phase converter questions

If you can't find cheap three phase motors, you probably are not looking in the right places. Try the scrap yards. I mean the yards that supply steel scrap to steel mills. Not salvage yards.

As far as looking at a motor generator, have you considered using a three phase induction generator?

Dan

Jim

I have a little dyno with which I can measure the efects of changing idler size on a 3 phase motor's ability to deliver power. The improvement in "tool motor power delivering ability" is almost undetectable for NO idler or for an idler 1 1/2 times tool motor HP when the tool motor is loaded up to 2/3 of its name plate Max HP.

It impressed me that the 3 HP 3 phase motor that I use produces 3 HP at

It is probably obvious to those who have thought about it, that a heavily loaded (near full name plate HP) 3 phase motor does run alot smoother when it is fed thru a big-big idler.

It might be concluded that 3 phase motors that arent loaded heavily for long periods can be run from single phase without an idler. But, a 3 phase motor will produce its full name plate HP even when fed single phase with no idler.

Jerry

Right, though they can't start on 1 ph. You need either a rotary converter, a static converter, or a pony motor to start the 3 ph motor. It also won't run smoothly on 1 ph power because torque will go through zero twice a cycle.

It may, for a short period, but it will be drawing current in excess of its ratings on the driven phase. A 3 ph motor driven by 3 ph power has its current divided

That's why there's a rule of thumb that a rotary converter needs to be sized

Example, 1 HP load motor should be driven by a 1.5 HP rotary converter,

Of course the rotary does the *transformation* from 0.5 + 0.5 to 1/3 +

Gary

Bad plan. Spin it up, *then* throw power to the rotary. With your wiring scheme, you'd face a starting surge of well over 800 amps. By spinning it up to nameplate RPM first, there will be no monster starting surge when you throw power to the rotary.

Throwing belts isn't a very good idea either. Just kill power to the pony once the rotary is started. It'll just freewheel then, with only a small amount of extra windage drag over the windage of the big rotary itself.

This is how I have my 20 HP rotary set up. The biggest single load motor it ever sees is the 7.5 HP motor in my lathe, which gives it plenty of headroom since it is capable of driving a 13.3 HP motor (20/1.5).

Ideal 100% efficiency current draw with that load should be 23.28 amps. Real world, it runs off a 30A motor rated breaker with no nuisance trips. Of course I did tune it, and use power factor correction caps. Without them, it'd need a much larger breaker.

At first, I had it wired your way, and it blew a 50A breaker every time I tried to start it up. Based on my research, I now know it would have blown the 200A main breaker if I'd wired it straight to the main. Uncontrolled starting surge on a 20 HP motor is

Gary

This is an unexpected, yet fascinating result. It may be that the hp ratings on some motors is fairly conservatve ("anti-sears power rating...") which could account for some of this.

I wonder if the same 3 hp motor would actually do 4, if it were run on utility 3~?

Jim

Right Gary! You explained it very well: "It may, for a short period, but it will be drawing current in excess of its ratings on the the driven phase" -- and further, when you said, "If you try to draw full nameplate HP from it while it is driven by 1 ph power, the driven windings will be overloaded."

This apparent ability to produce full power on "2/3 of a motor" is a function of the motor's service factor. Surely a motor driven to full name plate output on 2/3's of windings will draw excessive input current, eventually overheating to failure..

Bob Swinney

Jim

All the 3 pjase motors I've tested will produce more than their name plate rated HP when fed good 3 phase. A 3 HP motor will deliver 5 HP to the load when fed good 3 phase. It does get exactly as much hotter as you'd guess so you'd want to put an external fan on it. Heating of the motor is the principal factor that limits the amount of power that an induction motor can safely deliver. That could be improved by external fans and aditional heat sinks. Since the motor conducts heat away from the windings thru its frame, the entire frame becomes a significant factor in determining the max power rating. The 2/3 name plate HP 'rule of thumb' for guessing at the max power available from a 3 phase motor, fed single phase, must be altered to allow the fact that the heat sink and fan is working as though it is a 3/3 HP motor.

It might be interesting to note that many 3 phase motors that are used in Rec Craft applications are often commercial tool motors that will not be loaded to their original ratings so the need for producing 'good 3 phase' is often misunderstood.

A tool motor can easily spun up with an idler much smaller than the tool motor's name plate HP, contrary to the implication that Big idlers are needed. Most tools that use 3 phase motors can be unloaded while being started, so even a switch and capacitor can spin up most 3 phase tools.

As I noted above, the 3 phase motor will provide the load with very nearly the same HP when fed either single phase or 3 phase, when the motor is loaded to only 1/2 its name plate HP. At those "lightly loaded" conditions, I've been unable to descern the difference between the performance of the 3 phase motor fed either 3 phase or single phase. The instanteous loss of torque every 1/120th of a second exist in either a single phase motor or a 3 phase motor fed single phase. That "pulsing" doesnt become measurable when the motor is loaded to less than about 3/4 its rated HP. For loads under about 3/4 its name plate rated HP (2 1/4 HP for a 3 HP motor) a 3 phase motor fed single benefits little from the use of in idler. The pulsing is probably the first noticable result of having no idler (or small idler) when using single phase to drive a heavily loaded 3 phase motor.

I plotted (with Excel) the HP vs RPM for the 3 HP 3 phase motor, fed single phase, with a 5 HP idler and with no idler. I'll send you a copy if you'd want to see it..

Jerry

Agreed.

