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GMC 1440 Lathe w/Phase Converter

Bakerprecision

Plastic
Joined
Aug 20, 2022
Hi All,

I just wrapped up purchasing a used GMC GML-1440HD (6hp/3ph), and I purcahsed a 7.5hp Phase Perfect to run it out of my home garage. I've been exchanging emails back and forth with tech support over at GMC Machines, and they are telling me that a "rotary phase converter" does not work (or work well) with this machine, and that you will not be able to achieve full RPM (1800). They went on to say that they had an American Rotary 20hp unit, and could not achieve full performance.

This has me perplexed a little, as I was not aware that motors and/or 3 phase power has a bias?!?! Now the unknown part of this is, will my PP have the same issues as a rotary phase converter? Has anyone experienced an issue like this? Worst case, I'm SOL and have to find a different machine. Best case, this is just some silly story, right?
 

dalmatiangirl61

Diamond
Joined
Jan 31, 2011
Location
BFE Nevada/San Marcos Tx
Need more details on the motor they use in that machine, and which model AR they tried it with. I don't know enough about PP to say anything more than to call them for sizing. A 20hp rpc should handle 6hp just fine.
 

Bakerprecision

Plastic
Joined
Aug 20, 2022
I'll have to dig up specs on the motor, but it's a 4p/8p, 6hp, 20A. So the PP should work as it's rated to 26A. I'm also wondering if they were testing with the AR vs ADX. I've just never heard of an issue like that, but that's not saying much.
 

thesjg

Plastic
Joined
Nov 4, 2021
Location
Sturgis
I'll have to dig up specs on the motor, but it's a 4p/8p, 6hp, 20A. So the PP should work as it's rated to 26A. I'm also wondering if they were testing with the AR vs ADX. I've just never heard of an issue like that, but that's not saying much.

I just dug your order up and saw you went with our new Phase Perfect Simple. If you had talked to our sales dept. I suspect they would have steered you toward our 7.5 Horsepower rated Phase Perfect Enterprise model, because you will likely be marginal on the PT-S.

The difference with our PT-S (which is a very new product for us), is we reduce the size of the IGBT's in the unit (they are a major driver of cost). In order to start a motor like the one in your lathe with the current capacity of the IGBT's we use, we drop the voltage on the generated leg to 150V briefly during motor start. After we get the motor moving we quickly bring the voltage back up to match the input, and we then maintain the 2% or better voltage balance we are known for.

A rotary phase converter does exactly this same thing, when you start a large load the leg generated by the rotary sees a significant voltage drop. Typically, we can recover voltage much, much faster on our PT-S than a rotary would, and thus start a hard starting motor much faster than a rotary.

Now, as this applies to your machine, our 7.5 Horsepower PT-S is, from the website: "The converter is rated to start single motor loads up to 5 horsepower and run up to 7.5 horsepower worth of loads concurrently." This puts you right on the margin, IF it will start your lathe without tripping it should run it very well. A PTE007 is more well-suited to the application.
 

Bakerprecision

Plastic
Joined
Aug 20, 2022
I just dug your order up and saw you went with our new Phase Perfect Simple. If you had talked to our sales dept. I suspect they would have steered you toward our 7.5 Horsepower rated Phase Perfect Enterprise model, because you will likely be marginal on the PT-S.

The difference with our PT-S (which is a very new product for us), is we reduce the size of the IGBT's in the unit (they are a major driver of cost). In order to start a motor like the one in your lathe with the current capacity of the IGBT's we use, we drop the voltage on the generated leg to 150V briefly during motor start. After we get the motor moving we quickly bring the voltage back up to match the input, and we then maintain the 2% or better voltage balance we are known for.

A rotary phase converter does exactly this same thing, when you start a large load the leg generated by the rotary sees a significant voltage drop. Typically, we can recover voltage much, much faster on our PT-S than a rotary would, and thus start a hard starting motor much faster than a rotary.

Now, as this applies to your machine, our 7.5 Horsepower PT-S is, from the website: "The converter is rated to start single motor loads up to 5 horsepower and run up to 7.5 horsepower worth of loads concurrently." This puts you right on the margin, IF it will start your lathe without tripping it should run it very well. A PTE007 is more well-suited to the application.
I'm not worried about the PP at all, just has me concerned that the lathe manufacturer advised not to run a rotary. My understanding that phase converters work more off total amp rating rather than HP rating, correct? Because now I'm starting to doubt if what I'm buying is adequate. The lathe max amp rating is 21A, and the PP max amp rating is 26A, so I figured I was good. I'd sure hate to spend that kind of money to roll the dice. I may end up cancel the order until I can fund something suitable (and affordable).

