Amp-clamp: inductive clamp-on meter. I've seen very strange effects with those. Put on the input lines of my converter, they read
the sum of real and imaginary currents. Put on the manufactured leg, they seem to read only real current.
OK, I was not sure if you were separating the old "Amprobe" style meter from newer ones. The old ones usually read average current, newer may read either RMS or average.
As for the "sum of real and imaginary", I don't quite get why/how that would happen. What would suggest that to you?
For troubleshooting and ballancing a rotary converter, I suggest a voltmeter only. One can be fooled into thinking the converter is
wildly out of balance based on an amp-clamp reading, in spite of voltage balance being very good.
Volts may or may not result in a given amount of amperes..... In a motor, that depends on a number of things.
One thing for sure, however, torque in an electric motor depends on amperes. so the torque contribution of each wire is going to be related to the amperes flowing into the motor.
In BOTH cases, the phase is going to affect the result. Voltage at the wrong phase may produce less current, or current that is not helpful. Current at the wrong phase may produce less power, or may result in torque that is not helpful.
Most motors produce back EMF that is pretty much in the correct phase relation for all three wire pairs, so that is usually OK. Adding capacitors on the generated leg will change both voltage AND phase, by amounts depending on the added capacitor and the characteristics of the motor.
Relying on just voltage, or just current as a means of checking operation is probably unwise.
Remember what the goal is: It is to provide an output that is as close to perfect power company 3 phase as possible. And you know that if you succeed, the voltages on all phases and currents on all phases will closely match each other, assuming the powerco does their job (a very good bet) and that the motor maker also did a good job (certainly a decent bet).
Ideally, you would set the adjustments of an RPC to get a close voltage match, AND a close current match on all three wires.
The issue is that you start out behind the 8 ball to the extent of perhaps 5 to 10% on voltage, because the back EMF of the motor is always lower than the incoming line voltage by that general range of voltages at full power.
At idle, a motor has very nearly 100% match of back EMF and incoming voltage, which you can see with a tachometer, since speed will be close to synchronous. Very little power used, so current is minimum, just the magnetizing current plus a small power current.
Under load, the back EMF drops due to the motor slowing (allowing more current). The actual applied voltage also drops due to resistance of the motor windings.
That happens in the idler also, which means the output is not perfect, There is an inherent imbalance with the usual setup of an idler motor, and the smaller the idler in relation to the load motor, the worse that is. The "direct" wires are just that, they have nothing extra in series, while the "generated" phase has a motor with resistance and inductance, plus magnetic effects, in series. With the same current flowing, the generated leg is going to be lower in voltage coming out of the idler, than the direct legs.
In an effort to "fix" that, capacitors are often applied. That does correct voltage, but in the process it alters phase somewhat, and it is necessary to look at both voltage and current to determine when the best compromise is found, to fix on the values of added capacitor which will allow that idler and that load to best approximate powerco 3 phase.
There are some alternate methods for correcting voltage imbalance which may have less of a double effect, but the true best system is generally the largest idler combined with one or another means for correcting voltage. I think it is fair to suggest that sufficiently larger idlers will in general have an "uncorrected" output (no capacitors etc) that is closer to the ideal than most implementations of smaller idlers combined with correction schemes.
But, since the end result is to closely match "regular" 3 phase, there is a need to use both voltage and current measurement, along with phase angle if that capability exists, to get the optimal setup of "balance capacitors" or other schemes for balancing.
The other way to go is to have an idler considerably larger than the maximum load (perhaps 5x). That may actually give the best results if it can be done. The large motor has less demand on it relative to maximum load, so voltage is higher to begin with, and both resistance and inductance are lower.
Alternately, the method originally suggested in this thread, the motor-generator set, is likely to provide the best balance overall. That is natural, because all wires from the generator pass through the same resistance and inductance etc. There may be an overall voltage drop, but it will remain decently balanced at all loads. It is also the most expensive way to go, and can be right up there with a Phase Perfect, for instance, unless you have access to surplus equipment.
The VFD approach will give very good balance, with other benefits. But you need more of them.
