Choosing the correct motor wind

miuan

10 kW
Joined
Nov 26, 2009
Messages
992
Location
Slovakia
Let's face it, many of us never get beyond the fuss of achieving our desired speed using a given voltage and motor wind.
Which is fine of course, as long as we accept the reality of being limited to a certain choice of winds and voltages.

But it's always raised a question to me, am I more efficient going with a low resistance motor at lower voltage or the other way round?

Sadly, my knowledge is limited to ohms law, Justin's hub motor explanation at ebike.ca, and endless tinkering with his simulator. As for the theory, I've gathered that any motor wind is equally efficient at given speed and power, as long as copper losses are the same. Yet, anytime I play with the simulator, the outcome is that faster winds are less efficient.

I've tried to narrow it down to the motors only, using the simulator with extremely low battery and controller resistance, but even so the 5304 still has slightly higher torque than the 5302 at half voltage and double battery current. As much as 5-10% difference at low speed under acceletation, but gets smaller gradually the faster we go, reaching only 1-2% at high speeds.

Which leads me to believe that all the difference comes from voltage lost in phase wiring under higher current, and perhaps being unable to fit the same amount of copper using thicker copper wire.
 
Every motor has a different copper fill with various windings. Riding the mountain with 26 in wheels, I found that choosing the winding that fills the motor with a greater copper mass is better. Then, pulling the best of it has to be achieved by adjusting voltage and current.
 
The number of turns per pole of copper winding sets the rpm band. Lets assume that you have X amount of copper for motors A, B, and C...they all use the same winding size. You wrap 5 turns per pole for motor A, 7 turns per pole for motor B, 10 turns per pole for motor C. Motor A MUST be the largest motor with the highest theoretical kV, motor C would be the smallest motor with the lowest theoretical kV and motor B would be somewhere in the middle...remember that they all have the same amounts of copper just wrapped differently per phase. It all depends what you intend to do with it. Motor A has the largest surface area to cool but will theoretically reach its peak efficiency at a higher rpm than the other motors, forcing the rider to maintain a higher speed at a given voltage to avoid overheating...lugging any motor is a bad idea but it will hurt these wind types the most. Motor C has the smallest surface area to cool but will theoretically reach its peak efficiency at a lower rpm than the other motors, you are less likely to overheat by lugging it say up a steep hill for 5 minutes. Manufactures don't do this though they just add or reduce the amount of copper to alter the kV and stay confined within the physical boundaries of the hub and rotor themselves.

Controller-side configuration such as max phase amps, mosfet resistance and response time adds even more variables to the mix. Something I'm still learning myself, if someone else can chime in....
 
JoramsWeapon said:
The number of turns per pole of copper winding sets the rpm band. Lets assume that you have X amount of copper for motors A, B, and C...they all use the same winding size. You wrap 5 turns per pole for motor A, 7 turns per pole for motor B, 10 turns per pole for motor C. Motor A MUST be the largest motor with the highest theoretical kV, motor C would be the smallest motor with the lowest theoretical kV and motor B would be somewhere in the middle...remember that they all have the same amounts of copper just wrapped differently per phase. It all depends what you intend to do with it. Motor A has the largest surface area to cool but will theoretically reach its peak efficiency at a higher rpm than the other motors, forcing the rider to maintain a higher speed at a given voltage to avoid overheating...lugging any motor is a bad idea but it will hurt these wind types the most. Motor C has the smallest surface area to cool but will theoretically reach its peak efficiency at a lower rpm than the other motors, you are less likely to overheat by lugging it say up a steep hill for 5 minutes. Manufactures don't do this though they just add or reduce the amount of copper to alter the kV and stay confined within the physical boundaries of the hub and rotor themselves.

Controller-side configuration such as max phase amps, mosfet resistance and response time adds even more variables to the mix. Something I'm still learning myself, if someone else can chime in....

Let's make it more simple to understand the basics.
Example 1: 2806, 36V, 40A
Example 2: 2812, 72V, 20A
For now, let's presume the batteries and controllers have zero resistance, and phase ratio is the same.
Same input power, same copper loss, same rpm = same efficiency.
That's theory. But in practice, it seems higher winds seem to win. Why?
One of the reasons may be additional copper loss of high speed winds caused by more wires going from one pole to another, since these sections don't produce any useful torque.
Another reason is high amps thru controller and battery/phase wiring, but I let's talk about motor cores for now.
 
