RC Drives ... why such high voltage?

[LI-ghtcycle]

I'm curious, I'm sure there is a perfectly good answer, and I am missing something obvious, but, if you're using the favored Turnigy 63 74 -200Kv Outrunner http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=3890

why is it better to run say 44V on a 200 Kv motor (8800 RPM) and then have a three stage reduction, than just to use the same motor on 10V (2000 RPM) and use less reduction?

I understand voltage sag, so maybe if you want consistent performance at 3S (10V) you might run 4S instead so that even with cold weather, old batteries etc, etc, your still going to have that solid performance you want?

I understand that most RC motors are happier the faster they spin, and I know there is a sweet spot where a particular motor is at it's most efficient, but I'm just curious why it seems that most everyone is using higher volts and then gearing down instead of the opposite, or something in the middle?

It would also seem that this particular motor is an anomaly in that it is happy in a pretty wide range (3s - 8s or 10 - 36V) and most people seem to be pretty happy with it's all around performance/cost benefit value?

I know that there are definitely some motors that are NOT happy with much other than WOT, but for the ones that can handle partial throttle quite well, why not say (this is totally out of my hat just for example) you could either:

A) run 30V and a two stage reduction requiring a jack shaft and 90T final drive sprocket or

B) run 10V and a single stage reduction with a final drive sprocket of 60T? (just for example, no idea if I am even close here)

I'm guessing since it's all about watts in the end, I'm going to have to run higher amps to accomplish things at 10V vs 30V, but if I have the right controller/ESC, shouldn't matter right?

Thanks!

 

[amberwolf]

Your last sentence basically answers your question.

You need larger wires, bigger caps (or more of them) , bigger FETs (or more of them), higher battery currents, better cooling, etc., to run a particular motor at lower speed at the same power level as the same motor at higher speed.

 

[adrian_sm]

The higher the volts, the more power you can get out of the motor. Simple as that. A higher voltage lets you over come the back EMF.

So if you are chasing a certain power output, the you will effectively be able to use a smaller motor, by running it at higher volts.

Have a play with http://www.ebikes.ca/simulator/. Pick a random motor, and look at the power output at one voltage, then try it again with a higher voltage.

Here is an example. Same motor first at 36V, then at 72V.



- Adrian

 

[Jeremy Harris]

Another way to think of this is to remember that all motors have a maximum current, that is more or less independent of supply voltage.

If, say, that maximum current was 50A and you ran the motor on 12V, then the maximum power you could get would be 12 x 50 = 600 watts. Now increase the voltage to 48V, but keep the current at the maximum of 50A and you get 48 x 50 = 2,400 watts.

The motor will barely get any hotter delivering 2400 watts on 48V than it will when delivering 600 watts on 12V, as motor heating is largely a function of the current flowing through the windings.

As long as the motor can withstand the higher rpm, and as long as you have a means to gear down the additional rpm to a useful speed, you can get massive power increases from a motor by running it at high voltages.

Jeremy

 

[LI-ghtcycle]

Ok, I know I have seen charts, but I have never fully understood them, will the ebikes.ca calc or some other tell me where a specific motor's point of acceptable voltage would be? I'm asking since my Turnigy 63 74 200Kv claims it's happy 3s - 8s. I'm sure it's probably happiest somewhere around 4-6s as far as efficiency and heat management is concerned.

Thanks!

 

[liveforphysics]

Ok, I know I have seen charts, but I have never fully understood them, will the ebikes.ca calc or some other tell me where a specific motor's point of acceptable voltage would be? I'm asking since my Turnigy 63 74 200Kv claims it's happy 3s - 8s. I'm sure it's probably happiest somewhere around 4-6s as far as efficiency and heat management is concerned.

Thanks!

No.

A lot of wrong things in this thread so far.


First, if you're dumping 10amps from a 100v pack, or 100amps from a 10v pack into the same motor operating at the same RPM, then guess what? The motor's average voltage and phase current are identical. Yes, the motor has no idea if the controller had 10v 100amps or 100v 10amps going into it, because what comes out of the controller going to the motor ends up being identical for identical power levels (at the same RPM). Only the battery feels battery current and voltage. The controller feels battery voltage, and motor current. The motor only feels motor current/voltage, and these values will be identical if the power going to the motor and the motor RPM are identical, regardless of what the battery started with.

