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QSIMDO
05-14-2006, 12:02 AM
I know next to nothing about electricity so could someone please explain VFD vs. phase converter and which would be most suitable for running a 3 phase motor?
TIA
Len

torker
05-14-2006, 12:26 AM
Len, this is funny! I was just thumbing through the KBC catalogue looking at the same thing.
The old universal mill I just got has a 5hp/208/3ph motor on it and I was wondering if I should use a rotary phase converter or one of those little box looking Phase-A-Matic type converters.
The Phase-A-Matics cost about !/4 what a rotary unit costs.
I know I could build one...if I knew absolutely ANYTHING about 'lectricity!
Well, I've wired a couple houses but that's not quite the same.
Russ

J Tiers
05-14-2006, 12:54 AM
VFD
True 3 phase
Variable speed
Motor protections
can get over normal speed
can go from 115 in to 230 out, for some models
full 3 phase motor power out
full 3phase benefit of "phonographing reduction"


Phase converter, Rotary
can be made from a 3 phase motor.
not quite "real" 3 phase, but close
full 3 phase motor power out (almost)
Single speed only
most of the 3phase benefit of "phonographing reduction"


Phase converter, "Static", Phase-A-matic box type
single phase only
"3 phase" when starting only (actually 2phase)
about 1/3 to 2/3 motor power available
"regular" and heavy-duty" available, difference is in start circuit
Single speed only
no "phonographing reduction", same as any single phase motor

EDMTech
05-14-2006, 02:03 AM
A VFD is not quite "true" three phase. It's more of a pulsed type power rather than a true sinusodial waveform. Absolutely not CNC friendly at all. They do deliver a nice, balanced output which is nice. Downsides are whatever motor is being driven must be started by the VFD, NOT the machine switch. This can be a problem if the machine has multiple motors, like a spindle motor and a power feed motor, or spindle and coolant.

Rotary's can deliver very balanced three phase but they must be matched to your application to see full potential. Advantages are you can run multiple machines within the current limits, and operate the machines normally by their own controls. So you can use a machines on/off switch, turn on pumps and accessories and such as you would on a real 3phase line.

As for the little "Phase-A-Matic" boxes; generally not advised. They do not deliver three phase, just a "fake" leg to start the motor, but the motor actually runs on single phase. Serious reduction in power and increase in motor heat. They are OK for very light duty work but you'll never be able to even approch the machines full potential.

J Tiers
05-14-2006, 02:09 AM
The "true" part on the VFD is correct phasing, which is only for one power draw on a rotary, and not applicable on static.

Some VFDs are now starting to appear with a more sinusoidal output even than teh six step type.... theoretically it would be possible to provide a good sinewave output, and some may now do that.

Phase perfect claims the third phase is done with an inverter on their device, and that it is true sine.

sch
05-14-2006, 02:11 AM
If you do a google search on rotary 3ph convertors you will find a couple of good essays on how to build these very simple devices. Static convertors are even simpler, but as J Tiers says produce severe reductions in motor power. Aggregating the necessary pieces for a good rotary convertor is a bit trying for those not familiar. A lot of stuff is available in surplus markets but you have to have some understanding of how this stuff works to select the most appropriate pieces for your application. Depending on your sources, rotary can be dirt cheap or very expensive to make. Cheap or free 5-15hp 3ph motors are the basis of most of these. If you have to buy one then a VFD becomes more attractive. Right now the price leader on VFD for small motors <3hp is www.factorymation.com Their prices on TECO FM50/100 are very good, $50-75 less than I paid 2yrs ago.
Steve

torker
05-14-2006, 08:24 AM
Thanks guys! I knew it wouldn't be as easy as buying a cheap box and start making chips but was hoping.
I USED to be pretty ballsy with electrical stuff but got electrocuted pretty good once while rebuilding an old house. Grabbed a hidden wire that someone had cut off and just left it laying on the top of a heating duct. Now I'm scared sh--less of the stuff :D
Oh well, I'm a ways away from even thinking of starting the motor, have to get the rest of the mill home first.
Be lots of time for studying up on it.
Russ

thistle
05-14-2006, 09:27 AM
A RPC is like having 3 phase service to your shop,add a breaker box or some other means of distributing it and given a big enough RPC you could power
all your machines running at once,with out modifacation to controls and auxiliary motors.



