Crazy thought -- converting 480v drives to 240v

I am thinking about something. 480 VAC drives can be bought for next
to nothing. What they have is an inverter circuit and control
circuit.
They probably would not run on 240v because the internal transformer
that would supply voltage to the control circuit would supply only 1/2
voltage. They also might have an indervoltage sensor. I cannot think
of more things that depend on 480 volts being there.
So, if a hack could be done and a 1:2 transformer inserted between
480v inputs and the control circuit, then the drive would become a
240v drive with the same amp rating (and half HP).
What am I missing?
i
Reply to
Ignoramus12852
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Where can I get a 480 VAC drive for next to nothing? Used? Ebay? Please explain. I think your concept is valid though.
Reply to
Mike
What about the output voltage? The one that you're bidding on is 480v out. Is it programmable?
Bob
Reply to
Bob Engelhardt
The way VFDs work is, they rectify input voltage and then supply the same voltage as "output", turning it on and off several times per second. So the output voltage is, more or less, the input voltage.
i
Reply to
Ignoramus12852
Iggy dont most three phase motors relink to be a 240 or 480 so you wouldnt even need to play with it?
What happens if you just feed the 480V VFD 240V?
other than that would you be able to find a three phase step down transformer
Reply to
Brent
The control logic power supply is one thing. It would probably be worth opening up the box to see if there are jumpers on the control power primary that would allow the control electronics to run from 240V. You already mentioned the undervoltage sense.
A few other things to consider would be the high voltage rectification to DC and the output driver biasing.
If the high voltage rectification is power factor corrected which I think is fairly likely, all bets are off. The power factor correction is a boost converter that syncronously rectifies the incoming AC and regulates the resulting DC to a voltage above the peak to peak AC input voltage. This would have to be modified to boost to a different voltage.
If the high voltage recifier is a simple bridge, then the filter caps on the high voltage DC supply and the recifier will still be sized for half the current.
The output driver transistor biasing might or might not be OK at the lower voltage. I assume that VFD's use IGBT outputs, so there is no gate/base current to speak of like a bipolar transistor would require.
I would not bet even one beer that this would be as simple as just getting the control electronics to run.
Good Luck, BobH
Reply to
BobH
Bob, I will try to find a cheap 480 v VFD and will play with it just for the kicks. For maybe $15 or so, it will be a valuable educational experience. I will try to find a 2 HP 480v VFD and will try to modify it to run a 1 HP 240v motor. Worst case is, I will waste time and money.
i
Reply to
Ignoramus12852
It might be an interesting experiment, but since new drives up to a couple HP are in the $200-300 range and have a warrantee it would seem to not be worthwhile if you put some reasonable value on your time to make the modifications and on having a drive with a warrantee operating within it's design parameters.
If you have enough machines that could use VFDs, you're likely better off strapping their motors for 480V, buying the super cheap used 480V VFDs, and finding a single good sized transformer to setup up the shop 240V to 480V to feed the drives.
Reply to
Pete C.
I get flyers from automation direct that list single phase drives on sale for less than 200 bucks new for a 1 hp drive. Unless you are picking up the used drives for almost nothing, its not worth screwing around with transfomrers and introducing more electrical losses and potential problems in your system.
John
Reply to
john
I am thinking more about 40+HP drives. But the experiment has value. I placed a few snipes on 480v VFDs and will report my findings.
That's definitely another practical possibility.
i
Reply to
Ignoramus3938
That's my point. Unless you have say a half dozen 3ph machines in the 5 HP range, new individual 240V drives are a better idea. With those big machines you could go the step-up and use 480V drives and save some money potentially. Either way trying to modify the drives is almost certainly not worth the effort.
Reply to
Pete C.
Trying to run a 40 HP VFD on your 200A residential 240V service is sure to let magic smoke out somewhere, not to mention pissing off the wife when the house power drops.
Reply to
Pete C.
Please let me know how it works out. I did a component level repair on a 5 HP 240 volt unit, but it was only a matter of finding the roots of the visibly exploded cap on the control board and replacing it. I did not have a schematic and I suspect that they are hard to get.
The reason that I am expecting power factor correction on the high voltage DC supply is that the Europeans have been raising a huge stink about harmonic currents and power factor correction for about 5 or 7 years now on switching supplies and a motor drive is basicly a switching supply. I think power factor correction is a legal requirement to sell the drives in Europe for motor drives over a horsepower or so now.
Good Luck, BobH
Reply to
BobH
Most of the bigger drives use a transformer in front of them to switch the input voltage.
Just as a side note, with vfds the motors don't always come with the smoke alarm. It seems that the vfd limits the current and the smoke doesn't release. I had one fail friday nite that didn't smoke. 25 hp dc Reliance motor that just wanted to rotate to one position and stop. The drive limits the current so the coils don't get too much of an overload. On this motor there was a dead short to ground on the rotor. The operator said that it had been rumbling for a while.
John
Reply to
john
You will piss off the whole block, or whoever else is connected to your pole transformer, as well as the electric company .
John
Reply to
john
The 480v drives can be *really* cheap. E.g.:
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$23 *shipped* for 3hp at 480 or 1 1/2 at 240v
Reply to
Bob Engelhardt
In my area that is one neighbor at most and in many cases it's one pole pig to one residence. I share a 25KVA rated pole pig with one retired neighbor and given the ability of a standard pole pig to handle 100% overload (50KVA in my case) for 24hrs without damage I think I'm in good shape.
Reply to
Pete C.
A 40 HP 480 VFD would at most produce 20 HP off 240v, and 33% less when derated to run from single phase. That leaves us with about 12 HP at most.
i
Reply to
Ignoramus3938
Its not so much that you will damage anything but your neighbor may get pissed at his lights flickering. :) If the neighor is that old he'll problaby think it was his eyes going bad. :)
The power company doesn't like sudden starts of motors or any load that draws excessive current, especially if you are located on the end of the supply line. It means that they have to upgrade their lines, and if they know it's you doing it, you will learn about how a demand meter works.
Reply to
john
That is another of the nice things about VFD's. They can start up a motor slowly so no excessive current.
Dan
Reply to
dcaster

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