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3 hp motor conversion
By windy, Section Wind Posted on Sat Jun 03, 2006 at 04:05:36 AM MST
Testing of motor conversion

Finally got my 3hp 3phase motor back together again and it seems to work. There is very little cogging and I can turn it with two fingers. There is a slight tug-tug-tug as I rotate the shaft but if I turn it fast there is no cogging.
In the 1 wye connection I can get a voltage reading of 50 volts with a quick turn of the shaft, and can get a 100 watt light bulb to glow. It does the same on the other phases, so it should all work. I will be testing it at a constant rpm from 200 to 800 rpm's using the 4 different wiring combinations and will post the results.
I plan on using this generator for hot water heating. My question is, when testing the generator do I need equal loads on each phase or can I test it using one phase only. I have a 240 volt,1500 watt, and a 4500 watt water heater element that I plan to use for testing.
Another question I have is, what would be the maximum RPM of a 12 to 14 foot diameter blade before it starts flying apart? That's the diameter that I'm thinking of using.
Motor specs.
3hp 3phase
1750 rpm
8.6 amps at 240 volt
4 pole
12-.75 inch diameter .375 inch thick Grade 45 neos per pole with 10 degree offset
One more question then I'll be finished.
When testing how many amps can I safely go to before I start overloading the motor. The motor before conversion drew 2064 watts per phase so does this mean I can load it to this level or isn't this possible?
Thanks for any comments and will keep you posted on my progress!
windy

3 hp motor conversion | 9 comments (9 topical)

Re: 3 hp motor conversion (3.00 / 0) (#1)
by zubbly on Sat Jun 03, 2006 at 04:51:21 AM MST

hi windy!

sounds like it is going to work good.

first i would check your rpm/volt unloaded to see what you will have.
next, i would not run the elements from any of the 2 wire phases. you would be loading just 2 of the phases in star connection and just one phase in delta connection. either connection is just putting needless excess load on it.

unless you have three elements which can be connected in star or delta to equally load the genny, i would rectify the 3 phases and make dc. then you could feed the output from rectifier to the elements and keep the loading equal on all 3 of the phases in the genny. using dc voltage on single phases elements shouldn't make any difference to them.

keep in mind that if you load the genny with one kilowatt of load, your driving system will have to be at least that strong. i think water heating elements have to be in water in order to load them to their full potential or they burn out within seconds.
electric cook stove elements might be a good alternative for testing or some old electric baseboard heaters.

you may well get some heating from the genny on the test bench. as long as you can stand keeping your hand on the outer frame you will be ok. once it is up and running, you will have considerable air from the prop to cool it. you will be able to exceed the original amperage rating on the name plate of the motor considerably.

my 1.5hp conversion has the same amount and size of mags as your. 48 of the 3/4 round by 3/8 thick. i wound it for 12 pole, 4 mags per pole. if i remember correctly, the magnet wire ended up a little bigger than original. it often does 40-50 amp in good winds. a few occasions i have had a little more than 70 amp from it. i think it is in the 2 delta connection (can't remember, been a while since i did the connection on it). but if you will be using elements and no batterys, your gen will never see a high loaded amperage rating like that.

your testing results should give you a better idea of what size prop you will need. my current 7.5hp conversion will first get a 12 foot prop for starters. yes it sounds small, but it is also a bit different design and am not sure what to expect from it. i may end up extending it out to 14 foot or just make a larger what ever is needed. the connection you use will also have a great effect on the prop and its running speed.

i used to run an eleven foot on my 1.5hp conversion. caught it doing just over 700 rpm one time (that was damn scary) lol, i now have an eight foot on it with a star/delta switch which works well and slowed my heart down a bit :).

i think i would like to see the 12 foot prop do no more than approx 450 rpm on the 7.5hp conversion. i use steel plates and bolts to hold mine together. i wouldn't trust wood screws on a prop that size to stay together (just my opinion). if it did hit 600 rpm, i wouldn't worry about it either.

once you have some bench test results, post them here and let some of the very experienced people help you to match up a prop with your load. props are really not a strong point of mine in understanding (bet you guessed that already) lol!.

good luck and let us know what you find,

zubbly

Re: 3 hp motor conversion (3.00 / 0) (#2)
by Nando on Sat Jun 03, 2006 at 08:22:08 AM MST

Zubbly:

Question, would not be better to report watts instead of amps ?.