A VFD in general will service one motor.You might have to rework the controls as any contactors will not work with VFD.
I like the vfd ,as i have set it to slowly accelerate to full power and can see in that few seconds if i have left the clutch in , or if there is going to be a disaster i can shut down.apparently they are very economical on power.

although i have a RPC for big stuff , I am putting vfds on my small motors as funds permit as i like there advantages.



search this forum for vfd, also the PM site - I know this came up a few weeks agohere.

there is a very good article in HSM or MW by Forrest Addy,alsoon PM and I think copied here.

pstephens
05-14-2006, 03:15 PM
I ...got electrocuted pretty good once while rebuilding an old house.

If you did, you wouldn't be here telling us about it. You just got shocked. I know the feeling, by the way -- it does tend to make one a bit more cautious!

Michael Moore
05-14-2006, 04:16 PM
A Phase Perfect from http://www.phasetechnologies.com/ is an easy, but not inexpensive, solution to getting 3 phase power when you only have single phase available.

cheers,
Michael

QSIMDO
05-14-2006, 05:14 PM
Almost "phased" off my chair when I read the prices.

Whole darn lathe isn't worth that much!

torker
05-14-2006, 05:20 PM
If you did, you wouldn't be here telling us about it. You just got shocked. I know the feeling, by the way -- it does tend to make one a bit more cautious!
Hahaha...you're right! I only got killed a little bit :D Wouldn't have been so bad if I hadn't fell off the ladder to boot. Probably helped though, the step ladder peeled out from under me and I fell breaking the contact. Got a nice burn on both hands.
Lesson learned! Keep one hand in your pocket!

Michael Moore
05-14-2006, 05:20 PM
I said it wasn't inexpensive. :)

On the other hand, the PP isn't a permanent installation (move it with you everytime you move), and it doesn't care what you switch on/off downstream from it. You can plug a bunch of small 3 phase loads into it, or one big one, so it is pretty versatile that way. If you've got several devices that need 3 phase and have to buy and configure a good-sized VFD for each one that won't be terribly cheap.

It isn't something that fits everyone's budget, but if you are looking for clean and stable 3 phase it seems to be an option that should be considered. People who have a PP seem to uniformly be happy with them.

cheers,
Michael

HTRN
05-14-2006, 05:21 PM
A RPC is like having 3 phase service to your shop,add a breaker box or some other means of distributing it and given a big enough RPC you could power
all your machines running at once,with out modifacation to controls and auxiliary motors.

This is what I plan to do - I would suggest putting a switch between the single phase source, and the RPC. No sense running it when you ain't using it. I would also suggest using twist lock connectors between the two panels and the RPC, this way upgrading the converter is very straight forward, just unplug one, and plug in another. This is most helpful when you plan to upgrade to a larger phase converter in the near future.


HTRN

J Tiers
05-14-2006, 08:52 PM
Phase perfect is only one of many, and is over-rated and way too expensive to boot (but works)

Ronk, Phase-A-matic and others make RPCs also, around $400 or so at cheapest.

thistle
05-14-2006, 09:08 PM
This is what I plan to do - I would suggest putting a switch between the single phase source, and the RPC. No sense running it when you ain't using it. I would also suggest using twist lock connectors between the two panels and the RPC, this way upgrading the converter is very straight forward, just unplug one, and plug in another. This is most helpful when you plan to upgrade to a larger phase converter in the near future.


HTRN

yes you need a disconnect switch up stream of the rpc onthe single phase line.
I have a 50 amp fused disconnect switch, I am (on advice) going to run
the feed through a 70 amp circuit breaker as well.
to start stop the rpc I put in a contactor .

Leigh
05-14-2006, 09:50 PM
The "huge" power reduction attributed to a static phase converter is mostly marketing hype on the part of the RPC and VFD vendors. Yes, there is some reduction in capacity, but it won't be noticeable unless you're loading the machine heavily. I use a static converter on my 1J Bridgeport and it works just fine. Although this is a hobby environment, I've frequently done 8- to 12-hour shifts with no objectionable heating or other adverse consequences.

