Siemens SED2 VFD / Quincy 10 HP compressor troubles

I tried connecting the VFD to the compressor. The VFD is a 18.5 kW (25 HP) rated Siemens SED2 VFD. Its target market is HVAC. The compressor
is a 10 HP motor.
It was kind of unsuccessful. I made quite a few observations.
1) The motor, when VFD turns on, only spins at maybe 60 to 90 RPM. And struggles to turn the compressor.
2) The VFD has no troubles spinning up an unloaded 1 HP motor (just for test).
3) I measured DC bus voltage when the VFD was powering up the 10 HP motor, it was about 220v.
4) I used a straight piece of 10 ga wire instead of a DC choke to connect the DC bus.
5) If I measure voltage on the output, with my nice multimeter (knowing that I should not trust the result too much due to square wave), I get something like 70-90 volts instead of 220. That's what the read on panel reads if I set it to display voltage.
Logically, a 25 HP VFD should spin up the 10 HP motor.
I looked into the manual, available at
http://igor.chudov.com/manuals/
and found reference to parameter 1300, V/F curve. It talks about different selections, such as linear V/F, parabolic etc. It says that linear V/F is better for constant torque and "positive displacement pumps".
Due to family stuff, I had no time to actually try changing this, does it sound as though this could solve it? The wikipedia Pump article seems to imply that my piston compressor is a positive displacement pump.
Anyway, it was a somewhat frustrating day in this respect. I will followup to this once I try it in an hour or so.
i
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On Oct 29, 1:13 am, Ignoramus32225 <ignoramus32...@NOSPAM. 32225.invalid> wrote:> I looked into the manual, available at

I assume V and F in that case refer to voltage and frequency. You have to reduce the voltage as you reduce the frequency, because the motor has a lower impedance at lower frequencies as it is predominantly an inductive load.
A "positive displacement pump" is one where fluid cannot leak backwards through the pump. So a compressor is a positive displacement pump, but a fan would not be.
It sounds like you probably have a problem with the settings on the VFD.
Best wishes,
Chris
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    What is it when *not* starting the 10 HP motor? That sounds rather low -- I would expect the no-load voltage to be closer to about 310 VDC from a 220 VAC input.
    Can you hook an isolated (for safety) oscilloscope to the bus? I'll bet you see a *lot* of ripple.
    Essentially -- here is where you hit problems trying to run a three-phase VFD on single phase -- there is too much time when the voltage out of the rectifiers is low. With three phase input, the output from the rectifiers never gets anywhere near zero -- until the input power is turned off or disconnected.
    I'm rather surprised that it did not complain about input voltage being low.

    That won't help much while running the rectifier from single phase. Is your rotary converter big enough to hang the VFD on it and to have a chance to run the compressor?

    That sounds fairly reasonable, given how low the bus voltage is. Unloaded, it should be the square root of two (1.414) times the input AC voltage.

    If it has enough voltage full time. Is this one rated to run from single phase, or is it just one which happens to accept single phase?

    What you need is for the unloader to stay switched in until the VFD gets the motor up to speed.
    What happens if you remove the belts? Will your VFD spin the motor up under those conditions?
    Good Luck,         DoN.
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You need one of those "Energizer Bunnies" connected in parallel.
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On Sun, 28 Oct 2007 20:13:28 -0500, Ignoramus32225

Make sure parameters 0304 (Rated motor voltage), 0310 (Rated motor frequency), and 0311 (Rated motor speed) match the motor nameplate. If they don't, the slope of the V/f curve will be wrong, i.e., the motor will see an undervoltage or overvoltage condition which will get worse as the speed increases.
--
Ned Simmons

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Ned, yes, I did set it correctly. I remember. It also did not help much.
I also saw this:
http://www.sbt.siemens.com/HVP/Components/Documentation/SED2HarmonicsOverview.pps
See slide 16 out of 17.
Apparently, this SED2 drive has a very small capacitor compared to other drives. Only 20 uF compared to 1,000 uF for other drives, with the example of a 15 HP drive. (unspecified voltage)
That might, in my mind, account for voltage sagging when run from 1 phase, and low voltage delivered to motor.
So, a possibility to ponder would be to add some capacitance between DC+ and DC-. The terminals are available for connecting, but I am not sure of ramifications of, for example, inrush current greatly increased.
Any thoughts on this?
i
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Ignoramus10340 wrote:

Yes, this is a unit not designed for any holdup, and you have to add a LOT of capacitance!
jon
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On Mon, 29 Oct 2007 12:49:48 -0500, Ignoramus10340

I'm sure it does explain the sag, 20uF ain't much.

