Low line current on generated leg under load

I built an RPC using a 1140rpm, 220v, 2hp motor. I am powering a 850rpm, 220v, 1hp motor. Balance and power factor capacitors, etc. T1 and T2 are incoming/outgoing lines, T3 is generated line. Unloaded voltages and current are as follows: T1-T2=249V and 3.4A, T1-T3=247V and 4.8A, T2-T3=267V and 5.26A. Loaded (1hp equipment motor on): T1-T2=220V and 5.1A, T1-T3=220V and 5.0A, T2-T3=216V and 1.8A. The equipment motor amperes are: T1-T2=4.62A, T1-T3=4.93A, and T2-T3=2.6A Line current (input) is 2.3A unloaded and 4.6A loaded. I have tried various combinations of primary (T1-T3) and secondary (T2-T3) capacitors, and always get the low current on T3 with the 1hp equipment motor on. Has anyone some suggestions on correcting this, or is this normal? I am using the Finch Williams design. zuhnc

I really think you are good to go there. Getting the current balanced is always the problem, run it for an hour, then check the temperature of your motors and the capacitor, if they are all ok, use it!

Not knowing much about 3-phase electricity, would low back EMF, which is created by the idler motor, cause low line current in the generated leg? Would a larger idler motor help this? I have, at the moment, access to only a 2hp, 1140 rpm idler. I am somewhat concerned about the more-than-full-load current on L1 and L2 on the equipment motor. The full-load nameplate current on the equipment motor is 4.2A at 240V. Load current is L1=4.56, L2=4.87, and L3=2.46. It is a Baldor 1hp, TEFC, 850 RPM, SF 1.25, on a Millrite Powermatic mill. Voltages are L1-L2=248, L1-L3=227, and L2-L3=213. The idler motor gets to 120-130*F after 30 minutes; however, it is an air-over condenser fan motor, with no fan, and just convection cooling. Its current is about 80% of full load (6.2A @ 240V) I am not concerned with its temperature at this time. The equipment motor gets barely warm to the touch, after a similar period of time. The equipment motor is on the bench, not connected to the mill, so no way to put a load on it. Would the unbalanced line current cause any damage, in your opinion? This is a great site for those who want to know zuhnc

What instrument are you measuring the current with?

So... what you have is manufactured 3 phase, which is not perfect. But, actually, your voltages loaded seem to be very good, balanced within a low percentage error. (220 vs 216V). Seems OK likely, should work fine.

The current oddity is likely due to back EMF, yes

Where are you measuring? I am not clear on what you measured, since current is associated with one wire, and you gave 2 wires for each voltage, and did not show the wire. I assume the low current is on the generated leg.

If you are measuring current in the wires out by the load motor, then it can be back EMF.

The generated leg is just the back EMF of the idler. The balance capacitors are just an attempt (which will not work well) to boost the output by canceling some inductance. It would be smarter to boost the volts with a boost transformer, actually, since that addresses the basic issue (lower back EMF).

So, since the back EMF of the idler is lower than line volts, then it is comparable to the back EMF of the load motor, and the current will be low, at least when unloaded. The line is always higher than the back EMF, which is what forces current into the motor. Since the two back EMFs are similar when no load is present on the "load motor", there is not much to push current,

When you get into a cut, the load motor will slow, its back EMF will drop, and the current in the generated leg will be higher. The idea is that the smaller load motor will slow more than the bigger idler (which needs to supply only 1/3 the p;ower anyway), so more current will flow.

Bottom line is that you will probably be fine under a significant load. A 2 HP idler should run a 1 HP motor. Most would suggest as low as a 1.5 HP. A much bigger idler could dispense with the balance capacitors altogether, something like 5 HP.

zuhnc said:

Unloaded voltages and current are as follows: T1-T2=249V and 3.4A, T1-T3=247V and 4.8A, T2-T3=267V and 5.26A.

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A specification like "T1-T2=249V and 3.4A" does not indicate what T line the current is on. Therefore your entire description needs some help.

Thanks for the explanation of back EMF and other things. Very enlightening! Never worked with 3-phase before, and this is my first RPC. Learning a lot here. To clarify: T1 and T2 are the single-phase line/load wires; i.e., from 240v input and to the idler and load motor. T3 is the generated line, from the idler motor to the load motor. Please read post #3 for the currents on each load line. I was not very clear in my original posting of where the line current measurements were taken. Instruments are a Craftsman 400A AC/DC Clamp Meter 82369 (40A and 400A ranges for both AC and DC), and a Fluke 8060A True RMS Multimeter. The clamp meter I got primarily to measure DC charge and discharge currents in my solar installation on my horse trailer. zuhnc

The voltages you are getting under load are more than acceptable. Your RPC is working well. Go to work. Regards, Clark

I wouldn't call your voltage variation acceptable. The largest difference is 248 - 213 = 35VAC (14%). If I can get 1-2% so can you.
You should read up on what unbalanced conditions can do to your equipment over the long term. Looks like you are ok today, time will tell. This stuff is all documented.