In your case, 40-50 amps -- does not tell me a real output -- for 12, 24, 48, 120 volts output ???

So what is the REAL & TRUE output ?? 480-600, 960-12, 1920-2400, 4800-6000 watts ?.

Nando ;>}}.

[ Parent ]

Re: 3 hp motor conversion (3.00 / 0) (#3)
by TomW on Sat Jun 03, 2006 at 09:23:44 AM MST

Nando;

I tend to agree with you that watts is a preferred method of rating. No question.

However, as Zubbly and I both have noticed the amps from a conversion [at full output] seems to be constant.

For instance one of mine will do 10 amps and it doesn't seem to matter what voltage 12 or 24. It doesn't make sense but it is an observed phenomenon. Maybe Zubbly will explain further with some insight as to why.

My unproven private theory is the saturation in the laminates limits the amps that can be created but not the voltage increase. But I am not certain.

Just some observations.

Cheers.

TomW

The Truth is the Truth, even if no one believes it; and a lie is a lie even if everyone believes it

[ Parent ]

Re: 3 hp motor conversion (3.00 / 0) (#5)
by Ungrounded Lightning Rod on Sat Jun 03, 2006 at 12:56:40 PM MST

However, as Zubbly and I both have noticed the amps from a conversion [at full output] seems to be constant.

Good observation.

The max amps from a permanent-magnet alternator with a given coil connection ARE a constant. It's the current that would be needed for the coils to CREATE the magnetic field supplied by the permanent magnets (if they were replaced by a soft magnetic material of the same permiability).

It's easy to understand if you remeber that a genny is a transformer split in half, with the moving field from the primary replaced by moving magnets.

With a true transformer:
- Magnetization is input current times number of turns in the primary coil.
- Max output current is input current divided by the turns ratio (and derated by any flux leakage that lets flux from the primary bypass the secondary and by current "stolen" as eddy currents).
- (Output voltage is input voltage multiplied by the turns ratio (and derated by the flux leakage, resistive voltage drops in the windings, and energy dumped into core domain-flip hysteresis).)

With a permanent-magnet alternator the magnitude of the mag field is fixed by the magnets, and (except for losses) this fixes the maximum total current in the turns of your windings. Output current is this total current divided by turns per coil.

= = = = =

This has an interesting implication for PM alternator design: Heating is proportional to square of current. If your cooling is adequate to avoid damage from resistive heating at maximum current in ONE connection, it's adequate to handle it in ALL (balanced) connections and at ALL speeds. (Caveat: Hysteresis heating WILL increase - linearly - with speed. But air cooling will also increase with speed. Meanwhile resistive heating, including eddy currents, will plateau out.) Switching your connections (for instance: full Y <-> full delta to trade away voltage at higher RPM for increased output currnet) doesn't change the current distribution, and thus the current density, in your windings. (Note that a PARTIAL delta-wye switch: Inner delta, outer wye, WOULD redistribute current unevenly in the coils.)

Torque (loading the input shaft) is proportional to current. So once your genny reaches max current the torqe resisting the prop motion has reached maximum. So beyond that the genny has progressively less ability to keep the prop under control and you'll need to furl eventually to keep it from overspeeding. But if your design can handle the max current you don't need to worry about frying the genny under any operating conditions.

Good for motor conversions. A new engineering target for radial-flux designs.

Oh, BOY! An opportunity to improve the art!

[ Parent ]

Re: 3 hp motor conversion (3.00 / 0) (#6)
by zubbly on Sat Jun 03, 2006 at 04:27:25 PM MST

hi ungrounded lightning rod!

that is a very interesting and encouraging explanation. i had to read it about ten times to absorb it. still re- reading it (just the way i am-i have to keep reading over and over to absorb)

can you elaborate a little more on the "inner delta/outer Y", i don't follow you there.

how sure are you of your explanation?

just another observation. this winter during a wind storm (and cold outside) the genny had been running 50-60 amp for quite a while. wind came up more and starting hitting the low 70 amp on the meter. my thoughts were " its gonna burn". i went out to the tower, shorted the mill to stop it, climbed the tower and put my hand on the frame of the stator. it was cold.

yes the wind was cold also, but i really should have felt heat radiating from the stator.

thanks for your input on this. it has opened a very interesting can of worms LOL!

zubbly

[ Parent ]

Re: 3 hp motor conversion (3.00 / 0) (#8)
by Ungrounded Lightning Rod on Mon Jun 05, 2006 at 08:53:34 PM MST

can you elaborate a little more on the "inner delta/outer Y", i don't follow you there.

