Alternator Testing

[SamoaPower]

I've been thinking ahead to the day when I finish the current machine and it's ready for testing. I would like to characterize the alternator as much as possible to really know what I have which will mean, of course, a pretty comprehensive testing program.

Part of this need is dictated by the electronic controls I intend to employ and the rest is mostly verification of the design features.

The main reason for this post is to pick some brains for ideas on methods and equipment.

With nearly 30lb of copper and 96 in^3 of magnets, it's not a small machine. It's a 16/12 axial flux design with a 24" stator and magnet rotors of about 20". The design target is 1kW of 12V battery charging at 125 rpm and an operating limit of less than 200 rpm. Testing will be needed up to the 2-3kW region.

Basically, I need a test stand, a prime mover, a variable load and appropriate instrumentation.

Prime Mover.

I have a 5.5Hp Briggs engine, which I believe is rated at 3600rpm and is in good shape. I guess this means a step down of about 12:1, probably in two steps. I have a 6.3:1 gearbox that can handle the power so an additional step of about 6:1 is needed. If I use a belt and pulleys, should it go before or after the gearbox?

I could see that adjusting the throttle could be a tedious business particularly with a variable load. I'm thinking that a radio control servo could be used in a closed loop drive system to govern to a dialed-up speed. The stator has a built in Hall effect sensor for rpm monitoring. Any thoughts?

Loads.

I think it would be a good idea to test it with a three-phase AC resistive load as well as DC into an adjustable constant voltage battery simulator. I'd be interested is seeing the comparison of the two.

For a variable AC load, I'm considering PWM control of FET switches into fixed forced-air cooled resistors. Since we're looking at line currents of 100Amps plus, it'll mean a bunch of FETs but I think it's doable. I assume that since the alternator is connected in star, that the loads should also be in star.

The same switches and load resistors could be reconfigured into a PWM adjustable shunt regulator to simulate a constant voltage battery load for DC testing. Synchronous rectifiers are used so rectifier heating won't be much of an issue.

Instrumentation.

It looks like the following needs to be measured:

RPM - from Hall effect sensor built into the stator

Alternator input power - from load cell built into stator to measure reaction torque

Stator coil temperature - from built-in LM35 sensor

Load voltage

Load current

The data will be collected by a data logger with a one second or less sampling rate so the computer can massage it.

Methodology.

Since the variables of rpm and load will be under electronic control, it seems reasonable to automate the process somewhat. Rpm could be stepped in increments and for each increment, the load could be cycled through a range of values with data collected throughout. This would be a lot quicker and easier than trying to read and record a bunch of meters. This could be done with a fairly simple controller.

I believe this approach would result in a pretty complete characterization picture of the alternator, which would then allow tailoring of the boost and buck converters for matching. Retesting with the converters in place would then be needed for final results verification.

Any comments and/or ideas will be appreciated.

[SamoaPower]

Oops, already see a goof. Multiple step down ratios multiply, not add so, I would need a 2:1 in addition to the 6.3:1 gearbox.

Sorry about that.

[Walt]

SamoaPower

What type/model of load cell are you using in your stator?

.

 

[zapmk]

SamoaPower,

Quote:

I have a 5.5Hp Briggs engine, which I believe is rated at 3600rpm and is in good shape. I guess this means a step down of about 12:1, probably in two steps. I have a 6.3:1 gearbox that can handle the power so an additional step of about 6:1 is needed. If I use a belt and pulleys, should it go before or after the gearbox?

With the Briggs I wouldn't run it over 3,000 rpm, if you want it to live very long. 3000 at 12:1 should net you 250 rpm at the Alt.

sounds like the gear box is a stand alone? or is it one that can be bolted to the engine?

using a belt and pulley set, I would not recommend anything smaller than a 3 inch pulley, the smaller pulleys do not have enough contact area for the heavy loads.

With your gear box (if its direct drive to the engine) a 3 inch and 18 inch pulley will be close to the 12:1 your looking for.

Any kind of disengagement system between the Alt and engine?

As far as the other gizmos, Iam still working on that myself. Iam also interested in finding some solutions for the proper ways to setup and test on a run in stand.

 -Zapmk

[SamoaPower]

The load cell and its associated electronics were removed from a cheap ($25) digital bathroom scale. Can't afford the expensive industrial versions.

[zapmk]

Don't know where I came up with the 18 inch pulley. Must of been thing of some thing else.

Should of been a 3 inch and a 5.5 inch.

 -Zapmk

[SamoaPower]

Zapmk,

"With the Briggs I wouldn't run it over 3,000 rpm, if you want it to live very long. 3000 at 12:1 should net you 250 rpm at the Alt."

