Single Phase Converted Motor Works Under Load

[CraigCarmichael]

I ran my mini sawmill for the first time Aug 1, and have now cut a few boards from logs.

I'm pleased to report that the motor, converted from 3-phase to single as per my previous diary entries, seems to run very well under load. I can't tell if it actually has its original rated 7.5 HP or not, but it sure has way more oompf than my ~2 HP radial arm saw! Doing a 3 or 4 inch deep cut through a piece of relatively soft hardwood with a 12" blade I could push it along at a good speed and it didn't seem to take much notice of it. Today it cut a few 2"x2"s from an ornamental plum log with the 16" blade with no hesitation.

I ended up using a single 33uF run capacitor in the "start" winding. I'm using the starting technique someone suggested earlier which I pooh-poohed at the time: starting it spinning with a rope and then turning the power on. Ugh! It works... though just barely with the 16" blade, with about 15 feet of rope wound around the shaft, and a hard, steady pull, then a quick run back to the switch to flip it on.

I have stalled the motor twice. Once was when I had started it backwards by mistake and it probably wanted to stop anyway. It was actually cutting - but the blade was smoking, and I couldn't figure out why the carriage wanted to pull me along! Then once when a lever catch (that I knew was too weak and was about to replace, but a neighbor wanted to see it run) bent and the 16" blade jammed. Not before it flung the small log  aside, cutting right across the end of it an inch or more deep, in a fraction of a second. Luckily, no injury or damage resulted.

The couple of the times when I was cutting and it was running a few minutes, I remembered to check the heat as it spun down. There wasn't any I could feel. The blade was cold when I checked shortly after stopping, too.

So, the motor runs great - the mill needs more work. (So much for it being my "June project". August, and it's still holding up my wave power generator "July project"! Hah! Don't have me do your job labor estimates!)

Cheers,

Craig

[Flux]

If you can mount the motor on a hinge so you can slacken the belt you may be able to start it off load, then tighten the belt.

Flux

[Nando]

CRAIG:

Your 3 phase needs higher capacitor values and in a sense you need the same values that are used to convert the motor to an induction generator

Send to me the motor name plate parameters and I calculate the capacitor values for C & 2C arrangement to obtain maximum power.

Include your area GRID frequency and voltage.

33 UF seems to be a too low value which places the motor at low Power Factor and low HP.

The non- driven phases need to be resonated at the working frequency

Nando

[CraigCarmichael]

Hi,

The blade is attached directly to the motor shaft. (This simplifies pivoting the blade from vertical to horizontal - just tilt the saw platform, motor and all.) I suspect self-starting with no load is probably easier with an 1800 RPM motor than with a 3600.

I did convert the motor to single phase when I rewound it (see my previous diary entries), so any calculations by formulae for 3-phase would probably be off. With the 33uF, which I agree Seems small, I don't get the capacitive braking and heating that seems apparent with higher values.

Nevertheless, I think I'm getting somewhere around the original rated 7.5 HP, and I'm glad. Today I did some 3 inch deep cuts through some Cambodian Rosewood (read: Really Hard wood!) and it didn't seem to faze it a bit. I think with 5 HP I'd have noticed it lugging and had to feed slower. (My 2 HP radial arm saw takes forever to go through one short board, if it does it at all.)

Cheers,

Craig

[CraigCarmichael]

One further (final?) comment on my results:

It's really hard to start the motor with a rope with the 16" blade on it. I fail more often than I succeed. Too much inertia - since E=MV^2, it takes twice the speed and four times as much energy to get a 3600 RPM motor going as an 1800. Too hard with the extra M of the blade - the 16", being also wider, must weigh 4 times as much as the 12" and much of that is farther from the center!

So today I got a ~175 uF start capacitor, a bushbutton and a box for them, and wired it in. The result: the thing starts to spin up so slowly (with the 16" blade on) that the breaker trips.

So the technique now (100% successful on several tries so far) is to spin it with the rope, then run back to it and flip the switch, pressing the button until it gets going, which takes much less time as it's already spinning.

Conclusion: A single phase 7.5 HP motor pushes a 30 amp, 230 volt dryer circuit to the limit... and beyond for starting! But it sure cuts wood!

--Craig

[oztules]

Craig,

I would expect to use the 175uf on a 1hp 240v motor. I think you need considerably more than that and you will probably have success... maybe 500 uf or more.

least.

It will draw massive current for a very short time, but should get by without blowing the 30A fuse, as it should get up to speed in less than a second.... hopefully...

I'm impressed with your success thus far, as it is something I would not have done.. (i built a phase converter instead to achieve the same thing, but it now uses 1000uf to start the 7k5 motor, and 150ufand a 30uf to run it as a rotary converter. It starts on a 25 amp breaker no problems, and drives the mill or the 40 ton brake press... not both)

oztules

[CraigCarmichael]

Thanks Oztules,

I took the blade off and tried it (with the 175uF + the 33uF run cap). It started up, but it hesitated for the better part of a second before it spun up. With that sort of capacitance made up of several 'run' capacitors in parallel in my earlier tests there was no such hesitation. So maybe 'start' capacitors aren't quite the equivalent of 'run' capacitors.

But that's a side issue. That 16" blade is heavy, but a much bigger cap is definitely worth a try!

--Craig

[CraigCarmichael]

Yah, that works!

With ~325 uF, the 12" blade spins up in - what was it again? - a couple of seconds or less.

With ~500uF, the 12" blade spins up in about 1/2 second and the scary 16" in about 2 seconds.

With ~600uF, the 16" blade (weighs 6 pounds) takes about 1-1/2 seconds.

That was made up with 5 smaller (used) caps borrowed from my friendly neighborhood motor repair shop, and I ran out of aligator clip leads. I couldn't find any such big caps with 300+ VAC rating, so I bought a new electrical box (8"x8"x4") big enough to hold the 3 largest (the ~500uF combo), the run capacitor and the button, and bolted it to the motor. (It appears to be a heavy automotive button -- I couldn't find a heavy duty pushbutton at an electrical supply.)

I might never have thought of trying such a large capacitance, but (especially with my bad back) pushing a button sure beats pulling hard with a rope. which also seems undignified! Thanks again Oztules and all who've posted!

--Craig

[dinges]

Oztules,

I was going to ask you how you could calculate the value of those caps. Did a quick google search (lots of rubbish, booksellers, etc.), but, then this turned up:

http://www.utterpower.com/powerfactor.htm

On a fieldlines-related website, of all places.

May be interesting for anyone else dealing with capacitors.

[dinges]

I'm not really sure this is what you're doing, but maybe it is. It states a rule of 20uF/HP for running caps, and 80uF/HP for start caps.

http://abana.org/downloads/education/threephase.pdf

[oztules]

Hi Peter,

I use method no.2.... suck it and see with the meter, and find the one which gives the best match.

This applies to the start cap as well.... gut feelings first and smooth out the wrinkles from there.

In my phase converter, when i start loads up directly (static converter duty), i need heaps of capacitance (1500uf),

If I only use the caps to start the idle motor in rotary converter status,(no load on it), 400uf or less is all that is required start it.  (1500uf in this instance will make the motor nearly jump off the bench it starts so fast) (7k5 watt motor)

.........oztules