Thanks for the feedback and input on this!
 

thesjg

Plastic
Joined
Nov 4, 2021
Location
Sturgis
I'm not worried about the PP at all, just has me concerned that the lathe manufacturer advised not to run a rotary. My understanding that phase converters work more off total amp rating rather than HP rating, correct? Because now I'm starting to doubt if what I'm buying is adequate. The lathe max amp rating is 21A, and the PP max amp rating is 26A, so I figured I was good. I'd sure hate to spend that kind of money to roll the dice. I may end up cancel the order until I can fund something suitable (and affordable).

Thanks for the feedback and input on this!


This is a test we ran in house comparing our Phase Perfect Enterprise to a Rotary. It shows very clearly what the lathe manufacturer is concerned about. Getting all that inertia spinning is tough when the generated leg doesn't recover.
 

Bakerprecision

Plastic
Joined
Aug 20, 2022
Thanks for the information, very helpful. And to clarify, the lathe manufacturer wasn't concerned with ramp up, they said the lathe might not run at full RPM. Which I found to be odd.

I feel like I need to do more homework before spending this kind of money on a paper weight.
 

rons

Diamond
Joined
Mar 5, 2009
Location
California, USA
This has me perplexed a little, as I was not aware that motors and/or 3 phase power has a bias?!?! Now the unknown part of this is, will my PP have the same issues as a rotary phase converter? Has anyone experienced an issue like this? Worst case, I'm SOL and have to find a different machine. Best case, this is just some silly story, right?
Those people have their own benchmark. But for what you did with purchasing a PP. If that doesn't work that what else will? Your have the best unit you can get.

A rotary and a PP have a difference. The PP will generate the third to close tolerance. The rotary may not if run capacitors are not correct.
But close to correct for most people is good enough.
 

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
The manufacturer response is a bit odd. I am not sure why you would not get full rpm.

Is the machine run by a VFD internally? Or possibly by a DC drive of some kind? If so, they may find that the DC supply does not get to the full design value with matched-size RPCs. That could affect top speed, particularly with a DC drive. However, looking at the manufacturer data, it looks as if the thing is powered by a standard 3 phase motor.

They may be referring to the RPC having problems starting the machine in top speed. Some motors, including some WEG products, are real current hogs, and people report issues in top speed with lathes powered by such motors, typically not being able to get the motor to start with the controls set to top speed.

That should be solvable with a larger RPC. I don't know what the implications are for the PP. With that particular line of PP using lower current IGBTs, and consequently having a lower current limit, a minimum size PP in that line might have similar issues. Most of the traditional PP line will start anything that does not draw more than the nominal rating.
 

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
They may be more similar in some models. The "simple" PP as described above, appears to start off with the same issue as an RPC, a voltage drop on the generated leg. That will be done to limit inrush current, and is a valid approach, certainly.

Any motor begins to generate back EMF as soon as it starts rotating. As the back EMF gets higher, the current goes down, until it finally reaches the current needed to supply the load.

The RPC will have a voltage drop, which will be less as the motor speeds up, so that the output voltage of the RPC goes up (load is reduced). The "Simple" PP seems to emulate that, although they are probably utilizing the short-term IGBT ratings (probably heat-limited) in order to keep the maximum starting current. They know just what current they are allowed, and can switch to the normal voltage at an appropriate safe time.

There would be loads that the "Simple" cannot start, just as there are loads that an RPC cannot start (although a "full" PP would start them). There are loads that some "matched-size" VFDs cannot start, because they time-out on the overcurrent rating before the motor gets up to a speed that reduces its current due to back EMF. The "Simple" will have the same issue.

An RPC of a given size may not start some loads because the generated leg drops too far in voltage to provide enough current to accelerate the load. But, an RPC might start some that A PP or VFD cannot, simply because it does not have as "hard" a limit on output current.

None of the three devices is free of limits. But it may be cheaper to provide a considerably oversized RPC for hard-to-start loads.

The PT video in the link above shows that both devices can start the blower load. The PP does so in somewhat less time, probably about the same start time that "dropping the load on the line" would. Loads like that are typically not started very often, due to the heating of the motor from high start currents.

The OP's lathe appears not to have a clutch, so the motor has to be started every time the spindle is started. In high speed mode, the drive train has a significant inertia, and it might require an RPC that is more than 2x the load motor rating.

Larger idler motors have the capability to produce higher generated leg currents.

I might mention that the "pass-through" legs of the "Simple" PP, or an RPC, provide no torque without the "generated" or "manufactured" leg. So the current available on that wire is the real limit to starting a load.

