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Old June 22nd, 2017, 02:36 PM   #1
lesmar96
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Supply vfd with dc power

Hello,

I am working to design a simple system where I would like to power the VFD directly off of a DC power supply rather than 3PH power. The reason being because all I have available is 230V 1PH and rather than going to a much more expensive drive to get enough single phase input rating, I thought it may be interesting to use a rectifier and input the power directly on the DC bus off the drive. Has any one else built a system like this? One question is what is the best way to get the required 330VDC off of the 230VAC source?
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Old June 22nd, 2017, 02:59 PM   #2
rankhornjp
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Quote:
Originally Posted by lesmar96 View Post
I thought it may be interesting to use a rectifier and input the power directly on the DC bus off the drive. Has any one else built a system like this?
I have not done it personally, but I have heard of others doing it successfully. One case was a guy that used a drive to power his electric car that had an AC motor in it.

Quote:
One question is what is the best way to get the required 330VDC off of the 230VAC source?
Step it up to 367vac then it should be 230-ish rectified.
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Old June 22nd, 2017, 03:02 PM   #3
rankhornjp
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here's link to 2 electric car builds that used this idea.

http://www.diyelectriccar.com/forums...ted-70662.html


http://www.diyelectriccar.com/forums...ad.php?t=15363
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Old June 22nd, 2017, 03:37 PM   #4
GaryS
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to answer the question Yes it's done all the time
but why wen a single phase VFD it no more expensive than the 3 phase
they are available in 120VAC single phase in with 240VAC 3 phase out or you can get 240VAC single phase in with 240VAC 3 phase out or you can just use a 240VAC 3 phase input VFD and hut connect it to a single phase source you just need to derate the VFD by 1/3
you can connect then to a DC source but then you have the expense of the DC converter as well as the cost of the VFD I don't see any value to that
this is usually done where the DC source is already available on site and AC power in not available
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Old June 22nd, 2017, 04:26 PM   #5
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Originally Posted by GaryS View Post
... or you can just use a 240VAC 3 phase input VFD and just connect it to a single phase source. You just need to derate the VFD by 1/3...
The VFD already has diodes and caps inside. Turn off the "missing phase" detector for the input, and run it.
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Old June 22nd, 2017, 07:31 PM   #6
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The issue at hand is a part of a VFD system that is seldom discussed, called a "pre-charge" circuit. This is absolutely necessary for all VFDs, because the DC bus caps, when FIRST energized, will instantly charge themselves up, and will do so at the AVAILABLE FAULT CURRENT level, even though that might actually damage the caps themselves. So for the first few seconds, the VFD puts a current limiting resistor in series with the caps as they charge, then once charged, a relay (or contactor depending on the size) shorts around that resistor so that it's not in the circuit constantly. Some VFDs however use SCRs instead of Diodes on the rectifier, so they "soft start" the DC rectification to get the same effect. Yet others use an NTC thermistor in series with the DC bus, which has a high resistance when cold and low resistance when hot, so it inherently soft starts the DC into the caps. There is no way of knowing by looking at them how any particular VFD accomplishes this.

The problem with feeding a VFD directly with DC, is just that; you don't know HOW the VFD mfr designed that pre-charge circuit and where it is in the unit. So if you tap your DC into it on the down stream side of the pre-charge, you might blow the caps the instant to energize it the first time (or some future subsequent time if you dodge the first bullet).

So some drives are designed to allow you to do this, some are not, there is no way to tell in advance without asking the VFD mfr if it's possible, and if they will stand behind the warranty in writing if you do (always a good way to filter out the idiot inside sales people who say yes to everything without really knowing).

One way around this is to build the pre-charge functionality yourself into the DC power source. There are companies that actually MAKE DC power sources specifically for this purpose and offer them WITH the DC pre-charge in their package. One that I have used is Bonitron, it's a great unit. One issue with using a VFD for single phase input is that you must at LEAST double the size of the VFD with respect to the motor size, because the drive needs more DC bus capacitance to handle the added ripple from rectifying the single phase. At 230V 3HP and below, this is often designed into the VFDs anyway, because the added device cost at that level is negligible. But if your motor is 5HP or more, that's where you must at least double the size of the VFD (if it has a DC bus choke, triple the size if not). That's where using a DC input from a unit like the Bonitron pays off. When you double the size of the VFD to get the added capacitance, you are also doubling the size of the transistors and although you don't need that, you are paying for it so it's a waste. Feeding it (correctly) with DC avoids having to over size the VFD, assuming that the DC is already smoothed out (another thing the Bonitron unit does for you).

