100HP Rotax cranking amps?

[Downunder]

Anyone know how many amps is required to crank one of these over?

 

Just want to make sure my cable is adequate......

 

Cheers.....

 

 

[eightyknots]

Anyone know how many amps is required to crank one of these over?Just want to make sure my cable is adequate......

Cheers.....

Would the Rotax manual have this information?

 

 

[Downunder]

Good point, I'll check...........

 

 

[Guest Maj Millard]

They generally recommend a 17-18 amp hour battery to suit the charging coils.......Maj...

 

 

[Downunder]

They generally recommend a 17-18 amp hour battery to suit the charging coils.......Maj...

Yep, got that but the distance of the battery cables from the battery to engine and the amount of amps drawn at starting determines the cable diameter I believe.

I have a chart which calculates this for a 12V battery but needs the input of the amps needed.

 

I have the distance figure.

 

 

[Guest Maj Millard]

The leads on the Lightwing are about as long as they get with the battery situated just ahead of the tail. Make sure you have a ground lead between engine and engine mount and the mount grounded well to the fuselage and you'll be ok. Some people run a dedicated ground wire direct from the batt neg terminal, to starter body.

 

I have never had any cranking problems with the standard set-up, and a good 18 amp hr standard lead/acid battery which I recharge (fly!) once a week..Also fit a battery isolator switch so the battery is disconnected when the aircraft is parked....KISS theory applies...................Maj...

 

 

[Downunder]

Yep, got the isolator. I have a neg wire from the starter body to the starter solenoid mounting bolt/firewall and then from there to the battery.

 

Do you know what gauge/size wire you have?

 

Smaller dia wire will have more resistance reducing power to the starter.....obviously a larger cable is better but is also heavier, so it is a compromise.

 

 

[Guest Maj Millard]

Don't know the gauge of cable offhand, it's about 3/8" in diameter......Maj...

 

 

[Guest Maj Millard]

I still have the original starter fitted, however I have heard good things about the newer (longer) starters for a more efficient start. They are easily retro-fitable in most installations...................Maj...

 

 

[bushcaddy105]

Use arc welder cable - has huge number of very fine wires, so is flexible and easy to route and has a tough (but thin and light weight) outer covering for insulation. Will handle any starter current with minimal voltage drop.

 

 

[Guest Andys@coffs]

The real answer to the question asked:-

 

http://www.flyrotax.com/portaldata/5/dokus/d04967.pdf pages 196 (912 installation manual)

 

300A for 1 second then 75A for remainder of applicable duty Cycle

 

On page 195 Rotax declare that the cable needs as a minimum to be 16mm^2

 

Supply at starter motor needs to be able to supply 700W or optionally (I read this to be preferably) 900W

 

Andy

 

 

[Downunder]

Thanks Andy, that is exactly what I wanted to know.

 

I suspected 16 mil was about right.

 

I currently have what I suspect is 10mm2 from the lug markings.

 

The battery is behind theseats, so an upgrade should give me better cranking power and perhaps improve the life of the battery.

 

 

[Guest Andys@coffs]

You may already know this but what ever you do don't buy cheap jumper leads.....they look significant but are all insulation having less copper in them than fig 8 lamp lead......A real con....but realistically what you should expect if you only pay $20 for 4m of what should be thick cable knowing that the alligator clips alone would be more than $5ea.....Battery world have good quality cable on the roll....if stuck

 

Andy

 

 

[Downunder]

Yeah, on to it. Looking for a "flex" high strand count cable.

 

Double insulated if I can get it.

 

Could you tell me what you think of the 16 mil cable at "EV Works" Andy?

 

On a tablet so cant copy and paste atm.

 

 

[Guest Andys@coffs]

Ok so the specs for that say that the resistance is 1.21 ohms per km. if we assume that the run length is worst case 5m (x2 for +ve and -ve lead) then when we are cranking for that 1 sec at 300A the voltage drop across the cable will be 3.6v meaning that the starter motor only has about 8v to work with..... So I don't like it. but then I don't know if 5m is way over the top...straight line distance isn't generally achievable, but even if the distance is halved then cranking voltage is still circa 10v

 

if, as is the case for something like a jab where the cable length in total would be less than 1m a voltage drop of 0.36v becomes less of an issue.

 

The next size up 25mm^2 has a resistance per km of 0.78ohm (circa 64%) for a growth in weight over the 10m of 890grams.......

 

The one after that 50mm^2 is around 0.386ohms per km but weight is 528g per meter so for the same 5m (x2) we are talking 5.3kg's vs 2.02kg or 2.91kg but the voltage drop will be around 1/4 of what it was which is much more acceptable.... Like everything in aircraft its going to be a compromise.

 

The other thing I'd be wanting to consider is the insulation material. What happens when or if a short or a heavy current circumstance occurs. will the insulation melt/smoke and if so is it toxic?

 

Andy

 

P.S all the resistance measurements above are just cable measurements. The crimp lugs will add their own crimp to cable resistance and then again their terminal to lug resistance. It all has to be considered...Ideally make the run as short as possible and to do that the battery needs to be on the firewall

 

EDIT: just reread and saw you currently have 10mm^2, if that is getting the starter to spin and the motor to then run, then 16 will be a significant improvement! for not too much weight gain....

