Two questions for the electricians.
|David Davis||28/08/2018 17:23:56|
3425 forum posts
While I'm not a beginner with electric flight and while I hold a Grade 6 O Level Pass in Mathematics, (remember them?) maths has never been my strongest suit and more scientifically adept aeromodellers have, in these columns, helped me get to grips with the mathematical problems involved in electric flight, so here's a couple of new questions for you, one theoretical, the other practical.
Why is it that when describing electric motors that suppliers recommend using a smaller propeller with higher voltage packs than they do with lower voltage batteries? For example the recommended propeller for an E-Flite Power 15-950 is a 13x7 on a 3s LiPo (11.1 Volts) and a much smaller 10x6 if you use a 4S LiPo (14.8 Volts.)
To me this is counter intuitive. More Volts should give more power. It takes more power to turn a larger propeller than it does to turn a smaller one, so why do they always recommend a smaller propeller if using packs of greater voltage?
The second question is more practical. Many years ago a colleague gave me or sold me an Axi 2820/10 electric motor. I can't remember at this distance in time whether I gave him any money for it but Axi motors have an excellent reputation, it was new and unused and has remained in its box in a drawer until last week. The input wires had bare soldered ends enabling the user to chose different types and sizes of connector. I planned to remove the physically larger Twister 19 motor from my Junior 60 and replace it with the Axi so that I would have more space for noseweight. My 50 Amp speed controller had 2mm female terminals soldered to its output wires but I found that I had only two male connectors in stock so the next time I went flying I took the motor and connectors and asked a clubmate to solder three male connectors to the motor's input wires. My clubmate is a retired ship's engineer and he did a superb job of the soldering.
I fitted the motor to the model without the propeller just in case, switched on and nothing happened. The elevator and rudder worked so it was not a fault of the radio. I checked the LiPo and found that it was well-charged but changed it anyway, still nothing. I disconnected the Axi and fitted another outrunner. By carefully holding the static part of motor in a pair of plumbers grips and slowly advanced the throttle, the motor turned. I did the same with another motor with the same result so there is obviously something wrong with the Axi.
I consider myself to be mechanically pretty competent, I spent my youth and early maturity struggling to keep faithless British motor cycles from the 1950s, 60's and 70's on the road and the bikes included a free motorcycle mechanics course with every one sold! (Note to younger readers; Not Really but older aeromodellers may know what I'm going on about!)
I dismantled the motor by removing the two grub screws at the rear of the casing and gently tapping out the central spindle. I could not find anything wrong with it. No broken wires, no sign of any burning, nothing. I reassembled the motor, connected everything back up but still it refused to turn.
If it had been one of my i/c engines which had stopped working, I would have known excatly where to start looking but what should I be looking for in an electric motor which fails to turn?
|Biggles' Elder Brother - Moderator||28/08/2018 17:52:32|
15748 forum posts
Dealing with your first question David:
The short answer is that each set up attempts to extract the optimum power from the system. So your 13x7 at 11.1 v will have been selected to draw X amps and so deliver Y Watts - both of which might be 90% of the max the system is capable of.
Now, when you switch to 4s your voltage goes up as you say, but it's the same motor, with the same kV, so it will attempt to run faster, at higher rpm. This will require more power, the motor must work harder. We will indeed get more Watts - for short time! You see the amps will have to increase to give that power to get these extra rpm, and as our set up is already close to the limit we will now be over the limit, too many amps and the magic smoke emerges!
The solution is of course give the motor less work to do! Given that we are now stuck at 4s and more rpm the only way to do that is to reduce the prop load - go to a smaller prop.
Now that's the explanation, but I must admit I find the step down in prop size your motor manufacturer recommends rather large!
Edited By Biggles' Elder Brother - Moderator on 28/08/2018 17:54:36
|David Davis||28/08/2018 18:00:43|
3425 forum posts
Thank you BEB.
Do I understand therefore that the motor produces the same power with a 3S battery and a larger prop as it does with a smaller propeller and a 4S battery? If that is indeed the case, there seems to be little point adding the additional weight of a 4S LiPo to the airframe.
The figures incidentally were taken from the Weymuller catalogue. Weymuller are as well known in France as Steve Webb Models are in England.
|Mike Etheridge 1||28/08/2018 18:07:25|
|1535 forum posts|
I got caught out the other week when my electric YAK 55 which I had not flown for 4 years would not start. Rob from Avicraft suggested that the transmitter trim on channel three had not been set to zero so the speed controller would not sense that the control stick was at zero. He was right !
