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1937 Chief
10-22-2011, 06:13 PM
I would like to build my own rotary phase convertor. I have a 5 hp 440 v three ph motor, and can probably find a pony motor. Problems is I have no idea how to connect the whole thing, and end up with three ph to my lathe. I guess I need making a rotary phase convertor for dummies. Anything out there? Stan

MaxHeadRoom
10-22-2011, 06:52 PM
I based mine of the old Fitch Williams design.
http://metalworking.com/dropbox/_1998_retired_files/
page down to FRW files.
I use a start capacitor rather than pony motor.
Max.

SGW
10-22-2011, 06:54 PM
At one time (quite a few years ago) Lindsay Books sold a pamphlet that explained how to build a rotary phase converter. I have no idea if they still offer it or not.

Probably somebody on this forum can explain it as well. I'm afraid I can't, with any reliability of correctness.

flylo
10-22-2011, 07:26 PM
Booklets are cheap on ebay on how to do it.

Mike Burdick
10-22-2011, 07:37 PM
Here's a article to start with ...

http://www.metalwebnews.com/howto/ph-conv/ph-conv.html

bruto
10-22-2011, 07:38 PM
I did this years ago and it was quite simple. Once the three phase motor is up to speed, you put single phase input to one pair of the wires. Connect your three phase load to all three of the wires, and it will receive imperfect but passable three phase. It's really that simple.

I used a pony motor and simply switched the pony off as I switched the converter on. My converter was pretty hefty (an ancient ~7 1/2 horse monster), so I geared the 1/2 horse pony motor down a little and it didn't come up to full speed, but the converter picked up the remaining speed all right. I used an old repulsion-induction motor for the pony, so it could take its time spinning up without burning out.

Ideally you'd want a double throw switch with off in the center, so that you could switch over with one hand, but two switches work fine.

lakeside53
10-22-2011, 07:43 PM
440 motor? is it dual voltage - 220/440?

There's an entire forun on PM devoted to RPC/VFDs etc, and a sticky with designs. We'll worth reading.

1937 Chief
10-22-2011, 08:35 PM
It is more than likely it is a 220 440. If my memory serves me correctly 6-9 together 5-8 togehter 4-7 together 1-2-3 is the line, for 440v. Can't remember our anniversary date but that number stuck with me as well as my service number from the Navy for some reason. Thanks for the info I will check some of these sources. stan

chipmaker4130
10-22-2011, 09:17 PM
While you can sometimes get by with simply starting a 3Ph motor on single phase and using the raw output of the third winding, you'll have a unit that vibrates, is noisy and you'll also be getting a poor approximation of line voltage on that third wire. Putting a little time into balancing the output with capacitors will do a lot to reduce all of those problems.

gnm109
10-22-2011, 10:03 PM
I built a 5 hp self starting RPC about two years ago. I used the schematic below from a site called Electro-Tech. It works quite well. Line voltage (L1-L2) is of course 240 V and under load the other two legs (L1-L3 and L2-L3) are usually 242 V and 244 respectively.

Capacitors that I used for 5 hp were dual 250 mf in parallel for 500 mf and dual 50 mf run caps wired as shown in the schematic. The start caps make use of bleeder resistors of 15K ohm, 5 Watt to discharge them after startup.

Since you want to use pony start, which is a good alternative, you can leave out the Steveco potential relay. (In the schematic, this is normally closed and connects the start caps into the circuit. Once L3 is generated, it drops out and the RPC contuinues running)

These units are quite reliable. I recommend a 1,750 rpm motor since they are quieter. My first RPC was a Phoenix which had a 3,600 rpm motor and it was unpleasantly loud. I therefore sold it and built my own. It used the above schematic as well.

I also use a definite purpose contactor with a 120 V coil and fuses on each line. If you use 120 VAC for a contactor, you will need a neutral since you should't use one side of the hi voltage to ground for 120 V unless you have a neutral. ( I know you say you have 440 V) I added a control transformer to get 120 V with neutral so that I could have a red power light and 120 V with neutral.

