On 2007-10-29, Jon Elson wrote:
Ignoramus32225 wrote:
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.

Hmmm, should be 340 V for a 220 V input. Is it 340 when the VFD
is idle, but powered?

It is 340 when the VFD is idle, but powered, but drops to 220 when it
tries to spin up the compressor.

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.

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

4) I used a straight piece of 10 ga wire instead of a DC choke to
connect the DC bus.

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.


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.

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.

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

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.

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

i