I don't *advise* throwing the belt -- it is just the way that I remember Jim Rozen describing in the past how he used his, and since the original question was about the wiring and use of his setup, that is what I described as well as my memory allowed.

Enjoy, DoN.

As three generators at Hoover Dam stall and half of CA goes dark...

"Did I do that??"

;-)

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On 26 Aug 2004 20:06:25 -0700, snipped-for-privacy@krl.org (Dan Caster) vaguely proposed a theory ......and in reply I say!:

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Actually I just a 30" circular saw that I am going to drive from my tractor, and it has a 6hp 3PH motor attached, which I don;t need. I also remembered I have a 3PH motor in the other shed. However that one's 20 HP! and I believe they suck juice just to run.

eerrr..no?

I will chase it up. This will probably start a whole new _chain_ of back and forths. I think I saw some before about these.

But really..what the dickens are those caps good for? Ive got a number of them.

Gunner

No 220-pound thug can threaten the well-being or dignity of a 110-pound woman who has two pounds of iron to even things out. Is that evil? Is that wrong? People who object to weapons aren't abolishing violence, they're begging for the rule of brute force, when the biggest, strongest animals among men were always automatically "right". Guns end that, and social democracy is a hollow farce without an armed populace to make it work. - L. Neil Smith

On 26 Aug 2004 22:56:36 -0400, snipped-for-privacy@d-and-d.com (DoN. Nichols) vaguely proposed a theory ......and in reply I say!:

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No they are US. I would think frieght would kill me. It just surprised me what was available. I bet I can't come _near_ it locally.

Actually one problem here is that I am not allowed to do _any_ electrical work, so of course the market for bits'n is nil. You can buy completely built RPCs, which would then have to be wired by a sparky. So basically you DIY from scratch and keep schtum (sp?).

On 24 Aug 2004 19:25:25 GMT, snipped-for-privacy@w-sherwood.ih.lucent.com (Charles A. Sherwood) vaguely proposed a theory ......and in reply I say!:

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So to penetration, lubricant, heat, flow, affected zones, I can now add "amount of current in the third leg". No wonder metalwork is more fun than sex.

On 26 Aug 2004 16:31:22 -0400, snipped-for-privacy@d-and-d.com (DoN. Nichols) vaguely proposed a theory ......and in reply I say!:

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Now you see, _this_ is where the idea of a normal old 1 PH motor or 2 driving an alternator start to kick in again....>>I was going to use a 20HP motor too. I kicked it into gear with a

The obvious reply--put them on Ebay.

Steve

Jerry, I have a hard time believing a bigger fan will get you very far. If this were true, motor manufacturers would do it, especially on Sears compressors. I'm sure extra air will get you somewhere, but before long the hot spot will move inside the windings, where your fan can't reach. Extra air (and lower than rated ambient temps) might get you 10-20% (just a guess), but not 50-100% continuous duty. Intermittent, sure.

I agree. I've started an unloaded 20HP motor using a 7.5HP rotary converter. My gear head mill, on the other hand (significant starting torque needed) was a 1HP motor and needed a 3HP idler in my rpc. 2HP wouldn't start it in the high speeds.

Steve

On Sat, 28 Aug 2004 18:32:28 +0800, Old Nick calmly ranted:

He's obviously never peed on a 'lectric fence.

Ooh, you _are_ getting old, Nick.

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You don't have to chase it up. I will tell you about induction generators. They are just that same three phase motor that you would use for a RPC , but with that big single phase motor driving it and one leg of the three phase motor connected to the single phase power. When you do this it is easiest (in my opinion ) to use a belt drive with one of the pulleys being one that can be varied in size. And you adjust the drive so that the single phase motor is trying to drive the three phase motor a little faster than synchronous speed. In doing that you are putting in mechonical energy into the three phase motor and it comes out as electrical three phase power. When ( and if ) you do this measure the input current of the three phase motor and adjust the variable pulley so that the input current is close to the rated input current while driving whatever it is you want to drive with the three phase power .

This needs no special start up circuits and no protection circuits in case you lose the single phase power and it comes back up. It can be further tuned with caps to reduce the imaginary current and therefore the size of wire and circuit breaker on the single phase power. In my opinion this is the best way to get three phase power that is the same frequency as the single phase power. If you want variable frequency to control the speed of a motor, use a VFD.

Induction generators will work driven by a gasolene engine and not connected to the single phase line but that is a bit trickier. They will also work with a single phase motor and a gasolene engine. Again trickier ( needs the correct amount of caps across the line and maybe some DC voltage to get it started.

Dan

If they're rated 16V to 20V, you have a small gold mine - but as you describe the ones you have they would be too small, the 1 Farad 16V they use is the size of a cleanser can, 3" diameter by 8" tall.

Car Stereo "Boom Car" nuts (the AH's you can hear three blocks away with your windows rolled up tight - because they're driving with their windows down and wearing earplugs...) use the 1 Farad caps as power supply boosters right in front of the amplifier, it will act as a huge battery to supply the momentary burst of power for each "Boomp!", then it recharges as the alternator valiantly tries to keep up.

Wanna get them back? Get a nice stereo in your car, 100 WPC minimum, which doesn't have to be expensive if you can settle for last year's closeout amps and speakers...

And when they pull up next to you booming (c)rap at a long red light, drop your windows and crank some Jazz, Classical or Big Band right back at them. Let Doc Severinsen or Don Dorsey's Beethoven "Rage over a Lost Penny" drown 'em out. I get dirty looks, but they get the message. ;-)

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