Progress in running a 10 HP compressor from a VFD

For the last two nights, I moved the compressor into the garage and
have been trying to find a good way to run it. This is a Quincy
compressor with a 5-10 HP rated pressure lubricated pump.
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I bought it without the motor, it had a 7.5 HP 58 year old motor that
I did not care for anyway, which gave me a nice price reduction from
the seller, who happened to be a super great guy.
The compressor is now powered by a 10 HP Reliance motor that I had in
NOS condition. I reused the old pulley, which I had to bore out, but
with this pulley it only produced 7.5 HP of output since the pulley
and RPM stayed the same.
Installation of the motor is shown here:
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I have been working on powering this compressor from a VFD. I am
hoping to accomplish several things with a VFD:
1) Running from single phase
2) Quiet starts
3) Being able to vary speed a little bit, so as to either run super
quietly when I do not need much air, which is the usual case, or in
case of high demand to use full 10 HP at higher speed. According to my
simple calculations, it would run at 5 HP output at 40 Hz, 7.5 HP at 60
Hz, and at 10 HP output at 80 Hz. (the top limit is a little more
iffy, but should be close at pressures lower than 145 PSI)
I used the following VFD: Omegapak 10 HP AC Drive bought here for $100
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I decided to solve one problem at a time, and tried to use it from a
real three phase input (phase converter).
It was a little bit of a PITA, to put it mildly, as it had a very old
fashioned control interface. But after some pain, printing out the
manual, and going over things a dozen times, I am now at the point
where the drive comfortably runs the motor, gives smooth acceleration,
deceleration and all that.
As a result, this setup is exceptionally quiet and starts nicely and easily.
The next step would be to try to run it from single phase. (the drive
is 3 phase rated)
My plan is as follows. This drive has two separate inputs: one for
control circuit H1 and H2, and another for three phase input L1, L2,
L3. It also has full amperage contacts for the DC bus, + and -.
So what I want to try, is this: feed DC bus directly, through + and -,
from a completely separate rectifier and a huge 2,200 uF, 700V
electrolytic capacitor. (two 2200 uF, 350V caps in series
actually). Feed the control circuit H1/H2 from regular household 220v.
If I am right, and if it works, it would give enough juice to the DC
bus to not phase fault during zero crossings.
I will see.
Reply to
Loading thread data ...
Umm, that's only 1100 uF with the wto in series.
You can look up the rectifier or power block in a datasheet and see what the rectifier ratings are. on the VFDs I have used, they sure looked more than adequate for single phase operation. The capacitor bank might be the marginal part, so adding more caps outside would be a good idea for such a large load.
Reply to
Jon Elson
You are right about the capacitance. I goofed.
Usually, for single phase operations with three phase rectifier, there are warnings about overloading the rectifier, so I would rather not take a risk.
Reply to
Maybe I missed it, but why not run the 220 single phase into the mains inputs and put the pressure switch on the control inputs? It seems like the simplest way to do it. Also, it sounds like you have the capability to run the motor at the reduced load which would give you nearly the recommended de-rate for single phase operation without hanging a high current, high voltage kludge on the VFD box.
Reply to
The drive fails with "phase fault" if I feed it single phase in its inputs mand have a motor connected. That's why I wanted to provide power to the DC bus by separate means.
I will definitely connect the pressure switch to the VFD "forward" terminal. If everything works according to plan, I will have two controls:
1) Switch to prevent starts even if pressure switch is closed 2) A potentiometer to select running speed, in the range of 40-80 Hz. I would then select the potentiometer setting to select quiet, slow operation vs. a little louder, faster operation.
I wish it was the case.
Reply to
Seems that it might not work, but worth a try, I will try it. Thanks. In any case, if I need to get full power out of this drive, which I do, I would need to supplant the rectifier.
Reply to
Indeed...a jumper from L3 to L2 often works. Not always..but often
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Reply to
Gunner Asch
The reason why I think that it will not work is this: the drive reports "phase fault" only when connected to the motor. If it is not connected to the motor, it does not phase fault. This means that they detect phase fault based on dips in the bus voltage. This is also corroborated by the manual.
Reply to
Hmm ... what have you done with the third input? Often, it is necessary to strap together two of the three inputs when running from single phase input to prevent such faults. It is complaining that one of the power input leads is open circuited. Usually, the first two are used for actual power input and the third is jumpered to the adjacent terminal.
Hmm ... why not make that a three position switch with a string of resistors to emulate different pot settings. Maybe make it a double deck four position switch, so you can have stop as one of the positions?
Good Luck, DoN.
Reply to
DoN. Nichols
This one phase faults when it sees voltage dip on the DC bus.
Why, a potentiometer is a better match for this application. I would mount it right on the pressure switch.
Reply to
Why not use a modulating control to adjust the speed of the motor? Fed from the enable of your pressure switch, the control would adjust a 0-10vdc or a 4-20mA signal, which most VFD's can use. The greater the deviation from setpoint, the faster the motor runs. An L91A1035 Honeywell or a P7810. They are used to control the firing rate of modulating burners. I think I have one of each molding in the basement. If you would like to try one let me know.
Reply to
SnA Higgins
One issue is, what happens when the pot momentarily opens? (Many pots are ...noisy...). One approach is a number of resistors in series, and switches to short them out.
Reply to
David Lesher
Steve, it is a very interesting thought. I also considered something similar, but I decided that manual control is more suitable for my needs. In my instance, it boils down to how much air I need vs. how much noise I want, as well as pump life, which is a human judgment.
For example, if I was using an air ratchet, I would set it to lowest speed, if I was using an air hammer continuously, I would set it to highest.
It is a more idiot proof approach. But your thought has a very solid foundation and would be very appropriate in an industrial setting.
Reply to
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Here's a thread-tie:
If you use your PTO generator to drive your VFD, set to 60 Hz; it won't matter nearly as much that the engine speed/output frequency are not stable.
You could in fact use a throttle-opener that was in series with the output; the more current you draw, the more RPM's you get...
Reply to
David Lesher
[ ... ]
O.K. Perhaps a bit more capacitance?
But this *is* a potentiometer -- just with three fixed settings.
You only want to select three speeds don't you, not just any speed? The switch would be like this:
ref V --+ from VFD |
R1 <
< | +------------+ | |
< | R2 > O < | +----------O to control input on VFD | \
< O \ R3 > | O \ < | \ | | \ +------------+ \ | +------O \
R4 < | \
< | on VFD +---------------| ___ Control +------O - ground O
The sum of R1 through R4 is the same value as the recommended pot value (5K or perhaps 1K), and the individual values are selected to give the specific speeds which you want -- so there is no problem with the knob on the pot vibrating and changing your compressor's speed during a long run.
And the second deck is used to switch on the motor FWD in the top three positions, (circuit here assumes that you need to ground the pin to start the motor -- change as necessary.
This gives you a single control which does all that you need, and is unlikely to change setting during a long run.
Enjoy, DoN.
Reply to
DoN. Nichols

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