Good luck on your build.


http://i144.photobucket.com/albums/r188/gnm109/Machinery/3ph20converter.png

Abner
10-22-2011, 10:22 PM
I built one a year or two ago, it worked, I got the legs "balanced", but I was finally fed up with the seeming lack of power and bought a VFD, I will never go back.

gnm109
10-22-2011, 10:55 PM
I built one a year or two ago, it worked, I got the legs "balanced", but I was finally fed up with the seeming lack of power and bought a VFD, I will never go back.

VFD's are an alternate method. I used an RPC because I have a variable speed mill and I can fix any part of the RPC.

If you had a seeming lack of power, perhaps your idler motor was not sized appropriately. I use a 5 hp for a 3 hp induction mill motor. It has plenty of power.

I know that VFD's are popiular, though.

wooleybooger
10-22-2011, 11:00 PM
i built a 5hp to run a 3hp lathe and now run a 1 1/2 hp mill with the same RPC. with an RPC the converter motor needs to be about 1/3 larger than your largest machine motor.VFDs are popular but you need one for each machine. with a properly sized RPC you can run multiple machines at the same time if your able to move fast. i simply unplug one and plug in the next.

dvo
10-22-2011, 11:14 PM
Can someone explain the power up and power down sequence when using a machine that is plugged into a rpc? Another word what do you do when you want to use the machine?

J Tiers
10-22-2011, 11:20 PM
From an engineering perspective, the "balancing" is overrated, and probably most folks who swear it is required have no idea WHY.

If your idler is considerably larger than the load motor, balancing is pretty optional.

The more "matched" they are, the more important the balance capacitor is.

Now..... obviously, two lines are unchanged input lines....... the "manufactured leg" is the troublemaker.

As a motor, a three phase motor generates a "back EMF", which is nearly, but not quite, equal to the input voltage from the line. The difference between them is what makes current flow in the motor.

When a motor is unloaded, it generates a high back EMF, and has low "slip'. Speed is nearly synchronous.

The more heavily loaded it is, the slower it goes, the higher the "slip", and the lower the back EMF, which allows higher current to flow through the motor impedance to provide the added power.

Having "back EMF" is the same thing as the motor acting as a generator. Motors do that. In an RPC, the "generator" action is what provides the third, or "manufactured" output.

from the above, you see that that output will be slightly less than the input voltage, and equal to the "back EMF". If any current flows, the output will be even lower due to the "impedance" of the motor being in series with the output.

But, with a larger idler, two things are in your favor.

First, the full load current of the smaller load motor is only a fraction of the idler's full load, so the idler has only a small voltage drop in it's impedance.

Second, with the idler loaded only lightly, it is running closer to synchronous, with high back EMF and thus high output voltage.

With those two things working in your favor, you may have no need to balance to get a perfectly good performance from your machine motor. Slight imbalances only reduce the max power the load motor should be allowed to produce, limiting max loading of the machine a bit. If you have a flat belt lathe, you probably can't get full power at many belt settings, and it won't matter much. Ditto for other situations.

But there is nothing wrong with balancing the output, if you do it right.

Now, balancing can ONLY be done perfectly for ONE load. All other loads are going to be in a state of imbalance. Therefore you should pick your oad for best balance.

The worst idea is to balance at no load..... that guarantees poor balance at heavier loads, where you need decent balance.

balancing at full load can lead to a "reverse imbalance" at lower loads, where the manufactured leg is HIGH. Not terrible, but possibly not the best if you don't load to max very often.

Probably the best idea is to balance for your most common heavy load. If you do not know that load, perhaps about 2/3 power is best..... good even if not perfect at high power, reasonable everywhere else.

So, what is "balance" and how do you know it?

Balance just means the voltage between any pair of wires is equal, in whatever loading condition you have when measured.

Why does a capacitor "balance" the RPC?

The idler impedance is resistive and inductive. Adding a capacitor from the manufactured leg to neutral would cancel some inductive reactance, with the effect of raising the voltage. The amount of raise depends on how much capacitance is used.