Good advice. Each test run will probably be only a few minutes long so I don't think I'll stress things too badly.

Yes, the gearbox is stand alone.

Thanks for the minimum pulley size. This is the kind of stuff I'm looking for.

No disengagement is planned. Do you think I need one? There won't be any alternator electrical load at engine start-up.

Thanks for the interest.

[RP]

Two things will eat a gearbox: Torque and rpm.  Whether you put the additional 2/1 belt before or after the gears depends on what the gearbox was designed for.

If the gear box is designed to be driven by a step down belt like in a riding lawn mower then do that since the gears are designed for torque but lower rpm.  These systems are typically driven by 1/3 - 1/5 belt ratio before the gears.

If the gearbox was originally direct driven by an engine like a tiller then do that since the gears are lower torque but will stand the rpm.

As to the throttle control: If the engine still has the govenor assembly then keep it and use your servo to set the govenor speed.  Let the govenor handle the micro-variations.  This will be much simpler than getting into damping algorythms, etc. in your control system.

On RPM logging:  Consider putting the hall effect sensor on the test stand instead of the stator of the unit under test.  For instance, put the sensor next to the engine flywheel and multiply X 12.  That way you don't have to change anything if you ever test a stator with a different magnet arrangement.

On the AC resistance load test:  Could triacs be used (since it's AC) and skip the whole PWM FET thing?  Seems like maybe a Walmart light dimmer could handle almost a Kw.

My two cents...

[zapmk]

SamoaPower,

We must have been posting at the same time.

I have tried it both direct drive and with a clutch, direct drive work very well with my 24 inch rotors turning at 15:1

I did like the clutch because I could stop the rotors to make some changes,but had problem with the rpm at which the clutch would ingage (using a go kart clutch a lot of extra work to set the ingagement up right)  

Really be interested to see what your test results are.

My Alt also has the same magenet area and copper weight! but have not yet run any tests.

 -Zapmk

[RP]

One more thought:  Do you (or a friend) own a tractor with a 540 rpm PTO?  This could simplify things a bit!

[Flux]

I like the tractor with decent controllable speed but you would need extra reduction.

You will love building the servo controlled throttle control, in many ways it is a waste of effort but will be a fun project to see if you can stabilise it.

If you get the drive ratio right, the engine governor and speeder spring should be good enough to cover the range you want.  With the data logger you should get readings done quickly and for each load the engine should hold speed near enough.

I am not entirely sure that a pwm chopper into a fixed resistor will be the same as loading with variable resistors, you would need to confirm that.

Loading on the dc side with the pwm regulator should be fine with a decent size input filter capacitor so the windings don't see any chopped current.

You will find that these axial flux airgap alternators are quite predictable and you will not need many readings. Things are a bit variable near cut in with dc load but away from cut in 2 points will give the characteristic, the rest will verify your accuracy and let you weed out the rogue points.

I am not sure how much trouble you will have with the temperature readings from induced ac voltage ripple. Thermocouples in large ac machines always gave us trouble with filtering good enough for a data logger. At low voltage and with a detector with greater output it may not be a problem but be aware of it.

Open circuit volts/rev and dc resistance will tell you most of what you need.

look forward to results.

Flux

[ghurd]

I don't think you will get to 3KW with a 5.5HP.  1.8KW will probably be pushing the limit.

It would be interesting to see how and where a 5.5HP B&S gives up on more power, in case you need one more thing to try. We wouldn't want you getting bored

G-

[Flux]

You are probably right. With a highly efficient alternator and screaming the nuts off the thing it may do 3kW for a few seconds to take the readings.

Unless the engine is sacred I would gear it for 3500 rpm at expected alternator full speed at 3kw.

It won't last long at that speed but a few seconds to take a few readings shouldn't hurt.

Flux

[dinges]

Zap,

I don't think you could get 2-3kW electrically out of a 5.5HP Briggs & Stratton, not even for short times while testing.

I've got a 3.5HP B&S one that I'm turning into a powergenny; I can barely get 750W output, and that's with the motor bogging down. By playing with pulleys, I may get it a little higher, but not by much, I'm afraid.

Also, at the upper load range, RPM goes down from the 3000-3600 that you're calculating with.

I don't think achieving 1kW electrically would be a problem with it, but, depending on your requirements, the 2kW may be. The 3kW definitely will be a problem.

You could check out my IRC photo gallery for my project. When it's finished, it'll end up as a diary in fieldlines.

http://www.anotherpower.com/gallery/album45?page=2

For more info on this strange 'difference' between electrical and mechanical HP (rather: the marketing ratings that gas motors have), see this thread:

http://www.fieldlines.com/comments/2006/7/14/165245/306/16#16

Succes,

[dinges]

Oops. It's addressed to SamoaPower, not Zapmk.