[/quote]

Let's make it more simple to understand the basics.
Example 1: 2806, 36V, 40A
Example 2: 2812, 72V, 20A
For now, let's presume the batteries and controllers have zero resistance, and phase ratio is the same.
Same input power, same copper loss, same rpm = same efficiency.
That's theory. But in practice, it seems higher winds seem to win. Why?[/quote]

Same rpm and same efficiency for those motors and power levels are impossible, the 2806 must spin faster to generate enough Back Electromotive Force (BEMF) to limit the phase current and enter the higher efficiency ranges. When your motor is at a dead stop it is generating zero BEMF until you hit the throttle. Hit the throttle and you are drawing huge amps per phase, as your rpm increases BEMF begins to take over and you go from a motor condition of say 20V85A to 48V15A at cruising speed. What happens when you're not generating enough BEMF in a lower wind count motor is the phase amps drop less quickly and you remain in an inefficient state, you cook the windings over time. The idea is to reach the unloaded kV rating, which nobody has been able to do...BEMF becomes self-limiting after a point. Dropping the 2806 in a smaller rim lowers the top-speed required to limit the phase amps and enter the higher efficiency state, but, that is already done for you with the 2812. Even though both motors are receiving the exact same peak power they react to it in different ways.
 
JoramsWeapon said:
Same rpm and same efficiency for those motors and power levels are impossible, the 2806 must spin faster to generate enough Back Electromotive Force (BEMF) to limit the phase current and enter the higher efficiency ranges. When your motor is at a dead stop it is generating zero BEMF until you hit the throttle. Hit the throttle and you are drawing huge amps per phase, as your rpm increases BEMF begins to take over and you go from a motor condition of say 20V85A to 48V15A at cruising speed. What happens when you're not generating enough BEMF in a lower wind count motor is the phase amps drop less quickly and you remain in an inefficient state, you cook the windings over time. The idea is to reach the unloaded kV rating, which nobody has been able to do...BEMF becomes self-limiting after a point. Dropping the 2806 in a smaller rim lowers the top-speed required to limit the phase amps and enter the higher efficiency state, but, that is already done for you with the 2812. Even though both motors are receiving the exact same peak power they react to it in different ways.

The no load speed is 48 kph for both systems. Same wheels, different voltages and currents. Ideal batteries, controllers and wires
At 24 kph, the 2806 makes a BEMF voltage of 18V, while the 2812 produces twice as much: 36V.
Thus the 2806 is "fed" with 18V and 2812 with 36V. Assuming the 2812 has 4x the resistance, their input power (V times A) will always be the same. And since both motors utilize the same total number of equal copper strands, the torque at that particular speed will be same too.
 
JoramsWeapon said:
Let's make it more simple to understand the basics.
Example 1: 2806, 36V, 40A
Example 2: 2812, 72V, 20A
For now, let's presume the batteries and controllers have zero resistance, and phase ratio is the same.
Same input power, same copper loss, same rpm = same efficiency.
That's theory. But in practice, it seems higher winds seem to win. Why?

Same rpm and same efficiency for those motors and power levels are impossible, the 2806 must spin faster to generate enough Back Electromotive Force (BEMF) to limit the phase current and enter the higher efficiency ranges. When your motor is at a dead stop it is generating zero BEMF until you hit the throttle. Hit the throttle and you are drawing huge amps per phase, as your rpm increases BEMF begins to take over and you go from a motor condition of say 20V85A to 48V15A at cruising speed. What happens when you're not generating enough BEMF in a lower wind count motor is the phase amps drop less quickly and you remain in an inefficient state, you cook the windings over time. The idea is to reach the unloaded kV rating, which nobody has been able to do...BEMF becomes self-limiting after a point. Dropping the 2806 in a smaller rim lowers the top-speed required to limit the phase amps and enter the higher efficiency state, but, that is already done for you with the 2812. Even though both motors are receiving the exact same peak power they react to it in different ways.

JoramsWeapon, you forgot to account for PWM so your above statement isn't accurate.

I just want to add this as most people seem to leave this little bit of info out on hub motor discussions and focus too much on wide open throttle performance only when in reality most riding is done at varying states of throttle unless you are a binary throttle user (good luck trying that on any high powered ebike, nothing but sky then a big shock as you end up on your back).

