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Suggestions please on this strange RPC test reading.

imported_brian_m

Cast Iron
Joined
Jun 25, 2006
Location
Oregon
I've reached the stage of building the RPC for my lathe renovation. I ran a hacked together version of the basic RPC today and obtained a strange current reading for the generated phase of only 1.2 amps (measured with a Fluke clamp-meter).

First the system. I have left out several items that will be in the completed system. The 220V supply is from a 200 amp main panel. The RPC idler is an older Dayton 1700 rpm 15HP industrial motor. The machine motor is a new Marathon 1700 rpm 10HP motor. The idler motor starts easily from a 1 HP pony motor (this is a temporary arrangement). The idler motor runs smoothly and surprisingly quietly. Each supply leg from the main panel connects to a terminal block with several outlet ports. The idler motor feed is also connected to this block as is the output leg for that phase. To be clear, the input, the output to the 3-phase contactor and the supply to the idler motor share the same terminal block. This applies to both of the supply legs. The generated phase of the idler joins the two basic phases at another terminal block and the three legs are fed to the output contactor. When the output contactor is closed the machine motor starts with no hesitation at the idler motor then it continues to run smoothly.

These are the supply leg currents. (all readings with the machine motor running but no belt on the motor).
Line 1 41.2 amps. Line 2 41.6 amps.
Output current to the machine motor.
T1 16.4 amps. T2 1.2 amps. T3 16.1 amps.
Output volts to the machine motor.
T1 to T3 238.3 volts. T1 to T2 214.4 volts. T2 to T3 207 volts.

Do I have a fault on the system, is my wiring method wrong or is this just a matter of balancing?

Strangely the whole system works very smoothly. The idler supply does not even notice the machine motor being connected and the machine motor starts and runs very well.

Should I panic or just proceed with the project?

Thanks, Brian
 
That will change when you load the machine motor. A setup like that will only be right at one load. You make a compromise that works fairly well over the range. The idler motor improves the range compared to a simple static converter, but it still varies.

The best thing to do is look at it with an oscilloscope. You can play around until the last proton disintegrates changing capacitors and comparing readings to solve a problem that only takes minutes with a scope.

Bill
 
You should EXPECT strange results when powering a lightly loaded motor, unless you "balance", which means "canceling the inductive drop and compensating for lower back EMF of the idler"....

The load motor, when idling, is just like another idler, it is spinning and generating about the same level of back EMF on the "generated leg" as the intended idler. Their voltages will nearly balance out and only a low current flows.

When you ask for power from the "load motor", its back EMF drops as it slows down, and the actual idler now has a higher voltage than that back EMF, allowing current to flow.

If you do the "balancing" you raise the actual idler's output voltage, and you get more nearly the usual idle current on the load motor. And you get more current under load also.
 
This exact issue comes up all the time here, and the answer is the same as it always is.

1) don't bother trying to tune your converter by measuring current - with an amp clamp meter.

2) amp clamp meters read the sum of in phase and out of phase current.

3) the line terminal currents on your converter are HIGH because most of that reading (from the
amp clamp) is REACTIVE current.

4) if you could measure only the REAL current in those lines, they'd be just about the same, and be
nearer to the small value being read by the amp clamp on the manufactured phase.

5) if you want to convince yourself all is well, figure out a way to provide mechanical load to your
load motor, while monitoring with your amp clamp. I suggest the two utility legs will stay the
same value you see, unloaded - and the manufactured leg reading will increase as the mechanical
load is applied.

The single worst measuring tool for setting up a rotary converter, is exactly an amp-clamp meter.
 
The only thing missing with your explanation is that the load motor should pull the same current on all wires under ANY conditions.... reactive current and all. There is nothing about the manufactured leg that should cancel out reactive current and only draw "real" current.

If the source were a "Phase perfect", the currents WOULD be as equal as the motor can draw (the phase currents are probably never EXACTLY equal). Same for ANY true balanced 3 phase source.

So the existence of a current imbalance on the manufactured leg is PROOF that in some way the RPC is falling short of imitating a true 3 phase source. The action usually called "balancing" fixes a lot of that (not all of it).
 
The only thing missing with your explanation is that the load motor should pull the same current on all wires under ANY conditions.... reactive current and all. There is nothing about the manufactured leg that should cancel out reactive current and only draw "real" current.

If the source were a "Phase perfect", the currents WOULD be as equal as the motor can draw (the phase currents are probably never EXACTLY equal). Same for ANY true balanced 3 phase source.

So the existence of a current imbalance on the manufactured leg is PROOF that in some way the RPC is falling short of imitating a true 3 phase source. The action usually called "balancing" fixes a lot of that (not all of it).