[Murlin]

16/12....does this mean 16 pole 12 volt?

Murlin

[SamoaPower]

RP and Flux,

The gear box is from another old wind machine with a fairly large rotor (18' I think) so I think it should handle the rpm range and torque okay although it will be running backwards from the stepped up original. It uses spur gears so should work backwards. Now that I think about it, it's most logical to use the belt step down before the gear box and direct drive the alternator from the gear box.

You mentioned tiller and that rang a bell since I also have a Troy Built tiller with a two speed gear box and if I recall correctly, the tine speed is in the right range to drive the alternator. Hmmm, maybe, remove the tines and belt couple to the alternator. Have to look into that.

Tractor? What's that? Don't think I've ever seen one on this little island.

The Briggs doesn't have a governor but I see where you're comming from. Yes, feedback control systems can be fun, but I've done a number of them so think it's doable. Just have to get the poles and zeros in the right place.

The rpm sensor is already built into the stator since I will be using it operationally and yes Flux, signal conditioning on all the stator sensors is a definate consideration because of the environment. Low-pass filters, here I come!

I agree that chopping the AC is a bit iffy, but I think perhaps by chopping had a high rate compared to the AC, the stator inductance will help flatten out the current. Like you say Flux, needs to be confirmed.

I'm afraid that triac losses at these low voltages would be an issue. Dimmers won't handle a kW at 12 volts.

Flux, I'm not sure that OC voltage and resistance tells enough of the complete picture. Maybe I'm just being picky. Seem like you need to know the coil temperature to get the actual coil resistance. By the way, I'm down to about 3 milliohms per coil now.

Thanks guys.

[SamoaPower]

No, it's 16 magnet poles and 12 coils, but it is for a 12 volt system.

[zapmk]

Peter,

Have a setup like that to, Kohler 6 hp, GM large frame 1 wire Alt, I have a little over 1000 hours on it. Running it with a 1.8:1 over drive, it will push 80 amps into a 24 volt battery bank all day long with a engine speed of 2600 rpm. pushing 90 to 95 amps will start bogging it down.

In SamoaPower case were talking about a 12:1 gear down torque multiplier input to a slow RPM Alt. I would think that it would be able to do 3 KW for testing.

But then again I was worng once, OK maybe twice. ;>)

 -Zapmk

[dinges]

VERY interesting.

I was not sure whether I was going to play with pulley size. But now, I'm almost certain I will have to. Will try to gear it up to about 2:1... At the moment, it's slightly geared down.

[ghurd]

Peter must have taken only half his meds today.

He must be saving gas money for his 50MPG car.

I believe this is the link he must have intended to post...

http://www.fieldlines.com/story/2006/7/14/165245/306

[dinges]

Nope Ghurd, I took my medicine. And lots of it too, I needed it, considering the blindness, stubborn-ness and hypocricy of a certain person (no one on this board).

I narrowed it down to the interesting part of the discussion (w.r.t. gasmotors & HP output), considering that someone posted some wide pictures that made the entire thread unreadable.

Now that I regularly use the bicycle, I hardly need the gasmoney. Now I can spend it on alcohol, drugs and wild women... <rolls eyes>

[finnsawyer]

"Open circuit volts/rev and dc resistance will tell you most of what you need."

Right on.  The accuracy of the model using those measurements depends on the linearity of the alternator.  One factor affecting linearity is heating of the coils at high currents.  One quick way to get the sense of that effect would be to short out the phases and measure the phase currents as a function of rpm.  The extent to which these currents differ from those predicted using the open circuit voltage/resistance ratios will indicate the divergence, if any, from linearity.  One can start with low power, low rpm and increase the speed until the maximum available power is reached.

[SamoaPower]

ghurd, Peter, Flux and zapmk,

Yes, I realize that that the 5.5 Hp Briggs is a bit iffy to cover the range of interest, but since I will probably limit the machine to <2kW in the real world, anything I can get above this in testing is gravy.

Test sessions will be short, so perhaps it will make it. With a fast data sampling rate, I only need peak power for a matter of seconds.

Thanks for the input guys.

Love the banter, by the way.

[bj]

    Samoapower--I agree with Dinges comment further on, in that you

won't get enough power from the motor.  Plus if you have to run at

full throttle at less than rated RPM for long, it will cause severe

overheating.  (I am assuming air cooled)

   I have built several chargers for farmers around here based on a 5hp

and a GM alt.  80 Amp and a 5hp is really working.  These are great

though when your tractor battery is dead in a field 15 miles from home.

   bj