The statements by others regarding how a static converter works are correct. It "manufactures" a third phase briefly, just long enough to start the motor. Then the motor runs on regular single-phase current with the third phase disconnected.

Leigh
05-14-2006, 09:52 PM
I have a 50 amp fused disconnect switch, I am (on advice) going to run the feed through a 70 amp circuit breaker as well.
What kind of a machine are you running??? If your breaker is significantly larger than required by the load, you might as well not waste the money buying it in the first place. It won't do anything. A properly-sized breaker will protect the equipment and the operator from some types of failures. An over-sized breaker does not afford the same degree of protection.

J. Randall
05-14-2006, 10:11 PM
Another thing not mentioned is to use the cheap static to start an idler motor and use some run caps to balance your load motors and you basicly have an RPC. James

thistle
05-15-2006, 08:00 AM
What kind of a machine are you running??? If your breaker is significantly larger than required by the load, you might as well not waste the money buying it in the first place. It won't do anything. A properly-sized breaker will protect the equipment and the operator from some types of failures. An over-sized breaker does not afford the same degree of protection.


apparently the circuit breaker must be sized that way ,other wise the starting load of the rpc will trip the breaker.told to do so by the manufacturer
of my rpc.cant really argue with them.

EDMTech
05-15-2006, 09:18 AM
Almost "phased" off my chair when I read the prices.

Whole darn lathe isn't worth that much!

Very nice units but not really worth it unless you are going to be running a CNC. The good things about them is that, like an RPC, you can run as many machines on them as the current rating allows. The smallest unit can handle 50 amps, more than enough for a small shop.

J Tiers
05-15-2006, 09:18 AM
The "huge" power reduction attributed to a static phase converter is mostly marketing hype on the part of the RPC and VFD vendors. Yes, there is some reduction in capacity, but it won't be noticeable unless you're loading the machine heavily. I use a static converter on my 1J Bridgeport and it works just fine. Although this is a hobby environment, I've frequently done 8- to 12-hour shifts with no objectionable heating or other adverse consequences.


I am unaware of any statements as to "huge" power reductions. The inescapable FACT is that between torque reductions and heating, you are limited to something between 40% and 66% of full power. Period.

The reputable companies making static converters put that information out plainly and simply. As for what you "read on the internet", that is at your own risk.

SGW
05-15-2006, 10:55 AM
Having used a static phase converter on my milling machine, and having replaced it with a VFD...I can report that the VFD is IMO vastly superior.

With the static phase converter, it wasn't too difficult to stall the motor when drilling large holes.

Leigh
05-15-2006, 11:48 AM
I am unaware of any statements as to "huge" power reductions.

As referenced below


Static convertors are even simpler, but as J Tiers says produce severe reductions in motor power.

Phase converter, "Static", Phase-A-matic box type
about 1/3 to 2/3 motor power available

I'm not going to get into a stupid discussion about what constitutes "huge" or "severe". The fact remains that there is a reduction in power, but not so much as to impair the operation of the machine nor reduce its capabilities by any significant amount. The documentation with the static PC says to expect about a 1/3 reduction in horsepower, which appears to be correct, not the 2/3 reduction which you stated.

DR
05-15-2006, 11:50 AM
What are the approximate prices on the PP units? I didn't see that on their site.

A 10hp would be enough for my purposes. Are there extras you have to buy or is it a more or less complete setup as shipped?

Leigh
05-15-2006, 12:01 PM
"phonographing reduction"
What is "phonographing reduction"?

You put it in quotes, indicating that it's a made-up term, but don't explain it.

pcarpenter
05-15-2006, 12:25 PM
What worked well for me is to build a nice balanced RPC. It uses the equivalent of a static phase converter to start the idler motor and then automatically disconnect the start circuit. What is inside a static converter is a potential relay and a capacitor. If you want to start your RPC that way, don't waste the money on a purchased static converter. The parts that it took to make this self-starting portion of the circuit are maybe $25....not $100 or more for the static converter. I can post a link to a site with specifics if you need them.

I built my RPC using a used motor starter and this potential sensing start circuit. This spins up the idler motor and it just runs as a generator. I used balancing caps to balance the 3HP idler motor for best balance with a 2HP motor running. In my case, I have three machines that use 3phase power and each have motors right around 2HP. I will likely never run more than one simultaneously, although I could. Out of my RPC electronics enclosure, I run power to another box with a terminal in it...and power from there to each of the three machines.