When I was working we had a case where a motor generator set that provided emergency power at a Council's Parks department would not supply electricity. It turned out that the generator had lost it's residual magnetism and fortunately my Team Leader showed me how to to flash 12 volts DC from a battery across the stator terminals of the generator to restore the magnetic flux. If the generator did not then function it was necessary to to reverse the positive and negative flash connection. I wonder if our model electric motors rely on residual magnetism to function, and of course brush-less motors have artificial three phase connections, far more complex than the simple DC brushed motors ?.
|6050 forum posts|
a 4S battery will take less amps than the 3S on the same motor & prop, so you might use a lower amp ESC which is usually cheaper. And a 4S could be smaller in amp hours yet give the same flight time ( the smaller 4S might weigh the same as the 3S )
Edited By kc on 28/08/2018 18:19:13
|Geoff Sleath||28/08/2018 18:26:53|
3418 forum posts
Mike, the brushless motors we use have rare earth, very powerful magnets, usually in the outer rotating part. The generator you were using for an emergency supply were probably compound wound which rely on the generated energy to create the magnetic field rather than permanent magnets. In order to start there needs to be a small residual magnetic field which flashing provided. The permanent rare earth magnets retain their magnetism for a very long time.
If you remember school physics there were a couple of rules called Flemings left and right hand rules (the former for motors and the latter for generators. Look them up via Google
|Biggles' Elder Brother - Moderator||28/08/2018 18:45:14|
15748 forum posts
That is what we aim to achieve yes. It doesn't happen automatically. The system consists of: battery, ESC, motor and prop. Change any one of these and you'll need to work to restore the same power. For example, change the prop for something bigger, so change the motor for a lower KV to restore the same amps draw. It's not so much the power that matters - its the amps, then do the damage if you get this wrong.
Get a power meter - you can't really do electric powered flight without one! Theory is OK, but it can only put you "in the ball park" - the meter tells the truth! OK, on the ground is not the same as in the air - the prop partially unloads in the air, but a) it's a worst case scenario (if it's OK on the ground it will definitely be OK in the air) and b) it's better than no idea at all!
|Biggles' Elder Brother - Moderator||28/08/2018 18:49:44|
15748 forum posts
Regarding your second question: when you had the AXI in place and connected everything did the ESC arm?
[You can tell if it does because usually you will get a number of bleeps from the ESC which matches the battery cell count. So, 3 cell battery equals 3 bleeps and so on.]
If the ESC didn't arm you see then the problem lies in one direction, but if it did then the problem is in the other direction! Hence the question!
|David Davis||28/08/2018 18:56:52|
3425 forum posts
No it didn't beep with the Axi but it did with the other motors.
|Mike Blandford||28/08/2018 19:01:31|
532 forum posts
An electric motor is driven by the current, not the voltage. A larger prop needs more current then a smaller one at the same RPM.
Giving a motor a voltage specification is a bit arbitrary, it is the current that really matters, in particular the maximum current. If you are using a 3S battery, and are at the maximum allowed current, then going to a 4S battery requires you to fit a smaller prop to keep the current at that value, however, since power is voltage times current, you will have more power.
When a motor rotates, it acts as a generator (its called an alternator in your car!). Specifically, if say the kv of a motor is 900, then it means if it rotates at 900 RPM, it generates 1 volt. This happens even if the cause of the motor rotating is a battery. So, if you apply 10 volts to this motor, it will rotate at 9000 RPM (with no load). In practice there are some losses so the motor will rotate a bit slower. If you put a load on (fit a prop), the motor will then run slower.
|Martin Harris||28/08/2018 19:20:41|
8868 forum posts
What hasn't been mentioned is the greater efficiencies possible with a higher voltage source. Someone will no doubt explain this better but the current required for a given amount of power reduces proportionally to the voltage increase. Losses due to "wasted" resistance in such components as wiring and connectors reduce out of proportion with the decrease in current achieved by raising the voltage. You can also save weight in connecting cables or simply take advantage of the lower losses if weight is not a problem.
Those of us driving in the 70s and earlier will no doubt recall that you could identify older VW Beetles at night from their dimly glowing 6V headlamp bulbs straining to drag enough current through their wiring and switchgear and dropping the voltage to well below what was being supplied by the battery/dynamo!
Edited By Martin Harris on 28/08/2018 19:33:51
|Max Z||28/08/2018 19:35:09|
527 forum posts
Not being helped by the fact that both generator and battery were in the rear of the car....... I could tell you more about it (I had one), but that would be seriously OT
|Dave Hess||28/08/2018 19:45:44|
|303 forum posts|
Here's how I would explain question 1 in a simple way: the faster a motor turns, the less current will be pushed through it, so a smaller propeller will cause it to use less current, which will stop it from burning.
Now a bit more detail. say you have a 3S battery driving a motor and propeller at 10,000 rpm and drawing 10 amps at that speed. When you go up to 4S, the 33% higher voltage will increase the current to 13.3 amps at 10,000 rpm, but the increased power will turn the propeller faster, though not 33% faster, so you end up with a higher current. By using a smaller prop, you can increase the motor speed up to the point where it's drawing 10 amps again (maybe around 13,000 rpm), which might be the safe current limit for the motor. The end result is that you get 33% more power (3S x10A vs 4S x 10A) and higher rpm.
