Understanding QS205 efficiency and power

Darren2018

100 W
Joined
Aug 18, 2018
Messages
223
Could someone please help me to understand what parameters and variables effect efficiency and performance?

I have tried to use the data that I have for the QS205 on the Grin motor sim but I have a feeling the results are not accurate. I used the Cromo5004 and adjusted the Kv to suit the 3T, 4T and 5T values that QS have given but are there not other variables that matter like resistance or is it simply the amount of copper when assuming that both motors will be driven with the max a controller can supply and the same voltage. I am assuming due to the winding resistance it is essential to choose a motor that is wound correctly for the battery voltage you will be using or you will not be able to access all of its potential. I have read hundreds of forum posts and I might be completely wrong but it seems people are choosing their QS205 based on "torque/speed" when it should be based on voltage and wheel size providing your battery can supply the current required. Please can someone clarify if I am right or not :? Also if you understand how it works please could you explain it as simply as possible as I don't really understand it properly.
 
To make it really simple, Power out = BEMF*phase current. BEMF is linear with speed, so with a higher KV motor, at the same RPM, for the same power out you put more current in your motor. For the same motor a higher KV also means more copper cross section of windings and theoricaly everythings is linear (kV increase and cross section increase) but in practice increase in current means more losses in the controler and resistance isn't decreasing linearly so more copper losses in the motor. For the better efficiency possible you should choose a motor giving you the max speed you want with the battery voltage you got.
I'm not that much informed about those motor but this is the basics i've come to understand.
I'm sure some pros here will answers for the specific one you are talking about and hopefully correct me if i'm thinking wrong ;)

Envoyé de mon Redmi Note 3 en utilisant Tapatalk

 
So what you are saying is there is no gain as the resistance losses will be the same regardless to get identical power. Would a 5T be able to make the same power as a 3T when restricted to 21S but with identical controller settings?
 
Thecoco974 said:
To make it really simple, Power out = BEMF*phase current. BEMF is linear with speed, so with a higher KV motor, at the same RPM, for the same power out you put more current in your motor. For the same motor a higher KV also means more copper cross section of windings and theoricaly everythings is linear (kV increase and cross section increase) but in practice increase in current means more losses in the controler and resistance isn't decreasing linearly so more copper losses in the motor. For the better efficiency possible you should choose a motor giving you the max speed you want with the battery voltage you got.
I'm not that much informed about those motor but this is the basics i've come to understand.
I'm sure some pros here will answers for the specific one you are talking about and hopefully correct me if i'm thinking wrong ;)

Envoyé de mon Redmi Note 3 en utilisant Tapatalk

Does this data look correct for these QS205 windings? If this is right then the 3T makes almost double the hp and for 83% of its RPM range it has more power.
5T 3T.png
 
I'm not sure how it come up with that, if it is setup with the same current the thrust off the 5T should stay higher longer. But efficiency Makes sense, you can see the 5T is more efficient at lower speed. It all come up to your use case. If you need range and city rinding 5T might ne better, if you need more max speed 3T it is

Envoyé de mon Redmi Note 3 en utilisant Tapatalk

 

Does this data look correct for these QS205 windings? If this is right then the 3T makes almost double the hp and for 83% of its RPM range it has more power.
5T 3T.png
[/quote]

That is only power at peak load.. ( full throttle).... and the 5T runs into its loaded speed. The 3T keeps pulling power until it gets there ( to its.. higher, loaded speed) .. and therefore needs more power to make it. .... . Get the MPH ( load ) the same on (3T) the top of the RPM range ( by wheel dia) and you will see them both pulling the same power (the 3T and 5T.. ) .
 