I was fortunate to get an enclosure, start and stop buttons, the ldler motor and motor starter cheap, used. The motor even appears to be NOS. I bought a potential relay, starting capacitor, and run capacitors as well as a fuse holder, all new. The complete parts list likely totalled $150 which is a fraction of the cost of a commercial RPC, and mine is specifically balanced for my load motors. Each phase is within about 4-5 volts of one another.

If you decide to go this route, I can provide the links to the pages I used for information in building this.

VFD's are nice, but you can end up with quite a bit in them as you add machines as you will need one-per. You also need to insure that your machine controls can start and stop the VFD, as disconnecting the load as you normally would, from a powered VFD is a no-no. On something like a lathe that might have the motor control on the apron that lets you reverse the motor etc, this may be a good bit of work.

VFD's are also a bit hard on motors as running them "out of band" (at other than their rated frequency) can increase heating in the windings. Keeping the frequency range reasonable helps, but you hear guys talking about keeping their speed set to one setting on the machine and using the VFD for all speed changes and that is probably not very wise. It is a bit like driving a car in one gear and just revving the motor up or down as your only means of varying speed. Still, motors are somewhat forgiving and you make use of this forgiving nature when using an RPC, as well, since you still provide power on two phases that is 180 degrees apart and not 120 as it should be.

Paul

J Tiers
05-15-2006, 01:28 PM
What is "phonographing reduction"?

You put it in quotes, indicating that it's a made-up term, but don't explain it.

Phonographing is the sort of moire pattern produced by chatter on a part in the lathe.

because the single phase motor has severe torque ripple, that produces a "drive signal" if there is any resonance near a harmonic of line frequency.

3 phase has far less torque ripple, almost none, and in any case it is at a higher frequency, more likely to be out of range of the mechanical vibration frequencies of heavy work or lathe parts.

As far as power reduction, according to Phase-A-Matic, who make BOTH static and rotary converters, the power reduction is to 2/3 with Y wound motors and 50% with delta. Those are related to how motor currents flow. I overstated if I said 2/3 reduction.

Because of the single-phase operation, the torque ripple means the motor may have an overload stall point quite a bit different from what it would have if driven by 3 phase power.

True 3phase has "zero rpm torque", and single phase does not. So a single phased 3phase motor will experience "torque breakdown" with a rapid stalling if overloaded. Because the iron isn't utilized as effectively as in a single phase motor, stalling may occur faster.

If the same motor is run 3 phase, it will not "break down" the same way, torque will reduce with increasing slip, but it will continue to have torque all the way to zero rpm.

Mark Hockett
05-16-2006, 04:18 AM
What are the approximate prices on the PP units? I didn't see that on their site.

A 10hp would be enough for my purposes. Are there extras you have to buy or is it a more or less complete setup as shipped?



The 10hp unit is about $2600 plus shipping. It hooks up like a RPC 2 wires in and 3 out.

I am running a PP in my shop for a few reasons, it is very energy efficient, it produces clean 3ph power for my CNC machines, it doesn't take up much room and it is quiet. I make my living in my shop and spend 10-14 hours a day, 6-7 days a week running the machines. I got very tired of hearing the RPC run all day not to mention the cost to run it. I figure the PP has paid for itself in reduced energy costs.

When I was shopping for a new VMC many of the manufactures (Haas, Hurco, Fadal) recommended the PP and not an RPC.

I run my entire shop which includes a Fadal VMC, Milltronics CNC mill, Haas CNC lathe, 3hp manual lathe, 2hp bport mill, 1.5hp saw and some other small stuff with 1 PP. When I had a 7.5hp RPC sometimes the Milltronics mill would shut down when another machine was turned on, that doesn't happen with the PP.

I think a RPC is a fine way to go for most home shops, much better than a static unit. I was looking at a Wholesale tool catalog and many of their machines had a disclaimer saying that it would void the warranty if run on a static phase converter. I've also seen that in other catalogs, that tells me they must not be good for your machine. I also think that if this is a long-term hobby and the budget affords it the PP is a great way to go.

Mark Hockett
Island Tech Enterprises