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Odd currents using American Rotary AD

rke[pler

Diamond
Joined
Feb 19, 2002
Location
Peralta, NM USA
I'm getting a new machine working (K&T Model B Autometric) and in order to do so I got an American Rotary AD 5HP phase converter. When running the spindle for a while (half hour or more) my overload tripped. When I checked I found it had what were probably the 480V heaters and so I replaced them with what I expect should be the 240V heaters (I have CR123C466A in there now, 4.4A heaters as the motor is sized 4.4A per the nameplate). Even then the overload tripped after a while of spindle time.

I dug into the electricals and the voltages look reasonable - L1 & L2 have 245V (line) and with the rotary there's 255V L1 to L3 and L2 to L3. Looking at the current shows something odd - there's about 3A on L1 and L2 (subtracting the .6A to the control transformer and DRO) and 4.7 on L3. Running the spindle up to 2500 rpm the L3 current jumps to 5.2A.

So am I looking at some aspect of the current phase angle or something? I'm measuring with a Fluke 87 and trust the readings (still think the idle current on the control transformer is high, but industrial).

If this is a constant/normal thing I'm probably going to have to go to a C592A or something up there just on the wild leg. Not really a problem, just unexpected.
 
It is likely due to your higher voltage, if that holds up under load. Taking out some of the "balance" capacitors should bring it down.

Normally, the generated leg is lower. The idler does not naturally put out as much L3 voltage as it is supplied on L1/L2, just due to how motors work. The "balance" capacitors are used to "compensate' that voltage, bringing the voltage up by a resonance and inductance cancellation effect.

Usually, the heavier the load, the less well the capacitors work. Your 255 vs 245 input does not seem overcompensated, so I suspect that your motor is much smaller than the RPC can handle.

Your current of 4.4A suggests a 1 HP load motor on your 5 HP RPC, which can easily have such effects.

Removing some of the compensation (balance) capacitors should fix the issue. Aim for about the same volts out as are coming in, with the 1 HP motor loaded.
 
If the machine was wired for 480V previously, evidenced by the original heater sizes, did you check the motor and control transformer for the proper voltage connections?

The motor may run OK at half voltage, but will have less HP, necessitating more current.
 
The voltages stabilized nicely at the higher currents, so I'm thinking that this isn't any of the 'traditional' RPC issues. Just to confirm I found a 1.3HP motor with the same 4.4A current spec. Running it using the mill panel I see currents:

L1: 5.00A
L2: 4.15A
L3: 3.67A

So here the wild leg is the low current leg. Kind of unexpected.

So at this point I'm going to chalk it up to some weird thing with voltage/current phase angles and motor slip angle. If the overload pops again I'll toss another amp or so at the wild leg and see what happens. For now it's OK.
 
And some of those reasons would be...


The idler normally puts out a voltage several percent lower than line volts, because it is the back EMF of the motor, which is always less than line volts.

When you boost it with capacitors, you change both voltage and phase, which can undo some of the effect of voltage in allowing increased current.

The effect of capacitors is changed by adding a load. That "damps" the boost, and so the voltage is closer to the raw back EMF, lower than line volts.

The capacitor method* is limited by the need to not go too much higher than line volts with no load. That limits the boost under load, so the loaded condition is not as "corrected" as it might best be. (You can add the correct boost caps at the load motor, which will be better, since you can add as much as needed, with no worry about unloaded condition. Basically nobody does this.)

The load motor has a back EMF also, which is comparable to the idler, and at no load on the machine, it is the highest it will be. That tends to reduce current draw unloaded. The currents usually get more balanced as the load motor slows under load.... The idler being larger, it is less affected by load, and its load is only 1/3 of the power needed by the load motor, so it slows less, tending to hold up its back EMF, and improve the overall balance of currents.

Did you want more? :)

*There are other boost methods.

One is using a buck-boost transformer to make the voltage change. This has the advantages of 1; being less load sensitive, 2; having minimal effect on phase, and 3; having essentially no issues with the unloaded condition, since the voltage boost is a known, stable percentage. There is a small voltage drop and extra power use penalty, but that can be accounted for in design. Virtually no RPCs use this method because while it definitely has advantages, it is a considerably more expensive method than simply relying on the "balance" capacitors. The various "Online Sources" rely on low price for sales to the small shop and hobby users, and are not prepared to add expensive parts for performance enhancements their customers may not appreciate.

