Using RC motors on E-bikes [Archive]

[TylerDurden]

I think that their setup is fine for a motor that is pulling but when coasting the small wheel will need to rotate at about 7000rpm. And at this speed the clutch bearing will put quite some resistance.
Not good when batteries/gas run out. I would use the ring and cut out the mounting part and glue it onto a big pulley with a freewheel. That is the idea.

Good thinkin on the FW.

(The narrowing of the stays gets tricky, tho.)

 

[MitchJi]

Hi,

A few weeks ago I looked at a bike that used a beefed up rear fender as a structural part of the frame. It just occurred to me that on the right bike something similar might make a good place to mount a small motor.

I have some questions about a motor I am considering:
One of the Hacker A60L series (Peak Watts 2,600):
http://www.hackerbrushless.com/motorDetails.aspx?series=A60

Is this Motor a good choice? Hacker's are supposed to be high quality, this motor is supposed to have a lot of torque and the Kv is relatively low. It also has a built-in fan which seems like a good feature to have. Its a little expensive ($269.99) but I would rather pay a little more and get high quality.

My inclination would be to use 12S lipo for power with the A60-14L (192 RPM/V). Is that the best choice of the three (A60-14L, A60-16L or A60-18L) and is 12S a good plan?

40v (for simplicity) and 192 KV yields an RPM of 7,680. Is there a simple rule of thumb or formula to determine the range that RC motors in general or this motor in particular are most efficient? At 7,680 is it near its sweet spot or beating itself to death.

I don't know very much about motor selection or the best RPM range for a particular motor so I would really appreciate any advice or help.

Thanks!

Mitch

 

[John in CR]

Mitch,

I don't know anything about RC, but how are 10 Lipos and a peak amperage of 60A supposed to develop 2600W? Something smells funny.

John

 

[MitchJi]

Mitch,

I don't know anything about RC, but how are 10 Lipos and a peak amperage of 60A supposed to develop 2600W? Something smells funny.

John

Hi John,

Peak Amps - 90 amps

x 33v = 2,970

Operating Current(Amps) - 45 amps

x 33v = 1,485 (continuous watts?).

One review stated they are underrated.

Thanks for your input!

Mitch

 

[John in CR]

If you click on the detail it says peak current 60. Maybe the 90 is right and the 60 is the same typo 3 times. When similar spec'd motors are 1/5 the price elsewhere, which enjoy similar reviews, I know which way I'd go, but I'm cheap and only time will tell if the cheapies will hold up.

John

 

[Miles]

Mitch,

Changing Kv by varying the number of turns in the armature is just a way of matching rpm to voltage, it doesn't have any direct affect on torque output or efficiency.

This is a good program for learning about electric motors: http://www.drivecalc.de/

John,

Often, the cheaper the motor, the more inflated the spec.....

Case in point: http://myhobbycity.com/showpost.php?p=2187&postcount=10

 

[John in CR]

Mitch,

Changing Kv by varying the number of turns in the armature is just a way of matching rpm to voltage, it doesn't have any direct affect on torque output or efficiency.

Since when doesn't number of turns affect torque? I thought it affect torque and rpms in opposite relationships, so power out remains the same.

John,

Often, the cheaper the motor, the more inflated the spec.....
Case in point: http://myhobbycity.com/showpost.php?p=2187&postcount=10

Just not nearly as inflated as the price on the other, and inflated price definitely doesn't prevent inflated specs to go along with it.

WRT the testing of that big HXT. I just got my sensorless controller today, so from the looks of those tests results it seems to just need to run at 72 or 84V to get the big power. Too bad the RC controllers don't do high voltage, or I'd use one on my bike.

John

 

[Miles]

Mitch,

Changing Kv by varying the number of turns in the armature is just a way of matching rpm to voltage, it doesn't have any direct affect on torque output or efficiency.

Since when doesn't number of turns affect torque? I thought it affect torque and rpms in opposite relationships, so power out remains the same.