The advantage of voltage, is if the motor is mechanically capable of higher RPMs, you can double the voltage, and have 1 motor perform as 2 motors, because the motor itself is only limited by the continous torque it can produce.

Lastly, every RC motor test for low KV motors (like under 400kv), they only perform more efficiently at the higher RPMs enabled by higher voltage. There are loads of tests on RC groups with folks measuring thrust from props on RC motors on different numbers of cells in a pack. When people take motors that say "4-6s" or whatever, and run them at 10s or 12s, they generally always perform more efficiently, have a wider power band, and make a ton more power output.

 

[LI-ghtcycle]

Well, ok I think I am understanding better now thanks LFP, but can you tell me, in the interest of balancing the need for a higher pack voltage, and a motor's "sweet spot" I haven't been in terribly warm weather yet, so maybe that is also a reason I have gotten away with 15V, however, the highest temp my CC Phoenix ICE 100 has ever recorded is 90F (this is in probably 40F weather, but I have also put similar loads on it in 60F weather) I have never noticed the motor it's self get more than slightly warm after about 1 mile climbing a 4-8% grade hill at 12 - 16 MPH (Early on I was cautious and didn't go WOT) and these days I just run WOT with moderate pedaling and nothing gets hot, and amps peak at 70 (both my watt meter and the CC ESC agree on this, so I guess it's safe to believe?) and nothing even gets warm.

Maybe I should ask this a different way, how do I calculate the reduction using my Kepler Drive (63 MM outrunner to 700 x 25c Wheel) and translate that to a chain drive?

I would like to get more than 16 MPH up this 8% grade, and with no pedaling, closer to 20 MPH on a 10% (pedal start of course) so if I can stay at about the same reduction, I would think running 6S with the same motor would get me closer to my goal?

I'm hoping to get to a "sweet spot" where I can just use this same ICE 100 ESC and get this performance. Is that unrealistic? If so, I will happily up-grade to something like a HV160, but I'd like to see what I can do with what I have and keep the cost down if possible. I'm not as worried about efficiency as I am about heat, so if it would run less efficient, but still be ok on heat, and I would just have to carry say a 6S (22.5V) 40AH battery at 900 Kw as opposed to something like 12S (45V) 20 AH battery, and I would stay on the lighter side of batteries (less voltage, more capacity = more torque & range if I understand correctly, just at slower speed?) since I would be at the lower voltage, and maybe take this up to a 50AH battery for better touring range.

Does any of that sound close? :lol:

 

[recumpence]

High RPM that is geared way down gives an advantage over low RPM without much reduction because you will see a higher number of motor rotations per wheel rotation. That gives more power at the wheel. You get more noise and, normally, more frictional losses through the higher RPM, however.

Matt

 

[MrBoots]

I won't even pretend that I'm technically knowledgeable but from first hand experience, running this motor at lower voltage generated noticeably more heat (I'm assuming from the higher current draw). When I had this linked to a 6-fet hall sensor controller and running at over 30v, it barely got warm. Now running at 18.5v the motor heats up very quickly, especially when climbing hills. I made the mistake of diving head first into purchasing components before I really understood the technical underpinnings of an ebike system and it's good to see you're poking around and boning up on things.

Some folks have had issues with turnigy motors but I've ran mine hard for a year now and close to 2k miles without issues. Of course I replaced the bearings and reinforced the glue material but I would certainly recommend this motor for those looking for an inexpensive RC motor.

 

[Hillhater]

2 simple reasons..
1) POWER is a function of voltage and current. so if you want maximum power potential, you need to run higher voltages.
2) TORQUE is a function of gearing. If you run a motor slow with a gear ratio to give you say 20 mph you will have only half the torque ( at the drive wheel) as you would if you ran the motor at double the speed (double the voltage) and ran twice as much gear reduction to give you the same 20 mph. Gearing multiplies the torque.

 

[LI-ghtcycle]

Ok, new Q, what is the minimum "high" voltage for a Turnigy 63 74 - 200 Kv? Is 30V (8s) high enough to put it into this better "high" voltage?

Thanks! 8)

 

[liveforphysics]

Ok, new Q, what is the minimum "high" voltage for a Turnigy 63 74 - 200 Kv? Is 30V (8s) high enough to put it into this better "high" voltage?

Thanks! 8)

If you check RC groups, efficiency/power etc all just increase as people climb to 12s (with this and other similar motors). The only reason they are capped at 12s is due to crappy RC controller limitations.