Another is using a motor with a custom winding in the motor, essentially adding turns on the generated leg, which will provide a higher "back EMF" on that leg, and directly raise the voltage to balance with the direct legs. The advantages are essentially the same as with a boost transformer, but of course does not require that extra part. Very few, if any, RPC manufacturers use this method because it requires a custom motor, an investment many of the "Online Sources" are not prepared to make as mentioned above.
 
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The idler normally puts out a voltage several percent lower than line volts, because it is the back EMF of the motor, which is always less than line volts.

When you boost it with capacitors, you change both voltage and phase, which can undo some of the effect of voltage in allowing increased current.

The effect of capacitors is changed by adding a load. That "damps" the boost, and so the voltage is closer to the raw back EMF, lower than line volts.

The capacitor method* is limited by the need to not go too much higher than line volts with no load. That limits the boost under load, so the loaded condition is not as "corrected" as it might best be. (You can add the correct boost caps at the load motor, which will be better, since you can add as much as needed, with no worry about unloaded condition. Basically nobody does this.)

The load motor has a back EMF also, which is comparable to the idler, and at no load on the machine, it is the highest it will be. That tends to reduce current draw unloaded. The currents usually get more balanced as the load motor slows under load.... The idler being larger, it is less affected by load, and its load is only 1/3 of the power needed by the load motor, so it slows less, tending to hold up its back EMF, and improve the overall balance of currents.

Did you want more? :)

*There are other boost methods.

One is using a buck-boost transformer to make the voltage change. This has the advantages of 1; being less load sensitive, 2; having minimal effect on phase, and 3; having essentially no issues with the unloaded condition, since the voltage boost is a known, stable percentage. There is a small voltage drop and extra power use penalty, but that can be accounted for in design. Virtually no RPCs use this method because while it definitely has advantages, it is a considerably more expensive method than simply relying on the "balance" capacitors. The various "Online Sources" rely on low price for sales to the small shop and hobby users, and are not prepared to add expensive parts for performance enhancements their customers may not appreciate.

Another is using a motor with a custom winding in the motor, essentially adding turns on the generated leg, which will provide a higher "back EMF" on that leg, and directly raise the voltage to balance with the direct legs. The advantages are essentially the same as with a boost transformer, but of course does not require that extra part. Very few, if any, RPC manufacturers use this method because it requires a custom motor, an investment many of the "Online Sources" are not prepared to make as mentioned above.

Interesting, so are you saying I can basically just ditch all my balance caps and use a multitap autotransformer just on the manufactured leg and the voltage will be more stable through the load range? Or is there more to it?

I have oodles of autotransformers laying around and over 50HP worth of rotaries. Perhaps I could run more machines at once if the leg didn't get pulled down so much under heavy load. One of the VMC's I run has a 30 hp spindle. The leg doesn't get pulled down enough to trip anything, just more than I would like to see when other machines are running, and I rather not burn up a 35Kw power supply :crazy:
 
And some of those reasons would be...

Main reason being that it's pretty tough to measure 'currents' accurately on a rotary converter setup. Practically none of them are entirely real, they tend to have very low power factors in general, so unless you watch the phase angle (power factor) it's real easy to get fooled. Amp clamps in particular do poorly at this. Any current measuring technique that doesn't also take into account the phase angle difference between the voltage and the current will give misleading results.
 
Main reason being that it's pretty tough to measure 'currents' accurately on a rotary converter setup. Practically none of them are entirely real, they tend to have very low power factors in general, so unless you watch the phase angle (power factor) it's real easy to get fooled. Amp clamps in particular do poorly at this. Any current measuring technique that doesn't also take into account the phase angle difference between the voltage and the current will give misleading results.


The "fooling" is in the interpretation. The current is real and factual, actual electrons are flowing. Do not make the mistake of assuming the current is not real.