You said "to neutral", but the rpc has no neutral, what's up?

that is true, but a capacitor to one of the other legs can act similarly, except that the value can be smaller by a factor of just under 2:1 (1.73 : 1), and that is what is often done.
Sometimes two smaller capacitors are used, one to each of the other wires, which does the same thing in a slightly more "perfect" way. One can show that the line-to-line capacitors are equivalent to capacitors to neutral.

If a smaller cap is good, how about a bigger capacitor?

You can end up raising the voltage too much if you add too large a capacitance, so you should use as much as you need, but no more. The voltage will usually be the highest with no load, or the lightest load.

gnm109
10-23-2011, 12:14 AM
Can someone explain the power up and power down sequence when using a machine that is plugged into a rpc? Another word what do you do when you want to use the machine?


You simply start the RPC and then turn on the machine that you wish to power. When finished, shut off your machine and then shut off the RPC. If I am doing several different things on the Mill like moving a part or changing a collet, I just let the RPC run. It doesn't hurt a thing.

It's best to try to build an RPC that is reasonably quiet. That's why I suggested a 1,750 rpm motor. They are usually quieter than a 3,600 rpm motor. Some people will mount the idler outsiide to isolate the sound.

flylo
10-23-2011, 07:32 AM
I have a 10HP RPC, which is very quit just a low hum & just got a 7.5HP, a 440 transformer & the guy built me a panel to switch to 440 when needed. I can use them seperate or together.This guy will build you one that works & looks Great just like it's out of the factory cheaper than you can buy the parts. PM me & I'll send his number. He really knows his stuff. I bet he's built 500+

dvo
10-23-2011, 08:09 AM
You simply start the RPC and then turn on the machine that you wish to power. When finished, shut off your machine and then shut off the RPC. If I am doing several different things on the Mill like moving a part or changing a collet, I just let the RPC run. It doesn't hurt a thing.

It's best to try to build an RPC that is reasonably quiet. That's why I suggested a 1,750 rpm motor. They are usually quieter than a 3,600 rpm motor. Some people will mount the idler outsiide to isolate the sound.


Thanks, just what I was looking for. Thanks for the build info too.

lakeside53
10-23-2011, 11:15 AM
It's not always true that larger motors don't need balance caps. I have two motors that put out very low L3 (195-210) at low load that raised somewhat as the load increased.... and needed caps to raise that. As Jerry points out - balancing is only for a specified load so you need to figure out how to "load up" your destination motor(s) while adjusting. You can use Fitch's method to get "close enough" but even 5% unbalance at full load requires significant derating of the target load motor. If you want to worry yourself, put two cheap 300 volt panel meters (the type with needles!) and mount them L2-L3 and L1-L3. Watch them out of the corner of your eye as you work, then cover them up forever ;)

Another aspect of "balancing" is to reduce idle (or low output current) current with power factor correction. On 240 volt single phase a 15 hp idler motor (or my 8 pole 10hp) will have about 25 amp idle current without power factor correction caps. Place caps across L1-L2 near to the motor, and it goes down to about 3 amps. This of course has no effect (for most users...) on your domestic electricity bill as the power company doesn't penalize you for power factor, but it make wiring and breaker calculation easier.

gary350
10-23-2011, 11:40 AM
I would like to build my own rotary phase convertor. I have a 5 hp 440 v three ph motor, and can probably find a pony motor. Problems is I have no idea how to connect the whole thing, and end up with three ph to my lathe. I guess I need making a rotary phase convertor for dummies. Anything out there? Stan

Don't build your phase converter larger than the motor you want to run, it wastes electricity. It is best to use equal size motors. If you want to run a 1.5 HP Bridgeport mill use a 1.5 rotary motor. You can buy used 3 ph motors for scrap metal price at most electric motor repair shops. Get one with a damaged or bent shaft or broker motor mounts. If the shaft is bad I like to cut them off with a hacksaw.

There are several ways to build a phase converter. A static converter will make your equipment motor run at 2/3 it power rating. A rotary converter will many your equipment motor run at full power.