The extra current drawn on the "pass-thru" legs for an RPC is a combination of the current to power the RPC, and the current drawn by the load. So, the load motor is not heated by all that current. Both the motor powered by an RPC, and one powered by a PP will draw "about" the same current on the "pass-thru" legs at any given motor RPM during startup. (there will be "some" difference depending on the voltage supplied by the manufactured leg in each case).
 
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Bakerprecision

Plastic
Joined
Aug 20, 2022
Well... I have read and read to no end, and I'm still in the same place as I started. I'm still stuck on the comments from the lathe manufacturer "RPC won't achieve full RPM"???

Is it irrational to start looking into single phase motors? Possibly 5hp? It's probably cheaper to buy a new motor than dump money into a PP or RPC that may or may not give me full performance from a 6hp motor.

Has anyone ever had issues powering their lathe (or other machines) with a RPC or PP?
 

dalmatiangirl61

Diamond
Joined
Jan 31, 2011
Location
BFE Nevada/San Marcos Tx
Where is that lathe made? Maybe something got screwed up in translation? I cannot think of any reason it would not get to full rpm with an rpc that is sized 2x, 3x to be sure, the lathe motors rating. I seem to recall something about some Brazilian WEG motors not playing well with rpc's, iirc there is mention of it on the Phase-A-Matic website, but no hard details. If its a 2 speed motor I seem to recall some mention of them not playing well with rpc's too, but iirc its the low speed where they have a problem. Going single phase has its own set of problems.
 

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
As I noted, it may not matter, because the "Simple" PP has limits which are similar to a rotary converter..... It starts the load using reduced voltage, just as an RPC would.

So, some of the limitations may cross-over between the types.

As for why... if the motor is a current hog, it may require much more current than the typical "generated leg" can supply. The solution is either Powerco 3 phase, a more powerful PP, or a larger RPC.

As for the RPC issue, you might need to go to 4x the nominal power, but there IS an RPC that can start their machine.
 

Bakerprecision

Plastic
Joined
Aug 20, 2022
As I noted, it may not matter, because the "Simple" PP has limits which are similar to a rotary converter..... It starts the load using reduced voltage, just as an RPC would.

So, some of the limitations may cross-over between the types.

As for why... if the motor is a current hog, it may require much more current than the typical "generated leg" can supply. The solution is either Powerco 3 phase, a more powerful PP, or a larger RPC.

As for the RPC issue, you might need to go to 4x the nominal power, but there IS an RPC that can start their machine.
Well again, this is what had me confused. I'm told by the manufacturer that the lathe will start and run just fine, it just doesn't achieve full RPM. Which makes me wonder if it's an issue with on RPC's, a specific brand of RPC's, or just phase converters in general.

The subject RPC is a 20hp Noth American rotary (not sure which model), but a 20hp should be plenty to run a 6hp motor.

Like the variations with the PP models, they are not all alike. I just need to figure out what, if any, difference there is that would effect a motors ability to reach full RPM. I'm hopeful that I can get away with an American Rotary ADX 15hp setup. I'm certainly not dropping $4k on a 10hp+ PP unit that eliminates the issue if I can help it. For $4k I'll put a single phase motor in and call it a day.
 

dalmatiangirl61

Diamond
Joined
Jan 31, 2011
Location
BFE Nevada/San Marcos Tx
If what they have is an AR-Pro, the cheapest model, it is only for light to medium loads, so in high speed, with a big chuck, it might not be able to start it. On startup, a 3ph motor will pull 3x or more of the current rating on data tag. And AR says a 70A breaker is good enough, whereas another mfr says their 20hp rpc requires 125A breaker. Without more info on seller/dealer/mfr rpc used and how its wired, its hard to say why it did not work for them.

If the seller/dealer/mfr cannot provide more info on why an rpc will not work, maybe you should be talking to someone else.
 

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
Communication issue.

They may mean it starts on lower speeds, and runs fine, but will not start properly at high speed or speeds.

The "runs fine at lower speed" may mean exactly that.... just is not referring to a high speed setting.... instead they may be meaning that in low range there is no issue.
 

thesjg

Plastic
Joined
Nov 4, 2021
Location
Sturgis
I'm hopeful that I can get away with an American Rotary ADX 15hp setup. I'm certainly not dropping $4k on a 10hp+ PP unit that eliminates the issue if I can help it. For $4k I'll put a single phase motor in and call it a day.

At a $76 retail price difference (shipped) vs the PTE007, I would take the Pepsi Challenge with the ability to start a load against the ADX 15 any day of the week.

pt007-motor-start.png
 








 
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