No, if that were the case, it would be the equivalent of mechanical gearing.......

 

[John in CR]

Mitch,

Changing Kv by varying the number of turns in the armature is just a way of matching rpm to voltage, it doesn't have any direct affect on torque output or efficiency.

Since when doesn't number of turns affect torque? I thought it affect torque and rpms in opposite relationships, so power out remains the same.

No, if that were the case, it would be the equivalent of mechanical gearing.......

Isn't that the case? I must need to do some reading. I thought for example on the Crystalite models in the same series, it is just the difference in winding count that changes the kV and torque.

John

 

[Miles]

No. Have a look at specifications for the same motor with different windings eg. http://www.maxcim.com/pdfs/MEGAMax3.7Cat.pdf Or, use Drivecalc to superimpose graphs of the same motor with different windings.

 

[docnjoj]

So Miles! The only way to increase torque for a given motor is to >Volts?
otherDoc

 

[Miles]

No, the only way is to make it bigger, or gear it down....

That's why direct-drive hub motors weigh half a ton

 

[docnjoj]

Gotcha!
otherDoc

 

[HAL9000v2.0]

"Futaba J standard" is 0-5V control or something else?

 

[johnrobholmes]

Increasing voltage will increase torque via amp draw. Of course it doesn't make the motor have more torque, you just run the motor closer (or beyond) the ratings and more torque is produced.

Bigger motors have more torque than smaller motors, easy enough to understand.

Two identically constructed motors will have the same torque and power if run at max RPM. Double the magnet count on one and you get twice the torque per amp at half the speed. Overall power is the same however, and if gearing is doubled to keep final wheelspeed the same between the two motors you get no net increase in torque or power (ignoring switching losses that may change). We must be careful to keep in mind the difference between motor torque and torque produced at the wheel too.


To get maximum power (and torque) from a motor, it must be run at a voltage to allow max RPM. Changing the wind count only changes the voltage and amperage that max power occurs. It does not change the torque or power of the motor.


Glad to see somebody helping to dispel the myth that higher turn motors have more torque.

 

[John in CR]

Are you guys saying that at low speed, ie taking off where torque really matters, that the winding count doesn't matter? I'm having trouble with that pill. I'm not talking about torque at max power out, which is obviously at higher rpm. Maybe you guys are thinking in terms of propellers, not bicycle wheels. Of course, maybe I just don't have a clue.

John

 

[docnjoj]

I agree it aint intuitive for me either! The damn math keeps proving it, though! :?
otherDoc

 

[John in CR]

I agree it aint intuitive for me either! The damn math keeps proving it, though! :?
otherDoc

Then please explain the difference in a X5303 and X5305. I thought it was just the windings. AND the motor I got rewound with thicker wire to handle more amps, but it got 1 or 2 less turns, and now is very high speed but no torque, so it's useless to me.

John

 

[johnrobholmes]

Wind count doesn't matter. Geardown and motor size determines torque at the wheel, assuming things like batteries are up to the amp draw task.


Here is where wind count would matter- You run 48v and go from a 10 turn to an 8 turn motor. Keeping all else equal the 8 turn will produce more torque (faster acceleration) as long as it is not being overpowered (stalled) and the batteries can take the extra amp draw. You will also get more power since the top speed is faster and total wattage will have increased. The 10 turn produces more torque PER AMP, but draws less amperage.


John- are your batteries sagging in voltage under the new load? Is your ESC up to the task?

 

[John in CR]

Wind count doesn't matter. Geardown and motor size determines torque at the wheel, assuming things like batteries are up to the amp draw task.


Here is where wind count would matter- You run 48v and go from a 10 turn to an 8 turn motor. Keeping all else equal the 8 turn will produce more torque (faster acceleration) as long as it is not being overpowered (stalled) and the batteries can take the extra amp draw. You will also get more power since the top speed is faster and total wattage will have increased. The 10 turn produces more torque PER AMP, but draws less amperage.


John- are your batteries sagging in voltage under the new load? Is your ESC up to the task?

