48v vs 72v for racing go kart

Hi everyone!

I'm posting this question here because it really doesn't fit in anywhere else, (moderator, please move this question to whatever you feel it would be a more appropriate - thanks!).

I'm in the planning stages of an electric go kart for my 12 year old daughter for competitive go kart racing and I'm trying to figure out what would be the best ah to go with. She she would like to compete in the 5.5 hp racing class and want's to "see the look on the boys face when they get smoked by a girl with an electric motor!", (her words, not mine!). According to the local racing club, as long as the frame meets regulations and the motor isn't more rated at more than 5.5 hp, she can use an electric motor if she wants to.

Although an electric motor will unquestionably take a gas engine off the line, some races are 45 min to 1 hour long, (unfortunately, they don't have 1/4 mile races for go karts, where an electric motor would have the maximum advantage). Since pit stops are allowed on the longer races, swapping out a properly configured battery packs could be done faster than refilling a gas tank, IMO.

So my real question is should I go 48v or 72v. I know 72v will give me a longer run time, but at the cost of doubling the weight of batteries, (I will be going with SLA's in the begining, when I see that she is serious about it, I'll spend the coin on lithium packs). 48v would require more pit stops, but it would also save about 50 lbs in weight. (As we used to say in the Army, ounces equal pounds - pounds equal pain!).

Any suggestion?

Thanks!

zog

There is no such thing as a 5.5hp electric motor unless they make you lock down the battery - controller "supply" side. They would be handing you a free ride only specifying rated motor power. There are tons of threads to take a look at here that may explain it much better than I can.

For voltage determination, you will need to pick your motor, kv and gearing to get the rpm you need to go the max speed you can sustain with the power you have on tap. Too low a gear or voltage and you will run out of rev's. Too high and you will run out of power before your motor is in the efficient rpm range. FOC controller will get you a few more RPM but at a bit of a efficiency hit. Justin had a great post a few years back taking about this topic. See this section of his site on the topic which explains it well. http://www.ebikes.ca/learn/power-ratings.html I also hope she kicks butt!

Just pointing out the voltage doesn't matter really... You could use 4 15ah, or 6 10ah and it works out to about the same total energy and weight. What's going to determine it more is what motor you get, and getting it to run in a nice efficient rpm range for the sprockets and gearing you have available. For example maybe you find a great deal on a motor, but it turns too slow at 48v, so you need to volt up, or the reverse where it spins too fast, and you don't want a giant reduction stage, maybe you volt it down etc.

Voltron said:

Just pointing out the voltage doesn't matter really... You could use 4 15ah, or 6 10ah and it works out to about the same total energy and weight. What's going to determine it more is what motor you get, and getting it to run in a nice efficient rpm range for the sprockets and gearing you have available. For example maybe you find a great deal on a motor, but it turns too slow at 48v, so you need to volt up, or the reverse where it spins too fast, and you don't want a giant reduction stage, maybe you volt it down etc.

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Hi Voltron, (cool tag, BTW!),

I'm thinking in the beginning to use a 48v Unite 1000w with some cooling modifications to keep the motor from burning out. Then maybe upgrade her to a permanent magnet motor similar to this:

http://www.ebay.com/itm/Imperial-Electric-Permanent-Magnet-DC-Motor-Model-P66LR006-36V-3-6HP-/192280564776?hash=item2cc4d09428:g:XJIAAOSw4TNZleED

I use one on a heavily modified Razor Dirt Quad. It's rated 36v DC but I run mine at 48v @ approx. 2800 rpm and approx. 9600 watts with no problems. It has a heavy duty internal fan that is attached to the shaft, so the faster the motor turns, the more air it blows, although how well that would work at 72v remains to be tested! :lol: Eventually you start hitting a wall with air cooling, (although I do have some thoughts on a water cooled DC motor). The major drawback to the motor is that it weighs in at 54 lbs. Throw in 40 - 90 pounds of SLA's and the kart might be good for a golf course, but somewhat to heavy for the track.

Another thought that I had for the future would be a brushless motor similar to this:

http://www.hyperpowersports.com/products/brushless-2000-watt-60v-motor.html

Obviously, a lithium pack is in the future, it would really help with the weight. Hopefully, there is some new battery breakthrough soon, (come on Elon Musk! :lol: ).

Too bad 6 hp continuos would not be ok. Something like this would be my choice if they would pass it.
http://www.electricmotorsport.com/me1305-pmac-motor-24-48v-6-hp-cont-15-hp-pk.html

This looks interesting and likely to pass.
http://www.volcanomotor.com/products/48v_high_power_brushless_dc_motors-en.html

zogthegreat said:

I'm thinking in the beginning to use a 48v Unite 1000w with some cooling modifications to keep the motor from burning out.

Click to expand...

That 1000w motor is only a bit over 1hp, and people have destroyed a number of these motors abusing them in variuos ways with significantly less than the 4kW or so that 5.5hp represents.

Forced-air cooling will help some, but they're still cheap brushed motors. There's a recent thread about a burning man vehicle using these that might have some tips for you.

It has a heavy duty internal fan that is attached to the shaft, so the faster the motor turns, the more air it blows, although how well that would work at 72v remains to be tested!

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Keep in mind that brushed motors have significantly more waste heat from brush arc on the commutator, the higher you go in voltage. Wears the brushes out faster too. May not matter for this application, plus the bigger the motor physically is, the more surface area everything has to shed heat.

Since the fan spins faster too, it may compensate for that.

The major drawback to the motor is that it weighs in at 54 lbs. Throw in 40 - 90 pounds of SLA's and the kart might be good for a golf course, but somewhat to heavy for the track.

Click to expand...

Well, if the weight is all low enough, at least it'll stick to the track in hard turns.

The other motor that looks to have some potential is the 48v 3kw golden or the 5kw if you can get away with that. They look to rev pretty high also. Liquid cooling options also.
https://www.goldenmotor.com/

Also just noted that QS has a new 3kw mid drive now. Looks to geared near ideal for a kart as is. http://www.cnqsmotor.com/en/article_read/QSMOTOR%203000W%20138%2070H%20electric%20bike%20mid%20drive%20motor,%20under%20developing/612.html

Did you build the kart?!