No. This is a classic complaint for those who test rotary converters using an amp clamp. It comes up here twice a year:
'my amp clamp says my converter doesn't work. But it works.'

I leave it to the student to figure out why the manufactured leg on a converter only shows real current - but it does. Mine does,
and for those who run un-balanced rotaries, check it, it's true.

As you apply mechanical load, the utility legs show the same large current, but the power factor on the current being delivered
to those two wires gets closer and closer to one. Net result is the amp clamp pretty much stays unchanged.

For the OP, does the converter run your load motor, in all respects, as you want it to? Or is there some functional deficit there?
From his voltage numbers, the balance on the setup is not that bad. If you go back and read the fitch williams tretise on this stuff,
there's nothing about balancing using an amp clamp. Reason for that, there is.

If you folks want I could simply amp-clamp my setup and show the readings for the three wires. Scaled (this is a 5 hp motor), those
will probaly match his numbers pretty well.
 
The explanation(???) came from a balancing non-believer.


Mr Rozen is I guess a non-believer in balancing


I'm a "half" non believer, so I can see his points about it...

But he is correct in what he often says, that the motor inherently creates the correct phasing. The problem is that the back EMF of the motor CANNOT ever be equal to the line voltage. The difference is what allows current to flow to the motor.

He also believes in using a very large idler. That substantially reduces the issue of series resistance and impedance, as well as reducing the load that the driven motors are as a percentage of idler power, So it allows the idler to avoid slowing as much, which ensures the voltage drops less than a smaller idler would allow. and it reduces the need to compensate-out the reactive voltage drops

I agree it is a good approach, and works.

What most call "balancing" is really just compensating out inductive voltage drop, which brings up the voltage somewhat above the value of back EMF. best done with capacitors to both incoming lines. Some moving around of capacitors can be done if the phase needs a minor correction, which it may because there is no inductive reactance in the original single phase line.

An alternative would be to insert inductive reactance in the original two lines. That would do a similar job of balancing, but would still leave the basic voltage problem that back EMF (the generated voltage in the idler) is less than line voltage. I do not recommend it, it is expensive, and less effective.

One might boost the voltage input to the RPC, which would also boost the back EMF output, assuming the motor can take it. That would correct the generated leg voltage, and I believe it is done in some units from the UK. Either before, or more likely after, the idler.
 
No. This is a classic complaint for those who test rotary converters using an amp clamp. It comes up here twice a year:
'my amp clamp says my converter doesn't work. But it works.'

I leave it to the student to figure out why the manufactured leg on a converter only shows real current - but it does. Mine does,
and for those who run un-balanced rotaries, check it, it's true.

As you apply mechanical load, the utility legs show the same large current, but the power factor on the current being delivered
to those two wires gets closer and closer to one. Net result is the amp clamp pretty much stays unchanged.

For the OP, does the converter run your load motor, in all respects, as you want it to? Or is there some functional deficit there?
From his voltage numbers, the balance on the setup is not that bad. If you go back and read the fitch williams tretise on this stuff,
there's nothing about balancing using an amp clamp. Reason for that, there is.

If you folks want I could simply amp-clamp my setup and show the readings for the three wires. Scaled (this is a 5 hp motor), those
will probaly match his numbers pretty well.



Actually, the reason that appears to happen is just what I said. But I suggest to you that there is still reactive current in that reading, in about the usual proportion. Just enough less that it is not noticed, because the two motors basically cancel each other's output when the driven motor has no load.

Think about it, and what you yourself (with others) have said.... It's common to say "turn on some other machines with no load to get hard starting machines to start". They become idlers, because they are connected just like an idler, they run just like an idler, and they generate just like an idler. Two opposing idlers generating roughly the same voltage (unless the true idler has its output compensated to boost voltage, either with capacitors, or a transformer).

Since they have very similar output voltages, which are opposing, the current is low. On the order of the losses, but NOT purely real power. That is illusory, a coincidence. if you do not think so, then explain how the system can separate out just real power current. The same factors are operating that cause a motor to draw real plus reactive current when connected to real 3 phase.... explain why you think it is different.

As I said, and you cannot disprove, the lack of current simply proves the RPC is actually NOT acting as a balanced source.

Since you often insist, rightly, that the idlers produce good 120 deg 3 phase, it cannot be due to some phase shift between them.

When you LOAD the idler, its voltage drops, as does the back EMF of the driven motor. The current should go up because those two tendencies are not equal. And it usually does, but not as much as if you had real 3 phase. The larger the idler in relation to the driven motor (lower impedance), the more the current goes up.

AND, if the same motor is connected to real 3 phase, it will draw normal reactive current. Measure voltages in the two situationsand you will see the truth of the matter. The differences needed to give these results are only a few percent between line volts and back EMF.

