Help Please - Undervolt RC motor for low kV - Direct Drive??

[mwkeefer]

Hello all,

Just thought that since my initial reasons for posting this question have been satisfied through help from people like Grinhill, Luke, Methods, DocBass, etc.

Getting your RC motor KV is lower is usually a quite simple process. Most (not all but most) RC out runners (not sure if In Runners are the same, could someone confirm this / rule this out for me) are wired by default in a method named Wye. This saves some time in assembly (not more than a few seconds maybe) and allows these motors to spin so much faster or to have a higher kV rating.

An alternate method of wiring the motor (it's 4 wires that need to be unwrapped and 3 new phase wires brought out from the motor) is known as Delta. In Delta configuration the motor actually has a kV of a little more than 1/2 of the original Wye terminated motor. The exact reduction (or increase if going from Delta to Wye) is a factor of 1.73.

Using my Tower Pro 5330 9T as an exemplar:

[Stock Wye Terminated]
kV: 235 rpm per volt
Voltage: 10S or 42v max (really with lipo it's more like 41.6v)
Idle Current: 1.33A

[Modified Delta Terminated]
kV: (235 / 1.73) = 135.83815028901734104046242774566 - just use 136 for the calculations, it will be accurate enough.
Voltage 10S or 42v Max (I need to check this, it is possible this is increased when running in this configuration
Idle Current: .76 A

By changing the default wiring of the RC brushless motors to Delta from Wye you acheive a kV reduction of 1.73 times so in my case a tower pro 5330 9t @ 235kv becomes 235 / 1.73 = 136kv

If combined with a minor under volting, this will acheive an effective 2:1 reduction in the motors max RPM. If configured with switching between the Delta/Wye modes this would effectivly be a 2:1 transmission you could activate with the switch of a button.

I will setup a new thread to detail this further in a few days.

-Mike

 

[Grinhill]

mw

Can't help on the thory side, but I can offer some practical experience with my 230Kv motor.

I used to run it on 6S, then moved to 8S. The revs increased proportionally as expected (giving higher top speed).

The maximum power was definately higher at higher voltage. My impression was that on a given road (slight uphill incline), comparing the two batteries, at the same speed (let's say full throttle on 6S), the amps drawn and power consumption were very close, but running on 8S I would only require 3/4 throttle.

Overall, the motor ran warmer on 8S, because I was using all the extra power!

Although a lot of people discourage you from running motors on lower voltages, I think it can be useful in some situations. I still like to get the revs up to max as quickly as possible though. I would recommend gearing for the speed you will spend most of your time riding at.

Perhaps the efficiency of the motor will be lower at lower voltages? (tech help required please?)

 

[mwkeefer]

Grin,

Thanks for taking the time... This is what I have assumed but... I am searching for a way to use Higher KV motors with a max of a 7:1 or 9:1 at the rear in a similar fashion to your setup.

Remember this is for my mom so about 14mph top speed is fine and break neck torque isn't required.

Strange... my motor is 239kv so I guess were in the boat.

I had wanted to simply reduce the battery power thereby reducing weight even further and my motor is rated for 239 but at 10S or 42v hot. Thats dual reduction for direct to rear wheel. So reduction might be the only way it's just such as loss of power needlessly.

Thanks for the help man!

-Mike

 

[Grinhill]

Hills are the real killer, because they prevent the motor from getting up into the revs where they are happy. On flat ground, shouldn't be as much of a problem.

I've had similar ideas of doing a build for my wife using my smaller (270Kv) motor, but I would only do something with a higher gear ratio, e.g. 17 or 18:1, so that she wouldn't have to worry about overheating the motor and this would also limit the top speed to a lower value where she would be more comfortable.

 

[northernmike]

She needs a trike, with a cargo section. And possibly reverse.

Why not use a golf-cart type controller and a mobility scooter motor instead?

If this thing is going to be hauling those kinds of weights around, you want some power in reserve.

With a trike, weight won't be as much of an issue anyway, right, since it holds itself up

 

[mwkeefer]

Thanks to all for the input...

I don't expect her to be hauling more than the bike, batteries, herself and maybe a paper and some smokes from the store.

The hill is a murderous thing.... I actually used to have to go in reverse in light snow in an 81 camaro back in the day just to get up the hill! No, im not kidding.

What she really needs is a 20mph billygoat = )

Going larger sprocket in the back could prove difficult, I think (not sure) the Port Runner conversion uses a 98t (not 100% sure) and I could go down as far as perhaps 9 teeth with the right pitch on the motor for a single stage reduction of 10.889:1 but that is one huge rear sprocket... I was trying not to make this look like a mini motor cycle if possible.

I'm thinking 235kv @ 10S but through a 10:1 would give me 639 at the rear wheel (at nominal 3.7v per cell) and 726 rpm @ full charge (4.2v per cell). For a 20" wheel that would give 38 - 43 mph top speed.

Since that's a bit high, I could just build a throttle wedge to curtail (governer) the speed above 30mph and also to manage maximum current draw.

I'm just not sure it would be able to make it up the hill geared for that high of a top speed. (But the idea all the sudden sounds SWEET to me without the limiting for me).

-Mike

 

[northernmike]

This sounds like a scary scenario, man.

Frailty + large hill + heavy bike = high risk.

 

[mwkeefer]

LOL,

Maybe I gave the wrong Idea.... My mom is 5'4", maybe 145 now but would (and has) lifted a hot water heater (20+ year old, and she forgot to drain it) up 2 stories to the ground floor to toss it.

She's getting older but... I have watched her with a dolly and mahogony 4 drawer filing cabinets, no problem.

I just know she won't use it if it's not light enough to be easily picked up by 1 hand which means 30-35lbs max. The folding is a huge plus.

When you think about it (or well when I do) what are people in NYC or older people supposed to do for an eBike? Lord knows they need the extra "assistance" from time to time (heck, I do too) but I can't see my mom picking up 50-65lbs of folding bike and putting it in the trunk of her car.

Actually... when I think about it... I want a uber lightweight bike also = ) Why should I have to lug 60+ lbs onto Septa or a bus?

-Mike

Sorry if I had given the wrong impression of my mom... the reduced power is only because she doesn't need TOP SPEED > 20-25mph but she will need torque for that hill so... who knows
In either case, thank you all for the suggestions and insight.

 

[mwkeefer]

** Removed

 

[Grinhill]

Mike, does this mean that the rpm for each different voltage would be fairly similar, since a similar amount of power would be required in each case? So on the lower voltage, the motor was really struggling to turn the prop?

Using a prop as a load seems a little different to my experience with pushing an ebike, as I found power used increased a lot running on higher voltage (bike was going faster also).

 

[mud2005]

I used an astroflight 3210 10 turn 135Kv at 24 volts and had good results with a top speed of 18mph. that was dual stage reduction though. http://www.endless-sphere.com/forums/viewtopic.php?f=28&t=9838&p=160779&hilit=mud+bmx#p160779

GGoodrum managed to fit a 94t sprocket on his rear wheel http://www.endless-sphere.com/forums/viewtopic.php?f=28&t=12267&hilit=folding+3220

seems like you could get by with a single stage reduction, but it is a huge sprocket on the rear wheel. good luck

My mom is 5'4", maybe 145 now but would (and has) lifted a hot water heater (20+ year old, and she forgot to drain it) up 2 stories to the ground floor to toss it.

impressive, I just watched the Simpsons episode where Marge carries the water heater on her back up to the attic so this cracked me up