When running an optimiser should you change gears depending on light conditions? for instance increasing the gear ratio in low light and reducing it in high light? or should you stick to the same ratio no matter what? Cheers

Redlands,

If you are using a maximiser of some sort and you have the right gear ratio and wheel size, you should rarely have to change gears. It is true that there may still some benefits in doing so, but for most of the time it is not essential if you have a well set up car.

The last few years that I competed, I don't think I ever made a gear change. There was never the need to, as the conditions either suited the set up that I was running, or the car was still good enough to win.
In saying that, the level of competition of the top few cars has however really stepped up in the last 2 or so years so it might now be something to begin consider.

The electronics extends the panel voltage range (or motor rpm range) at which maximum power is received. This is illustrated in the following graph from Ian Gardner's design hints.



Note: The plots will obviously vary for different sunlight levels, but the general layout is the same.

As you can see when looking at the plots without electronics, the output power varies greatly with varying output voltage (or rpm). The plots with electronics display a much greater voltage (or motor rpm) range at which the output power is at a maximum.

If we look at the dyno motor output plot, having a gear ratio that will have the motor operate anywhere across this max power range would be sufficient.

One thing that I must however add that is not really evident straight off here (the power consumed by the flywheel needs to be added) is that the electronics become slightly more efficient and resistance losses are smaller at a higher voltage range and so a higher gear ratio (higher rpm) rather than a smaller one would probably be your best bet. This is so that once the car has reached its top speed, it is operating at a higher voltage rather than a lower one. A higher gear ratio will give the car better acceleration.
Also remember that, at lower voltages, the current is higher and there are therefore also higher resistance losses. On the other hand, don't make it too high or else you'll be start entering the high end drop off region.
To start, and if you aren't going to be changing gears, the gear ratio that one should be going with will be determined by your motor, wheel size, the solar panel's maximum power voltage and the car's top speed in full sun (8 or 9 m/s). In full sun, and at top speed, you want to be operating at just before the panel's maximum power voltage or entering the high end drop off region.

It may be worth to set up a lower gear ratio than normal (or increase wheel size) for when there is particularly good sunlight and/or you are racing over 2 laps since cars sometimes reach their top speed too quickly and start to operate in the high end region mentioned earlier. This is even more the case if you are using John Jeffery's engelec maximiser since it drops of slightly earlier than some of the other units like the Easymax or the Boxhill units.

It may also be worth setting up a couple of higher gear ratios than normal for when the sunlight % begins to drop off so that you are able to, firstly, still make it up the hill (at extremely low sunlight), and secondly, bring your motor voltage back up again to reduce your electronics and motor losses.

Just so that you have something to go off, the last few years of racing I used a gear ratio of 6 to 1 (6 revolutions of the motor to every 1 wheel revolution) with a wheel size of around 50mm to 55mm and a panel max power voltage of around 14V.

miseli

Ok thanks a lot for all this information. Much appreciated

I have a few questions:

You said that it is better to run a higher gear ratio because the electronics work more efficiently at a higher voltage. How much higher? If you started with a 6:1 ratio would you go up to maybe 6.25:1 or 6.5:1 ?

You also mentioned the high-end drop off region, what kind of gear ratio would you have to run to reach this?

My last question is about poor weather. How high should you make the gear ratio in very poor light? 9:1 ? or is that too high?

Cheers

Excellent post Marc, you explained that bit perfectly.

Basically, we want to finish the race, with the motor running at the top of, or around, that second peak, at around 18 - 19 thousand RPM. But, we need to understand that that graph is for a Solarex Panel, which put out higher voltages. If we lower the panel voltage, to say 10 DSE segments, your optimum operating area, or finishing RPM, would be around 14 -15KPRM.

How do you know that the gear ratio you are running is within the right panel power v volts range? how do you know you if you are operating in the drop-off zone? do you need to make your own graph?

Thanks for that little addition Tony.

Below is a chart of electronic efficiency vs motor voltage is based on tests that Stan Woithe in South Australia undertook a little while ago. These were done before the Easymax unit was around. I'd imagine that this unit would be somewhere close to the Boxhill unit.



To answer your questions with regards to gear ratios (and wheel sizes), like Tony mentioned, it really depends on what kind of solar panel you've got.

