Phase converter won't spin compressor

I built a rotary phase converter using a 25HP motor. I am using 400uf Cp and 20uF Cs with no Cpf. It just barley starts my brand new Ingersoll Rand 10HP
compressor with an empty tank but will pump it up to 175psi. It won't start the compressor when it cycles on at 135psi. The compressor turns slowly and after about three revolutions it slows down and the overload trips. I checked the unloader and it is working. I tried starting the compressor motor without the belt on and it snaps right on, but apparently with the inertia of the flywheel and pump, the motor draws too much current. It pulls down V1-3 and V2-3 to about 140 volts before it kicks out the overload.
Do I need a different or bigger idler motor or is my compressor motor just that hard to start?
I thought that a 25HP idler would be big enough but evidently not. The specs on the idler are: Linclon A.C. Motor, Lincguard Dripproof, Frame 284T, 1765 RPM, 25 HP, Insulation B, 230/460V, 62/31A, 40 Max Amb, 60 Hz, S.F. 1.15, Time Rating Cont., NEMA Code G, NEMA Design B. I have heard that some motors do not make good phase converters, maybe this is one of them.
Any insight on this will be greatly appreciated. I bought the large compressor specifically to do sandblasting and now I can't get it to work.
On a positive note, the phase converter seems to power up my 1HP Bridgeport OK. 8>)
Jeff Borning
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If I ever decide to do sandblasting, a gas-powered compressor on a trailer is what I'm going to get. I had one once and even *it* could barely keep up.
I think you need to check current and voltages on your 3 legs. And your 10 hp motor *is* wired for 220, right? You may need to borrow a peak-detecting ammeter.
Grant Erwin
Jeffrey M. Borning wrote:

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Grant sez:

peak-detecting
Agreed! What is the function of the 400 ufd Cp? Is that the start capacitor for the 25 HP (idler) motor? What about the "20 ufd Cs"? Are they run capacitors? If so, they aren't nearly large enough for a 25 HP idler and 10 HP load motor. Theory has it that run caps should be at least 25 X 30 ufd or 750 ufd. Insufficient amount of run cap will give the symptoms you mentioned, esp. when trying to run a loaded air compressor.
IMO you should:
Install aprox. 750 ufd of run cap between 1 side of the input 240 V line and the 3rd leg of the idler motor. Make sure the run cap is connected across the same leg (phase) as the temporary start capacitor. Check voltage across the 750 ufd cap with the idler running unloaded. If that voltage is more than 15 % higher than the input line voltage, reduce the cap amount until it comes down to aprox. line + 10 to 15%. Check the voltage with the load motor running. It should then be aprox. the same as line in V or slightly lower. This arrangement gives an "unbalanced" rotary phase converter.
Get it up an running satisfactorily in the unbalanced mode. Then, later you can manipulate the amounts of capacitance in both legs to achieve the Fitch style balanced rotary phase converter. This will probably require more capacitance in a 60/40 arrangement between the two phases - that is from both sides of the input line to the 3rd leg. As before, check voltges and strive to achieve aprox. line voltage across all 3 "phases"; making sure neither phase goes higher than 10 - 15% greater than line in with the load disconnected.
Bob Swinney