If the amp-clamp measures a current, that current is present and flowing, there is nothing "not real" about the flow of current.

What may be "not real" is the POWER that "appears to be" drawn.

If the voltage is not in phase with current, then for part of a half cycle, voltage is positive, but current is negative. So a portion of the "volts x amps" is subtracting from the rest of it, meaning the "apparent power" (the volts x amps) is not the true factual power.

For a motor that is running unloaded, or nearly so, the current is lagging because most of it is "reactive current", "magnetizing current" that produces the magnetic field in the motor iron. That is true of ALL THREE PHASES. The current is actual current that is in fact flowing, regardless.

So all three phases are drawing the same type lagging current, and the concept of "balance" in that condition is perfectly valid.

There is a fundamental flaw in your reasoning.... You are confusing "imaginary power" (which is a "thing") with "imaginary current" (which is NOT a "thing"). The currents are completely present and factual-real. The "not real" part of it comes ENTIRELY from the concept of power = volts x amps. That equation is true, but it is true only if the measurements are simultaneous.

If a genuine, factual, current is flowing, but is not "in phase" with the voltage, then that straight multiplication does not work to derive "power". You have to multiply by the cosine of the phase angle to get "real" power.

It just happens that the cosine of zero is one, so a shortcut of not bothering with the cosine is possible so long as the current is in-phase with voltage (as in a resistor, such as a heater).

However, do not be fooled.... if a properly functioning meter says the current is flowing, it IS flowing. You may not be able to use that current to derive power, but the current is actually present, and it can be used to determine "balance". This is true since if "real" and "perfect" 3 phase were provided, an ideal motor would have completely balanced current on all three wires under ANY condition of lagging or leading current.
 
Main reason being that it's pretty tough to measure 'currents' accurately on a rotary converter setup. Practically none of them are entirely real, they tend to have very low power factors in general, so unless you watch the phase angle (power factor) it's real easy to get fooled. Amp clamps in particular do poorly at this. Any current measuring technique that doesn't also take into account the phase angle difference between the voltage and the current will give misleading results.

Not this argument again...

The current is there. It's flowing. It's heating up the cable and motor windings, and will operate both the thermal and magnetic trips in breakers, as well as lead to volt drop.

Whether it's in phase or not really doesn't matter except to your revenue meter.
 
Not this argument again..

Yep. Was summoned from the demon depths of the internet when sombody said 'how come I can't figure out the weird currents in my rotary converter.

The current and voltage are hardly ever in phase. This makes it really tough to interpret whatever magic value has been tested. Complex plane. Call the voltage
on the real axis. The currents are then on the imaginary axis.

Imaginary. Yep. Tough concept. But useful!
 
Oh wait. Let me check the thread title, maybe I got it wrong or something.

Ah, nope. Odd currents. AKA low power factor for some of them. AKA reading with a meter that doesn't care about the phase angle.
Complicated. Tough to interpret. Because of all that imaginary current stuff.
 
Oh wait. Let me check the thread title, maybe I got it wrong or something.

Ah, nope. Odd currents. AKA low power factor for some of them. AKA reading with a meter that doesn't care about the phase angle.
Complicated. Tough to interpret. Because of all that imaginary current stuff.

Jim, nearly ALL of the current flowing to the motor is "out of phase" for a motor that is not loaded..... the current from the generated leg, as well as the current from the "direct" or "pass-thru" legs. It is "magnetizing current", and lags due to inductance.

As the motor is loaded more heavily, the percentage of "out of phase" current is lower, because more in-phase current is drawn. The full load current has perhaps 30% out-of phase current, but that does not affect its "reality".

The measured currents are going to be similar on all legs under any condition of loading, as long as the SOURCE for each of them is similar.

The output of an RPC (the generated leg) is intended to act, and be, as similar as possible to the voltage/current provided by the other two wires.

The motor windings are the same for all wires, and the motor conditions are the same. One is entitled to presume that if the currents (which is what operates a motor) are the same or within a reasonable balance, and the voltages are likewise similar, the RPC is doing its job of "imitating the power company".