Thecoco974 said:
I'm not sure how it come up with that, if it is setup with the same current the thrust off the 5T should stay higher longer. But efficiency Makes sense, you can see the 5T is more efficient at lower speed. It all come up to your use case. If you need range and city rinding 5T might ne better, if you need more max speed 3T it is

Envoyé de mon Redmi Note 3 en utilisant Tapatalk

Would it though? It looks like the 5T is limited by the voltage where as the 3T is capable of fully utilising the controllers bA and pA limits. Providing this data is correct 20S is no where near enough to make the 5T a "torque" version just look at the amount of usable power and torque that the 5T sacrifices just to make on average ~25 lb of thrust more from 0-10mph. The 3T makes over 13hp at the wheel and the usable RPM range is massive when compared to the 5T. I think the efficiency at a regular speed of say 30mph is also not really a problem because these pulls are based on WOT and once you modulate the throttle even a difference of 5% will not be a problem because you will only be pulling ~1200W.

I wonder if 5T with FW is better than maxing out the controller by using a 3T. The controller has plenty of room for extra current plus the inefficiency that comes with it but only once the rpms have increased beyond the point at which the controller is capped. If the controller and motors capabilities are able to be manipulated then maxing these out would be ideal and choosing the motor with the highest amount of torque natively might actually be a benefit rather than a sacrifice. I read a post from doctorbass regarding this but I can't find it now and I don't think there was any direct data comparing the 3T and 5T with FW but I know he said that the 5T with FW was good.

Has anyone actually got data or an explanation as to how this exactly works?
 
The lower turn count has more copper per turn, and shorter coil-runs. This means a 3T has less resistance than a 5T.

If all other things are equal (same tire diameter, same voltage, etc) is it better to attain "X" speed with a 5T at full throttle, or 3T at a partial throttle?
 
spinningmagnets said:
The lower turn count has more copper per turn, and shorter coil-runs. This means a 3T has less resistance than a 5T.

If all other things are equal (same tire diameter, same voltage, etc) is it better to attain "X" speed with a 5T at full throttle, or 3T at a partial throttle?



From the data there is not much of a difference. What I would like to know is what can be exploited with FW? How inefficient is it? Some people say it causes the motor to get hot and some say they only get the increased speed without much of an impact to efficiency. When does the controller start to apply the FW and what are the limitations?
 
Field weakening is useful. Its not a bad feature to have, but even though it uses more watts per minute than when the controller is back in it's designed normal operations profile, that's not a big concern to me since I would only use FW on rare occasions.

My biggest concern over FW is that it generates more heat than normal. That's also not a big problem for someone who rarely uses it, but...after considering the big picture, using a low-turn-count motor (3T vs 5T) also provides some headroom for an occasional high-speed burst on rare occasions. Between the two, I'd prefer a lower turn-count motor over FW, but...why not have both?
 
spinningmagnets said:
Field weakening is useful. Its not a bad feature to have, but even though it uses more watts per minute than when the controller is back in it's designed normal operations profile, that's not a big concern to me since I would only use FW on rare occasions.

My biggest concern over FW is that it generates more heat than normal. That's also not a big problem for someone who rarely uses it, but...after considering the big picture, using a low-turn-count motor (3T vs 5T) also provides some headroom for an occasional high-speed burst on rare occasions. Between the two, I'd prefer a lower turn-count motor over FW, but...why not have both?

Which controller are you using? What do you think of the Nucular 24F? I was going to use the BAC4000 but it looks quite difficult to get the software without having to pay ridiculous amounts of money and the 24F has lots of genuinely useful features. I am not really sure how Vasili has implemented FW but I am going to assume it is an extension of torque and not an increase. What is this?
aBLF41.jpg
 
Which controller are you using? What do you think of the Nucular 24F? I was going to use the BAC4000 but it looks quite difficult to get the software without having to pay ridiculous amounts of money and the 24F has lots of genuinely useful features. I am not really sure how Vasili has implemented FW but I am going to assume it is an extension of torque and not an increase. What is this?

Field Wakening (FW) is an extension of speed/RPM not torque.

As Thecoco974 said on his first post: "For the better efficiency possible you should choose a motor giving you the max speed you want with the battery voltage you got."
Please note that the motor has to be able to maintain that speed (enough power) without overheating.

If you want to add FW to be used very occasionally that is find, but if you pretend to run a lot using that higher speeds using FW feature then choose a higher KV motor.
 