Imagine a dual-voltage motor / generator (two coils for each phase, hook in series for high voltage, parallel for low).

Hook it in Y.

Now switch the inner coils of the wye to delta and leave the outer coils hanging from their original connecion to their partners. (Picture an equalaterial triangle with each side having a spur extending, say, clockwise, by the length of the triangle's sides.) Outputs are the hot end of the outer coils.

Another way to do it (getting a different voltage) is again to reconnect the inner coils of the series-wye into delta, but then rearrange the outer coils by moving their connection to the inner delta to the terminal that doesn't connect to their partner coil. (Picture an equalateral triangle with three more equalateral triangles sharing one side each with it. Then on these outer trangles erase one outer side, ending up with a three-sided swastika-like affair with the inner lines being a triangle rather than a wye. Sort of a triangular spinning firework.) Again outputs are the hot ends of the outer coils.

These connections do a current drop in the inner coils like a delta-wye conversion, but leave full current in the outer coils. The inner coils lose a lot of self-heating by sharing current and thus keeping the peak current low. The outer coils don't get a lower self-heating benefit at all - though if they're two-in-hand or shared-slot with their partners they get SOME benefit from their partners' reduced heating.

I'll work out the exact voltages later. In each you get a voltage intermediate between a delta and a wye connection of the series pairs of coils. But because of the current imbalance you get little, if any, of the delta's increased current handling capability. So the connections end up with a less efficient machine. They would appear to have little utility (beyond possibly giving you an additional voltage choice if you're willing to accept a higher percentage of power lost to heating.)

[ Parent ]

Re: 3 hp motor conversion (3.00 / 0) (#9)
by Ungrounded Lightning Rod on Mon Jun 05, 2006 at 08:55:17 PM MST

Hook it in Y.

That is: Hook it for high voltage (phase-partner coils in series) and hook the series-connected coils in Y.

[ Parent ]

Re: 3 hp motor conversion (3.00 / 0) (#4)
by zubbly on Sat Jun 03, 2006 at 11:57:48 AM MST

hi Nando,

good point. sometimes i am just talkin to myself and forget the reply is for others to understand.

i have run this 1.5hp mill both connected to 12 and 24 volt batt system and it seems to be limited to low 70 amp, even at the same connection of the genny.

there may be one or more reasons for it, but i have not put my finger on it exactly. actually, i have not even put that much effort into thinking about it. so this mill seems to be limited to 1kw coupled with a 12 volt batt system and 2kw conected to a 24 volt batt system.

i sometimes tend to think in output amps (reguardless of batt volt) and not on watt rating as my thoughts are centered on whether the winding will stand up to it or not.

10 amps on a 12 volt system is no more stress on the winding than a 10 amp load at a 48 volt system (given the same size magnet wire).

tomw as he mentioned seems to have noticed this also.

glad you brought this question up Nando. for now, all i can say is that conversions seem to be current limited rather that wattage limited. as time goes on with more units being made by myself and others, we will see if the same thing seems to apply to them also.

zubbly

[ Parent ]

Re: 3 hp motor conversion (3.00 / 0) (#7)
by tecker on Sat Jun 03, 2006 at 04:45:18 PM MST

The Transformer comparison is a good one .If you consider the alternator windings as exited this makes the power available somewhat a function of the wire size . That's a little redundant there I know
but to continue the magnet will saturate the core and with a big load down line would be more useful to measure VA (apparent power ) than watts (True power) just like a transformer. Given the load is at the cosign ( other end ) of a work function and is dictating the power resolved and the copper on the stator is large enough to provide that push the load is the true power item we want to overcome.
So to simplify It's important to see your loads in watts so you can get a device with enough VA to
cover those .

3 hp motor conversion | 9 comments (9 topical)

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