Unloaded vs Loaded Speed?
- Thread starter Username1
- Start date
Username1 said:
The motor develops torque in response to being loaded and slowing down below its free speed. Usually, for any given motor, you can find a power and torque chart that identifies these values at given RPM. Max torque is almost always at zero RPM. Max power usually occurs near 50% of the unloaded RPM, and max efficiency at close to 80% of the unloaded RPM (but with much less torque and power).
How fast you go with a given motor-battery-controller combination is a function of how much power it takes to push you and your bike at a given speed. Your top speed is always less than the motor's unloaded speed.
I like to match 80% of the motor's free speed, and the power the motor makes at that speed, with the power I need to travel at my target speed on flat ground. That way, I get good efficiency almost all the time, and there's a deep well of surplus torque and power available for hills, headwinds, etc.
Play with the Grin Technologies motor simulator to get a better sense of how this stuff works.
They will both be fairly close on level ground and at the lower speeds. As you go faster, wind resistance increases. As a result, your speed will sag, and the watts used per mile goes up (less range).
There used to be a type of bicycle "top speed" run, where the competitor pedaled his bike behind a truck that had a large wind-break,
It's the same when you add more cargo weight, or begin factoring-in the incline of a hill.
If you want to keep up with car traffic, you'll need a lot of power for safety, and you might consider selecting the unloaded voltage and kV that is rated for at least 10-MPH more than the average car in that road.
There used to be a type of bicycle "top speed" run, where the competitor pedaled his bike behind a truck that had a large wind-break,
It's the same when you add more cargo weight, or begin factoring-in the incline of a hill.
If you want to keep up with car traffic, you'll need a lot of power for safety, and you might consider selecting the unloaded voltage and kV that is rated for at least 10-MPH more than the average car in that road.
Username1
100 W
- Joined
- Nov 26, 2013
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- 168
Thanks for the replies. So if my desired top speed is 20% below my kv, that should basically guarantee i can actually reach the top speed, and operate near peak efficiency? This is the specific example i'm looking at doing...
I want to use a large 6s LTO battery to go 20mph on 20 inch wheels. I want to maintain the same speed throughout state of charge, so i'm calculating kv requirement using the lower cutoff voltage (13.2v)...
I'm considering the 1000w leaf motor with a 2T wind. The kv would be 32.2, so unloaded top speed would be 25.3mph. Taking 20% off would give 20.2mph.
Assuming they'll sell me a 2T version if requested (lowest i'v seen is 3T), does this seem to accomplish what i'm after?
I want to use a large 6s LTO battery to go 20mph on 20 inch wheels. I want to maintain the same speed throughout state of charge, so i'm calculating kv requirement using the lower cutoff voltage (13.2v)...
I'm considering the 1000w leaf motor with a 2T wind. The kv would be 32.2, so unloaded top speed would be 25.3mph. Taking 20% off would give 20.2mph.
Assuming they'll sell me a 2T version if requested (lowest i'v seen is 3T), does this seem to accomplish what i'm after?
Drunkskunk
100 GW
What do you plan to use for a controller? With such a low voltage system, you are going to need to pull massive amps.
Some would consider 6S LTO to be a 12V replacement. Calling it a 14V pack might be the max.
I am a huge fan of 48V packs, because you could achieve the same watts with fewer amps than when using a 36V pack. Plus...there are affordable inverters that can take a 48V pack and convert it to a 120V AC output during a temporary power outage.
I would even prefer 24V over a 12V pack. That being said, best of luck with your build...
I am a huge fan of 48V packs, because you could achieve the same watts with fewer amps than when using a 36V pack. Plus...there are affordable inverters that can take a 48V pack and convert it to a 120V AC output during a temporary power outage.
I would even prefer 24V over a 12V pack. That being said, best of luck with your build...
Username1
100 W
- Joined
- Nov 26, 2013
- Messages
- 168
I know 5s is a better "12v" match, but surely that doesn't matter as long as i''m not exceeding a controllers limit with my extra few volts. It seems like a 2T leaf motor matches perfectly with 20mph on 6S. I"m happy to use that motor because of it's good efficiency which helps offset the shitty energy density of this battery chemistry a bit.
Was originally planning to go 10s with a nuclear controller, but i'm now strongly considering 6s. I'v been too busy to look into controller options for this voltage yet.
This isn't a typical build where the battery configuration is highly flexible. I'm going to be using giant 66260 cells, so space limitations are going to dictate the voltage to a large degree. The amps won't be massive because this is a low power build. I only need 30 amps for cruising at 20mph (max speed), and i guess about double that for peak power.
Drunkskunk said:
Was originally planning to go 10s with a nuclear controller, but i'm now strongly considering 6s. I'v been too busy to look into controller options for this voltage yet.
