Comparing Idle Power Draw of Rotary Phase Converters

Looking at the specs for a 20hp American Rotary and North American Rotary; the PL-20 is advertised having an idle amperage of 7.37 and $.14/hr and the ADX20 having idle amps at 2.48 or 0.06kW and $0.06/hr. Is American Rotary doctoring their numbers? Even so any idea if one is much more efficient or better than the other?

Lots of phase converter threads here and I've read a lot of them but if anyone has any comments on my plan feel free to chime in. I have a 2016 size Dyna Mechronics machining center with a 7.5hp spindle and a Fadal 4020HT coming. The shop has 200A panel of single phase power. For now I plan on buying used ADX or PL on ebay or buying one of their DIY panels new and getting a used motor locally for an idler. For now I'm thinking a ~ 20hp RPC for both (not at the same time), the Fadal of course with a 22hp spindle I will have to baby on limited power. In the future I'd leave the 20hp RCP on the 2016 and get a ~40hp RCP for the Fadal or a ~25hp Phase Perfect.

have you checked on VFD pricing lately ?
you really should, single phase 240 in and 3 phase out
Much better performance than any kind of converter

the differences in idle current will be the result of how well the capacitors are tuned and whether or not power factor correction is applied by using caps
(by the way any converter you buy will not be as well tuned as you hoped, if you cant do it, you wont get the best results)

further more tuning with caps means you need the same steady load all of the time for best results
the effects of caps changes with amp draw i.e. changes in load
a heavily loaded converter will supply sagging voltage particularly on the generated leg

you will want a converter of at least 1.5 times HP of your load HP for a decent supply
the larger the converter HP above your load HP the better it will perform
start adding up the cost of all your components and the knowledge you will need to implement them
and then there is trouble shooting a home made device, here is one of only a few places i know of that will be able to help you with that
in other words converters are not plug and play. you will spend a lot of time before you can use your new machines.

none of these problems occur in a VFD
if you have any electronics in your machines you will be MUCH happier with their performance on a vfd

goldenfab said:

Looking at the specs for a 20hp American Rotary and North American Rotary; the PL-20 is advertised having an idle amperage of 7.37 and $.14/hr and the ADX20 having idle amps at 2.48 or 0.06kW and $0.06/hr. Is American Rotary doctoring their numbers? Even so any idea if one is much more efficient or better than the other?

Lots of phase converter threads here and I've read a lot of them but if anyone has any comments on my plan feel free to chime in. I have a 2016 size Dyna Mechronics machining center with a 7.5hp spindle and a Fadal 4020HT coming. The shop has 200A panel of single phase power. For now I plan on buying used ADX or PL on ebay or buying one of their DIY panels new and getting a used motor locally for an idler. For now I'm thinking a ~ 20hp RPC for both (not at the same time), the Fadal of course with a 22hp spindle I will have to baby on limited power. In the future I'd leave the 20hp RCP on the 2016 and get a ~40hp RCP for the Fadal or a ~25hp Phase Perfect.

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The Phase Technologies Cost Savings Calculator (https://www.phasetechnologies.com/explore/reference/phase-converters/calculator) is based on the "average" published idle consumption of those you mentioned and other brands (not everyone publishes the idle consumption figure, and we haven't validated their figures). I don't ever really try to make the case for the price difference based on energy consumption for a small/home shop. If you are powering an elevator or similar and are running your phase converter 24/7, the equation ends up looking vastly different.

One thing we (Phase) don't talk about enough when it comes to home shops is power factor correction. The difference between a power factor correcting phase converter with near unity power factor and an RPC at nowhere near unity might mean you end up with 10, 20, ... percent more three phase power available to run equipment. This can easily be the difference in being able to run, or not, one extra machine simultaneously on a 200A single-phase service.

We see a lot of customers opt for a PT030 (30 Horsepower rated) Phase Perfect when they have a 200A service available. Max steady state current draw of the PT030 is 165A, so it fits into a 200A service, without a ton of room to spare, and sort of maximizes what you can do with three phase power, given that size of service.

The PT030 uses ~175W at idle.

A PT030 would be great but costs nearly as much as the running Fadal I just purchased it would power. If and when the machine starts making decent money I can see upgrading but until then just wondering where American rotary is getting 2.48 idle amps from. Seems way too low.

OK, a motor typically draws about 40% of full load amps at idle. Looks like a lot of power, doesn't it?

But it draws that at a power factor of 0.1 to 0.15, usually. The current is largely magnetizing current, and as such, is "lagging", meaning the current is not in phase with the voltage.

That means thw "power producing" current (the "in phase" part) is only 10% to 15% of the idle current. The rest is the lagging "magnetizing current", or "reactive current".

Since by definition, you are powering the RPC from single phase, maybe at a residential location, you are not often charged for the lagging "reactive" current. I don't know if any single phase installs are charged for that, generally the charge is for actual "power" used, although the new meters have the capability to measure and report "reactive current".