Something to start with.

https://www.progress-energy.com/assets/www/docs/business/motor-protection-voltage-unbalance.pdf

Their example which demonstrates the problem has a unbalance of 4.2%.

According to the easy read above, you have a voltage unbalance of 8.3%.
Vavg = (248 + 227 + 213) / 3 = 229
% unbalance = 100 x (248 - 229) / 229 = 100 x 19 / 229 = 8.29 %

I've seen this effect before. Probably the current (which you are measuring with an amp-clamp, yes?) is balanced just fine. The utility-supplied legs have lower
current than you think because the amp-clamp is measuring a combination of real and reactive current. For some reason the current in the manufactured leg
into the load motor, shows up as all real.

A test: while measuring the manufactured leg current, put mechanical load on the machine being driven. The the amp clamp will start to read upscale. Switch
over to the utility phase, and repeat the test. Likely the amp clamp reading will stay just about the same.

This is one of the reasons that Fitch suggests to not use current readings to tune a converter. Voltages only.

I have tried various combinations of T1-T3 and T2-T3 capacitance to balance the voltages when loaded. My T1-T2 voltage is "always" 248-249. That does not change regardless of balance capacitors or PF correction capacitors. I can get balanced loaded voltages in the 220-230 range on both T1-T3 and T2-T3, but then the voltage on T2-T3, unloaded is in the, in my opinion, unacceptably high 270-280v range and T1-T3 is around 260V. In that instance, loaded voltages drop to T1-T3=225, T2-T3=220, T1-T2=248. I have never been able to get the T1-T3 or T2-T3 voltages anywhere near T1-T2, without unloaded voltages (other than T1-T2=248, always) being close to 300V. I believe I will stay with the unbalanced load voltages, and monitor voltage, current, and temperature when using the connected equipment. This discussion has been very enlightening, and I appreciate everyone's thoughts. I am looking for a larger idler motor, which may alleviate some of the voltage and current issues. Time will tell. Thank you all. zuhnc

To clarify you are measuring directly at the load motor, after all capacitors, right?

In my 10hp RPC I have 15 amps of reactive current flowing around unloaded, even though it only draws an amp or two from the line.

Yes, measuring after all the capacitors. Unloaded input line current (L1), is 1.23A. Loaded with the equipment motor, 5.58A. I have been able to get unloaded line current to 0.87A, but loaded line current goes up 7A. T1, T2, and T3 voltages and currents don't change as the PF capacitor value is changed. I think I will just leave it as is, and monitor parameters. I don't expect to use this 8 hours a day, but situations change, as all here know. I will rig up a prony brake of some sort to apply a load to the motor; should be interesting and informative. Can't measure reactive power (I think); no instruments to do so. I do, however, have a Fluke 123 Industrial Scopemeter. Can it be used for any meaningful measurements? Again, thanks to everyone for your insight. Is there any complete treatise by Fitch I can reference? Did the Google (tm) thing; all I found was a short paper with a diagram and tuning instructions. zuhnc

zuhnc said:

Yes, measuring after all the capacitors. Unloaded input line current (L1), is 1.23A. Loaded with the equipment motor, 5.58A. I have been able to get unloaded line current to 0.87A, but loaded line current goes up 7A. T1, T2, and T3 voltages and currents don't change as the PF capacitor value is changed. I think I will just leave it as is, and monitor parameters. I don't expect to use this 8 hours a day, but situations change, as all here know. I will rig up a prony brake of some sort to apply a load to the motor; should be interesting and informative. Can't measure reactive power (I think); no instruments to do so. I do, however, have a Fluke 123 Industrial Scopemeter. Can it be used for any meaningful measurements? Again, thanks to everyone for your insight. Is there any complete treatise by Fitch I can reference? Did the Google (tm) thing; all I found was a short paper with a diagram and tuning instructions. zuhnc

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I think the scope would need to be dual channel to be useful (for checking phase angle). What value capacitors are you using?

