VFD used with either a 3~ Rotary Converter or Phase Inverter

Jan 21, 2006 16 Replies
M

I'm a newbie who's trying hard to do my research before simply asking the dumb newbie questions; despite my efforts I still need to helpful coaching.



Here are my 3 key goals:



  1. Using a 5 or 10HP 3~ motor, I want to control the full 40-4000 RPM range of a Monarch 10EE directly from the machine, without swapping belts, and with minimal torque loss/variance; and possibly increase the RPM to 4500 or 5000.
  2. I want to convert/provide 3~ power throughout my shop from my standard 1~ grid power.
  3. I want to be able to run 1 or more 3~ machines at a time up to a combined maximum of 10 or 15 HP, and to control their RPM speed directly on each machine.

STEP 1: SAFELY/CONVENENTLY PROVIDE 3~ SHOP POWER



The two best shop options seem to be a Rotary Converter or Phase Inverter.



ROTARY CONVERTER If I build one myself here is what sounds like the best recommendations I've read (please let me know if you disagree or have any other suggestions):


  1. Size the idle motor 1.5x my maximum HP needed (e.g. 15HP x 1.5 =

22.5 HP+).
  1. Automatically power the converter with the shop lights or with a sensor detecting that power is desired by a machine. Power should be turned off automatically with a drop in grid power, when the shop lights are turned off, and possibly after a certain delay from non-use (e.g. 30-minutes).
  2. Automatically self-start the idle motor by compromising (staggering) the capacities of the tuning capacitors during the startup process, and then once running, relays can engage additional capacitors to restore balanced capacitor capacity ("tuning") between the 3 phases, in order to optimize the balance of voltage/current in the generated phase to match the other two phases. BTW, I really would like to learn more about this in greater detail.
  3. After the tuning has taken place, I've read that you can also add a power-factor correction capacitor across the line power leads coming into the converter to minimize any out-of-phase current flow into the idler motor. This apparently should not affect your home electricity costs, but will probably affect commercial rates. Also, it is important to note that this can trip circuit breakers if you become close to the maximum rating. I would like to learn more about this from 'DoN. Nichols' (who first wrote about it) or others if possible.

PHASE INVERTER



I've not read much about this other than I know that a modified sine wave is more accurate, but it I don't know if that really matters to a motor. Are phase inverters more expensive than building your own rotary converter? What are some good sources?



STEP 2: SAFELY/CONVENENTLY VARY MACHINE SPEED WITHOUT TORQUE LOSS



It sounds like the preferred option, using the latest technology available today, seems to be VFD (variable frequency drives). Most people use a single VFD to control power for the entire shop and locate this near (or on) the machine that would most benefit from having variable speed control nearby.



If I want to have this convenience on every machine then it appears I will need to purchase a VFD for each machine rate at the power for that particular machine.



Am I wrong with my understanding? Is there a better way to adjust the RPM for the full range of the Monarch 10EE without severe torque impact?



Can I modify a 4000 RPM rated machine to run a 4500 or 5000 RPM?



What are great sources for new or used VFD's?



Thanks to all in advance for your help.



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D

Bear in mind that when you change pulleys or gear ratios, you increase available torque when you reduce output speed. A VFD does not do this. The torque an induction motor can deliver does not increase when speed is reduced by reducing drive frequency.

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K

A proven design for the 10EE is to use a VFD and 10 HP motor. No need for backgear (lower gear) if you go this route. You can run a VFD off single phase if you oversize it. In this case a 15 hp. VFD would run your 10 hp. motor. Belt your unit so that you get max RPM at two times motor nominal speed. For example a 1750 motor will go 3500 RPM top end with a VFD. You'll need to step it up a bit if you want 5000 at the spindle for example.

You have to be careful that you don't buy a VFD that senses for all three phases present. I bought a Hitachi S300 and found out I couldn't use it. But the Hitachi S100 doesn't test for three phase. I'm using a 10 hp unit to run my 7 1/2 hp. lathe. I bought from Automation Direct web site.

I used the back gear on my Monarch 10 EE. I already had it on the machine. Then I could use a 5 hp motor and 7 1/2 hp. VFD. WAY LESS BUCKS!

For your other queries. look up a web site "Metal Web News" There's a huge section on 3 phase build your own converters.

Karl

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M

VFD would run your 10 hp. motor.You have to be careful that you don't buy a VFD that senses for all three phases present.

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R

If you opt for a rotary phase converter just remember the idler motor / total load ratio of 1.5 X is on the low end. A reasonably larger sized idler won't be a problem but you will probably have to consider a pony motor for starting. It is usu. recommemded that idlers of 10 HP and larger be started with a pony to reduce "start-up" current surge.

Bob Swinney

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M

Thanks Bob. I was doing some more reading and found the recommendation of 2X max load being a better minimum size for the idle (converter) motor. I've trying to find a 40 HP 220/240V 16 pole (450 RPM) motor (or an 8 pole/900 RPM if I can't find the slower). Is it possible for me to oversize the starting capacitors enough to avoid the pony motor? If not, any suggestions on how to automatically engage the pony during startup and then disengage it once stated?