If you have a higher voltage panel with a lower current, it would be more suited to a higher gear ratio since the extra volts will mean that the motor (and therefore the wheel) will be able to spin faster. For this set up, too low a gear ratio would require more current and therefore result in a lower motor voltage and greater losses. Too low a gear ratio would also slow car acceleration.

On the other hand if you have a lower voltage panel with a higher current (or just a lower powered panel), a lower gear ratio would be required since there are less volts to work with. For this set up, too high a gear ratio will mean that the motor voltage will approach the panel's open circuit voltage, or exceed the max power voltage at which the power begins to drop off.

Because the electronics efficiency begins to level out towards higher motor voltages I wouldn't put too much emphasis on trying to get the perfect gear ratio (in terms of electronics) for a particular condition - all you have to do is be in the right area. You will only ever gain 1 or 2 percent in electronics efficiency (if at all) above a certain voltage and there are a lot of other things that are far more important to the overall performance of the car. No load motor losses also come into the mix of things with increasing motor rpm and will may in fact outweigh any increases in the efficiency of the electronics.
Remember all setups will still be subjected to the low power at the low voltage end of the spectrum at the start of a race. Just make sure you that you don't remain in this region.

As long as your setup doesn't drop off at the high end too much either (some top speed might be sacrificed for better acceleration) then there shouldn't be a problem.

I suggest that you go with a ratio of somewhere around 6:1 ratio with a 50 to 55mm wheel diameter (or the equivalent speed reduction) for most of your races if you are running with 14V to 15V max power voltage panel or similar (this is what I had).
The 6:1 ratio that I have was originally used for the older faulhaber 2233 4.5 volt motor that I had, but this had a smaller torque constant and higher speed constant than the new 2232 6V that replaced it so a 5:1 may actually be the way to go in good sunlight (or you could increase your panel voltage). I don't think that there will be a great deal of difference over 1 lap, but there may however be some difference over 2 laps, depending on the type of panel or electronics and conditions.

To maximise the overall efficiency of the cars in lower sunlights a gear change may be worth looking at but if, and to what extent, you will get an improvement in performance remains to be seen. The type of electronics that are being used will influence this result. It would probably be easiest to just run some on-track tests.

You can roughly calculate what kind of gear ratio would best suit a given sunlight by using your panel voltage, wheel size, motor speed constant and approximating your top speed in those conditions.
With the setup that I used, a gear change to an 8:1 when the sun drops to below maybe 40% would bring the motor voltage back up and closer to the panel's max power voltage again. Similarly a 10:1 would do the same for sunlights below maybe 15-20%.
As mentioned a little earlier, these changes should improve electronics efficiencies and lower resistive losses. The increase in motor rpm and no load motor losses must however also be considered.
It would certainly be worth going to a higher gear ratio if the cars are beginning to really struggle up the hill but those kind of conditions are not seen all that often at an event.
One risk that you take when changing gears (as occurs when you are running without electronics) is that you might gear for bad sunlight but then have the sun suddenly come out. This is particularly bad as you will then max out and have a much lower top speed. Being geared a bit on the low side is certainly better then on the high side.

I created a simulator based on some of Ian Gardner's 10 dicksmith cell panel (the type of panel that I used) dynamomter results in my last year of competition and it turned out that a gear ratio of around 5:1 to 6:1 for that type of panel was pretty close on the money. I still have the simulator and will be getting it up on the web once I get an accompanying document alongside with it explaining certain assumptions and calculations. This may still take a little while though.

If you want to somewhat accurately simulate the performance of your own car, you will probably need to run your own dynamometer tests with the panel and electronics that you hope to use. You'll also need to perform air drag, rolling resistance tests, etc.

An easier way to evaluate the performance of you car and my suggestion to you is to run some actual on-track car tests if you are able to. Having 2 similar cars to test against one another would be ideal.

miseli

Redlands,

What type of panel do you have? What are its characteristics?

One way you could approximate a gear ratio is by working backwards and working off the typical top speed of a fast car in good sunlight as a start.

A good car in full sunlight will approach speeds of about 8m/s. With a wheel size of 50mm diameter, the circumference is Pi*D or 3.1416*0.05 = 0.157m. This equates to 60*8/0.157 = 3057.3 rpm.

