[query] 3 phase converter monitoring setup

Greetings all,
I'm working on balancing my rotary converter and need a sanity check.
(Basic statistics for the record. 1125RPM 15HP idler.
Feeds off a 125A breaker in the shop panel, through a 100A fused disconnect. L1 and L2 from the disconnect feed L1 and L2 in a 3p panel on the rails (I might pull single-phase 220 from this subpanel for other uses in the shop). The idler and start circuit feed out, and back in through the 3P panel's original 100A Main breaker. Starting is the standard push-and-hold auto starting setup, with a Grainger voltage-sensing relay to cut out the starter switch when L3 comes up. I've forgotten how much starting capacitance I have in the box, but it spins the (kinda chunky) 15HP idler up in less than 2 seconds. When running, it'll currently spin up a 5HP high-pressure blower (Invincible vacuum), in about 5 seconds, pulling about 45A on L1 and L2, and 23A on L3. Once spinning, it pulls about 25A on L1, 19A on L2, and 5A on L3. I don't currently have voltages on it.)
Just for gits and shiggles, rather than just tweaking the balancing caps with an amp-clamp and DVM, I thought I'd build a full-blown current and voltage monitoring system for it.
For current monitoring, I've stuffed 3x 100:5 current transformers into the 3P subpanel, and brought them out to current-sensing resistors.
For voltage monitoring I need voltage in the range -1V -- 1V to match my scope inputs, so I need to build a voltage-divider. This would be no big deal, except I just tossed myself a puzzler when getting ready to solder the bits together.
One could build 3 bridges (say 2MOhm and 2KOhm roughly, just to make the numbers reasonably neat), and bridge L1->L2, L2->L3, and L3->L1, thereby being able to measure the between-leg voltages.  Alternatively one could bridge L1->C, L2->C, L3->C (and possibly tie C to ground/N). I've doodled an ASCII version of this below. Hopefully it survives being posted. L1, L2, and L3 are as traditional. L1->a is 2MOhms, a->d is 2KOhms. The other legs are symmetric.
L1 L2 . . _ _ | | _ _ | | _ _ . a b . | | _ _ | | _ _ . d \ / \ . c \ / \ / \ / . L3
This setup appears allows metering of both the between leg voltages by metering across both middle .1% resistors on a leg pair ( ie, a to b, b to c, c to a) and the Leg->Neutral voltages if the d is tied to ground/N.
I hadn't originally thought of using the "Wye connected" meter setup, but I can't currently think of a downside.  I'm wondering if anyone who does this stuff more regularly might know some reason not to do this, that I haven't thought of.
Hmm - while I'm at it, another question -- Is there any reason that I can't tie together one leg of each current-sensing resistor for the current monitor part, and use this as a "common", for metering purposes? I'm suspicious that my scope inputs don't have isolated grounds.
Many thanks for your time, William Ray
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    [ ... ]

    Good -- that gives you isolation.

    Yes -- one very good reason -- have you tried measuring the voltage from ground to the center point? I'll bet that it is a nasty range above ground, since you have no isolation between the incoming power line and the rotary converter.
    Get three identical filament transformers with input voltage appropriate to the leg-to-leg voltage. You can then tie one side of each secondary to ground and safely measure voltages (taking into account the ratio of the transformer, of course.

    Yes -- since the current transformers give you isolation.

    Most don't. The probe ground is connected to the safety ground pin on the power cord.
    You want the isolation that the current transformers give you, and similar isolation on the voltage side with the filament transformers. (Those used to be cheap enough and common enough so you should be able to pick up three identical ones.
    Good Luck,         DoN.
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On 11 Jan 2009 07:03:37 GMT, "DoN. Nichols"

I see you're aiming for the luxury of both current and voltage measurement. When there is significant unbalance, the apparent 3 phase balance of the current set and the voltage set can be very different.
With both sets of figures available you may be tempted to aim for a rough average betweeen voltage and current balance.
Because motors are essentially current driven devices this is not the way. Balanced current should be your aim even if this results in larger voltage unbalance.
Jim
    
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On Jan 11, 5:52 am, snipped-for-privacy@yahoo.com wrote:

What do you think of displaying the current transformer output on the scope to watch the phase angle? A vectorscope would be really nice but I don't know how to show 3 phases on an X-Y display.
jw
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    [ ... ]

    [ ... ]

    [ ... ]

    That depends. Yes, if a motor is the only load. But if it is also powering something like a Bridgeport BOSS-3 through BOSS-6, a voltage imbalance will pop stepper driver power transistors like popocorn on the opening night of a big movie. :-)
    The transistors used in these systems were just barely high enough voltage rating to handle what was being applied. (Of course, really high voltage power transistors were very rare and very expensive when the system was designed. I think that the BOSS-3 hit the streets about 1977. :-)
    If you're using such a machine, it might be good to use two rotary converters -- one tuned for current balance for the spindle motor, and one tuned for voltage balance for the electronics boxes.
    Enjoy,         DoN.
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They sell three phase power monitors and they are not very expensive on ebay. I have one and am going to install it on my phase converter "one day". My phase converter already does a fine job, so I keep postponing it in favor of more fun things.
i
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...
Hi DoN - always nice to see familiar knowledgeable faces when I have time to drop in!
I haven't measured the center point, but I'm assuming it's off center. The "Wye Connected" bridge network was my solution to let me monitor the center - but of course you're right - if it's off center, that screws up the leg-leg measures.
Filament transformers it is. I probably have enough identical ones in a box somewhere here to put that together from spare parts. Finding that box, could take a while, so maybe I'll order some more...

I let the smoke out of the first scope I discovered that on... I "manually float" most scopes that cross my path now. I'm pretty sure my Tek. scopemeter has completely isolated inputs, but I don't trust that "pretty sure" enough to bet the magic smoke on it, and I'm less-sure about the 4-channel Tek that I'm planning to use for monitoring the 3P converter.
Ok. Back to the barn to search for transformers...
Thanks much! Will Ray

        You want the isolation that the current transformers give you,

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| Voice (all times): (703) 938-4564