 

 

[Downunder]

Cable length is about 3m for each cable. Battery behind seats for balance.

 

Currently getting up to 5V drop during cranking, so a bigger cablecan only improve things.

 

Your calculations would probably be better based on the 75 amp draw.

 

Basing the cable size on the 300amp draw (for 1 second) is probably overkill.

 

However I do apreciate the info on the ohms....never really got this deep intoit before but find it interesting.

 

Good points on the toxicity of the insulation..........cheers.

 

 

[Guest Andys@coffs]

Cable length is about 3m for each cable. Battery behind seats for balance.Currently getting up to 5V drop during cranking, so a bigger cablecan only improve things.

Your calculations would probably be better based on the 75 amp draw.

 

Basing the cable size on the 300amp draw (for 1 second) is probably overkill.

 

However I do apreciate the info on the ohms....never really got this deep intoit before but find it interesting.

 

Good points on the toxicity of the insulation..........cheers.

Keeping a starter motor spinning is much easier than the initial get it moving. As such I think you do need to consider the 300A surge current. Especially as the 1sec timeframe mentioned assumed a source that was capable of providing what it needed. If not then that 1 second is likely to be longer, albeit at a lower than 300A load....

 

I chose 5m because the total cable length was 10m and mathematically very easy to deal with. In your case 6m is still a very long run and as you've said the voltage drop is significant. If you are measuring 5v then that's likely to be at once the thing is cranking and not the initial drop which to all intents is likely to be a very significant proportion of the total 12v.....

 

Check out the standard that the page calls out, "complies with AS3808 V90HT (105°C)" that may well tell you what the insulation properties are and the question of toxicity may well be not an issue. try this one for interest.... http://www.vision.net.au/~apaterson/aviation/wire_types.htm

 

Andy

 

 

[cscotthendry]

Another factor in the equation is the internal resistance of the battery. Generally, the higher the CCA rating of the batery, the lower the internal resistance of the battery. The internal resistance becomes significant in that even with a battery cable a metre square, you will still get a voltage drop under high loads because the internal resistance restricts the battery's ability to supply current.

 

Another way to look at it is that a battery with 0 ohms internal resistance which has a short circuit across it of 0 ohms would theoretically supply an infinte amount of current. Even a 1.5 volt battery should theoretically do this. But in practice the internal battery resistance limits this in the case where the external resistance of the circuit is less than the battery's internal resistance.

 

 

[damkia]

http://www.engineeringtoolbox.com/awg-wire-gauge-d_731.html

 

http://www.engineeringtoolbox.com/amps-wire-gauge-d_730.html

 

To help in wire selection...

 

Note that these are actual copper wire cross section NOT insulator cross section measurements, as has been noted elsewhere there is definitely different thicknesses of insulation around the same sq mm copper.

 

Be aware of the real killer, voltage drop. go for the biggest gauge wire you can tolerate with W&B. From what has been indicated from the Rotax data supplied in the above posts (16mm^2) that would equate to about 4 Ga AWG if the battery distance to the starter is less than 5m (from above links). I would think this would apply to most Rotax electrical installations.

 

But what do I know, I'm an OHS Advisor/Registered Nurse.............

 

 

[Downunder]

Well....to put this thread to bed.

 

I have installed double insulated 16mm2 multi strand (flex) wire. 6m in all.

 

Hydraulically crimped lugs and used conductive grease on the connections. This is supposed to give better conductivity while protecting the copper from corrosion.

 

I have used aluminium nuts on some connections as it conducts far better than steel. ie the isolator.

 

Engine turns over faster and feels easier doing so. There is no longer a wind up over a few seconds to full cranking RPM, it cranks fast instantly.

 

Volt drop is 1 to 2 volts less. I think the battery should be less stressed and last longer.

 

 

 

 

 

 

[facthunter]

AMPS is what upsets your battery if it is not made for it. Starter circuits have LOW resistance. Thick leads , the same. This makes the starter work and loads the battery more. You should not just crank and crank if the engine does not fire. Put a time on it? Say 15-20 seconds Max... let starter cool and battery settle down. Nev

 

 

[Downunder]

Fires faster now.....within a couple of seconds.....no ramping up to ignition speed, no winding up......virtually instantly spins like a top. Like it should.

 

Good for the battery. Good for the starter. Good for the engine.........

 

 

[facthunter]

If it starts better it will be better for everything. You want it to get to a good cranking speed because until you do that you don't have ignition. Motors with "impulse" need very little rotational speed. Not such a problem. Nev

 

 

[Downunder]

So the rotax has no "impulse" ignition?

 

I did think a magneto system had to "spin up" to a set rpm to produce a spark.

 

I'm not really sure how the "modules" intergrate with the rotax system?

 

Magneto's are used because they are "self generating" but the modules fail?

 

Seems slightly hypocritical.......

 

My magneto knowledge is basically limited to small 2 strokes......magnets in the flywheel.......

 

 

[frank marriott]

Imitation magneto - think hand start