This isn't a typical build where the battery configuration is highly flexible. I'm going to be using giant 66260 cells, so space limitations are going to dictate the voltage to a large degree. The amps won't be massive because this is a low power build. I only need 30 amps for cruising at 20mph (max speed), and i guess about double that for peak power.
fourbanger
100 W
Interesting. I'd been wondering the same thing myself lately but was too embarrassed to ask. Actually, I'd been going on the premise of calculating top speed at 75% of no load rpm. But now that I really think about it what SM says makes a lot of sense. On the flat having had the throttle pinned for a while, the combined weight of rider + bike don't really mean much since you've already attained all the momentum you're going to. Wind resistance is a very real thing, however.
I'm still not going to wear lycra, though.
I'm still not going to wear lycra, though.
John in CR
100 TW
Username1 said:
There's very little loss in efficiency that way, and that occurs in the controller only as long as you increase wiring size commensurately, so you don't incur additional copper losses due to the higher current requirements. Assuming equal wire fill in the motors, the same motor wound to a different Kv will have the same torque potential and same efficiency as the lower Kv version.
In reality though having a bike capable of much higher speed will result in you using the extra performance for less range for a given energy capacity.
My statements above are factual and backed by the fact that I have 2 ebikes capable of over 100mph top speed, though I rarely exceed 50mph. I do emphatically advocate building for a top speed greater than you use for safer riding in traffic...quicker passes and easier to create safe riding spacing well away from vehicles. In addition, I can't imagine using any vehicle used at WOT throttle since even the slightest hill or puff of headwind will slow you down.
DogDipstick
100 kW
Username1 said:
The great debate. Here. If you want to know more:
Read THIS:
[urlWhoopsie! See below for amended link[/url]
John in CR said:
It’s safe, quiet, comfortable, energy-efficient, and trouble-free to configure a bike to cruise at or near the maximum legal operating speed where you ride. Remember if you’re running at top speed and full throttle, power increases dramatically with small decreases in speed, so you really don’t slow down all that much on hills (if you have surplus power available).
For any given battery and controller, you get more initial and maximum torque from a slower wind motor, so there’s that too. You can have more capable climbing and better acceleration from a stop, without having to spend more or carry more weight.
John in CR
100 TW
Balmorhea said:
That kind of restriction is like putting a gas tank and carburetor from a lawnmower engine on V8.
John in CR said:
If you do that, the engine will have less power, less torque, and less range. If you substitute a slow winding for a fast winding in a given system, it will have the same power, more torque, and more range. So I think it isn’t a good comparison.
fourbanger
100 W
So, um, preference aside, am I to understand it that the best motor of choice for a given form factor will ALWAYS be the one that achieves top efficiency at the highest RPM withstanding pole count, lamination width etc?
I've always found it curios that so many sellers advertise their highest rpm models as low volt/low amp versions.
Eg. 36v 350w usually spins faster than the 48v 500w model.
Thoughs?
I've always found it curios that so many sellers advertise their highest rpm models as low volt/low amp versions.
Eg. 36v 350w usually spins faster than the 48v 500w model.
Thoughs?
John in CR
100 TW
Balmorhea said:
No, if was optimized before, then swapping in a slow winding motor will burn it up. Also, power = torque x rpm , no you can't get the same power with more torque simply by changing the winding to a lower Kv with the same battery and controller. It's a bad idea to look at changing just a motor unless you've already invested in a controller you can grow with that is more capable than your current motor. In that case the route to across the board improvement (unless you're talking about off road riding using a hubmotor where a large diameter wheel has real benefits) is a higher Kv motor running a lower gearing. Since the motor is capable of exactly the same torque at exactly the same power rpm and efficiency, you will get more thrust at greater efficiency without sacrificing speed.
The myth that lower Kv motors have greater torque and/or are more efficient and/or are better at climbing hills should never be perpetuated.
Are you oblivious? Same power, more torque implies lower RPM. Lower as in target speed, not 2X target speed.
I think I’m in accordance with most here in thinking “pushed to the limits of operability” does not equal “optimized”.
I think I’m in accordance with most here in thinking “pushed to the limits of operability” does not equal “optimized”.
John in CR
100 TW
fourbanger said:
I don't know about "highest rpm", but it does make sense that lower voltage models of the same motor often have a higher Kv so they can reach the same speed at the lower voltage. In that case the lower wattage rating wouldn't make sense other than that the seller is selling them with controllers with similar current limits at the 2 voltages. Without buying both motors and measuring actual Kv and phase-to-phase resistance at a minimum, then you can't be sure exactly what you're getting unless you're purchasing in sufficient volume that you can obtain detailed specifications and motor test reports, and even then it can be hit or miss even when dealing directly with the manufacturer.
fourbanger
100 W
Thank you for the reply. It's tough talking with sales reps without getting something along the lines of "yes yes, you buy now?" and it seems they keep their engineers and techs locked in the basement or something where I can't talk to them. One thing I've learned is that "yes" or "no" questions are generally best avoided if possible.
Thanks again.
Thanks again.
