F&P stators are >82% efficient as a PMA.

[DaveP68]

Today setup a new test rig inspired by SparWeb to more accurately measure the power output efficiency of a Fisher & Paykel smart drive stator as a PMA.

The preliminary results are about 82 % efficiency which I already knew to be in that range.

My test rig is a bit of a crude set up using a Smart Drive motor control module to drive a stator connected on the same shaft to another stator used as a PMA into a resistive load.

Here is a photo of the test rig.


Power input meter reading.


Power output measurement volts and amps.


That's 286.2 W input for a net output of 179.1 W, a very good result for my first attempt. With further refinements expect to get an even better result.

The estimated efficiency of the motor controller is 93 % which leaves the 2 stators with an approximate 82 % efficiency each.

Hope this is of interest to anyone wanting to use the F&P stator(s) on a wind turbine build.

It is possible to extract >1250 W at 500 RPM from a single F&P stator. If more power output is required just add more stators to the same shaft.

[SparWeb]

Dave,
Great work!   

If you were to run it at several speeds, and plot a number of points, a graph like this should emerge:



I think it's pretty common for efficiency to peak and then drop above a certain speed for most motors and generators.  When the power goes up, the resistance losses go up.  Since the R*I^2 is increasing squared, it's going up faster than the power output, eating up a larger portion as the speed increases.

You could be close to the peak power point at 82%, but trying some different speeds might allow you to find even better.  You're certainly doing really well compared to my typical motor conversions.  To make the graph above, I was testing in to a battery load (not a resistive one) which clamped the voltage down and there's a big penalty in efficiency doing that.  But that's the intended purpose (wind turbine charging a battery bank) so I was being as faithful as practical.  I believe a Smartdrive would suffer a similar penalty, but who's to say it would be as great as what I lose into my batteries?

A word of caution: I doubt that the efficiency of Smartdrive 1 (driven by the F&P controller) is close enough to the efficiency of Smartdrive 2 (driven by shaft as a generator to a resistance load) for them to be divided equally.  I can't guess which would be more, but I do expect them to be different - and for the difference to change at different speeds.

[DaveP68]

Hi SparWeb

Sorry for the delay in getting back to you. A close family member passed away a week after this was posted.

Have some interesting stats to add to this topic.

Have been able to evaluated the efficiency of the stator in different modes and over a wider RPM range as per what you have shown above.

Here is test rig v2, this time with a fully adjustable load (blue module below) that can simulate MPPT or a Battery being charged.


This is a graph of an F&P stators efficiency in 3 different modes, MPPT, Battery charging and Battery charging using a Capacitor Doubler.


Have taken into account the many losses that can also be measured such as friction, copper loss etc.

Here are 3 graphs of input power vs output power measured over similar RPM ranges. The motor controller used to drive stator # 1 as a motor is torque limited above 270 RPM so there is no data for above that RPM limit. My data logger was to take a video of each test from 92 up to 245 RPM and enter all the 4 readings displayed as they updated. Readings taken were RPM (white display module), AC power meter measuring input to motor controller, then Volts + Apms on stator #2 output at DC side of 3 phase rectifier.

First is a graph of simulated MPPT which results in the best overall efficiency up to 245 RPM.


Second is a graph of simulated direct connection to a battery to charge it.


Third is a graph of simulated direct connection to a battery to charge it but this time using a capacitor doubler.


** Please note in last 2 graphs, only a half stator used for on the output side. The motor torque required to drive a whole stator was almost double as the stator output current approached it's maximum limit due to stator voltage being clamped for battery charging. This allowed for a more equal comparison between the different set ups as the input drive torque requirement remained fairly constant over a similar RPM range for each test done.

So the conclusion I've reached here is that directly charging a battery is less efficient as the RPM increases above cut in. The MPPT system is a lot more complex to impediment, but is the most efficient way to extract power from a PMA.

[SparWeb]

Hey Dave.  Happy New Year to you.
There's a lot of work here so it's no surprise it took you a while to put it all together.
I saw your post in the Shed just a minute ago and posted a quick Q about torque curves - partly curiosity, partly the way that a torque curve makes a clear presentation.  Identical to the simple link between the power at one shaft coupled to the power at the other shaft.  Of course you may also have access to manufacturer's power curves but either way it means the same thing.  I'm curious about the "torque limit" you mention.  Is it actually a current limit of the motor controller?  If so then I think I understand better the details you added on the other post.

So the conclusion I've reached here is that directly charging a battery is less efficient as the RPM increases above cut in.

I'm tempted to joke about reinventing the wheel, but of course I know what's backing up your statement.  It's really hard to build up the data to answer the harder questions like "by how much" and "under what conditions".  Now you've got them answered.  This is an excellent stepping-off point to size up blades to match the generator, and tune the power scheme.

Don't sweat the presentation too much - I can figure it out - but a few other questions:  Am I seeing a sliding resistance potentiometer inside that blue unit, which you describe as your adjustable load?  This I understand was your way of adjusting the voltage "clamp" to simulate a battery.

[DaveP68]

Hi Steven

Thanks for the complimentary replies both here and on The Back Shed site.

First question is yes to the current limiting function in the motor controller. The motor control module trips out at 71 % maximum operating current of the stator. I already knew what it would be from testing to iron core saturation level as a generator using the drill you spotted in another topic.

Here is a photo of slightly different test set up that you may have seen on TBS.


Note the blue module in this photo is wired up different to the one already one shown in this topic. That version only clamps voltage with no current limiting function.
 
