Motor and Battery question
 

G'day guys,

New to this forum and it seems like a great place. Fantastic information in here and what appears to be a good crowd. Keep it up :great:

First up what are the pro's and con's to using low kv motor and high #s lipo's or higher kv motor and low #s lipo's? Is weight the only real issue or is there some efficiency gains?

Second, I'm running Scorpion 3026 1400kv, quark monster pro, kongpower 6s lipo in a Jammin X1-CR. Scored 61.16kph out of it and components only ever feel warm to touch after abusing it. Quark set to brakes only/no reverse. However, sometimes when I hit the skids and jump back on the gas it goes backwards. I flick the throttle quickly and we're flat out forwards again. Only happens say 2-3 times a pack and sometimes not at all.

Any Ideas?

Cheers,
Clip

First, welcome! In using a low kv motor, higher voltage set up you will gain some efficiency, but more importantly, your components will run cooler...more efficiently. I have a mbx-5 that I run on 4+5s and the 5s set up is more efficient and the components run cooler. On 5s, I turn my EPA throttle adjustment down quite a bit.

In answer to your second question, it sounds like a minor glitch in the programming. I could be wrong see it as I know nothing about quark controllers.

Cheers magman, efficiency makes sense as I was running 2 x 2s trakpower 4900's in series and it was running much warmer but I guess amperage would have been an issue with this set up? These packs punch pretty hard on their own in a 1/10th so does the amperage increase when running 2 in a series?

This whole amperage thing is just another thing to confuse me when it comes to working out the optimum set up......Cheaper and faster than nitro? yes, not if you make a mistake though.

Ohms law covers this...

Power = current X current x resistance

Power = current X voltage

So for constant a power output ( amps x volts) the loss is the square of the current. Hence to raise the power by a factor of 2 - raising the voltage by a factor of 2 keeps the loss constant - raising the raising current by a factor of 2 to achieve the same result raises the losses by a factor of 4 - i.e 4 times as much heat!

This is why power lines are at 100,000 volts....

UMMM....yep thanks Arct1k.

"This amperage thing is just another thing to confuse me."

That didn't help :oh: but I think there might have been some important words missing in your main paragraph or double ups? Not sure sorry mate....

Cheers anyway, but I still don't get it:oops:

amp(current) is the cause of heat. Amps x voltage = watts(power) If you increase the voltage your current goes down to make the same power. This is only true if you change your gearing as voltage goes up.

Ex. a setup using 4s 14.8V making an peaking at 1500 watts will draw a peak current of 101 amps - 14.8 x 101.35 = 1500. Now on 6s 22.2 volts to make the same power it only takes about 67.6 amps

If you have a truck geared for 40 on 4s and just throw 6s in it heat will increase, put the 6s in and gear it down so it's still at 40 and current draw goes down and so do temps.

Lower kv motors are used on higher voltages so that the RPMs are still in your target range, generally between 30-35000RPM. Also lower kv motors draw less amps at any voltage

As current increases losses go up by the square... very basically twice as much current = four times as much heat...

whereas for voltage it is more linear... twice the voltage = twice the heat...

I thought currents were raisin-like dried fruits... Apparently I have no idea!:lol:

Of course no thread can go without some input or a joke from Linc:lol: Gotta love it!

That's why he is the Pimp!:mdr:

:lol:

Awesome, now I'm starting to get it. Currents make your set up more regular :yipi:

The gearing pulled 61kph(40mph) when the pack was over 2/3rd's discharged. I only balanced it when I received it as it was at 22.4 volts. Charged it last night and had a run - albeit without the GPS on - and :gasp: Now I think the gearing will have to come down.....

.....and I'm going to have to stop driving it in my back yard. Both my shed and the outdoor aircon unit sustained buggy damage.....

OK, seen as though you're all so very helpfull - next question.

I read somewhere on here (I think) about a esc mod (resistor? - maybe while searching specifically for quark?) to reduce/stop arcing when connecting a pack. Well today I put the 5.5mm bullets on esc and motor and when I connected it up for the photo's (see buggy thread) I got an arc. I never had this before when I used 4mm. I understand the bigger connectors will allow more current/voltage/power through but is it really this big of a difference?

Tomorrow is the final test day before I fly out to work again so I'll be sure to post my latest speed figures. I'm positive it's going to need gearing down before it hits the track.

Cheers again,
Clip

Thats BrianG's 'no-spark' connector:

http://www.scriptasylum.com/rc_speed/_nospark.html

As for the size of the spark, that depends on if the caps are discharged or charged when yuo plug in the batts, and the size of the batts (voltage wise). Sometimes I get sparks, sometimes I dont- I wouldnt worry about it unless its being caused by reversed polarity between the batts and esc... :eyes:

Sparks are indeed dependant on the size (in uF) of the caps, charge level of the caps, and the voltage used.

When connecting the two connectors together, you get a bigger spark if you do it slowly. As the connectors get really close, the spark will jump. If you can do it fast enough, there will be much less of a spark. Deans are probably the hardest to do this with because you have to make two connections at the same time and can sometimes not get them lined up quite right intitially. Bullet connectors are easier since you make one connection and then the other.

As the page linked above says, do NOT use a no-spark resistor on any of the Mamba ESCs. That is straight from Mr Castillo himself. Although, as long as the switch is "off", I still don't see what the problem would be. If the switch was "on", the motor would immediately try to arm and the rapidly fluctuating voltage could possibly cause issues. But, I'm not gonna argue with the engineer after all. :smile: