Converting 3-faze motor to 1-faze

[CraigCarmichael]

My intended use is to power a 16" saw blade for a mini sawmill. I won't have 3 faze power available, and plan to plug it into a 240V, 30 amp dryer plugin. (No doubt it will be down to the rated 230V or less by the time it gets through the cable to the outdoors.)

The motor is a Baldor that had the following specs before a previous owner burnt it out with a seized air compressor:

Slots: 36

Rotor diameter: about 3-1/4"

Stator length: about 4-1/4"

RPM: 3450

HP: 7.5   DP

Volts: 208-230/460

Amps: 19-18/9

Efficiency: 85.5%

PF: 91%

Hz: 60

Phases: 3

Class: B

Service Factor: 1.15

40 degrees C ambient continuous

Cat#: M32-19T

Spec: 36B01-194

Frame: 184T

Windings: 6 coils per faze
 A: 3-16,2-17,1-18 (in series), and 36-19,35-20,34-21, with 22 windings per coil

 B: 4-27,5-26,6-25, and 7-24,8-23,9-22, like A

 C: 10-33,11-32,12-31, and 13-30,14-29,15-28

The pairs of 3 coils on each faze are connected in parallel for 240 V (66 winds per faze) and series for 480 V (132 winds per faze).

If it is of any significance, the wire throughout was 2 strands of #21 and 2 of #22, the 4 strands in parallel. I plan to use beefier wire with less strands. That should give the same amount of copper with less insulation volume in the slots, leaving a bit of elbow room, eg for starting windings. (I actually counted 88 wires in one coil, 87 in another (assumed I missed one or it was burned off), and divided by 4 to get 22 turns per coil)

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First, I'm not clear on how voltage is measured in 3 faze wye/star systems: if it's "230V", is that from each faze to common, or from one faze to another?

Then, I'm not sure how the 3-faze current is worked out. One can calculate what the single faze full load current should be as:

The motor power as watts = 7.5 HP * 746.7 W/HP = 5600 W

Watts = E*I*Efficiency, thus I = Watts/(E*Efficiency)
or in this case:
I = 5600/(230*.855) = 28.5 Amps

(Obviously no problem at all for a 30 amp dryer outlet! I'd have to use an extra 25' or so of cable to use the 40A kitchen stove outlet, and it's not very accessible.)

If one assumes the 3-faze rated current of 18 amps is multiplied by 1.5 for single faze, we get 27 amps, which is close but not quite right. (Closer than using square root of 2 or sine of 60 degrees, though!)

Let's just go with the 1.5 anyway. If we want 1.5 * the 3-faze current, do we use 2/3 of the windings between two fazes, ie 2/3 of 66*2 = 2/3 of 132 = 88 turns? That's probably wrong. How about 66 turns from one faze and half of each of the other 2 (33 turns) = 99 turns? That's probably wrong too. My gut feeling says to use about 110 turns. That's probably wrong too.

Then, are these turns to be distributed equally over the 18 coils? For 110 turns, that would be 6.11 turns per coil, if they're all in series. Or do we distribute them over, say, 9 or 12 coils, leaving slots for 9 or 6 free coils at 90/270 degrees just for the starting windings?

Then we get to the starting windings: how many, how thick wire, and do we put them over the main windings or use free slots?

Finally, the starting stuff - I presume a centrifugal switch at say 1200 RPM, but how big should the capacitor be in uFarads, and what other ratings does it need?

You'd think these things, while not trivial, aren't exactly rocket science. Surely, for example, there is a single ratio for the number of winds for single faze operation versus the number for 3 faze at a given voltage, current rating and frequency. But in the absence of any such general info, guidelines or formulas seeming to exist, it's no wonder the guys in the motor repair shops back off like it was a hive of bees when you start asking about modifiying a motor!

Maybe I knew some of this stuff after my power electrical course at BCIT in 1974, but somehow I can't seem to remember any of those sorts of details.

--Craig

[JW]

Hi Craig,

"my intended use is to power a 16'' saw mill, i wont have 3 phase power available, and plan to plug it in 2 a 240vac dryer outlet."

 although its less efficient than utiliy grade 3phase, id recommend checking into a 'phase convertor' thru a search on the www. this will allow you to run a 3phase motor from a 240vac source. heck you you might even find one used if you look hard enough. There generally pretty cheap. have fun.

JW

[zubbly]

hi Craig!

to convert a single phase motor to 3 phase is easy. you throw away the capacitors,centrifugal switch, stationary switch that is mounted in the end housing.

to convert from 3 phase to 1 phase is "very" difficult and honestly, not worth the attempt.  please save your effort and money. because the shaft was not machined internally inside the motor to accept a centrifugal mechanism, and the housing does not have a cast in place mount for the stationary switch, you would need to go to a very expensive modular electronic device made for this. approx $100 plus. usually those electronic devices are made to replace obsolete switches and mechanism.

also, to design a winding for a single phase motor is much more difficult than for 3 phase. the responce in winding is very critical for starting torque and hp rating. there is no formula to convert 3 phase windings to 1 phase.

just as an example, in the Baldor farm duty single phase motors (5 hp and 7.5hp) both in the same frame and same core, there is only one wire size difference and 1 turn difference in the start winding. not much difference in winding but a huge effect on outcome.

again, not worth the time or trouble to try this.

just my 2 cents,

zubbly

[Nando]

A SHORT NOTE:

One can run a 3 phase motor with a single phase power BUT a circuit needs to be implemented with a set of capacitors to attain the phase angle for the third ( free phase connection).

There are two methods, one using capacitors alone and the other using capacitors plus a transformer.

The capacitor values need to be calculated depending on the input voltage and the HP size.

Also, most of the time the motor needs to be started annually ( some wrap a cord around the shaft and apply the power pulling the cord to initiate the rotation).

Nando

[Flux]

I have to agree with Zubbly on this. Converting a motor to an alternator is fine, the performance may not be ideal but it will be the best you can do.

Messing about modifying motors is very hit and miss, these things have been designed very specifically using years of experience. Things such as numbers of cage bars on the rotor have been determined to be the best for the design case.

You may produce something that will run perfectly well, but the starting characteristics may be poor and if you hit a harmonic crawl then it may refuse to start even off load. In this case a saw is not a difficult load to start so you may be ok.

It is a gamble that may or not come off, if you have the wire and time, ok otherwise it may be a costly mistake.

At best its single phase rating will be well below the 3 phase and you may do no better that using an un modified 3 phase motor with phase shifting capacitors as Nando suggests.

If it is a fun experiment, go for it, otherwise think twice.

Flux

[oztules]

Well Craig, your now in a position to wind yourself an alternator for a wind gennie

I can't help with the conversion of three to single phase and having seen Zubbly's and Flux's response, it looks uphill from here.

However, you have saved enough information to rewind it back to three phase. (only 1 more coil set than single phase anyway. (run + start 2 sets))


If you rewind it to three phase you will be able to successfully run it from single phase as I do.


Around 1000uf for the start caps and around 50-100uf for the run cap in this config. Power out is usually limited to around 80% of three phase output.


Auto start is a simple matter of using an open cage 240v normally closed relay, that you can get at the return spring...... use the tension to set the turn on voltage.


If this relay is placed in parallel with the phantom phase, at start (T=0) Volts=0. use the N/C contacts to drive the phantom phase with the output of  the 1000uf caps. As the motor picks up speed, the voltage of the phantom phase increases, and at about 170v the coil in the relay overcomes the spring tension of the return spring and pulls the solenoid in, opening the N/C contact and open circuiting the start caps. A 50 - 100 uf cap is wired permanently to the phantom phase.


In this config it is completly automatic, and I use this to drive my milling machine, lathe and brake press..... 5kw.   Start up torque is very good, certainly spin up 16" blade in less than a second if my set up is anything to go by (probably 1/2 sec)


So if your keen to rewind it, rather than finding a big single phase motor (although s/hand three phase seem to be dirt cheap.... wouldn't bother rewinding that one) then running three phase motor off single phase is a good option.... one I use every day.