Now say that you have a 17 or 18 volt open circuit panel with a max power point at about 14 or 15 volts. I'm not quite sure how fast the electronics power starts dropping right off as soon as the max power point (you want to avoid going too far beyond this point) has been reached (Tony, you might be able to help out here), but say you want the motor voltage around here at top speed. The faulhaber 2232 6V motor has a speed constant of 1190 rpm/V from memory, so that would mean about 15*1190 = 17850 rpm.

You therefore want a gear ratio of about 17850/3057.3 = 5.8 in these conditions.

Of course, you'd still require your panel and electronics to be actually capable of supplying the required current/torque/drive force to overcome the losses (air drag and rolling resistances, etc) at that voltage/rpm/speed, but it's a start. (about 500 - 600 mA short circuit current coming out of the panel will get you there abouts for this example)

Note that the voltage drop across the motor resistance has not been taken into account here and the gear ratio calculated above will in actual fact need to be slightly lower.

miseli

I'd just like to add that if you aren't already aware of it, cooling down your panel will increase the your panel power and, in particular, the panel's voltage. For 2 fast cars that are evenly matched, this can very potentially make the difference between you winning or losing a race.

If you don't know what you're doing and you have a maximiser that needs setting manually, cooling the panel down however has the potential to disadvantage you rather than give you the advantage.
If you are going to be cooling down your panel, I wouldn't set up your maximiser when the panel is at its coldest.

The reason for this is because of the open circuit and max power voltage drop with increase in panel temperature. This means that the rapid high end drop-off region visible a few posts earlier will shift to a lower voltage (or to the left in the earlier power vs voltage graph) when the panel heats up. As a result, if the electronics unit was set up when the panel was particularly cold and then the panel warms up by the time you start racing, you could find yourself losing a lot of power since the original set voltage is now in that high voltage region.

On the other hand if you do set it at a more regular temperature and then cool the panel, there should be slightly less of a problem since the panel voltage and max power voltage is increasing and your set voltage is moving away from the rapid high end drop off. This is slightly better but it's still not ideal though. Cooling the panel and no longer being exactly (or close enough) at the max power voltage may in fact be worse than just setting the the max power point and not cooling down the panel at all.

Of course, if you are right on max power then that will be optimal and you should see the benefits.

This is a reason why I feel that Boxhill with their auto tracking unit have a real (almost unfair) advantage over others at the moment, since you can do whatever you want to the panel, but can't stuff up your setting.

I would suggest not to go playing around with panel temperatures until you have fixed up everything else - in which case you should pretty much win any race without doing so anyway. Just set up your electronics for the panel that hasn't been warmed up by the sun and then try and keep that temperature constant (keep your panel shaded and out of the sun) for whenever you race.

The max power voltage also doesn't vary a great deal with sunlight percent for most sunlights (I have a chart somewhere that Ian Gardner sent me based on some testing that he did). I will try and find it, but this means that the maximiser doesn't really need to be re-set before every race with varying sunlight (as long as the panel temperature is kept constant). You may need to re-set things at low sunlights but apart from that you should be able to leave it.

When I was still racing, I basically set the maximiser up once and that was it and I didn't touch it again.

miseli

miseli wrote:I'm not quite sure how fast the electronics power starts dropping right off as soon as the max power point (you want to avoid going too far beyond this point) has been reached (Tony, you might be able to help out here)

Once the output voltage of the maximiser reaches the maximum power point (or the reference its been set to), the maximiser will just let the power pass through directly without switching (doing a DC-DC conversion). So, we can say that maximiser will only benefit you if you are operating your motor at a voltage below the maximum power point. If your motor volatge exceeds the maximum power point voltage, you wont be operating your solar panel at its maximum power, so you shouldnt run it there anyways.

If your panels maximum power point is at 15 volts, and your using an Easymax or Boxhill unit, you shouldn't run your motor over 15 volts, however, if your using the Engelec unit, you mustnt run your motor above 13.5 volts (because of the early drop off region shown on the curve). The controller IC, used in the Engelec unit is a tl494 IC, it can achieve a maximum of 90% duty cycle, so your input to output voltage ratio is going to be 1:0.9 at best, however, you will have 10% more current, so you have no power loss (theoretically). The Easymax and boxhill units are digitally controlled, and can reach input to output ratios of 1:1.

