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Old December 8th, 2010, 01:00 PM   #1
The Plc Kid
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Pre Charge and Common DC Bus

Can someone explain precharge in a ac drive to me? How it works in detail and what it is used for?

Also how does it come into play in a common dc bus system.

I am configuring some drives here http://raise.rockwellautomation.com/...CEB2FB57E45F45

And i have a choice of ac/dc input without pre charge and dc input with precharge.

I would like to get the ac/dc version because it is off the shelf and i need this in a hurry. But do i need precharge in a common dc bus circuit? if so how do accomplish precharge outside the drive correctly?
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Old December 8th, 2010, 01:18 PM   #2
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In short, an AC Drive has a rectifier front end feeding a capacitor stack.
Without some sort of pre-charge circuit, you will generally destroy input components in short order on power up (and/or blow fuses, open breakers, etc).

The basic precharge circuitry usually is something as simple as a contactor that shorts out one or more resistors in series with the AC input after the capacitor bank has charged up to some safe amount. More advanced drives will have a phase controlled front end to do the same thing, and bring the capacitor bank charge up slowly.

Active Front End units for a DC Bus are almost always phase controlled, but they don't need to be. Again, they will ramp up the DC Link voltage slowly, as with a common buss system each drive probably still has local capacitance, or there might be capacitor modules in the system. Some drives (DC Input) will still have a pre-charge circuit, but not all of them.

In many cases here with Common buss systems, I take an output from the drive itself (buss up to voltage) to pilot an external DC Link contactor shorting out a pre-charge resistor. Without that, isolating a drive and reconnecting it to the buss would be an expensive experience from replacing the semiconductor fuses!
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Old December 8th, 2010, 01:52 PM   #3
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The system in question has a home brew rectifier ( no Precharge) which makes the dc bus and all the drives connect to it.

There is a chopper module connected to a single very large braking resisitor that shunts the voltage should it rise aboove 750 VDC.

All of the current drives have built in precharge and the replacemnts i was looking at does not so i am assuming i would need to change out the home brew rectifier for a powerflex rectifier unit with precharge sized for the system ( ie all drives)? What would be the method to size that?

Or use a system active front end vs using a rectifier unit?

Am i on the right path?
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Old December 8th, 2010, 02:19 PM   #4
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Looks like i have choices of diode power supply http://www.ab.com/drives/commonbus/20t.html

SCR power Supply http://www.ab.com/drives/commonbus/20s.html

and active front end http://samplecode.rockwellautomation...p001_-en-p.pdf

Looks like AFe is the best but most expensive. What are the pro's and con's of the diode vs scr supply?
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Old December 9th, 2010, 04:55 AM   #5
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AFE's are definitely more expensive, but consider that usually they have the ability to regen power back to the source line, so you don't waste regen power as heat in a resistor.

Diodes are just that, a simple full wave rectifier, with or without a precharge circuit to reduce inrush surges on capacitor charges. These cannot be regenerative.

SCR Front ends also cannot USUALLY be regenerative, although there are some SCR Front end controllers that are. These have a built in precharge, based on phase controlling the input full wave SCR bridge to bring the buss up slowly.

If you are just putting new drives on an existing stupid buss, there is no reason not to roll your own precharge circuit; it's just a contactor (one pole will do) with a power resistor across the pole. On power up, the contactor is de-energized. Once the drive module reports that its buss is up to voltage (usually a configurable digital output), energize the contactor to short out the inrush-limiting resistor. Do make sure to interlock the contactor with the actual drive enable with an aux.
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Old December 9th, 2010, 06:25 AM   #6
The Plc Kid
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Bob

What formula would i use to size the power resistor correctly.

How do you determine the correct rate of charge or rate of allowed voltage rise on the buss?
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