I have some questions which have been puzzling me. Hope to get some idea.
I keep a few stock item of VSD, I was told before using them, if store for a long period of time I need to do a "pre-charge" by using DC supply to its DC bus before supplying AC from incoming mains?
1. Is there a guideline how long VSD can be stored?
2. Is it necessary to go through such proceedures? Myth or logic?
3. If powering from DC can prevent damages of VSD, why is there no proper connection terminal for this purpose?
K C Oh
I've been using AC drives for quite some time now, including some which came off the shelf after sitting for a while, and this is the first I've heard of this. The DC bus carries power rectified from the AC line anyway, so I can't see how energizing the bus from an external source is supposed to help anything. You could power up the drives from the AC line for a few minutes before running the motor if you want, but I don't think even that is necessary.
It happened to me many (>25) years ago when I repaired photographic flashes - a batch came with unformatted electrolytic (aluminum) capacitors - after applying an increasing DC voltage (up to 250 VDC in that case), the
capacitors where OK. At the beginning, they had a huge leakage current - until the oxide layer was formed on the electrodes. Ringing a bell?
The electrolytic caps in an old stereo amp which has been
sitting for a while - the kind with the big old linear power supplies - also need to be reformed when you start to restore one. It's a problem that's been around for years. In fact, I leave my old amp on 7/24 just for that reason, even if there's no signal going to it. Besides, the panel lights look good in a dark room.
Michael R. Batchelor - Industrial Informatics, Inc.
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I have not experienced any problems personally, however there has been some chatter in recent years about the dielectric in the capacitors tending to physically change without use. The dielectric as I understand will return to its normal state with exercize. I have spent numerous years specializing in VFDs and I can tell you that a capacitor that explodes will get your attention. I would contact your drive supplier or check on http://www.drives.com (yaskawa site) because that is where I believe I read about the problem.
The electrolytic caps dry out if unused for a long time. I don't think it is necessary to do a DC charge up because the AC is convetred to DC anyway. The main point is to slowly raise the voltage to prevent stressing and damaging the dry caps. Analogy is a dry sponge that can absorb water only slowly until it is wet. I recommend contacting the drive manufacturer for specific advice.
Most decent VSDs have a capacitor "forming" mode which will be initiated automatically when required. Pre-charge is quite another thing, if the VSD has been switched off for some time, as in hours or days, instead of months or longer, then the caps are discharged and the DC bus voltage is influenced by the need to charge the caps before simply starting up and running the motor. Pre-charge can take a few seconds, forming can take a few hours, forming is apparently not uncommon when a VSD is first switched on and seldom required after this. Pre-charge could be required every time the drive is switched on, depending on the duty cycle and nature of the control.
Questions and answers below.
STOP!!!!!!!!! and read this prior to starting up that drive.
After three years, the electrolytic capacitors may need to be "Reformed". Failure to do so can result in permanent damage to equipment and personnel!
On November 30, 2003, KC Oh wrote:
> 1. Is there a guideline how long VSD can be stored? <
Check with the manufacturer of the drive.... or the manufacturer of the capacitors within the DC bus of the drive.
> 2. Is it necessary to go through such proceedures? Myth or logic? <
It is not myth. If you power up the capacitors in the converter section of a VFD and the capacitors require "Reforming", the capacitors could literally blow up.
> 3. If powering from DC can prevent damages of VSD, why is there no proper connection terminal for this purpose? <
Some drives do have a direct connection to the DC bus.
This is absolutely necessary because the DC link capacitors should be "self healed". If you do not do this procedure (called "forming"), they may blow up. The forming current should be limited by a resistor to a certain ampere and this current and the duration of forming should be given by the VSD manufacturer. However as an example, Siemens Masterdrives require to be formed for one hour after one year of non-operational time.
You have to do this from the DC link and VSD should be off.
By the way, pre-charging is something different: it is current limiting of the DC link for a short time by a resistor at each power on of the VSD. This is done internally by the VSD.
Please I need more details about this point. There is York chiller with vsd drive panel reads :" SYS 1 VSD DC BUS PRECHARG". what does that mean please?
>By the way, pre-charging is something different: it is
>current limiting of the DC link for a short time by a
>resistor at each power on of the VSD. This is done
>internally by the VSD.