it that) is that the drive and transformer must be matched, thus sold together and be in the same cabinet. Other drive mfrs can have the transformer mounted separately, i.e. outside of the building to reduce the heat load inside. Robicon's take on that is that you need ventilation for the drive anyway, so having the transformer inside is not that big of a difference in exchange for the payoff of not having a precharge circuit.
Skogsgurra (Electrical)
23 Jul 07 14:56
That is not unusual at all. Only this Saturday/Sunday, I had to rush out to a paper mill to figure why they got failure "DC link precharge time", which means that the time to charge the DC link took too long time. More than 15 seconds to reach 85 % of rated voltage in this case.

I had told them to check the precharge resistor. They couldn't find it, so I had to go there and show them. Took me eight hours of driving to get there and same time to come home again.

No wonder they couldn't find the resistor. This drive had a thyristor controlled mains rectifier (not a true AFE) and the precharge was done same way as Robicon, using commutating reactor impedance and thyristor delay angle taken up gradually to charge the capacitors smoothly.

It is not uncommon. The actual drive was from 1989. So the technology has been used for a long time.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

sparkyMV (Electrical)
24 Jul 07 9:02
thanks, but one thing I know for sure is the Robicon drive I am talking about uses diodes on the front end of the low voltage power cells. not SCr's etc. I am quite familiar with those types of systems, or CSI drives that don't have a need for a precharge circuit. I am not used to a passive front end VFD not having a pre charge cicuit of some type.
This drive I am talking about is 6600V / 2000HP but as we know Robicon uses low voltage (690Volts)cells in series and phase shifted to make up the medium volatge.
Thanks again for all of your input.
Skogsgurra (Electrical)
24 Jul 07 9:34
Looks like it has been designed to take the peak charging current then. I think I would also prefer that instead of having several resistors and the potential fire hazard in a large inverter.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

supermacc (Electrical)
25 Jul 07 10:12
Not really a confirmation, but in patent US6087738 Variable output three-phase transformer Robicon claims that the main benefit of their invention is to achieve more precise control of motor starting voltage, so in IMO this would be a viable solution to limit dc cap inrush current. Appears to be a pretty nifty device BTW.
jraef (Electrical)
25 Jul 07 16:09
Sorry for the confusion.
My statement about the SCR front end was not directly related to your question about the Robicon design, but rather a confirmation of DickDV's response as it related to LV (Low Voltage) drive, which is what the AB 1336 drive is. It was meant to point out that there are a lot of different topologies out there.

Robicon's system does indeed use the high impedance of their matched transformer in their design, that's one of the reasons the transformer is integral to the drive.

With CSI drives, the motor and the drive must be matched for similar reasons. That is why people have tended to shift away from using CSI technology. It does make the drive somewhat bullet proof however; I always liked that aspect. I once worked on a 2300V drive in a pump station that had flooded. We cleaned out the muck, put all the PC boards through a dishwasher, dried everything out and it fired back up. I seriously doubt a PWM drive would stand up to that kind of abuse.
DickDV (Electrical)
26 Jul 07 9:17
jraef, seriously doubt!  Yes, I guess so!
jraef (Electrical)
26 Jul 07 12:55
LOL, everyone thought I was nuts to put the PC boards through the dishwasher, but I had worked in a PC board plant in High School, so I knew they went through that anyway when they came off the wave solder machine. Yes I know it would have been deionized water there, but I figured, what did we have to lose? It worked out. I just called a buddy who lives near that pump station and he told me they just recently took it out of service, so it lasted about 15 years after that "fix"!
Skogsgurra (Electrical)
26 Jul 07 17:02
Anyone that has saved production equipment after flooding can verify what Jeff says. Me for one.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

itsmoked (Electrical)
26 Jul 07 20:02
I make boards all the time and always feel strange washing them in a sink with Microtm and a tooth brush.  I always rinse them with RO water though.  Certain parts will freak out with this treatment.  Notably, certain switches and certain dips made with a funky lead frame plating (often TI).   The key is to really try to get the water off pretty quickly.  Something like a blower helps a lot.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

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