I do NOT particularly like the idea of using currents to balance the RPC. it's a case of "choose your error". and you really need to watch the voltages more than the currents. if the voltages and phases were correct, so that the generated leg was at all times equal to the input voltage between the other wires, and in proper phase, the current would take care of itself (and normal reactive current would flow).

iif you attempt to "balance" on currents, you will just get into bad trouble, trying to optimize a device that cannot BE optimized to that degree. It works well enough without being optimized, so no need.
 
My converter running a small (3/4 hp) load motor:

L1 = 2 amp
L2 = 2 amp
L3 = 0.2 amp

Mechanical load on the spindle has L3 heading up to 1 amp, that's as much brake
force as I wanted to apply with my hand.

There's nothing wrong with his converter. That's how they behave.
 
........

There's nothing wrong with his converter. .....

THAT I will agree with in general. It runs the motor, and supplies generated leg current. It works.

It's possible it could work BETTER, but that is another issue.
 
I like BETTER. BETTER is not the same as GOOD ENOUGH.

As the Hp increases the difference between BETTER and GOOD ENOUGH increases as well.
 
These are the supply leg currents. (all readings with the machine motor running but no belt on the motor).
Line 1 41.2 amps. Line 2 41.6 amps.
Output current to the machine motor.
T1 16.4 amps. T2 1.2 amps. T3 16.1 amps.
Output volts to the machine motor.
T1 to T3 238.3 volts. T1 to T2 214.4 volts. T2 to T3 207 volts.

Do I have a fault on the system, is my wiring method wrong or is this just a matter of balancing?

Strangely the whole system works very smoothly. The idler supply does not even notice the machine motor being connected and the machine motor starts and runs very well.

Should I panic or just proceed with the project?

Thanks, Brian

The voltages and currents you are getting are approximately what i have experienced with smaller motors and no balancing capacitors.

where you can run into additional difficulty is the degree of saturation of the idler motor. you may get better voltages, like say, 240, 216,212, if you wire the motor for 277 volts delta and run it on 240. the difficulty with delta windings is not that they are electrically inferior when run as a single phase motor, but they are thermally inferior. All of the current is going through one phase, rather than two phases in series. electromagnetically they sum to the exact same value from the point of view of the rotor! but thermally the heat is concentrated on a third of the windings instead of two thirds. however, the impedance of the motor will mostly follow voltage squared. so a 15 hp 240v idler configured for 277 will be about equivalent to a 10 hp motor. (its idle power consumption will probably be 50% lower though!)

The reason why the voltage at the generated phase is low, is because the rotor has to supply the power to magnetize the air gap of the induction motor.. and that energy is going through another air gap to get there! far as i know, the difference between (in your example) 214 and 207 volts, is the slip of the induction motor. If you supply power to the induction motor via the shaft (say you set the pony motor to drive the motor faster than synchronous) you should see those voltages equalize and then reverse. they will still be lower than the line voltage, and you may have to supply about 1 shaft hp to that 15 hp motor in order to get it at synchronous speed.

When you add capacitors you get a series resonant circuit going on with the inductance of the air gap and the voltage across the air gap. you aren't canceling anything out, you are just circulating current in the coil to get the voltage up. this is necessary if you want to use your idler to supply a three phase motor that will be run at more than 60% nameplate hp without exceeding nameplate temperature rise.

An idler with no capacitors supplies starting torque only to a three phase motor. when you see amps like 16/1/16, it means there is no torque being generated by that third phase and you should not load that motor beyond 57% nameplate hp or it will be running hotter than nameplate temperature rise (if you run the motor long enough to heat it up of course!)

Even if you see half as many amps delivered to the third phase of your load as line amps on the other two phases, electrically that is still mostly nothing from the motor's point of view.
 
...........

The reason why the voltage at the generated phase is low, is because the rotor has to supply the power to magnetize the air gap of the induction motor.. and that energy is going through another air gap to get there! far as i know, the difference between (in your example) 214 and 207 volts, is the slip of the induction motor. If you supply power to the induction motor via the shaft (say you set the pony motor to drive the motor faster than synchronous) you should see those voltages equalize and then reverse. they will still be lower than the line voltage, and you may have to supply about 1 shaft hp to that 15 hp motor in order to get it at synchronous speed.

When you add capacitors you get a series resonant circuit going on with the inductance of the air gap and the voltage across the air gap. you aren't canceling anything out, you are just circulating current in the coil to get the voltage up. this is necessary if you want to use your idler to supply a three phase motor that will be run at more than 60% nameplate hp without exceeding nameplate temperature rise.

.......