up.
and
10HP
start
I
just
specs
1765
1.15,
motors
work.
Bridgeport
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least
Woops, I made a typo on previous post. I should have said a 200uF Cs capacitor. (Cp is between L1 & L3, Cs is between L2 & L3, Cpf is between L1 & L2.) I have 1800uF worth of start caps between L1 & L3 which I momentarily switch on to start.
The voltages I measured with different caps are as follows: Cp Cs V1-2 V1-3 V2-3 0 0 238 211 204 100 0 238 222 210 200 0 239 237 217 300 0 239 246 220 400 0 239 256 230 400 100 240 256 244 400 200 240 257 255 400 300 240 258 265 400 400 241 258 276 500 200 239 267 261 500 300 240 268 271 600 100 241 276 259 600 200 240 276 268 700 100 241 287 267
I didn't go with all 800uF on Cp as the V1-3 was getting pretty high.
With Cp of 700 and Cs of 100, the compressor currents at 150psi are I1!A, I2)A, I30A. This is with the compressor starting with no pressure in the tank. It still won't start at 135psi.
I can email anyone an excel file of this if the columns don't come out right.
Thanks, Jeff Borning
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switch
Jeffrey,
I'm sorry to say I am pretty much drawing a blank, here. From your description it appears you are working a balanced (Fitch type) rotary phase converter with Cp between L1 and L3 and Cs between L2 and L3. L3 is the 3rd leg or "manufactured" leg. The 240-V "line in" is on L1 and L2. L1, L2 and L3 are parallel connected to the air compressor's motor.
Are you certain the wiring connections are all tight? Loose fitting temporary clip leads could be a problem. Are you using at least #14 wire between the rotary and the compressor motor? Use #10 here if running more than a few feet. Are you really sure the compressor is properly "unloading" for starting. 135 psi sounds like a high pressure to start against.
The next to last line in your chart shows Cp of 600 ufd and Cs of 200 ufd with V1-3 at 276-V and V2-3 at 268 -V. The 276-V is about 15% higher than line in V which is near the practical limit with V2-3 (268-V) a little lower. You might try increasing Cs to 300 ufd to see if it raises V2-3 up a little. Do not do this if V1-3 or V2-3 goes over 276-V (the upper 15% limit). The obvious reason for this is to keep the unloaded idler motor voltages from going high enough to damage insulation. Stay tuned - there may be an alternate solution.
What follows is opinion and is unsupported by any practical experience on my part: Loaded air compressors draw extremely high starting currents and your idler motor to load motor ratio is about 2-1/2 : 1, which may not be enough for a loaded compressor. Consider that more run capacitance may be necessary but that you are already at the practical limit with the amounts of capacitance you have tried. Consider addding permanent capacitance to the air compressor motor, connected across L 1-3 and L 2-3. Make sure this capacitance is connected directly to the compressor's motor, downstream from the compressor's on/off switch. Begin with, say, 200 ufd in each leg. Observe the compressor's starting "labor" and increase capacitance as indicated. If enough capacitance is added to facilitate proper starting, make sure that running voltages are no more than 15% higher than line in. Good luck!
Bob Swinney

I1!A,
the
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I removed all the wire nuts from the compressor motor, straightened the strands and redid them. Didn't make a bit of difference that I could tell.
Are you using at least #14 wire

I have the compressor connected to the phase converter with 10 ft. of 10-4 AWG SO cord. I wanted to use 8 AWG, but I had the 10AWG on hand and I was anxious to try out my new toy. Now I think I will buy some 6 AWG. The converter is wired with left over 8AWG THHN except the caps, which are paralleled with 10AWG to terminal strips. I am thinking of going to copper bus bars instead of terminal strips.
Are you really sure the compressor is properly "unloading"

The compressor is unloading. At first I thought it wasn't, but the unloader exhaust is plumbed into the air intake and is silenced by the air filter. I removed the line to the intake and I could hear the "pssshh" when the motor shut down at 175 psi. But...,this is a mechanical unloader which uses weights that are activated by centrifugal force to close the unloader valve (which is like a schraeder valve). After about 3 or 4 revolutions, the weights are advanced enough to close the unloader valve. I removed one of the two adjusting shims and it didn't seem to help. I will try removing the other. I doubt if it will help, though, as the motor needs to overcome the pump inertia and come up to full speed before pumping any pressure. A friend at work has a big 3 phase compressor running from a home made phase converter and he says his unloader takes a lot longer to close. I suspect he has a oil pressure activated unloader. I was thinking (and another poster has just suggested it), that I should use a time delay relay and a solenoid valve to delay the unloader's closing until the motor is up to speed.
The other thought would be to add another phase converter or use a larger idler motor. I read on the GWM phase converter site that ePact motors on compressors may not work very well with rotary phase converters and may require a 4X idler size. I even thought of using 3 separate identical phase converters with the generated leg going to a different terminal on the compressor. But this sound like a lot of complicated contraptions.
The last solution that I toyed with is to reduce the motor pulley diameter. The pump on the compressor is used with 5, 7.5 and 10HP motors with different sized pulleys. By dropping down to the 7.5 HP size pulley, my CFM would go drop from 35 cfm to 27 cfm at 175 psi. Hopefully the motor would have enough torque to spin against the load. This kind of negates getting such a big compressor, but at a distress sale, I got a new deluxe 10HP with intercooler and after cooler for the same price that I would have of had to pay for a 7.5HP single phase compressor with smaller tank.