If you run a motor in the PWM range it's effectively the same as running a lower battery voltage and you still get into BEMF no matter what wind the motor is. The only downside to this is the controller has to be able to handle the amount of heat dissipated. Hub motors are very efficient at almost all throttle ranges because of this except when going really slow. High load + going slow + needing a lot of throttle to maintain that speed is what will build lots of heat into a hub motor because it's in a poor efficiency range.

In the example above with the 2806 at 36V drawing 40A compared to the 2812 at 72V drawing 20A showing the max speed is the same, but that same result can also be achieved if you run the 2806 at 72V and run 50% throttle (50% PWM) at which point it will draw right around 20A as well.
 
I may be missing or just too ignorant of the motor tech, but it seems to me that all depends on how you ride.

Case in point, Zombies and I both ride dirt a lot. But the motor he likes and the motor I like are on opposite poles because the way we ride is so diffferent. The max speed I want is a lot slower than Zombies wants. But both do the same job, getting up a steep hill.

The only rule of thumb I have, is this. The slower you want to be climbing steep hills, the slower the winding you want. The name of the game is not stalling the motor. The two basic approaches to not stalling the motor are slower winding at 1000-2000w, or bigger motor so it can handle the 3000 or more watts needed to fly up hills.


The lower power, slower winding approach I like does make binary throttle work fine. I ride all day in Wot on or off mode. Flipping the power on 2 seconds, off 2 seconds, over and over. One nice thing about this, is it rarely PWM's and I never get a hot controller.
 
zombiess said:
In the example above with the 2806 at 36V drawing 40A compared to the 2812 at 72V drawing 20A showing the max speed is the same, but that same result can also be achieved if you run the 2806 at 72V and run 50% throttle (50% PWM) at which point it will draw right around 20A as well.

The result will be less torque because of PWM loss. This is the first toll you pay for higher speed potential. The second is that using higher voltage / less battery current controller leads to weaker low end torque once you go so slow that you reach the phase current limit. Ultimately, doing more PWM means more heat in the controller and less overall efficiency, especially after the FETs heat up.

dogman said:
I ride all day in Wot on or off mode. Flipping the power on 2 seconds, off 2 seconds, over and over.

Dogman, you realize that having your throttle on all the time instead of 50% of the time would make it much more efficient and thus even cooler...

dogman said:
One nice thing about this, is it rarely PWM's and I never get a hot controller.

Hell no. Unless you travel over about 70% of your no load speed, your controller always PWM's. The reason your controller doesn't get so hot is because you have a low speed rig and the PWM band is still fairly wide.
 
Hmm, I thought if you lugged the motor full throttle you just made heat in the motor. How does the controller know to pwm? I thought it was strictly by throttle position.

Perhaps I should explain the on off throttle use. If I held it on all the time, even at part throttle, the result would be implalement on cactus. To clarify, it's throttle on, then brakes on throttle off, then throttle on again. It's called trail riding.

You guys have fun figuring out why your motor is efficient. I'm perfectly happy to just look at my CA and know mine is.
 
The PWM story is incomplete in real world use though, because while riding we are constantly accelerating and decelerating. During acceleration the high speed wind motor will chew up current. My 2 speed motor that splits the windings in half and switches the 2 halves from series for slow to parallel for high speed, really enables me to see what happens with essentially 2 differently wound motors just by switching a lever. The controller even behaves somewhat differently between the 2 modes. What amazed me the most is how much lower the currents are in slow mode. Absolute peak current is quite similar and happens as near zero rpm, but after that the current tapers off far more quickly making acceleration virtually identical. Where the slow wind pays dividends is hill climbing as long as it can generate the power you need to climb the hill you want at a given speed, which is typically where they fall short for sizable people or really steep hills.

High speed winds need to go fast to reach peak efficiency, and if you accomplish that via a smaller tire then it will have greater peak efficiency. Otherwise the slower wind will typically be more efficient overall in real world use. The exception is to use fairly extreme currently limiting with a speed wind motor. It will give you less thrust on takeoff, but can greatly broaden the efficiency curve making it more efficient overall. To demonstrate this effect, use the Ebikes.CA simulator, and input the fast wind motor on there, an X5302. Give it a 74V batter, a 40A controller, and throttle from 60% to 100%. Slower winds only dream of those results.
 