Don't worry too much about the transformers being identical. It is easy enough to scale the outputs to be equal when the primaries are all connected in parallel to the same source. Any non-linearities will be insignificant if the primary voltages are within ratings.
Don Young
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William, I don't wish to rain on your parade, but if you value your time at all, you will find you can replace your entire setup with a new solid state sinusoidal VFD that will easily handle your loads. These are available both monitored and unmonitored at prices around $500 not made in China on eBay. These VFDs are far more balanced than your rotary converter with far better power factors. If you consider the money realized from the sale of your existing kit and the cost of your time, the VFD would be virtually free. Steve
Greetings all,
I'm working on balancing my rotary converter and need a sanity check.
(Basic statistics for the record. 1125RPM 15HP idler. Feeds off a 125A breaker in the shop panel, through a 100A fused disconnect. L1 and L2 from the disconnect feed L1 and L2 in a 3p panel on the rails (I might pull single-phase 220 from this subpanel for other uses in the shop). The idler and start circuit feed out, and back in through the 3P panel's original 100A Main breaker. Starting is the standard push-and-hold auto starting setup, with a Grainger voltage-sensing relay to cut out the starter switch when L3 comes up. I've forgotten how much starting capacitance I have in the box, but it spins the (kinda chunky) 15HP idler up in less than 2 seconds. When running, it'll currently spin up a 5HP high-pressure blower (Invincible vacuum), in about 5 seconds, pulling about 45A on L1 and L2, and 23A on L3. Once spinning, it pulls about 25A on L1, 19A on L2, and 5A on L3. I don't currently have voltages on it.)
Just for gits and shiggles, rather than just tweaking the balancing caps with an amp-clamp and DVM, I thought I'd build a full-blown current and voltage monitoring system for it.
For current monitoring, I've stuffed 3x 100:5 current transformers into the 3P subpanel, and brought them out to current-sensing resistors.
For voltage monitoring I need voltage in the range -1V -- 1V to match my scope inputs, so I need to build a voltage-divider. This would be no big deal, except I just tossed myself a puzzler when getting ready to solder the bits together.
One could build 3 bridges (say 2MOhm and 2KOhm roughly, just to make the numbers reasonably neat), and bridge L1->L2, L2->L3, and L3->L1, thereby being able to measure the between-leg voltages. Alternatively one could bridge L1->C, L2->C, L3->C (and possibly tie C to ground/N). I've doodled an ASCII version of this below. Hopefully it survives being posted. L1, L2, and L3 are as traditional. L1->a is 2MOhms, a->d is 2KOhms. The other legs are symmetric.
L1 L2 . . _ _ | | _ _ | | _ _ . a b . | | _ _ | | _ _ . d \ / \ . c \ / \ / \ / . L3
This setup appears allows metering of both the between leg voltages by metering across both middle .1% resistors on a leg pair ( ie, a to b, b to c, c to a) and the Leg->Neutral voltages if the d is tied to ground/N.
I hadn't originally thought of using the "Wye connected" meter setup, but I can't currently think of a downside. I'm wondering if anyone who does this stuff more regularly might know some reason not to do this, that I haven't thought of.
Hmm - while I'm at it, another question -- Is there any reason that I can't tie together one leg of each current-sensing resistor for the current monitor part, and use this as a "common", for metering purposes? I'm suspicious that my scope inputs don't have isolated grounds.
Many thanks for your time, William Ray
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Unless things have changed rather significantly in the VFD world recently, I think I'd either need something ridiculous in the VFD world, or a rather complex and extensive switching and control system to route power and control around the shop.
The current setup will replace Mains power for 8 to 10 machines, including hopefully allowing me to run a step-up transformer to power a 440V 5HP subsystem. To simulate Mains power with a VFD (as opposed to using either a separate VFD for the switching/control system of each machine, or one VFD that can be cut over to each of many machines), I believe the consensus has been that it required a monstrously oversized VFD.
Are there inexpensive VFDs out there now that can provide "Mains Service like" (capable of handling load-local switching, plug reversing etc) power to a subpanel for distribution?
(of course, half the reason I'm doing this at all, is for the amusement of it, so entirely reasonable suggestions along the lines of "do it the smart way", will end up deprecated in favor of suggestions for byzantine but functional :-)
Thanks! Will
Will
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Willray sez:
"(of course, half the reason I'm doing this at all, is for the amusement of it, so entirely reasonable suggestions along the lines of "do it the smart way", will end up deprecated in favor of suggestions for byzantine but functional :-)"
Depricate as you will . . . but the method of voltage balancing rotary phase converters as outlined in RCM over the years is probably the best way to go. See articles in www.metalwebnews.com which describe the voltage balance method in detail.
Bob Swinney
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This isn't the first rotary I've built or balanced - just the first that I decided to incorporate a full monitoring package into, so that it can draw pretty pictures on my 4-channel scope. The availability of 4 channels provides some visualization and monitoring capabilities that aren't typically taken advantage of by the RCM techniques.
Fitch and Jim R. seem to be the people who have done spent the most time staring at scopemeters hooked up to their converters, but they were stuck with the Fluke's dual-sweep inputs, and weren't really working for designing an available-full-time monitoring system.
In the end, it'll end up balanced just as normal, but it'll be more fun doing it, and it'll draw pretty, and moderately meaningful pictures for me :-)
Will
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Phase displacement can be measured with an oscilloscope. I use 1 turn of heavy wire around the winding core of a 12:120 Vac transformer and read the voltage scaled current waveform on either the primary or secondary of the transformer. The resulting waveform can be displayed on 1 channel of the scope at some conveint size determined by the scope's calibrated vertical amplifier. Place a voltage divider across the leg under measurement and display it on channel 2, with the two waveforms dispalyed in the same "size". View both waveforms on a common timebase and read phase displacement between the traces. Make sure the scope is powered through an isolation transformer.
Bob Swinney
wrote:

This isn't the first rotary I've built or balanced - just the first that I decided to incorporate a full monitoring package into, so that it can draw pretty pictures on my 4-channel scope. The availability of 4 channels provides some visualization and monitoring capabilities that aren't typically taken advantage of by the RCM techniques.
Fitch and Jim R. seem to be the people who have done spent the most time staring at scopemeters hooked up to their converters, but they were stuck with the Fluke's dual-sweep inputs, and weren't really working for designing an available-full-time monitoring system.
In the end, it'll end up balanced just as normal, but it'll be more fun doing it, and it'll draw pretty, and moderately meaningful pictures for me :-)
Will
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