Obviously the best thing would be to ask Siemens. But from my experience building unregulated supplies for servos, I doubt 1000uF would be a problem. Looking at the schematic for one system I built, there's: 100V/5A xformer secondary -> 25A bridge rectifier -> 2100uF/300VDC cap -> servo amps.
There was a never a problem with this or several similar systems I've built.
--
Ned Simmons

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Ned, thanks. I will approach this issue from two angles.
One is that I will use all caps that I can find at home that carry an appropriate voltage rating. I will try to see if it makes any visible difference.
Second is I will try to work with the unloader system and try to spin the flywheel without the compressor doing "work".
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Ignoramus32225 wrote:

Hmmm, should be 340 V for a 220 V input. Is it 340 when the VFD is idle, but powered? If so, then wither the rectifiers or the capacitors have a big problem. it could be the caps are in really bad shape, but still good enough for 3-phase input. If your mains voltage at the VFD was dropping to about 155 V (220/1.414) then I would expect the VFD to perform very poorly, as that's only a little above half what it should be.

Most VFDs come with the rectifier connected to the inverter, if it had been set up for use with input reactors, there may be configuration settings to put it back for use without.

Even if you were averaging the waveform correctly, if the drive has not come up to 60 Hz, then the applied voltage would be reduced.

Not likely, but I'd set it for linear, and 230 V at 60 Hz. Set it for a modest ramp-up time. If the VFD frequency "loses" the motor, ie. it ramps up frequency faster than the motor is spinning up, then the motor will bog, as the VFD can't provide the kind of line currents such a motor would draw on a real 3-phase line. Most VFDs are set up to stop the frequency ramp when the motor current hits the limit. What IS the current limit set to, by the way? Should be around 33 Amps for 10 Hp.
Jon
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It is 340 when the VFD is idle, but powered, but drops to 220 when it tries to spin up the compressor.

I thought that dropping to 220 under load was kind of normal?

Yes, except that it does come up to 60 Hz.

Jon, I will make a separate post about my last night's findings, so that things do not get lost.
i
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Ignoramus3971 wrote:

No, it is NOT normal. Now, of course, you are running the VFD on single phase, and it was not designed for that. So, I would expect a significant voltage drop, maybe to as low as 300 V or so. That's what the caps are for. But, 220 VDC is abnormal.
Jon
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So, it sounds as though I need more caps.
i
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    [ ... ]

    Beware -- more caps with the same rectifier may result in too much current through the rectifier and thus burn out the rectifier. Perhaps power up the VFD with the standard caps, then switch in an additional one, then another, until you have the full set in place. This will reduce the current surge through the caps -- at least until a short power outage shuts off the VFD and then the power returns. If this works, you may want to add a circuit to automatically add the caps as you start up the VFD. Or -- you can give each additional cap its own bridge rectifier, so you are not overburdening the built-in rectifier.
    Good Luck,         DoN.
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I found my 92 uF cap and will try it tonight to see if it makes any difference. The question of "switching in" caps and "inrush current" is relatively well studied. I am indeed concerned about burning the rectifier. I do have a rectifier of my own. I will probably need to get more caps.
i
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DoN. Nichols wrote:

Lots more, probably no way it will work at less than 1000 uF.

You can't do this! Connecting several hundred uF of caps (some charged, some not) together with a switch will cause a HUGE bang, as loud as a pistol shot, and the switch will be toast, the very fist time.
Larger VFD's, like my Tosvert 11 KVA drive, have a resistor and relay circuit that slowly charges the cap bank before making the direct connection. It wouldn't be real hard to make up such a circuit if it doesn't already have something like this. You turn it on with a big 240 V light bulb or resistor in series, then short out the resistance after a second, and before allowing the VFD to power the load.
Jon
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A double pole relay can be used to connect the cap directly, as well as to close the control circuit from the pressure regulator.
i
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I took two more 92 uF caps from my phase converter. For the total capacity of 351 uF added capacitance. It was 167 uF added capacitance yesterday.
This enabled me to raise the maximum achievable frequency from 20 Hz (yesterday) to 30 Hz (today). I am hopeful that with, perhaps, 1,000 uF this drive will be able to power the compressor.
i
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