John in CR said:
To demonstrate this effect, use the Ebikes.CA simulator, and input the fast wind motor on there, an X5302. Give it a 74V batter, a 40A controller, and throttle from 60% to 100%. Slower winds only dream of those results.

John, I definitely see the reason in fast motors and especially small wheels. You can reach similar results with a slow motor and double the voltage, given same wheel size. Yes you need high voltage fets, but less current, so thinner wires and/or less electricity turned into heat.

But let's abstract from other components. The misconception that I perceive is, that the fast motor itself is less efficient and generates more heat (as dogman has said many times), whereas my impression is, the heat and inefficiency come from all other components expect the motor itself, except maybe phase wires and wiring between copper windings. At least this is what I gathered from Justin's tech page.
 
Given our voltage limits, the faster wind motors are more efficient and capable of greater power. Thicker shorter copper means less copper losses, which are the primary loss in our motors at our rpms. You have to go to impractically high voltages to get similar operation from the slow motor.

It's all a kind of a moot point though without going to a smaller wheel, at least until we get someone to build a proper geared hubbie. The too steeply geared systems is why it takes a 15lb+ motor just to go 30mph, when a 2lb RC motor can do the same thing.

In the meantime this 2 speed motor is giving me the best of both worlds. 1. Low speed wind for crazy steep hill climbing and more efficient acceleration up to moderate speeds. 2. High speeds. Add in that peak efficiency is 93% in high speed mode and 90% in low, and I've got a hubmotor head and shoulders above other ebike and scooter hubbies. :mrgreen:
 
Fast wind motors seem to have a higher proportion of wasted copper between teeth to useful copper on the tooth than slow wind motors, so are inherently less efficient, if everything else is kept the same.

You can see this clearly in Farfle's rewind of a pair of Magic Pie's. Between the teeth there are 4 strands, and the teeth have just 2 turns. If the motor was wound with 2 strands of 4 turns the wasted proportion of the wire would go down by a large factor, but the motor would require twice the voltage and half the current.
 
For the record, I have never said a fast motor makes more heat all the time.

Maybe I just put it in the wrong words to communicate it right.

Here's the deal. ALL motors make heat when you lug them. They hate it. Under specific riding conditions, that is, riding up very steep hills, a slow wind motor will not lug for as much time, because it reaches it's efficient speed zone at a slower speed. Since it reaches a somewhat more efficent sooner it will make less heat. If the fast motor never reaches a moderately efficient speed, then the fast motor makes heat all the way up the hill. That's not so uncommon if the wheel is big.

This applies only to that specific riding conditions. Whether that top speed penalty is a penalty or a bonus is entirely up to you, and your particular riding needs.

Damn right 20 inch wheel and more power works great on pavement. The effect is still about the same, a wheel size that matches the winding enough to climb hills better. I'm stuck on 26" wheels because of the rocky terrain I ride on. So for my particular needs, the slow wind 26" works great for me. The fact that the max speed I'll fly over the bars is 25 mph is a definite bonus for me. Since I RIDE, I crash. I don't cruise trails, I ride em. But I have no desire to ride like that at 40 mph. I don't pwm my controller much, because I can handle full throttle.

Works for me. If you never tried it, you have zero real world experience with how it rides. You may well be correct about how a motor works in therory, but once you add the other variables, things get different. Add the 26" wheel, and specific terrain conditions, and then your motor may never get to run like it does on the sim.
 
Alan B said:
Fast wind motors seem to have a higher proportion of wasted copper between teeth to useful copper on the tooth than slow wind motors, so are inherently less efficient, if everything else is kept the same.

You can see this clearly in Farfle's rewind of a pair of Magic Pie's. Between the teeth there are 4 strands, and the teeth have just 2 turns. If the motor was wound with 2 strands of 4 turns the wasted proportion of the wire would go down by a large factor, but the motor would require twice the voltage and half the current.

Thanks Alan, I was trying to explain the same couple posts ago, but you've done it so much better. I'll have to check that thread out.
 
Is there any Math that can tell me the kV of a motor winding if another winding kV is known.

For Example, the Standard wind of the MXUS 45mm Motor is 4X15 with 400RPM at 48V or 8.33 RPM per Volt.
What kV would the same motor have with a 5X12 winding?
or with a 6X10 Winding?
 
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