The entire principle of resonance is canceling inductance with capacitance, and reducing the impedance to resistive only......... When you add capacitors, you are canceling part of the series inductance, and moving toward resonance at line frequency. You do not want to get there, but the closer you get, the more you raise voltage.

Same thing the power company does to fix bad power factor due to inductive impedances in loads and sources.

Generated voltage is lower than line voltage because of slip. The motor cannot ever run synchronous, because then no energy would be transferred to the rotor bars. And it cannot ever generate an equal voltage to the line, because then no current could flow into it to provide magnetizing current and induce rotor currents. Slip is REQUIRED in order to provide rotor bar current. Rotor bar current in turn is what induces the voltage known as "back EMF". The motor slips just enough to provide the rotor current that generates exactly the back EMF needed to allow just the required total power into the motor.

So the RPC without capacitors will always generate a little bit less than line voltage. Adding load lowers the generated voltage due to slip and losses. That further unbalances the RPC as a source.

Bottom line is still the same..... If you have a low current in one wire, the RPC is NOT acting like a true balanced 3 phase source. If it did, all currents would be "equal".
 
if you want to cancel the inductance out without burning up copper losses you need to fit the capacitor between the rotor and the stator.

we call that trick: magnets. permanently storing the amp turns in.. stuff like neodymium.

no way round it: you have to supply the energy needed to magnetize the air gap, from the rotor.. which is being driven by another air gap.

Same thing the power company does to fix bad power factor due to inductive impedances in loads and sources.

its the same thing when the resistance is negligible. in this case its not at all negligible, if it were, you could get a single value of capacitance to "cancel" out the air gap inductance and your induction motor would behave as if it were a synchronous one.. well, everyone here knows that doesn't work.

a synchronous 2 hp motor is a better "idler" than a 5 hp induction idler in my experience, and that "2 hp sync motor" started out as a 76% efficient induction motor as compared to the 92% efficient 5 hp induction motor. the nearly 3 fold reduction in losses with the larger motor (proportionately) somehow doesn't compare to canceling out the air gap reactance with a magnet, and its no surprise once you understand what is going on here.


ever measure the voltage across the start winding of a single phase induction motor and read nearly zero volts?
 
Thank you, Gentlemen, for a most comprehensive answer to my question.

The best course of action seems to be to continue on the present plan and look at the load balance problem when the machine is truly under power. My initial concern that the low current on the manufactured leg was caused by an idler motor fault has been removed.

Thanks, Brian
 
if you want to cancel the inductance out without burning up copper losses you need to fit the capacitor between the rotor and the stator.

we call that trick: magnets. permanently storing the amp turns in.. stuff like neodymium.

no way round it: you have to supply the energy needed to magnetize the air gap, from the rotor.. which is being driven by another air gap.



its the same thing when the resistance is negligible. in this case its not at all negligible, if it were, you could get a single value of capacitance to "cancel" out the air gap inductance and your induction motor would behave as if it were a synchronous one.. well, everyone here knows that doesn't work.

a synchronous 2 hp motor is a better "idler" than a 5 hp induction idler in my experience, and that "2 hp sync motor" started out as a 76% efficient induction motor as compared to the 92% efficient 5 hp induction motor. the nearly 3 fold reduction in losses with the larger motor (proportionately) somehow doesn't compare to canceling out the air gap reactance with a magnet, and its no surprise once you understand what is going on here.


ever measure the voltage across the start winding of a single phase induction motor and read nearly zero volts?



Sorry.... it's just physics, the canceling principle works regardless of resistance. Resistance alters the "Q" of the resonant circuit, but in no way changes the cancellation. You simply add up the VARS and see what the residual is.. And, you never want to cancel all of it anyway, or you run up the resonance curve and get high voltages. The powerco shoots for about 0.9 pf.

Naturally, the whole thing changes when you add an inductive load. That is more or less why Jim Rozen's approach is so good. It is pretty much independent of the load, because ALL the impedances are much smaller. You pay for that with increased idler draw to supply larger magnetizing current and iron losses, friction etc..

You cannot cancel the resistance with any component, so that is why the common wisdom is to use an idler rated at least 1 1/2 x the load power. It's OK as far as it goes, but using a much larger idler works even better. Both approaches are intended to lower the winding resistance in series with the load.
 
Regardless how I connect capacitors to the motor, the leakage flux and air gap inductance are Still there, causing the third "generated" phase to be of rather high impedance.

When you connect capacitors to the third generated leg, you have a series resonant circuit set up between the air gap and leakage inductance of the motor and the capacitors you added, and this increases the voltage across the coil, at the expense of increasing the current flowing through those coils.

There is really nothing more to say: the capacitors do not make the air gap inductance and leakage flux go away, and those two things are the reason why you get voltages on the order of 240/214/210 without capacitors.
 








 
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