What if motor watt ratings..

Burb, excuse me. What if manufacturer's advertised motor wattage was representative of how much heat it could shed rather than produce? Would that make sense? Do the lower wattage motors actually overheat more easily? Less copper?

That would account for the slightly lower price between a 1000w and a 1500w motor, all else being equal, but would it make sense if those extra few mm of stator and slightly thicker copper windings meant more heat dissipation. Like 500w more?

Thoughts? Or is it like "hey good for you, you finally figured it out."

Thanks as always,

v.

The amount of heat a motor can shed is extremely dependent on ambient temperature. Continuous ratings are fine the way they are, you just have to understand the profile of your motor and the environment you’re riding in. Internal temperature gauges make this pretty easy to manage on most motors.

fourbanger said:

Or is it like "hey good for you, you finally figured it out."

Click to expand...

Hey good for you, you finally figured it out.

Sorta. Power is power is power. Arbitrarily.

Motors rated on "power " in " W or Kw"... and that is taking the ( load) heat into account.

Based on motor frame, construction, and the universal "1" of an output... in power. Power loss is usually measured in watts, or btu. Any two motors may look very different but have the same rated output? Why?

W= F x D

-so-

Electrical work is the work done on a charged particle by an electric field. The equation for 'electrical' work is equivalent to that of 'mechanical' work: The electrical work per unit of charge, when moving a negligible test charge between two points, is defined as the voltage between those points.

With an electrical motor, you have a force (on the egge of the shaft, or pulley) in lbf ( pounds of force) in a vector perpendicular to the shaft center and ... and a distance from the center ( in feet, or inchesZ, radius). This gives you the ability to calculate W=FxD... You now have a force, some work being done, and a distance traveled circumference). You now know the work done in 1 RPM.

Now figure the power.

W=T x 2 (pi) Ft.Lb.F ( reads, "Work equals Torque times two Pi Foot pound force"... )

Now for the RPM...

(RPM) x T x 2 (pi) Ft. Lb. Force
---------------------------------------
1 min = what is known as "horsepower"....

and...

1 "Hp" = 33,000 ftlb force
---------------------
1min So...

basically if you cannot make the torque, you are constantly trying to (bogging) and the heat ( waste, inefficiency, loss) accumulates to the point it overwhelmes the frame and design for the motor... always trying to spin up. So they rate it as a lower power motor..... Remember electrical motors take power to spin up to speed, ( use the tq,, but not to hold an RPM.. load drops. Motors are sized to "handle " the load, not " be overheated trying"....

up to saturation, I was told. This is my take on it... anyone welcome to correct me or let me know anything I may have left out....

but... Bog = bad. Not enough power, so the drive is loping and lugging, loaded up and heating up. If it cannot support that dissipation ( of heat in watts) ... the drive is gong to be damaged by the heat loss accumulation. This is why a large massive motor can take the power longer... even if it is rated as the same power as a smaller motor... the RPM is certainly lower. Same power, lower RPM... = more T.. ( torque)....

The efficiency of a motor is easy to define.. ( power in W / power out W) and therefore is used when sizing. If you size your motor absolutely incorrectly.. you will either have a 50lb motor that can handle 100w.. or a 5 lb motor that can handle 100w... and their torque will be different at the same RPM. Or just overbuilt or underbuilt without purpose. You see alot of low speed high torque motors overbuilt. The torque is a force to reckon with.



τ is the difference.

fourbanger said:

Burb, excuse me. What if manufacturer's advertised motor wattage was representative of how much heat it could shed rather than produce?

Click to expand...

Would be nice. With some motors from some sellers, that might even "work" to judge differences.

But the advertised ratings don't have to have anything to do with reality, becuase sellers are selling things adn as long as it sells it doesnt' matter to them what they say about it.

The very same motor may be sold as 250w, 500w, or 2000w (less likely), or anythign in between, with absolutely no difference between them beyond what the seller happens to put in the ad.

Here's a good article:

https://www.ebikes.ca/learn/power-ratings.html

Thanks dogs. I'm finding it increasingly frustrating dealing with sellers who don't know their own products... an ordeal that I feel could easily be avoided.

Ex: Motor A is sold in three versions - 36v 350w, 48v 350w, and 48v 500w. After MUCH probing (almost more like a full on cavity search, really) it's finally revealed they are in fact 3 different windings. Whew.9t 10t and 11t. But wait 9t is 36v350w, 10t is 48v500w and 11t is 48v350w. And guess what? they want more money for the 10t version.

For a moment I thought there was something really obvious I was missing like the 10t had more copper pound for pound than the others, for example. But no, it really IS just about marketing.

It's just painful.

Thank you for the math lesson, DipStick. It was not entirely lost on me and I'm going to bookmark this post to come back to it later and my brain expands. For now, I'm going to re-read Justin's excellent article on the grin website.

OK. Thanks again.