Also, I was considering powering a small fan to cool down the enclosure and motor. If I use the idle motor to power the fan, is that going to be a problem so long as I keep the blade size small (e.g. 3" - 6")? I was thinking of rigging up some kind of furnace filter type setup to allow air to flow into the enclosure; I'd place some kind of metal protection that allows the filtered air to pass through, but still keep hands away from reaching inside when the filter is removed.

It seems that it will be more cost effective to make a nice 3 phase converter even if I decide to go with a VFD later on. It seems that oversizing a 3 phase converter is much cheaper than oversizing several

3 phase VFD's to run on 1 phase power. This is of interest to me because I could see myself adding a VFD to several machines... I really don't want to have a single VFD running the whole shop--it's a convenience thing.

Please let me know if my cost analysis is incorrect. Thanks.

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R

Why are you looking for an 8 or 16 pole idler motor? Generally, idler motors are made from 4 pole or 2 pole motors which are the most common and least expensive. The number of poles in the respective load motors will determine their speed, i.e. they will run near their "synchronous speed", what ever that may. It may be possible to start a 10 HP, or so, idler with capacitors but I think starting a 40 HP motor with capacitors alone is out of the question. Someone on RCM a while back said he was starting a 15 HP idler with no problems.

Bob Swinney

enclosure

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I

Regarding fans and cooling, regular muffin fans can push out quite enough air to remove heated air from your phase converter enclosure. In my own 17.5 phase converter

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I simply use a couple of muffin fans and they move out at least an order of magnitude more air than is truly necessary. A 230 CFM muffin fan is about enough to keep coils on my 9.6 kW from overheating. I will get a bigger fan out of abundance of caution, but it is an example to show that regular muffin fans have quite a bit of power if air flow is not restricted. Your cooling setup could end up costing you under $10, as smaller muffin fans are quite inexpensive.

Let's say that your enclosure is, say, 2x4x4 feet, or 32 CF. A muffin fan than pushes out 100 CFM replaces air in the enclosure three times every minute.

i
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I

Forgot to say, when I start the 10 HP idler in my 17.5 HP dual idler RPC, the starting current is 120 amps.

i
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3

think starting a 40 HP motor with capacitors alone is out of the question. Someone on RCM a while back said he was starting a 15 HP idler with no problems. Yeah, I'll probably have to go with the pony motor. If I can actually

I've used a 30 HP RPC for years, and always started it on just the capacitors. It was a 4 pole (1750 rpm) motor. It was fed from a 240V

100 amp panel. No problems for me. I don't see why you need anything more than a 4 pole motor. Unless you have a big grudge against your wallet. No pony motor, Just about 2000 MFD to start. THUMP!. Running. Cut metal. Pete
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I

Pete, I am curious, did you try to measure starting current on that 30 HP idler. The reason for my question is that my 10 HP, 1750 RPM idler draws about 120 amps at startup. I suppose that a 3 times bigger motor would draw 3 times more, or 360 amps. It would be interesting to see if that was true and if starting it dimmed the lights in your house, etc.

i
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3

It was ~around ~ 300 amps, but for such a short time, that it didn't much matter. Never even could tell it happened from in the house. Now I had two VMC's on that converter. If they both tried to go from zero to 10,000 rpm (20hsp each) at the same time the air compressor started, and air conditioner was on in the house and the shop, then you might hear it for about 1.5 seconds, but still no dimming of lights.

Now I have a real shop, with 480V 800 amp power. Only the power meter seems to notice what I do. Pete

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R

Perhaps my statement:

"It may be possible to start a 10 HP, or so, idler with capacitors but I think starting a 40 HP motor with capacitors alone is out of the question. Someone on RCM a while back said he was starting a 15 HP idler with no problems." should have been more clear. I should have pointed out that running motors of 10 HP or greater on ordinary residental service is likely to require larger circuit breakers than normally provided in residences. Circuit breakers are sized for full loads. A rotary phase converter "looks" much like a single phase motor in operation - but not an ordinary single phase motor. RPCs draw large leading currents in proportion to their size, and it is unlikely an RPC of much over 5 HP, with full load, could be operated from a 30 amp residential-size breaker.

Note that Pete's 30 HP, capacitor started idler, was fed from its own 240 V,

100 amp (dedicated) panel. I'm sure it started and ran OK but he probably never loaded it up to near its capacity. A fully loaded 30 HP RPC would require more than the 100 amp capacity of Pete's panel.

Bob Swinney

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R

One of RCMs frequent responders noted his starting current on a large RPC to be 120 amps. Get real! Locked rotor current (starting current) of a 17 HP motor on single phase would be in the neighborhood of 300 - 400 amps; probably more when starting a 3-phase motor on single-phase. The OP probably tried to read the maximum "kick" observed on a clamp-on amp meter, and was oblivious to the effects of meter "ballistics".

Bob Swinney

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R

"RPCs draw large leading currents. . . ."

Sorry; damned proofreader -- make that *lagging current*

Bob Swinney

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