Now, rather that insisting on some magical property of "imaginary currents" that meters don't measure correctly, I suggest you provide an explanation of just why you suppose they are different, HOW they are (to you) different, and how that causes an issue important enough to be mentioned and cautioned against.

It would not hurt if you explained what you are referring to when you mention "imaginary currents".

Or, alternately, just quit putting in posts that are just confusing folks who are asking questions. We have enough of that already, of a less technical nature.
 
The measured currents are going to be similar on all legs under any condition of loading, as long as the SOURCE for each of them is similar.
.

Actually not. The currents in the manufactured leg *look* a lot different than those in the utility leg. Sorry if that's confusing.

If readers are confused by: "it's tough to measure currents in a rotary converter and make sense of them, unless you look at the power factor in them" then yes, it's a confusing
situation. How about if I sugar-coat it and say, 'there be dragons here' and that might not be so confusing?

This topic comes up again and again. Simple hand-waving apparently does not work. Sometimes ya just gotta whip it out. Complex plane, that is.
 
This could use a different thread.... see last sentences below.

Actually not. The currents in the manufactured leg *look* a lot different than those in the utility leg. Sorry if that's confusing.

If readers are confused by: "it's tough to measure currents in a rotary converter and make sense of them, unless you look at the power factor in them" then yes, it's a confusing
situation. How about if I sugar-coat it and say, 'there be dragons here' and that might not be so confusing?

This topic comes up again and again. Simple hand-waving apparently does not work. Sometimes ya just gotta whip it out. Complex plane, that is.


You did not read/comprehend, unfortunately.

What I WROTE was that the currents in the LOAD MOTOR are going to be similar on all legs IF the SOURCE is similar on all.

Point being that the better the idler motor simulates the power company (this is the part you missed), the more similar the current in the generated leg is to the current in the "pass thru" legs. The "balance" will be better.

The inverse of this (it IS a "linear" system so that's fair) is that if the currents are similar in all three legs, the RPC is balanced and effectively simulating the power company output.

The load motor will draw current in the same way on all legs if provided with a similar impedance source of the same voltage on all legs.

So, it is now up to you to justify the statement that the currents "look a lot different" on the manufactured leg. It appears that you are really criticizing the less effective converters, rather than establishing a principle that applies to all of them regardless. But it is not clear just what you are saying.

Perhaps you are referring to a slight phase difference on the manufactured leg. I say slight, because the voltage is the same back EMF on all, and it is substantially in-phase with the source. It is generated by the rotor field cutting the windings of all the phases. There may be a slight phase difference, but if the voltages were not substantially the same phase and close to the same voltage, then currents would be very high. They are not, so that condition is well satisfied (the inductive impedance of the motor is low, as is the resistance, most of the opposition to current comes from the back EMF). Your own often-posted scope traces support this.

There is a higher harmonic content in the back EMF, as shown by the slight flattening of the sine wave visible in your 'scope traces. That will slightly affect generated voltages, and thus current.

There is an effect of resistance in the generated leg, which is not present in the pass-thru legs. That would be phase-neutral, except that the load is inductive, so there is some effect, which changes with load. The larger the idler compared to the load, the less effect there will be, as your own large idler shows.

There is an effect of inductance as well, also not present in the pass-thru legs. That DOES tend to be phase-neutral if the load is inductive, and will change with load because the load motors change their characteristics as they are loaded, becoming more resistive. This is also reduced by a larger idler.

The usually used capacitors can counteract inductance, and somewhat eliminate these effects. Phase changes are dependent on the amount of capacitance added, and on the split of that capacitance between the two incoming (pass-thru) legs (which are 120 degrees ahead of, and behind, the generated leg, and so can provide currents at different phase angles).

Part of the "balancing" process is to adjust those factors of phase and voltage (which are related), so as to minimize bad effects, although an RPC does not need any balancing to actually operate, as your own simple converter shows.

So, please provide a more detailed impression of what you are talking about, and the mechanisms you feel are causing the effects you claim.

I suggest we have a different thread to do it in, and if you are interested enough to start one, I will move the somewhat off-original-topic discussions from this thread to that thread.

If not, then I think I will clean this thread up to just remove them.
 
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