Now I'm more lost than ever. It just reminds me of my hatred of electronics. I don't plan on building any motors, so I'll leave it to you experts that like the stuff and not even try to understand, just like I do with controllers. That way I can stick to the mechanical stuff that comes naturally for me. Still, I thought the higher torque Crystalite hub motors had more turns in their windings than the speed hubs of the same series...Is that incorrect?

John

 

[dirty_d]

more turns in the same motor just means you need more voltage to get the same performance. if you had an x5305, then moved to a x5303 with the same voltage, youre gonna get more power. the power output of two motors is proportional to its winding resistance for the same voltage. increase voltage or decrease resistance and the power increases. the speed/torque relationship stays the same though, more iron and stronger magnets make more torque. just adding more magnets wont increase the torque, the only case it would was if there was space between magnets that was unused. 2 big magnets covering the entire bell would be the same as 12 magnets covering the entire bell, the only difference would be the commutation frequency.

 

[John in CR]

Now I feel compelled to simplify it further. I've got identical everything and two otherwise identical hub motors other than the winding count. Which one is going to have an easier time pulling a laden trailer or hauling my fat ass up a hill, the one with more turns in it's coils or not?

John

 

[johnrobholmes]

Now I feel compelled to simplify it further. I've got identical everything and two otherwise identical hub motors other than the winding count. Which one is going to have an easier time pulling a laden trailer or hauling my fat ass up a hill, the one with more turns in it's coils or not?

John

Assuming your voltage doesn't change, the higher turn motor will be better suited to pulling larger loads as compared to a lower turn motor. The higher turn motor will produce less power, consume fewer amps, run slower, and run cooler. The slower speed at a given voltage makes it better suited for pulling hills, merely by means of producing less heat. It will not pull you faster, it will not pull more load, it will not produce more torque.

The only exception would be if the lower turn motor were near or beyond stall pulling your hill. In this case it would pull too many amps and cause saturation, producing much more heat than power. In this case a higher turn motor would indeed produce more torque than the stalled out "fast" motor, by means of a higher efficiency.

 

[John in CR]

Now I feel compelled to simplify it further. I've got identical everything and two otherwise identical hub motors other than the winding count. Which one is going to have an easier time pulling a laden trailer or hauling my fat ass up a hill, the one with more turns in it's coils or not?

John

Assuming your voltage doesn't change, the higher turn motor will be better suited to pulling larger loads as compared to a lower turn motor. The higher turn motor will produce less power, consume fewer amps, run slower, and run cooler. The slower speed at a given voltage makes it better suited for pulling hills, merely by means of producing less heat. It will not pull you faster, it will not pull more load, it will not produce more torque.

The only exception would be if the lower turn motor were near or beyond stall pulling your hill. In this case it would pull too many amps and cause saturation, producing much more heat than power. In this case a higher turn motor would indeed produce more torque than the stalled out "fast" motor, by means of a higher efficiency.

If this is all true, then we should just have single turn coils with big fat wire, and just gear accordingly. In the above example, equal amps in at equal voltage (ie same power in) then less heat means greater efficiency, however, even at equal efficiency same power in means same power out. If the rpms are lower then there has to be more torque, just like changing the gear ratio.

Something definitely doesn't jive with the explanations given. Do the Crystalite torque hubbies have more turns in the windings or not than the speed hubs, or is it that they have more coils? They have a lower kV, and my impression is that at the same power in (equal amps and voltage) they will rotate slower but with more torque. I'm not talking about how much current is possible, just how much you provide.

I'm sorry if I'm being dense, but this really doesn't make sense. Once I see the light, I'll go back and delete my posts to keep the thread clean.

John

 

[lawsonuw]

"Futaba J standard" is 0-5V control or something else?

"Futaba J" is a 'standard' pin out and connector body for RC components. The actual signaling system uses is common to all RC components. The signal consists of a 1-2ms positive pulse 50-70 times a second.

Marty