This new one has a fully programmable circuit board interfaced into it that can closely simulate MPPT. Built this one up about 6 moths ago. This was the most complex part to interface it the test rig. It required lots of careful calibration. Without it most of what is published here becomes hard to prove. It sets the base line for tuning the system up and to make meaningful sense of the numbers.

This other board can be programmed to be constant voltage or constant current or a combination of varying voltage and current which is the core function of MPPT.

Thanks for all the constructive feed back recently, as it inspired me to put in the extra e effort required to get to this point. Was just a matter of having the spare time to put it all together and prove it could work.

David

[Adriaan Kragten]

If I look at the photo's I see no device with which the torque is measured. If you use two identical motors and one as motor and one as generator and if you measure only the electrical input and output power, you measure the efficiency of both together. If you assume that both efficiencies are the same, you can calculate the efficiency of one engine. It gives you a good impression but for an accurate generator efficiency, you have to measure the electrical output power and divide it by the mechanical input power which is the product of the angular velocity in rad/s and the torque in Nm. Accurate measuring of the toque is rather difficult. The test rig which I have used to perform such measurements is described in my free public reports KD 78 and KD 595.

[SparWeb]

Accurate measuring of the toque is rather difficult.

Indeed it is.  That is why it's acceptable for Dave to do what he can to account for electrical losses in his drive motor as best as possible.
If you want to directly measure torque, you need a "prony brake" or a set-up like this one I used when driving me generator from the lathe:



The shaft of the generator is driven in the lathe. 
The base of the generator is clamped to a long 2x4 board. 
The board presses on a scale. 
The length of the board X force on the scale = torque.

[Adriaan Kragten]

My older PM-generators have been measured on a test rig of the University of Eindhoven. The driving motor of this test rig is supported in air film bearings which have no friction and the torque can therefore be measured very accurately. Recently I have developed a test rig for small PM-generators which is used in my shed. The generator is supported at its own shaft for normal generators or at its housing for axial flux generators. The torque is measured by a lever, a rope and a balance. I will add a photo of this test rig when it was used to measure a Chinese axial flux generator. This test rig and the measurements are described in KD 595.

[DaveP68]

Accurate measuring of the toque is rather difficult.

I fully accept that, but I have a way of closely working out the electrical/mechanical losses of motor then generator and can come close to working out what the torque will be in Nm on the shaft. Close enough is good enough for me at present. 

Will later set up a system that is close to what you have both shown and validate that number more accurately. It will involve changing motor/generator mounting hardware to take weight measurements etc more easily than on the set up pictured above.

Thanks for taking interest in my set up though.

[SparWeb]

...Will later set up a system that is close to what you have both shown...

Ok, but remember run when the men show up with the white coat for you.

[DaveP68]

Managed to take a "torque" reading using an inline digital torque meter that I purchased from the local Hardware store.

Got a readings of 12.2 Nm for a current draw of 2.8 ADC from the test stator. The 12.2 Nm reading is a flat linear reading, independent to the shaft RPM. Calculating power in for power out using this meter I get just over 86% efficiency in the 500 to 600 RPM range. Above 900 RPM the efficiency goes to over 90%.

Here is a photo of the inline digital torque meter with the reading of 12.2 Nm.

[SparWeb]

Dave,
That looks like a very handy tool.  Great find!

I take it you put it in-line with your power-drill drive. 
Torque proportional to current is exactly what the textbooks say motors do, when operating in a linear range.
However, if your test is done with constant 2.8A current at variable RPM, then the EMF (voltage) is rising instead.

For 12.2 Nm at 500 RPM to be 86% efficient, maintaining current at 2.8A, then the output voltage has to be over 200VDC.
For 12.2 Nm at 900 RPM to be 90% efficient, maintaining current at 2.8A, then the output voltage has risen to about 370VDC.

Suitable for MPPT, which allows voltage to rise according to a set program, but out of the battery charging range.
It demonstrates what you've been saying before: clamping voltage with a DC battery really compromises the efficiency of the PMA.
You would be nowhere near the 80% range if you had it charging a battery during the same test.

[DaveP68]

Yes was a handy find and will come in very useful from now on.

I take it you put it in-line with your power-drill drive. 
Torque proportional to current is exactly what the textbooks say motors do, when operating in a linear range.
However, if your test is done with constant 2.8A current at variable RPM, then the EMF (voltage) is rising instead.

For 12.2 Nm at 500 RPM to be 86% efficient, maintaining current at 2.8A, then the output voltage has to be over 200VDC.
For 12.2 Nm at 900 RPM to be 90% efficient, maintaining current at 2.8A, then the output voltage has risen to about 370VDC.

Spot on Steven, you've reverse engineered the voltage readings and come very close to actual data collected during the tests. Example for a test at 600 RPM the reading out of the rectifier were 236 VDC at 2.8 Amps. My MPPT simulator can be programmed to keep the current constant no matter what the RPM is. The voltage can then rise in proportion to the RPM hence the reading I got.

All this comes as no surprise to me as have got to know those F&P stators very well. This of course applies to all PMA's.

As can be seen from the graphs already published, when the F&P stators rectifier output is directly connect to a battery the efficiency drops off very quickly compared to the "Simulated" MPPT system I use.

Now that the torque readings have been taken the efficiency of the battery is slightly better by 8 % than what I tried to guess it would be. But was close enough for at the time for illustration purposes. The MPPT efficiency readings were very close, with actual reading coming to 84% being only 2 % low is close enough to me.