The motor's kV determines the motor's maximum theoretical rpm, so the maximum theoretical unloaded speed of the motor will be 33% higher at 4S compared with 3S. The current is zero at that maximum speed, and you could consider it something very high at zero rpm, so you have a straight line graph, where the current is inversely proportional to rpm. It's not exactly like that but before all the smart people come along and tell us so, it's close enough to give you a good idea of what's going on. That's why your motor and ESC instantly burn when you stop the propeller under full power.
Counter-intuitive, but unlike IC motors, the faster you spin a DC motor, the better it is for the motor: less load and less heat, though if you spin anything fast enough, you can make it explode, so there's still a limit.
One more thing, by using an even smaller propellor, you can increase the rpm up to a point where there is less current. In fact, you're controlling the maximum current with the propeller size and pitch. With the a particular smaller propeller, you could get to the point where the new maximum current and the new higher voltage give exactly the same power as you had with the 3S battery.
Edited By Dave Hess on 28/08/2018 19:49:24
Edited By Dave Hess on 28/08/2018 19:57:00
Edited By Dave Hess on 28/08/2018 20:01:46
|94 forum posts|
Most likely reason that the motor won't run is one or more windings are open circuit. Check each for continuity with a multimeter or simple continuity tester.
|Frank Skilbeck||28/08/2018 21:39:11|
4486 forum posts
The power to turn a given prop is proportional to the square of the speed, i.e. to double the rpm you need 4x the power, so if you increase the volts the motor will turn faster and so will the amps if you don't change the prop.
|Biggles' Elder Brother - Moderator||28/08/2018 21:51:26|
15748 forum posts
OK, we're making progress! From this it seems that the ESC wouldn't arm, ESC's generally don't arm because they believe there is a fault or a possible safety issue.
Some candidate reasons for the ESC not arming:
1. The connections between the motor and the ESC are not perhaps as good as they look! One or more could be duff. I mention it as a possibility only!
2. The ESC thinks your throttle is partially open. This is quite a strong candidate so let's try to eliminate it. We do this by calibrating the ESC's view of your throttle stick like this:
a. Take the prop off.
b. Put you throttle trim where ever you would normally have it - bottom or middle etc.
c. Turn on your radio Tx and put the throttle to max (yes, max!)
d. Connect the ESC to the battery. No, I haven't gone mad, this puts your ESC in programming mode and allows us calibrate the throttle.
e. Once you connect listten carefully. The ESC will probably play a little tune then a short pause ( a second or two) then give one single bleep.
f. Once you hear that bleep immediately sweep your throttle to shut - ie zero.
g. Unplug the ESC.
Now try the the system normally, leave your throttle at zero, plug in the ESC. Does it arm now?
1900 forum posts
Did you try to turn the motor by hand, I used to repair a lot of washing machines and one of the probs were the water pump packing up. The cause in the majority of cases was rust build up between the stator and rotor.A strip clean and lacquer cured that. .Although you motor is NIB the clearances are smaller and even a layer of rust may have formed. However the lack of beeps seems to indicate an electrical problem.. Did you try swapping leads around? A possible break under the insulation maybe. there are little screwdriver type detectors that you hold against the insulation with the battery connected. Run it along the wire til the reading goes off the screen. That is where the break is. Here are some.
|Martin Harris||28/08/2018 22:35:59|
8868 forum posts
Will these will work with model aircraft motor wiring? I'd be interested to know if you've tried using them for low voltage pulsed DC successfully.
4220 forum posts
The ESC has already proved OK by swapping out the motor. It looks almost certain to be an open circuit in the motor.
The AXI 2820/10 is a 1200kv motor which isn't really a good choice for a Jnr 60 if ran on 3s as it would mean using a relatively small dia prop for this model. If the problem can be resolved, it would possibly be OK using a 2s lipo of greater capacity with a more suitable dia prop.
IMO best to stick with the existing motor & try moving the cg rearwards incrementally over several trial flights. The cg on my Jnr 60 is nearly 1.5 " rearward of the plan position. The model is still a pussycat but has a better glide & has been like that for about 25 years.
Edited By PatMc on 29/08/2018 00:25:43
|Biggles' Elder Brother - Moderator||29/08/2018 08:43:49|
15748 forum posts
|Yes Pat I am fully aware that the ESC functioned with another motor. But David is relatively new to electric power and, in my view, would benefit from a more systematic approach to fault finding. Rather than the random "here are 27 ideas it's probably one of them" approach! |
It could well be that his throttle trim was different or the throttle part open in previous tests. This eliminates all that and several other factors.
You may be right that the motor is open circuit, but with respect at the moment that is only a guess, albeit an informed one!
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