So you need to gear to stay within these limits. However, as Marc said, dont gear down too low, otherwise youll pass the MPP, and the power seen by the motor will drop off as it drops of on the panels power curve, once you pass the maximum power point. Also, dont gear up too far, otherwise your electronics wont run as efficient, and you'll have greater I²R losses in the motor.

We are running:

- 6W Panel

TonyB wrote:you shouldn't run your motor over 15 volts

How do you control this?

TonyB wrote:So you need to gear to stay within these limits

How do you know if your gear ratio is within these limits?

Redlands wrote:We are running:

- 6W Panel
- EasyMax unit
- 2232 motor
- 53mm driving wheel

Do you have the old easymax or the new one.
Send it back for a firmware update.

How do you control this?

You need to gear high enough to stop this. You will be reaching this point if your car is topping out.

How do you know if your gear ratio is within these limits?

Well if your car is fast, you would be with those limits.
If it accelerates too fast, its topping out, so you need a higher gear / lower gear ratio.
If its too slow off the line, your geared too high (pinion).

So you would say with or current configuration a 6:1 ratio would be in the right range?

Redlands wrote:We are running EasyMax III I believe. Competition is next Tuesday so probably no chance of sending it back.

PM me your address, ill send you a new one via express post, free of charge.
Then send the old unit back whenever you like.

Redlands wrote:So you would say with or current configuration a 6:1 ratio would be in the right range?

Well you'll be close, but generally, you need to get a feel of it when you see it running.

TonyB wrote:PM me your address, ill send you a new one via express post, free of charge.
Then send the old unit back whenever you like

So we get the new unit for free if we send the old one back?

One more thing, miseli suggested not setting up the optimiser when the panel is cool. What if you keep your panel cool until the race so that when you put it on the track it still should be cool?

Redlands,

If you have set up your car in such a way that you are able to set the Easymax on the track just before a race, then feel free to go ahead and cool down your panel. You should get a useful advantage if you get it right.

The reason why I advised against it earlier was if you were going to be setting up your electronics only once on the day or a little while before getting to the track and allowing your panel to warm back up again by the time you begin racing. Under these circumstances it probably wouldn't be worth the risk.

Please correct me if I'm wrong Tony, but the Easymax also sets the max power voltage slightly lower than what it actually is at the max power point. This is to compensate for the temperature increase experienced by the panel just before and during a race.

From the specs that you have mentioned a post or 2 ago (I am assuming that your max power voltage is at around 14 - 15 volts) and if you're running the Easymax then the 6:1 gear ratio would be a pretty good starting point. I suggest that you then run some actual tests with your car from there onwards. Try a 5:1 or 5.5:1 in good sunlight and see how they go. In half decent sunlight, I'd expect that the 5:1 will be better over 2 laps but there won't be a lot of difference between them over 1 lap.

miseli

That is correct, the easymax will find the maximum power point, and set itself slightly below.

Example.
For a panel with a mpp at 15 volts, easymax would set at around 14.8, and 10 seconds after power on, it will drop a further 100mV to 14.7 volts.

Its always safer to have a setting slightly below the mpp, because the drop off region is nowhere near as steep as the drop off once you pass the mpp.

During an actual race, under normal circumstances, your mpp shouldn't shift down more than 200 - 300mV. You can see this if you test the panel before and after the race with a nanometer.

ok thanks a lot for your help guys

Basically I'm concerned that we will be running our car outside the ideal range. But as you have already said you should start with a 6:1 ratio and then get a feel for it as you race and you should be on the money.

TonyB wrote:f your panels maximum power point is at 15 volts, and your using an Easymax or Boxhill unit, you shouldn't run your motor over 15 volts

I'm a little bit confused by this, if your car tops out early this means that your motor is exceeding the panel's maximum power point in which case you need to lower the gear ratio?

Redlands wrote:I'm a little bit confused by this, if your car tops out early this means that your motor is exceeding the panel's maximum power point in which case you need to lower the gear ratio?

Thats correct.
What im trying to say is that the car will top out even earlier using the Engelec unit (at 13.5Volts as mentioned, and as can be seen in Ian Gardners graphs). The Boxhill unit and the Easymax can regulate all the way up to the MPP, whilst the Engelec unit drops off about 1 - 1.5 volts before your MPP, thus, you will require a lower gear ratio with the Engelec unit.

However, you need to always ensure your gear ratio is low enough not to exceed the MPP, or tops out, as you say.

Ok thanks a lot