I am thinking about something. 480 VAC drives can be bought for next
to nothing. What they have is an inverter circuit and control
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?
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.
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
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 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.
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
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.
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.
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.
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.
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.
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
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.