Last edited by jraef; June 22nd, 2017 at 07:37 PM.
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Old June 22nd, 2017, 10:18 PM   #7
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Quote:
Originally Posted by lesmar96 View Post
Hello,

I am working to design a simple system where I would like to power the VFD directly off of a DC power supply rather than 3PH power. The reason being because all I have available is 230V 1PH and rather than going to a much more expensive drive to get enough single phase input rating, I thought it may be interesting to use a rectifier and input the power directly on the DC bus off the drive. Has any one else built a system like this? One question is what is the best way to get the required 330VDC off of the 230VAC source?
Sounds to me like an attempt to reinvent the wheel. Any losses that you would have encountered in the single phase input drive will simply be transferred to your rectifier.
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Old June 22nd, 2017, 10:29 PM   #8
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very good explanation jraef
most people don't understand much about the inter workings of a VFD
also keep in mind a standard power supply will have a voltage of about 250VDC
the buss voltage on a VFD will be about 325VDC due to the rectified voltage filtered current. most VFD's will fault out on under voltage when connected to a pure DC source so you will have to either disable the under voltage fault or boost the DC source to a level the VFD will except. on some VFD's you can disable the under voltage fault
And let's not forget the new line regeneration VFD's they send the buss over charge generated in a overhauling condition back into the AC supply line and can be used as a DC to AC line sync inverter.

Last edited by GaryS; June 22nd, 2017 at 10:34 PM.
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Old June 23rd, 2017, 01:07 AM   #9
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I agree completely with GaryS.

Its nice, but why would go into so much trouble? It will cost you a lot more than a simple solution, and safety isnt given if you modify a existing drive.

Simply use a vfd with single ph 230V Input, and 3 ph Output.
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Old June 23rd, 2017, 06:32 AM   #10
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Thanks for all your explanations. I have done some experimenting with DC precharge resistors and capacitors to build a device that will carry a VFD through small brown-outs, so I know a little about that.

I had not thought of the inrush etc. that would have to be taken into the calculations. This machine, which would require a 40hp motor, is still in development stages. I never personally built a system with DC input to the drive so I thought this might be a good project to research it on to see if it at all would be cost-effective. It sounds a bit more complicated that I first thought it might be so it will likely not pay off in this job.

The difference between a 40hp 230V drive and a 80hp 230v is quite substantial, though.
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Old June 23rd, 2017, 10:52 AM   #11
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Quote:
Originally Posted by lesmar96 View Post
...

The difference between a 40hp 230V drive and a 80hp 230v is quite substantial, though.
You will likely find that the cost of a 40HP VFD + a Bonitron DC source unit is less than the cost of a VFD capable of 2x the current of a 40HP 230V motor (more likely a 100HP rated VFD).

By the way, de-rating a VFD by 1/3 is a myth no VFD manufacturer (who knows what they are doing) will support that. The capacitors may survive, but not for long. Filtering out the excess ripple takes a toll and increases the temperature of the caps, leading to premature failure if not seriously over sized to compensate.

The generally accepted rule among VFD mfrs is 2x the motor current rating IF the VFD has a DC bus choke, 3x the motor current rating if not.

Last edited by jraef; June 23rd, 2017 at 10:57 AM.
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Old June 23rd, 2017, 11:35 AM   #12
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I am not real familiar with DC bus chokes. Our standard rule of thumb is double the size. We sell alot of Fuji Electric drives and they give single phase specs for them, and they are often larger than double the size.

Is a DC bus choke a standard accessory on VFDs? I have used DC reactors already.
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Old June 23rd, 2017, 12:15 PM   #13
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Sigle Phase VFD

Attaches is an copy of a table showing the part number you need to select a 3 phase VFD to be powered from a single phase source
it looks like the 2075 is exactly what you need for a 40HP
it also requires a 250A source
the problem is not difficult to solve just get the correct VFD
Attached Images
File Type: jpg Single Phase VFD.jpg (83.6 KB, 161 views)
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Old June 23rd, 2017, 12:18 PM   #14
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Here's the link to the full document on single phase source for VFD's
worth reading

https://www.yaskawa.com/delegate/get.../AN.AFD.15.pdf
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Old June 23rd, 2017, 03:25 PM   #15
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That is an good document. I'm am still interested in researching a bit more on the dc supply idea. We are a dealer for Fuji and they have a very cost-effective vfd for OEMS but this series of vfs only goes up to 40hp in 230v. So I am at the biggest size already. If I have to derate the drive for single-phase input, I will have to go their "Cadillac" of drives to get that rating, which is quite more expensive. Their OEM drive, the 115A version, I can sell for $1350, but if I have to go to their high-end drive, I can't sell it for less than $4500. This is basically an experiement yet.

I have checked into the Bonitron and they may be a practical way to go. Size of control panel to house all this and many other factors will need to be taken into consideration as well.
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