Motors, RPM, and Efficiency

[GeeReg]

I have a question that I couldn't seem to find an answer to. When people are talking about motor choices and kv values, sometimes you'll hear "That would put you at about 48,000 rpm and that's outside the motor's efficiency range, so it will heat up" or something to that effect. My question is, where do these numbers/ranges come from? How do people know where a motor is efficient?

[mack]

You can see it on the motor spec's What the rpm per volt is
so you can caculate this.

And I think that the guys here are talking from experince.
(I think by trying it there self)

And if you read this forum you find several time's that the best range wihl be around 30000 and say 40000rpm

So just caculate which engine is the best choice for you.
;)

[GeeReg]

I know how to calculate the rpm by kv value, I just don't understand how people know what's "efficient" and what's not. Like you said, I think it's experience, word of mouth, but I wanted to make sure.

[mack]

If I got it correct if you go for the high rpm/v the motor will be to hot and looses his power.
And if you go whit the low rpm/v motor you can gear it so you got the same top speed Bud more torq,
And a cooler motor and longer runtime.
Because less stress on your batts and controller/motor

[sleebus.jones]

What you really need is the torque curves for the motor, and gear the motor so it runs the speed you are looking for just a bit past the torque peak.

The trick is finding those curves. I've seen some a while back, but I can't seem to find them now that I need 'em! =/

Michael

[GeeReg]

That's what I thought, there must be the data somewhere. But I think now it's all experience and recommendations.

[Serum]

A high revving motor will suffer more lost than a low revving motor, just because at every pulse, the setup suffers more losses. But there also is a dead point too.. (on too low RPM) So to get in an ideal range, just use some sense..

I personally prefer a low kv big motor with a taller gearing.

A segmented rotor normally has got a higher efficiency as well. (on both WOT and at partial load.)

It depends on the setup (size of motor/voltage it's running on/gearing to determine the setups sweetspot. Use a tempgun, a few pinions and perhaps a few more cells to start the experiment..

[Gustav]

Originally posted by GeeReg
That's what I thought, there must be the data somewhere. But I think now it's all experience and recommendations.

I agree with this to a large extent,the collective knowledge we have here due to experiment is the most valuable source of data.

[MetalMan]

I have spoken with an electrical engineer on this, and he tells me that 40,000RPM is the most efficient point. This does not mean that a motor spinning at 40,000RPM will heat up less than if it were spinning at a slower speed, since more voltage is being put in that would translate to a loss (in heat).

40,000RPM being the most efficient spot is similar to how they say that one can get the best gas mileage at 40mph.

[Serum]

that just isn't the case...

that so much depends on the magnetic mass and so much other factors. You really can't say that 40k is the most ideal rpm.

This is an interesting discussion. It brings to mind something I tested. I've always felt that my 9L (on 14 cells) gets much hotter than it should. I've got it geared so that full throttle is reachable in several places on my track (so I guess that means I've got it geared low, quick) In any case I did a test. Inside, normal day, temp around 70 degrees. I took the motor out of my truck and pegged the throttle to 100% and held it there for exactly one minute. I did this with two 9Ls and one 10L, same speedo and 14 cell pack.

Temps were 141 for the 10L, 154 for 9L #1 and 162 for 9L #2...

All seem a bit hot for 1 minute of use at no load. During the heat of the summer I would often break 200 after the 5 minute races. I'm starting to think the "L" series may not be as ideal as an "XL" but I've not tested, nor ever run an XL.

[MetalMan]

Gearing properly is the key to keeping a BL motor in a good temperature range. An 8L that runs hot on a certain gearing can be replaced by a 9L (same gearing/cell count), and the 9L will be 40 deg. cooler or more! This is what I encountered with my 1/8 buggy. So, there really is a "sweet spot" for gearing to obtain the optimum BL motor efficiency.

[Gustav]

Sounds like your 9l may be geared too low?

These motors need a certain load to run efficiently.I was noticing how dramatic a temp difference i get from changing rear tyres on a ST(less traction less load).It was way hotter without decent traction compared to the same gearing with good traction.

I'm going to be trying a basic XL soon in a touring car on the basis that the very high gearing and air resistance will provide enough load for this motor.

[sleebus.jones]

Originally posted by MetalMan
I have spoken with an electrical engineer on this, and he tells me that 40,000RPM is the most efficient point.

Unless you two were talking about a specific motor, his statement is utter rubbish. That's like saying all cars get the best fuel economy at 42.5MPH. I'm hoping you were discussing a specfic setup, because I'd think an engineer would know better than to paint with that broad of a brush.

Michael

[Serum]

Yeah Michael, that is exactly what i mean too.. saying 40K is the most ideal range is complete rubish.

@mac II, you mean you let your BL motor run unloaded for 1 minute?! that is a bad test, that is just torturing the motor.