Six pole (=high leakage reactance) idler providing three phase to an eight pole (=even higher leakage reactance) drive. You are going to have problems with the generated leg getting an acceptable voltage with a sensible phase angle. Tune it to the best you can achieve and live with it.

The scope is dual-channel. Would a 3450 rpm idler be a better choice? Maybe a 3-5 HP? That would be somewhere in the medium future . My knowledge of reactance and motor poles is similar to a match in a very large cave! But, with the guidance and information here, I am slowly seeing more light. Capacitors are T1-T3=40uf, T2-T3=20uf, and T1-T2 (power factor correction)=30uf. No others. With these values I get T1-T3=259V, T2-T3=277V, and T1-T2=248V, unloaded. Voltages loaded are T1-T3=225, T2-T3=220 and T1-T2=247. Load current is T1=4.73 T2=4.23, and T3=2.67. Unloaded input line current is 1.23A, loaded is 5.58A. Unloaded voltages measured at the idler motor, and loaded are at the equipment motor. I think I can live with that, monitoring temperature of both motors. Great site! I have a 9" South Bend lathe being delivered tomorrow; don't know specifics of it, bed length, exact model number, etc. It, however has a single phase motor on it. Depending on HP, it might get changed to 3-PH. I will peruse the South Bend forum for more information, tips, etc. Thank you all. zuhnc.

The 3450 rpm idler is probably going to sound louder.

Did you calculate the power factor correction needed? Or just stuff it in as a guess? The power factor correction only means something with a full power draw from a running machine. Not just a motor on a bench. You should remove the 30uf PFC cap and balance the RPC.

zuhnc said:

The scope is dual-channel. Would a 3450 rpm idler be a better choice? Maybe a 3-5 HP? That would be somewhere in the medium future . My knowledge of reactance and motor poles is similar to a match in a very large cave! But, with the guidance and information here, I am slowly seeing more light. Capacitors are T1-T3=40uf, T2-T3=20uf, and T1-T2 (power factor correction)=30uf. No others. With these values I get T1-T3=259V, T2-T3=277V, and T1-T2=248V, unloaded. Voltages loaded are T1-T3=225, T2-T3=220 and T1-T2=247. Load current is T1=4.73 T2=4.23, and T3=2.67. Unloaded input line current is 1.23A, loaded is 5.58A. Unloaded voltages measured at the idler motor, and loaded are at the equipment motor. I think I can live with that, monitoring temperature of both motors. Great site! I have a 9" South Bend lathe being delivered tomorrow; don't know specifics of it, bed length, exact model number, etc. It, however has a single phase motor on it. Depending on HP, it might get changed to 3-PH. I will peruse the South Bend forum for more information, tips, etc. Thank you all. zuhnc.

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If nothing more than educational you should throw the scope on with one channel across the supply line and the other channel across the motor phase that is in parallel with the capacitors. As you add more capacitors you should see the phase difference shifting further from 120 degrees to 90 degrees.

You could try to match up capacitors to form a resonant circuit with the motor inductance but that can be dangerous for unloaded conditions due to voltage multiplication, unless motor inductance changes enough to self regulate, in which case you need to calculate the loaded inductance (can be derived from motor parameters and power factor rating) and design it to approach resonant frequency as the idler reaches maximum rotor slip. This would be the only way I'm aware of to get totally balanced phases, and it won't get you that perfect 120 phase angle motors tend to like.

I designed a static phase converter using the principle above except with a dynamically driven saturable reactor, but that will be its own thread I think. (Never built due to needing custom wound transformers that I didn't have the time to make).

Thanks for all the suggestions. Other things are intruding into my "play" time at the moment, so may be a few days before I can implement some of these ideas. Acquired the South Bent lathe today; turns out it is an 11" swing, 5' bed, QC gearbox, motor in pedestal, flat belt drive. Off to the South Bend forum! Thanks everyone. I have an ONAN generator that uses saturable reactors as the voltage control, so I am familiar with that arcane technology. zuhnc

zuhnc said:

Thanks for all the suggestions. Other things are intruding into my "play" time at the moment, so may be a few days before I can implement some of these ideas. Acquired the South Bent lathe today; turns out it is an 11" swing, 5' bed, QC gearbox, motor in pedestal, flat belt drive. Off to the South Bend forum! Thanks everyone. I have an ONAN generator that uses saturable reactors as the voltage control, so I am familiar with that arcane technology. zuhnc

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Saturable reactors introduce a lot of harmonic distortion if controlled by a DC signal. In my case I was going to drive it with the "opposite" of a sine wave to counter that effect and maintain a better distortion power factor. But that will be a different thread.