When the stated amp draw is very low, it should mean that the reactive current has been compensated by capacitors (power factor has been corrected to around 0.9), and the remaining current is for the actual "power" drawn (what you get charged for).

While it makes no difference to you in general for your bill, the compensation does reduce the current drawn. The breaker does not care what kind of current it is, all current affects the breaker.

So you can power a larger load from a given breaker if the idler is power factor corrected.

If you take the number they give for amp draw, and multiply by 7 or 8, you will probably get a result that is around 40% or so of the likely full load amps of the motor size used as an idler. It won't be exact, becaue we are guessing at the power factor, and we do not know the actual ratings for that motor.

goldenfab said:

Looking at the specs for a 20hp American Rotary and North American Rotary; the PL-20 is advertised having an idle amperage of 7.37 and $.14/hr and the ADX20 having idle amps at 2.48 or 0.06kW and $0.06/hr. Is American Rotary doctoring their numbers? Even so any idea if one is much more efficient or better than the other?

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If you have a way to see the idler motor name plates then you can see the FLA number and then estimate a number at idle.
To me the 2.48 Amp idle current appears low for a 20Hp motor.

" the 2.48 Amp idle current appears low for a 20Hp motor."

Agree.

Maybe not so low.

You would expect 54A @ 230V FLA. Taking 40% of that, is 21.6A.

If the net current is taken as a percentage of that 21.6, we find that 2.5A/21.6 = 0.11, or a reduction to 11% of the idle current. That may be a little optimistic, but the 54A is also higher than many motors, it is the UL "official estimate", which many motors do better than.

So I'd class it as anywhere from a little optimistic to within the ballpark.

Yes, it is using a motor "off data sheet", and so we cannot necessarily know what the actual idle current single phased will be. But it does not look ridiculous, in a first cut analysis

Thanks for the explanations. Sounds like I'm not alone thinking 2.48A is optimistic.

Question is the other brand is advertising as pulling 7.37A at idle. Is one actually going to draw that much different than the other or is one just fluffing the numbers? Most 20hp motors have about the same efficiency of high eighty or low ninety percent right? Is there really much difference between RCPs that would make one appreciably more efficient than another if they were both tuned correctly? I have an old school crt screen oscilloscope and plenty of multi-meters. I've never done it but it's not too hard to measure the power factor and use capacitors to tune for optimal performance right?

So my question is, should I pay attention to advertised idle current usage of a RPC or does it really boil down to taking the time to get the power factor right for the power supplied and application?

I would say the second. Unloaded idlers draw very little real current. The trick is to measure that number (small) against the reactive currents flowing (large) if you want the real number. If you only care about reducing the reactive current then minimizing an amp-clamp meter reading is a good way to go. In the case of a 5 hp unloaded idler (mine) the real curent is on the order of 1 or 1.5 amps.

My ADX30 shows 7amps at idle with a cheap amp clamp meter it is the same on both single-phase input lines.
The American Rotary specs call 3.73 amps
Regardless, cost is only a few dollars max if it idles all day long and the pay back of doing anything different doesn't add up.

What would a Fluke True RMS meter indicate.

not sure if you are serious or that was rhetorical so ...

even if it was rhetorical, then for the sake of others who may not know
it would indicate true rms amps
rms stands for root mean square
in simple terms it means the effective voltage and amperage we all talk about
in other words the portion of the sine wave that applies the most power to the motor or other load
the peaks and the near zero portions do not actually apply an appreciable amount of power
the actual peak voltage reading is above 120V, but it is so short lived it doesnt add much power

that meter would also read the reactive and real amps together
it would take an o-scope to separate the reactive and real amps and to read the actual peak voltage
almost any meter you buy these days is rms, unless they say otherwise
and fluke coming out with "true" rms probably means it is a better measurement according to the math formula
and not that other meters are not a good measurement any more

in fact i would not care to own a "true" rms meter, particularly if it is significantly different from the old rms
voltage on any meter is a measurement between any two points consisting of: phase, neu., grnd. or anything else including the air around you if you hold one or both leads in the air

it is not an absolute value of any voltage in particular, it is a difference between any two points the user chooses,
and it is up to the user to determine what information he wants to gather and then apply the meter accordingly

The way to measure real power does indeed require a multichannel scope, with the ability to do math on the waveforms. Basically one waveform is the voltage difference across the idler, and the other waveform is the voltage across a small, non-inductive resistor in series with the power to the idler. Note this requires three inputs to the scope, as both sides of the incoming line are hot with respect to ground. A photo of such a setup is shown below. Don't try this at home kids. (the resistor is the gold colored item underneath the mallet handle. Red is voltage, green is current.

I have the PL-20, yes it does draw 8.31A min under idle, but that is only with an amp clamp. so not really accurate.

BT Fabrication said:

I have the PL-20, yes it does draw 8.31A min under idle, but that is only with an amp clamp. so not really accurate.

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it will be as accurate as the meter itself

All I can say is I wouldn’t get hung up on a 8 cent per hour difference.