How to build a manual start static phase converter. Wire your equipment motor for 240 volts AC. Connect power to L1, L2 with a ON/OFF switch. You will need a start capacitor of 60 uf per HP. A 2 HP motor needs 120 uf. A 5 HP needs 300 uf. Connect the start capacitor between L2 and L3 with a momentary ON push button switch in series with it. To start your equipment motor hold down the push button switch then turn on the 240 VAC. When the motor comes up to full speed about 3/4 of a second release the push button switch. This is a manual type static converter it will make your equipment motor run at 2/3 power. A 3 HP motor will produce 2 HP.

How to build a manual start rotary phase converter. Wire your equipment motor for 240 volts AC. Connect power to L1, L2 with a ON/OFF switch. You will need a start capacitor of 60 uf per HP. A 2 HP motor needs 120 uf. A 5 HP needs 300 uf. Connect the start capacitor between L2 and L3 with a momentary ON push button switch in series with the push button switch. To start the rotary motor hold down the push button switch then turn on the 240 VAC. When the motor comes up to full speed about 3/4 of a second release the push button switch. L1, L2 and L3 of the rotary motor needs to be connected to L1, L2 and L3 of your equipment motor. Turn on your equipment motor it will run at full power. L3 of each motor act like a generator and generator electricity to each other that is how it runs at full power.

In both of the above you can replace the push button switch with a relay to take the phase converter be automatic start.

If you use a pony motor to start the 3 phase rotary motor you don't need any capacitors or a push button switch.

This rotary phase converter motor runs at 2/3 power until it is connected to another motor like a mill or lathe. When the machine is turned on then L3 of both motors generate power to each other and both motors will then run at full power. When your machine is turned OFF the rotary motor continues to run a 2/3 power. If you connect your mill, lathe, and other machines all to this rotary then you can turn on any machine you want L3 will always generator power to L3 of the other motors and both motors run at full power. You can also turn on several motors like the Lathe and Mill at the same time all 3 motors generate power to each other through L3.

Your rotary motor needs to be sized close to the largest motor in the system. In my home shop I have a 1.5 HP bridgeport mill, 2 HP surface grinder, 1.5 HP lathe and a 1.5 HP rotary motor phase converter. I start the rotary motor then I can run any machine in the shop even the larger 2 HP surface grinder. The 2 HP surface grinder works good most of the time but some times it is a little short of power so all I have to do is turn on the Mill or the Lathe and just let it run and not use it. The extra motor in the circuit generates the extra power the 2 hp surface grinder needs. If I were to us a 2 HP rotary motor the surface grinder would always have all the power it needs but running a 1.5 hp mill on a 2 hp phase converter all day would waste a lot of electricity being over sized by 1/2 hp. There are several crazy tricks to make this work I could have 2 phase converter motors one being 1.5 hp and the other being 1/2 hp. I could use the 1.5 hp motor on the lathe and mill then if I need to run the 2 hp grinder I flip a switch that connects the 1.5 and 1/2 together this will equal 2 hp for the rotary. 2 rotary motors running in parallel add HP. If you need a 10 hp rotary phase converter and all you have is a 5 hp, 3 ph and a couple of 1 hp motors connect all 4 motors together and you get 10 hp. You could even connect TEN 1 hp motors together to make a 10 hp rotary converter. LOL.

I use to use a rotary phase converter but I removed it from my circuit. I use a static phase converter and my 1.5 hp bridgeport mill does fine running at 1 hp. I have no problem with my lathe either. My 2 hp surface grinder is the only problem and if I turn on the mill and let it run the 1.5 hp mill motor acts like the rotor for the 2 hp grinder and the grinder runs fine, no problems at all. It saves electricity not having that rotor motor running all day plus I don't have to listen to it run. The 2 hp motor is probably running at about 1.95 hp from the reading I get on my amp clamp.

Photos show an automatic start rotary phase converter.

http://home.earthlink.net/~gary350/phase01.jpg

http://home.earthlink.net/~gary350/phase03.jpg

http://home.earthlink.net/~gary350/phase04.jpg

http://home.earthlink.net/~gary350/phase05.jpg

gnm109
10-23-2011, 12:28 PM
For the record, the RPC idler should be somewhat larger than the motor to be operated on three phase. If you use an idler that's the same size as the driven motor, you will not have enough power due to the electrical issues mentioned above in other posts.