If you do decide on this course of action, "http://home.att.net/~waterfront-woods/Articles/phaseconverter.htm">http://home.att.net/~waterfront-woods/Articles/phaseconverter.htm

will walk you through it.


...........oztules

[CraigCarmichael]

Thanks everybody for the replies!

I haven't run across any cheap used motors in this size range around here, so my best option is still to rewind this one... I think.

It's starting to sound like I should probably rewind it as it was (except I won't bother with the 480 V option) and find some capacitors and a relay, or a good phase converter.

That at least saves figuring out a new configuration!

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Oztules, I had some questions about the capacitor/relay system, but they're mostly answered on the web site, thanks for that link! I guess the relay is a good sub for the centrifugal switch or electronic control.

But I still have a couple of questions.

First, you're suggesting 50-100 uF for run, which looks like "50 to 100". The web site shows two running caps (one to each line), which would suggest using "a 50 uF and a 100 uF". Did you mean that, or to have just one, 50 to 100 uF? Sorry to nitpick, but these are the sort of details that (AFAIK) can mean success of not.

Second, don't they need to be high current capacitors? I should think a lot of smaller electronics caps might burn out.

and a couple of comments:

I presume polarized capacitors are out, unless they're connected in series back to back. I would also comment that 240 VAC RMS has a peak voltage of 339 volts, so capacitors with a rating of 250 V would be rather underrated: 500 V rated would be better, again unless they're in series (and identical).

I see the available power is only 80%, or 6 HP instead of 7.5. I'll trust that's enough. I'd probably lose it by rewinding it to 1 faze anyway.

BTW, I think I'll pass on starting a 16" saw blade that will spin up in 1/2 second with a pull cord - sounds really scary!

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Or I can always look for the used faze converter, I suppose.

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BTW, my original idea was to find about 3 cheap used electric lawnmowers and connect the motors to the blade mandrel, probably with v-belts. 3 pole switch, 3 extension cords to different circuits around the house. 12 amps * 120 V = 1440 Watts = 1.9 HP; 1.9 * 3 = 5.7 HP. If they're only say 80% efficient, though, that would be 4.6 HP, which is doubless pretty light. I was willing to try it (can always add motor #4!), however, I've only found one cheap mower since I've been looking, and I wanted one for the lawn anyway.

-----

Cheers,

Craig

[oztules]

Craig,

this is probably too late, but the answers are thus:

"First, you're suggesting 50-100 uF for run, which looks like "50 to 100". The web site shows two running caps (one to each line), which would suggest using "a 50 uF and a 100 uF". Did you mean that, or to have just one, 50 to 100 uF? Sorry to nitpick, but these are the sort of details that (AFAIK) can mean success of not.

Second, don't they need to be high current capacitors? I should think a lot of smaller electronics caps might burn out."

Well I use the single capacitor for my rotary converter.... however, it uses  an idler motor and that uses the start caps and the single run cap. It drives various machines, and so each machine has seperate extra caps to fine tune the usage that they may do. (the mill has no more caps on the "larger" of the two caps mentioned in the conversion, but has a 20uf on the other phase now built into the machine proper, The brakepress uses an extra 50 across the larger one, and a 50uf onto the other phase... this makes it run sweet.

Most people don't seem to use the second caps (the smaller set).

Use run caps from other motors or get hold of new ones for motor run specifications. You can use dc caps back to back, but you may as well use the ones designed for this application. I use 600v ac types for motor run application. DO NOT use motor start caps  for the run caps...they will self destruct as they are designed for limited time/use (20 start cycles per hour, that sort of thing) try here if your in the USA http://www.capacitorindustries.com/excessinv.htm

I agree with Underground lightning rod.... stick with three phase if possible, it will work better than single phase. My lathe i converted to single phase 5hp... but would hardly start on the long extension cord. but with three phase conversion... no problem.

As a curiosity, I found the rotor on a three phase is identical to the singlephase... in case you wondered...

..............oztules

[CraigCarmichael]

Oops,

Just found this!

Thanks for the reply. Of course we have moved on, but it's good to know the rotor is the same!

--Craig