_GonZo_ said:
Which controller are you using? What do you think of the Nucular 24F? I was going to use the BAC4000 but it looks quite difficult to get the software without having to pay ridiculous amounts of money and the 24F has lots of genuinely useful features. I am not really sure how Vasili has implemented FW but I am going to assume it is an extension of torque and not an increase. What is this?

Field Wakening (FW) is an extension of speed/RPM not torque.

As Thecoco974 said on his first post: "For the better efficiency possible you should choose a motor giving you the max speed you want with the battery voltage you got."
Please note that the motor has to be able to maintain that speed (enough power) without overheating.

If you want to add FW to be used very occasionally that is find, but if you pretend to run a lot using that higher speeds using FW feature then choose a higher KV motor.

Thanks for the advise. I am not a motor expert in any way and I am just trying to understand how these things work but could you explain how you can create extra speed/RPM without additional torque?

I have been planning my build for over a year now and I thought that I wanted efficiency but when you look at the graph you can see how much power is lost. I cycle because I like exercise and I have no intention to do frequent high power runs so I would rather have the extra power over a very small gain to the efficiency.
 
FW is similar to tune advance on an internal combustion engine. You get more revs. but the torque does not increases as you are not using all the potential from the magnetics fields.

Bicycles are very efficient by themselves, most e-bike motors if well proportionated for the task will perform quite similar and differences on efficiency will not be so remarkable. Sometimes you may end expending lots of money in order to get very efficient controller and motor, and probably could better invest that money on just a bigger battery that will give you more power and range.
 
Field Wakening (FW) is an extension of speed/RPM not torque.

Field weakening increases torque in order to get those higher speeds/rpms. That's the point.

DC motors lose the ability to produce torque the faster they spin. When you reach top speed on your bicycle the torque output of the motor is balanced against total drag on the rider and bike. By applying field weakening you are increasing the amount of torque available to the motor at higher RPM, thus allowing you to reach higher speeds.

Depending on motor specifics field weakening may not increase peak torque. But peak torque only occurs at very low RPM on these types of motors and usually there isn't a whole lot you can do about it. Torque output for DC motors is a factor of amperage. More amps means more torque, but also means more heat. Which means that heat is almost always going to be the limitation on peak torque.
 
sleepy_tired said:
Field Wakening (FW) is an extension of speed/RPM not torque.

Field weakening increases torque in order to get those higher speeds/rpms. That's the point.

DC motors lose the ability to produce torque the faster they spin. When you reach top speed on your bicycle the torque output of the motor is balanced against total drag on the rider and bike. By applying field weakening you are increasing the amount of torque available to the motor at higher RPM, thus allowing you to reach higher speeds.

Depending on motor specifics field weakening may not increase peak torque. But peak torque only occurs at very low RPM on these types of motors and usually there isn't a whole lot you can do about it. Torque output for DC motors is a factor of amperage. More amps means more torque, but also means more heat. Which means that heat is almost always going to be the limitation on peak torque.

Ok I think I understand but could you explain exactly where or when you will see the increase in torque? I will be using a Nucular 24F controller if this makes any difference to how the FW is implemented. Could you draw on the graph and show me where the extra torque would start and what type of increase I could expect to see with 30% FW applied? Does the increase start from the moment the controller is no longer capped by its battery or pA limit?

MXUS 250A.png
 
FW only kicks in at speeds (rpm) when (higher levels of) torque is no longer needed
 
john61ct said:
FW only kicks in at speeds (rpm) when (higher levels of) torque is no longer needed

Sorry John I don't quite understand :? Do you mean that it will start to take effect after the controller is no longer pegged at its full battery amp limit?

Would someone please draw it on the graph for me? :roll:
 
Voltage is rpm and current is torque.

Torque is mostly a low speed thing

FW is all about allowing rpm to go higher than otherwise possible, when there is no longer need for so much torque.

Thousands of graphs out there, GIYF
 
Theoretical curves, showing maximums, yes.

IRL actual performance of course very different, so many various factors come into play
 
Back
Top