Estimating Power from Induction Conversion

[Warrior]

Hello fellow DIYers. I have been working on a single phase induction conversion.

The motor is about 1/2 HP (sorry no nameplate) 220 volts. I have separated all 4 run windings and connected them in parallel for a total winding resistance of 0.8 ohms.

I can also series/parallel the windings for higher voltage @ 1.5 ohm total resistance (one pair in series, then parallel).

Testing the generator on the lathe, all windings parallel I get 28 volts DC open circuit at 1000 rpm. That would give a theoretical cut-in for 12 volts at 450 rpm, but it starts charging 0.4 amps at about 335 rpm at 9 volts DC open voltage (not sure why).

At 1000 rpms, it puts about 9 amps @ 15 volts into the battery.

If I use the formula amps = (Open Volts-12.6 v)/winding resistance it should give a theoretical current of about 19 amps.

Using the high voltage connection made about the same current.

So I assume impedance is limiting the output. How do I calculate the real current I will get taking impedance into consideration? Is there some formula I could use?

Thanks,

Warrior

[Warrior]

Correction:

I can also series/parallel the windings for higher voltage @ 1.5 ohm total resistance (one pair in series, then parallel).

Should read 3 ohms series/parallel, each winding is 3 ohms...

[stephent]

Cuz the voltages out of each phase winding  are out of phase kinda?

Only 2 of the windings (across from each other) can be seriesed with each other.

Other 2 can be seriesed and then both sets paralleled--but they are 180 deg out from each other--recitfy coil sets seperately then add the sets together.

[RP]

Actually they're 90 degree out of phase but yes Stephent is correct.

You have two kinds of windings in there.  The "run" windings are thicker wire and will have a lower resistance.  The "start" windings are thinner but there are ususally more turns with those.

Connect all the run windings together (series or parallel) and recitify these.  Then connect the start windings together and rectify those with a seperate rectifier.  Then parallel the output of the two rectifiers.

[Warrior]

Maybe they are out of phase??? i'm not using the start winding cause they are toasted.

I unsoldered all coils from each other and put leads on them. These were all brought out of the motor. To connect them in parallel, I took two leads from one coil and paralleled with two leads from an other, and so forth for the other two.

If I picked the wrong coils the magnet rotor would be very hard to turn (coils fighting each other). Reversing one coil connectionsolved the problem...but then again maybe I have it all wrong.

Would the voltage add up from 2 coils in series even if they were 90 degrees out of phase?

[Flux]

Slotted iron cored machines are normally reactance limited. There design is complex and without a lot of background knowledge that would have to be determined from tests on similar machines you might as well completely forget any of the equations I or Hugh or anyone else has posted here to predict the output of air gap machines, it just will not hold up.

If the start winding is burnt in your motor, remove it before it causes you trouble. You can rewind it and rectify it separately to add to the output if you wish but there will not be enough room to make it a true balanced 2 phase winding.

Most of the spectacular output that people get from these motor conversions is derived from the fact that they accept severe overload, they are generally not more efficient and often the increased output has to come from a considerably higher prop speed.

Generally you can get more power for the same volume of magnet but to keep the same speed and efficiency the margin becomes much less than you would expect. The single phase case is particularly disappointing.

Flux

[stephent]

The starting to charge at 335 rpm (9volts @ .4 amps reading) is probably cuz your meter is reading DC volts and not reading the actual AC peak.

Single phase motors are less efficient of space (motor size occupied and your time involved vs 3 phase) and mags but definitely do-able.

We can leave the wire efficiency in single vs 3 phase alone outside of those who know how to bust Ohm's Law.

Since you have it "done"--go_for_it!

[Warrior]

Guys thanks for the response. I really meant to say reactance limited not impedance.

So according to Flux, there's no simple way or formula to determin when the reactance limit will kick in.

I also agree that the high output figures from conversions is due to them withstanding overload because of better heat dissipation.

I haven't built an axial flux machine yet. I have machining capabilities at home and find the motor conversion much simpler, rugged and reliable.

Although I have to recognize that no motor conversion within reasonable size can make the same power as an axial at the same rpm. A well designed axial will make about 500 watts at 280 rpm.

I haven't seen any motor conversion in the 2-3 HP size make this much at that rpm.

Warrior

[dinges]

Is determination of the reactance limiting not just a matter of measuring the inductance (with a suitable meter), and plugging the value in the equation Xl = 2 * pi * f * L  ?

[Flux]

No, sorry it is not that simple.

You are looking at leakage reactance not the actual winding reactance.

The only method easily available is to plot the output against speed and see where the curve starts to level off. This takes place when Xl = R but it will be difficult to choose the correct point.

You can get a reasonable figure for the synchronous impedance but this also includes the effect of armature reaction ( Probably not that significant with neo and saturated teeth)

Flux

[dinges]

Thanks. Interesting.

Not sure what the difference is between 'leakage reactance' and 'actual winding reactance', but I already thought the rotor core would have an influence too.

As far as the flattening of the curve goes; I have observed this in the past as I was measuring the output of a 130 W conversion (which actually got to put out 130 W in a battery bank):

http://www.anotherpower.com/gallery/dinges/130W_genny_graph_output_001

(the graph is derived from this measured data:

http://www.anotherpower.com/gallery/dinges/130W_genny_table_output)

I thought that the flattening was caused by the batteries, but it most likely was reactance limiting, in hindsight.

[stephent]

And if you haven't taken out the "toasted" start windings --do so.

They can look like a shorted winding, and probably are.

[Warrior]

Regarding the start winding, yes it's pretty dark, but until it's last day as a motor, it worked OK. I don't have it connected as part of the conversion, it's just sitting it the slots useless.

Tks,

Warrior