Furthermore, a larger RPC idler can be used later to power larger machines.

lakeside53
10-23-2011, 03:39 PM
Don't build your phase converter larger than the motor you want to run, it wastes electricity. It is best to use equal size motors. If you want to run a 1.5 HP Bridgeport mill use a 1.5 rotary motor.


>>>>>>>>>>>>>

There are several ways to build a phase converter. A static converter will make your equipment motor run at 2/3 it power rating. A rotary converter will many your equipment motor run at full power.

>>>>>>>>>>>>>>>>

The 2 hp motor is probably running at about 1.95 hp from the reading I get on my amp clamp.





There are many problems with what you posted, but I just snipped out 3.

1) You need an RPC motor 1.5 to 2X larger than the target motor to start anything but the lightest loaded machine. OK, you can "start" with a lower rated idler, but how long does it take the target to get up to speed, and what is it doing to the target motor in the ramp-up period? I have seen many cases where you cannot start a 5 hp target with even a 7hp converter because the motor starters pop due to overload caused by a sagging T3. Bypass the current trips, and it will likely get there in some time, but that's trashing your motor. A lathe in high gears is a classic test -it will start in lower gears, but not in the upper speed ranges. Another great test... in a higher gear, snap your lathe from fwd to reverse and see how the rpc handles it.

As for "electrical waste" your lights likely use more than the RPC at Idle... A 5hp idler uses maybe 200 watts. Sure, you'll measure a high input current, but that's at a power factor of 0.1 to 0.2.

2) A static converter does not "make your equipment run at 2/3...". Your target motor will quite happly draw as much power as you load it until it stalls (all motor are like this, whether driven from a static converter or otherwise). If you USE more then 2/3 of the rated power, you will overload and overheat the motor. And...2/3 is theoretical ; in practise it's closer to 50%. This is the problem with static converters - although they have their place, if the target motor was just sized for the intend use (most are), then it's very easy to overload it.

3) Measuring the current of a three phase motor to determine power output is fraught with problems. Even full power (how are you determining that - did you load the grinder to develop 2hp?) the power factor is maybe 0.8...

chipmaker4130
10-23-2011, 05:51 PM
While the experts are on tap, could someone please explain both 'power factor' and 'service factor'?
Thanks.

gnm109
10-23-2011, 06:35 PM
While the experts are on tap, could someone please explain both 'power factor' and 'service factor'?
Thanks.

I found Power Factor on Wikipedia. Check it out for explanatioin. I know longer do Wikipedia links.

Here's a nce short explanation of Service Factor. I sort of knew what it was but I this is better than my explanation.

http://www.engineeringtoolbox.com/service-factor-d_735.html

P.S. I'm no expert. :)

J Tiers
10-23-2011, 09:48 PM
Power factor is just a number that relates the "actual" power drawn to the "apparent" power drawn. It is the fraction of the "apparent power" that is actual power.

Actual" power is power that can do heating or other "actual" work.

Because a motor has an "inductive" character, current is not drawn exactly "in phase" with voltage. Current is drawn related to "how long" a voltage has been applied (mathematically there is some calculus involved).

As a result, maximum current flows somewhat later than at maximum voltage. And current is still flowing when voltage is zero.

In a resistor, voltage and current are exactly in-phase, the maximum of one is the maximum of the other. Current changes instantaneously when voltage changes. In an inductor, current takes some time to change after voltage changes.

Power is the product of voltage and current. A current of 1A at 100V is 100W, but a current of 1A at 10V is only 10W.

So, with a resistor, you can measure the current with one meter, and voltage with another, and get actual power. The "apparent power" is equal to the "actual power".

With an inductive load (or a capacitive load), when you do that same measurement, you get "apparent power", but it is NOT the same as the actual power, because the current is delayed, it has a "phase shift".

As a result, the voltage is lower when the current is flowing. The voltage you measure is not occurring at the same time as the current you measure. The "apparent power" is larger than the "actual" power. One has to multiply the "apparent power" by the cosine of the phase angle between currrent and voltage. That is what the power factor is.