my
Consider addding permanent capacitance to the air

from
Adding more capacitance accross the compressor motor would be no different than having them in the phase converter as far as the compressor is concerned, right? But other connected loads on the converter wouldn't see the high voltage, so I guess that may be an good option. My only other 3 phase load at this time is the 1HP Bridgeport mill, which spins up nicely - even in high gear!
Thanks, Jeff Borning
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Jeffrey sez:> "Adding more capacitance accross the compressor motor would be no different

Adding capacitance directly across the compressor motor would be anagalous to adding enough capacitance to the idler motor - except - that amount of capacitance would not contribute to excessive voltage on the idler motor as it would if it was placed there. Your symptoms, I believe, indicate there is insufficient total capacitance to facilitate starting a heavily loaded compressor. However, we cannot add enough capacitance to the idler motor to handle the heavy load without excessive (over 15%) voltage across the idler motor during standby. Placing the caps directly on the compressor motor would (should) be a way of overcoming the excessive starting load, while not causing excessive strain on the idler's insulation during standby.
Bob Swinney

more
unloader
I
motor
valve
the
friend
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solenoid
phase
diameter.
CFM
with
to
on
starting,
in.
My only other 3

nicely -

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Addendum to previous post:
It might be necessary to connect the capacitance to the compressor motor with a separate switch or set of contactors, connecting extra capacitance only during the difficult starting interval of the loaded compressor. This in order to prevent possible excessive voltages after the compressor comes up to speed. Those voltages will be felt by the idler as well as the compressor. IMO, it would probably be well to limit this "running" voltage to 15 % over line in.
Bob Swinney

tell.
10-4
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copper
filter.
of
the
suspect
poster
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would
getting
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For wires short (less than 1/20th wavelength for practical purposes) with respect to a wavelength (5,000 km for 60 Hz), the voltage will be the same all along the wire (ignoring for the moment IR drop). So it doesn't matter which end of the wires has the capacitor.
Gary
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Were they thinking about having the C(extra) at the compressor motor, simply because of the compressor contactor? That way when the compressor wasn't running converter voltage would not tend up.
Jim
================================================= please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ================================================
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Yeah Jim, that was the plan. Evidently you read the posts more carefully than some.
Bob Swinney
Gary Coffman says...

not
same
matter
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Were they thinking about having the C(extra) at the compressor motor, simply because of the compressor contactor? That way when the compressor wasn't running converter voltage would not tend up.

Adding a second potential relay, at the compressor, with the hysteresis changed such that an additional run-type capacitor (A-B), or capacitors (A-B and C-B) were placed on-line whenever the B voltage dropped on account of the compressor starting would be a way to go.
Another way to go would be to add a state machine at the converter, one which has a starting state for the converter alone, a running state where normal loads would be supplied, and an overload sate which would handle the special case where the compressor was placed on-line.
This overload state would place extra caps, hence extra VARS, on the converter, thereby temporarily boosting the B-phase voltage.
(If the A- and C-phase voltages are sagging while the compressor is starting, then the motor feeder is too small).
Another possibility is adding an autotransformer for this case.
(In fact, one of the first published rotary phase converters did in fact use an autotransformer on the B phase).
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Jeffrey M. Borning wrote:

You can try a larger idler very easily. Instead of using a larger one, just add a second one in parallel. If you have another three phase machine, all you have to do is turn it on (idling...) and see if this helps your compressor start.
Steve Smith
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Are these (I haven't seen or read much but this ) the correct values ? The caps are based on the Inductance of the winding, not the HP rating... Naturally they are related in some ways.
THe concept is to have XC subtract from XL of the inductor so that the Copper resistance Rs or Rcopper + |Xl-XC| is large enough not to blow fuses / breakers, but allow enough in-rush current to turn the shaft. The shading winding gives the first kick with several hundred amp-turns so the direction is set if shaded.
Once the motor starts, and is up to speed without twisting one end of the shaft off - the switch is thrown and the capacitor is cut out and the run winding takes over.
So the point is this : capacitors are defined by the windings. For a given Horsepower, the voltage determines the inductance - (considering things like number of phases ...).
Martin
Bob Swinney wrote:

--
Martin Eastburn, Barbara Eastburn
@ home at Lion's Lair with our computer snipped-for-privacy@pacbell.net
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Assuming adequate wire sizes for the starting load, you might try three capacitors directly across the compressor leads. This could raise the power factor and reduce current enough to permit starting.
Don Young

and
10HP
start
specs
1765
motors
Bridgeport
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I have THEORIES on why this isn't working. They are UNTESTED and should be used as food for thought.
I have a 10 horse motor phase converter. I end up with 220 volts across two windings of my milling machine, and with 209 volts on the third leg. When an amp-probe is set on each leg of power to the mill, the two legs going to the 220 V single phase have large amounts of current in them, and the generated 3rd leg has essentially none. I think that the generated third leg is more startup than run. Any amount of 3rd phase is better than none, and gets most motors running. Motors running off these home made rigs run mostly on single phase, in one winding.
I suspect that putting a step up transformer on the home-made third leg, to boost the voltage where it matches the other legs would make a large difference in the amount of power delivered, and hence increase the starting torque. Never tried it, since my mill starts, I am happy.
I also suspect that the only hope you have on your present setup is a pressure relief valve which unloads your compressor until it gets up to speed. I think these devices are available commercially.
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Jeffrey, I have the same situation, 25 HP home made rotary converter, 15 HP IR compressor. Here is what I do to get things rolling.
First, the converter. No caps. I use a 1 phase starter motor directly coupled to the 25 hp motor shaft. Starting uses a time delay relay and a contactor. When power to the converter is energized, the starter motor comes on getting the main motor up to speed. Then the starter times out with a time delay relay, and the main contactor comes on.
Second, the compressor. Make it easy to start by installing a time delay relay to energize 2 solenoid valves. One relieves all intermediate pressure in the interstage pipe. The other applies air pressure to the unloaders. When the compressor starts the TDR goes on, opening the valves, then times out in about 30 seconds, after things are up to speed.
There may be 10 or 20 psi left in the tank. Not enough to open the unloaders and enough to add a load. If that is the case, dump all air pressure.
Also get some heat tape and wrap it around the cylinder bases to warm the pistons and cylinders. That helped mine to roll easier when cold.
After my rotary converter I tapped off to a step up transformer that feeds another panel. That gives me "home made" 480 3 phase. The compressor starts much better on 480 that 240. I switches all of my big motors over to the 480 panel.
Finally, start another motor. I fire up my shaper, (15 hp 480 3 ph) before starting the compressor. The added induction helps the compressor to roll over.
This last step is only necessary on the first start of the day when things are cold. From then on it is fine.
Good luck. This is real experience from someone that IS there and DOES that. Contact me if you need more info.
Bob
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Bob,
Do you have an oil pressue activated unloader valve? Mine is a mechanical valve activated by centrfugal weights. I had the same thought as you suggest after talking to a fiend that has a big 3 phase compressor running from a home mase phase converter. He say his compressor gets up to full speed before the unloader closes. I suspect he has a oil pressure activated unloader which is taking longer to close for some reason. What kind of solenoid valves and TDRs did you use? I kind of hate to hack up a new compressor, but if it doesn't work, it doesn't do me any good. Adding the TDR and solenoid valve may the best solution.
What kind of leg to leg voltages are you getting out of your converter?
Thanks,
Jeff Borning

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"Jeffrey M. Borning" wrote:

Indeed. And if it doesn't work, you've wasted your time and money on the "fix".
Can you hook a manual unloader, just to try it? Disconnect the unloader line from it's valve and attach a hand/manual valve to the line. Open the valve until the compressor is up to full speed then close it. Be sure to open it again when the compressor stops. Bob
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