Author Topic: another synchronous conversion.  (Read 3764 times)

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joestue

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another synchronous conversion.
« on: October 10, 2013, 04:51:09 AM »
I put this under hydro because that is the application.


Photo of the rotor: http://johansense.com/bulk/synchronous_mod7.JPG
I will replace that photo with an updated shot, i added two more coils to it.

I got rather lucky with rather low cogging, but it could have used a bit more math (i didn't do any) to find the optimum location for coil placement.
In any case the voltages and current you see in that chart correspond to WYE line to line voltage across a single 240 volt winding.
The frequency is 14.91Hz as the machine is driven by an 8 pole induction motor.
So multiply the voltage in that chart by 4 to find nominal 3600 rpm voltage.
I'm actually quite impressed that the saturation point seems to be nominal "rated" volts per hz.

some background on the motor:
It is a marathon 2 hp 3450 rpm 3 phase 240/480 volt 1.15 service factor.

The winding stack is 3.00 inches high, the diameter of the rotor is 3.000 inches, and there is about .75 inches of "back iron" behind the 24 slot winding.
It is wound with 20 gauge wire and IIRC 1.8 ohms per winding. so that's 3.6 phase to phase for wye, and a single 240 volt segment.
480 volt wye would be 7.2 ohms phase to phase, 240wye is 1.8 ohms
7.2 ohms comes out to about 420-440 turns, so that makes about 244 mm^2 copper per phase.
this means the peak flux at 240/480 volts is about 1.3-1.35T, which is the nominal knee for cheap E core transformer steel as well.
The winding arrangement is the worst possible.
If you go here:
https://www.emetor.com/edit/windings/
select 2 poles, 24 slots and the worst possible winding arrangement, the nominal winding factor is .83, and click on show winding layout and you will see how its done.

The rotor's main coil is 37 ohms of 26 awg wire and that's about 742 feet and that's about the number of turns. the two lesser coils are at about 150 electrical degrees (i think) and are both 10 ohms, so the total is 57 turns.. works out to about 1200 turns.
1200 turns is about 181 mm^2 of copper.

now that I've thought about this it makes sense to unwind the tertiary coils and mill the slots wider.. but its not worth the effort.
the main coil could also be milled wider as well, as i only have 75% of the copper area of the stator, on the rotor.

but at "nominal" volts per hz is .701 amps on the stator, that is 28 watts lost for magnetization.
so that's 5.1 amps per mm^2.

I don't know the relationship between stability angle and additional stator amps but i think its simple to see that at 1 amp of rotor current we have a surplus of about  400 amp turns to drive the stator coils..
which works out to about one amp at 480 volts/60hz.

480 volts, 480 watts
57 lost in the rotor
20? lost in the stator iron
7.2 lost in the stator copper
bearings? windage?
Probably on the order of 80% efficiency, at 3600 rpm and 480 volt output.
My wife says I'm not just a different colored rubik's cube, i am a rubik's knot in a cage.

oztules

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Re: another synchronous conversion.
« Reply #1 on: October 10, 2013, 05:27:01 PM »
I like this solution better than the magnet solution. It allows for better flexibility, and load matching.

Nice to see one done.

.....oztules
Flinders Island Australia

joestue

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Re: another synchronous conversion.
« Reply #2 on: October 10, 2013, 08:32:01 PM »
http://johansense.com/bulk/synchronous_mod9.JPG
http://johansense.com/bulk/synchronous_mod7.JPG

I've run it up to ~3800 rpm just briefly, seems to have an out of balance problem that shows up around 3500 rpm.
The magnet wire has an adhesive coating that sets in at 100C plus to form a rock solid winding. I have about 70 pounds of it left.
Thing is I read somewhere that it has to be activated by MEK or something. There are several coats of lacquer on it, and I accidentally left it hooked up to 120vac rectified. so it made it all the way to 140C (calculated by the resistance rise) before i gave myself a half inch square blister attempting to pick it up.
btw: that is indeed a brass plate that is fitted in the slots cut to hold back the main winding. I took my single point thread mill and ran it down the sides of the slot. the other ones have plastic shims from some other small motor that just happened to fit with a good 60 degree arc to them, so i stuffed two of them in each slot.

I cannot perceive or accurately measure bearing loss and windage losses at 3500 rpm.

At 600G forces.. i'm wondering if that brass shim can hold it back.. that's about 8 kilograms per cm of distance.. i think it needs to be steel.. rather surprised it didn't blow up already, but the copper is rock solid now. hmm...
My wife says I'm not just a different colored rubik's cube, i am a rubik's knot in a cage.

joestue

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Re: another synchronous conversion.
« Reply #3 on: October 10, 2013, 11:27:38 PM »
it seems to have a static .250 foot pounds required regardless of rpm.. increasing from .25 to .268 foot pounds from 1800 rpm to 3600 rpm.
but this might have something to do with the strain gauge.

That's 136 watts lost at 3600 rpm. much more than i expected. there is also a 20% ripple in the field current, which will soon be fixed.

cogging torque at nominal 2 volts per hz is about .55 foot pounds.
My wife says I'm not just a different colored rubik's cube, i am a rubik's knot in a cage.

joestue

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Re: another synchronous conversion.
« Reply #4 on: October 11, 2013, 06:32:24 PM »
seems the true saturation point is actually at 2/3rds rated volts per hz. about the point where the two slopes of that graph converge.
I have not yet collected no load iron loss data.

Running at 2400 rpm I was able to get about 80% efficiency not including the rotor field watts at 500-550 watt load.
the motor was connected wye and the field set up to deliver 120vdc into a 500 watt halogen bulb.
when only half the stator coils were connected, efficiency dropped to 67%. lots of copper loss at 6 amps line current through 1.8 ohm coils.
--not including field losses.
-----------
this photo was line to line voltage and current for 3.7 amps of dc current after the rectifier and inductor, and 95-97 volts dc into the light bulb.
yellow is line to line voltage, blue is ac line current. blue is 1 amp per division.

---------------

more accurate numbers for the following example:
field current and voltage was 47.44 volts, .716 amps and that makes 33 watts.
motor was spinning at 2440 rpm
load was 97.8 volts and 3.77 amps for 368 watts.
input torque was 1.26 foot pounds or 438 watts.
84% efficiency ignoring field losses.
78% including field losses.

turning off the load will launch the volts from 98 volts to 240 vdc, no load iron loss is around .224 ft pounds or 70-80 watts for 240vdc output no load at 2400-2450 rpm.
« Last Edit: October 11, 2013, 06:37:48 PM by joestue »
My wife says I'm not just a different colored rubik's cube, i am a rubik's knot in a cage.

joestue

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Re: another synchronous conversion.
« Reply #5 on: October 11, 2013, 06:53:02 PM »
prior oscope shot is line to line.

here is line to neutral for very similar conditions as the above mentioned test data at the end of the prior post.
line current isn't 100% accurate, the current transformer isn't designed for 40Hz
My wife says I'm not just a different colored rubik's cube, i am a rubik's knot in a cage.