In an unloaded motor, the inductive characteristic is the main effect, so the current is "phase shifted" quite a bit. The "power factor" or fraction of actual power, can be quite low, like 0.1 or 0.2. Only 10 or 20% of the power the motor *appears to be drawing* is actually being used.

As you load the motor, more actual power is drawn, and the "power factor" goes up. It can reach anywhere from 0.6 to 0.8, meaning that 60 to 80% of the "apparent power" is actually being used.

With a resistor, the power factor is 1.0, all the "apparent" power that you get by multiplying measured current by measured voltage is actually power being used.

chipmaker4130
10-23-2011, 10:47 PM
Thanks, Jay.
I think I actually followed that!

Thank you too, gnm109.
The S.F. link was very clear as well.
Gordon

bruto
10-24-2011, 12:09 AM
The simple one I did was based on the advice of a local motor rebuilder. His advice was, first, to run at least once size larger than the motor you need to power. The second thing he recommended was to use an old sleeve bearing motor, the older the better. At one time you could pick up old motors like this for a song, since they're obsolete and unwieldy, but a big old motor will be smooth and quiet and have lots of copper in it.

Abner
10-24-2011, 10:17 AM
As you guys have correctly decided my Idler was 2hp as was my driven. The VFD is sweet beyond words, but if I had to go bigger then the RPC would make the most sense. Used 3 ph motor seem readily available.

KEJR
10-24-2011, 08:34 PM
Putting a little time into balancing the output with capacitors will do a lot to reduce all of those problems.


This is what I did and it both starts itself and is well balanced just on a motor and some capacitors. The capacitors also force it to start in the same direction all the time.

I think it depends on your motor, mine would self start from 240 single phase so I just need capacitors to balance things and influence start direction. No relays, no startup motors, its simple. I'd try your motor on single phase as a quick test to see what it does.

Be safe, these are scary voltages.

KEJR

dvo
10-24-2011, 10:59 PM
Which potential relay should I look for or what spec do I need to look at? Will universal potential relay work?

gnm109
10-24-2011, 11:16 PM
Which potential relay should I look for or what spec do I need to look at? Will universal potential relay work?


Steveco 90-66 is good for 240 VAC usage. They pick up at 208-239 V and drop out at 90, normally closed. I used one and it's proven to be completely reliable.

They are now built by White-Rodgers. They are pricey, however.

J Tiers
10-25-2011, 12:35 AM
For what it is worth, the ARCO brand RPC which I have does not use a relay of any sort.

It has a double-conductor rotor, and a single starting/compensating capacitor.

There are thinner (higher resistance) copper conductors in shallow slots on the rotor, which respond to the high slip frequency at start, and whip it up to speed fast.

And there are thicker regular aluminum conductors buried deeper, that come into play when the slip frequency is low (rotor is near synchronous speed).

This is a standard technique in the induction motor world for getting good starting as a somewhat separate design consideration from the run characteristics.

darryl
10-25-2011, 02:17 AM
I just bought (locally) what they called a 'contactor' to replace the aging one in our RPC. Call it a relay, call it a potential relay, a contactor, whatever- it's got a coil that activates a pair of contacts. That, plus 12 ft of high temperature wire and a dozen spade connectors, plus taxes- less than $20.

Replaced burnt wires and connectors, replaced the 'device', all is well. This is a 20 hp RPC.

The burnt wires- the guy who built it used 14 ga wire to wire up all the caps, but he daisy-chained them. The result is that the first wire in the chain carries all the current shuttled through the capacitors. Each bank was wired like that. I think we were lucky that it didn't go on fire. Better would have been to make a local connector block which would fan out to all capacitors in each bank, and run a heavy wire from each block to the appropriate winding.

JoeLee
10-25-2011, 08:43 AM
VFD's are an alternate method. I used an RPC because I have a variable speed mill and I can fix any part of the RPC.


I know that VFD's are popiular, though.

What does having a variable speed mill have to do with the type of converter?? RPC or VFD??

JL.............

gnm109
10-25-2011, 09:41 AM
What does having a variable speed mill have to do with the type of converter?? RPC or VFD??

JL.............


The VFD will control speed through variable frequency fed into the 3 phase induction motor.

