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

On Oct 29, 1:13 am, Ignoramus32225 ignoramus32...@NOSPAM.
32225.invalid wrote:

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".

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

According to Ignoramus32225 :
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.

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.

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

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?

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.

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.

Logically, a 25 HP VFD should spin up the 10 HP motor.

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?

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.

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.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

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? 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.
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.
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

"Ignoramus32225" wrote in message
...
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

You need one of those "Energizer Bunnies" connected in parallel.

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

[top-posting to keep information better organized]

I spent some time with the drive last night.

1) Changing V/f curve does seem to change things a little, but not much.

2) I set all sorts of parameters like "acceleration boost", to the
highest values, without much success.

3) Under load, the input voltage (from utility) does not change
much. So voltage drop is not an issue.

4) When starting, the compressor starts spinning, then hits an
"obstacle", and comes to complete or almost complete stop.

I now think that at last part of the problem, as mentioned by other
people wrt phase converter, is with "unloaders". I do not think that
they are correctly functioning.

At the very least, I want to try altering their function so that the
compressor spins freely, such as when running with a gas motor that is
idling. It would, obviously, be for test purposes, but I want to now
try this with the compressor NOT pumping any air.

With the limited time and understanding that I had yesterday, I was
unable to do so. I even disconnected some air lines that led to the
unloader adjustment box. To no avail, nothing has changed.

I will read the manual today and see how I can try to disable
unloaders. (disable in the sense of preventing the compressor from
pumping air)

This story is more difficult than I thought it would be, but I stil
have some optimism.

i

On 2007-10-29, 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.

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

On Sun, 28 Oct 2007 20:13:28 -0500, Ignoramus32225
wrote:


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".


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

On 2007-10-29, Ned Simmons wrote:
On Sun, 28 Oct 2007 20:13:28 -0500, Ignoramus32225
wrote:


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".


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, yes, I did set it correctly. I remember. It also did not help
much.

I also saw this:

http://www.sbt.siemens.com/HVP/Compo...csOverview.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

Ignoramus3971 wrote:
On 2007-10-29, Jon Elson wrote:
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?


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

Ignoramus10340 wrote:

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.

Yes, this is a unit not designed for any holdup, and you have to add
a LOT of capacitance!

jon

On 2007-10-29, Jon Elson wrote:


Ignoramus3971 wrote:
On 2007-10-29, Jon Elson wrote:
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?


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.


So, it sounds as though I need more caps.

i

On Mon, 29 Oct 2007 12:49:48 -0500, Ignoramus10340
wrote:


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.

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


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?

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

On 2007-10-29, Ned Simmons wrote:
On Mon, 29 Oct 2007 12:49:48 -0500, Ignoramus10340
wrote:


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.

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


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?

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, 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".

According to Ignoramus10340 :
On 2007-10-29, Jon Elson wrote:


Ignoramus3971 wrote:

[ ... ]

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


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.


So, it sounds as though I need more caps.

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.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

On 2007-10-29, DoN. Nichols wrote:
According to Ignoramus10340 :
On 2007-10-29, Jon Elson wrote:


Ignoramus3971 wrote:

[ ... ]

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


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.


So, it sounds as though I need more caps.

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.


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

DoN. Nichols wrote:
According to Ignoramus10340 :

On 2007-10-29, Jon Elson wrote:


Ignoramus3971 wrote:


[ ... ]


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



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.


So, it sounds as though I need more caps.
Lots more, probably no way it will work at less than 1000 uF.



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.
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

On 2007-10-30, Jon Elson wrote:


DoN. Nichols wrote:
According to Ignoramus10340 :

On 2007-10-29, Jon Elson wrote:


Ignoramus3971 wrote:


[ ... ]


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



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.


So, it sounds as though I need more caps.
Lots more, probably no way it will work at less than 1000 uF.



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.
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.


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

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