The RPC provides 3 phase power to my variable speed mill which has a mechanical range of 60 to 4,000 rpm that is built-in.

So as long as I can change speeds im the machine itself, I can use my RPC to provide power.

Some people will like the convenience of a VFD even though they have a variable speed mill since changing speeds is very easy - just turn a potentiometer.

Others, like myself, like to be able to tinker and build their own phase converter, although changing speeds on my mill is not all that difficult, just turn a knob. Sometimes you have to change ranges to the back gear but that's not difficult.

I like the fact that with an RPC my motor is running at it's ordinary (near synchronous) speed all the time and is getting full cooling through its fan at all times. It must work since the mill is a 1987 model and when I had the motor off two years ago to clean and inspect it, it looked like new.

That's good because a new one would cost $900. :)

JoeLee
10-25-2011, 09:57 AM
gnm109, reason I asked is I have always run my BP mill off a static converter that I built...... it was one of those little kit deals. It's run the mill fine for years but the motor is noisy. I replaced the bearings and put new bushings on the sheaves etc. and it still just doesn't run as smooth and quiet as others I've seen, it's not the quill or anything in between. I've been thinking it's an unbalanced voltage related issue. I have a VFD but never tried running the mill off of it. I might just try it today.

JL......................

gnm109
10-25-2011, 10:06 AM
gnm109, reason I asked is I have always run my BP mill off a static converter that I built...... it was one of those little kit deals. It's run the mill fine for years but the motor is noisy. I replaced the bearings and put new bushings on the sheaves etc. and it still just doesn't run as smooth and quiet as others I've seen, it's not the quill or anything in between. I've been thinking it's an unbalanced voltage related issue. I have a VFD but never tried running the mill off of it. I might just try it today.

JL......................


If you already have a VFD then it would be an excellent idea to install it. I understand that they are an excellent concept. I'd like to try one myself but I don't want to spend the money on one right now. I've been spending my extra cash on tooling and so on.

If you have an oscilloscope or access to one, you could look at the output of the static converter. It's possible that the power is not all that clean.

lakeside53
10-25-2011, 11:34 AM
Static converters are missing one phase completely. The rotor/stator are designed for all three. That makes the motor vibrate or be noisy in exactly the same way as an rpc, which is why rpc idler motors are generally mounted on rubber feet - not bolted down hard. Bearings in motors on static converters have a lower life.

TexasTurnado
10-25-2011, 03:27 PM
I read through this thread and did not see where anyone mentioned the alternative design offered by Eurotech - they use SCRs to switch capacitors in and out on the fly depending on load conditions. The less expensive model uses three SCRs and offers three levels of 'balancing', while a more expensive model has four SCRs and offers 7 levels - in either design one SCR is used for starting.

Without explanation of why, they specify the motor should be delta wound, rather than Y wound. There is a seller on ebay, "single2threephase", that sells the special controller and/or plans for building converters using their contoller.

I built one of their designs using a 15HP delta wound motor and their four SCR controller with very good results.

They claim the balance is good enough to run three phase VFDs, but I did not test that claim.

Below is a pic of the converter I built - the motor came on the wooden structure shown and I used that to hold all of the parts. The large coils at the top are specified by Eurotech and are there to limit the surge currects when the SCRs switch under load.

http://i288.photobucket.com/albums/ll168/TexasTurnado/P3200037.jpg

This pic shows their controller:

http://i288.photobucket.com/albums/ll168/TexasTurnado/P3220044.jpg

I sized the unit to fit under the workbench shown.

gnm109
10-25-2011, 03:50 PM
You have done some beautiful work there. I can see the benefit of switching capacitors in and out to compensate for varying loads. The other side of that is the increased complexity.

In my case, I'm only running one 3 hp motor and that's probably the extent of my 3 phase work. I've got a very nice setup that's fairly simple and has been running nicely for two years now.

I do think that a good quality motor makes a difference in how much balancing is needed but I have no scientific proof of that. I was lucky and scored a NOS 5 hp Boston gear motor on eBay. I guess no one else wanted it because it was a flange mount. I got it cheap and built my own mount.

Nice job!