Difference between 250w and 350w motor

[colin22]

Hi all
I have been riding an ‘Unrestricted’ (that’s the manufacturer) ebike for a while. It has a 350W rear motor and I changed the controller when I first had it to one without restriction. (Yes I understand the ramifications of doing so). It accelerates quickly, and tops out at about 17mph - which is close enough to 15 for me.

I now also have a Batribike Sigma (which I bought because it has 7 speed Nexus). This has a 250W front motor and is restricted as it should be in the UK. It accelerates fairly slowly and still requires a lot of effort when going up hills (unlike my other ebike).
My question is: If I changed the controller to an unrestricted one, would I notice any difference in either acceleration of top speed?
Or to ask it another way: Is there a big difference between a 250W and a 350W motor in terms of acceleration or top speed?

Thanks

 

[Peter.Bridge]

It's the controller that "supplies" the power, not the motor.

The maximum current that the controller can supply is the limit to the power. This determines how easily it will go up hills and how heavy a rider it can accommodate.

The motor will have a maximum "no load" speed. Supplying more power from the controller won't affect this.

The controller will usually have a setting as to the maximum speed that it will assist. If you go over this speed then the controller will stop supplying power to the motor. Legally in the UK this should be set to 15.5 mph (or 17 mph if you want to take advantage of the 10% leeway)

Motors are most efficient at around 80% of their maximum "no load" speed, so it's best to have a motor that operates at this sort of speed for most of your riding

 

[colin22]

It's the controller that "supplies" the power, not the motor.

The maximum current that the controller can supply is the limit to the power. This determines how easily it will go up hills and how heavy a rider it can accommodate.

The motor will have a maximum "no load" speed. Supplying more power from the controller won't affect this.

The controller will usually have a setting as to the maximum speed that it will assist. If you go over this speed then the controller will stop supplying power to the motor. Legally in the UK this should be set to 15.5 mph (or 17 mph if you want to take advantage of the 10% leeway)

Motors are most efficient at around 80% of their maximum "no load" speed, so it's best to have a motor that operates at this sort of speed for most of your riding

Thanks for your answer.
It seems that changing the controller may well help me get some more power/acceleration then.

 

[colin22]

Why not just pedal harder or better still, if you want to go faster, buy a moped or a motorbike?

Thanks for spending your time answering - it’s very helpful.

 

[Ghost1951]

I'm glad it was helpful.

 

[thelarkbox]

A kt (ketung) brand controller is what is generally advised as an aftermarket upgrade/replacement. 2 reasons 1) a wide range of display lcd options, and 2) a configurable system tho the chenglish manuals are a pita..
( Controller and display generally come as a pair and buying as such avoids many issues tho KT brand is an exception to this otherwise general rule.. )..

For more power you will want a controller rated to handle more amps than your current controller, so if you have a 12a controller fitted?? look at a 15 a replacement, if its a 15a controller fitted? perhaps look at a 17a controller? Its the peak power rating in amps that we go by when discussing a controller rating..

Watts = Volts X Amps so im sure you can do your own calculations as to why more than 17a might warm up a 250w rated motor..

just be mindful to buy a sinewave controller unless specified a cheaper squarewave controller could be on offer..

Once fitted setting the speed to the legal limit is no problem.

 

[saneagle]

Tell us your weight and what sort of hills you have, then I can recommend a specific controller.

Whichever controller you get, there will probably be a bit of soldering required to make connectors match.

 

[Az.]

What is important for you? Speed or how easy it is to ride up hills?
If you derestrict e-bike and remove 25km/h speed limit, you will have illegal bike.
Power delivered to the motor is not limited by the law. You can have very powerful and legal motor which will accelerate quickly and climb hills easily on condition it will be rated 250W.

 

[dlwest]

It's the controller that "supplies" the power, not the motor.

I would partially agree with that. The controller can be limiting the phase current it drives the motor with. Most of the motor softwares available use torque control algorithm where torque is proportional to stator current. Hence, the maximum torque depends on maximum phase current defined by the controller. But the motor's inductance dictates how much current the winding can develop at a given input voltage. So, with a high inductance (high voltage) motor, the controller might never be able to reach its current limit if supplied from lower voltage.

 

[saneagle]

I would partially agree with that. The controller can be limiting the phase current it drives the motor with. Most of the motor softwares available use torque control algorithm where torque is proportional to stator current. Hence, the maximum torque depends on maximum phase current defined by the controller. But the motor's inductance dictates how much current the winding can develop at a given input voltage. So, with a high inductance (high voltage) motor, the controller might never be able to reach its current limit if supplied from lower voltage.

Every hub-motor I've tried could reach the max current given by the controller. The only time when the back emf affects the current is from mid to high speed, depending on how many turns in the motor's coild and the applied voltage. Your theory is correct, but it hardly effects our types of ebike.

 

[Sturmey]

It's the controller that "supplies" the power, not the motor.
The maximum current that the controller can supply is the limit to the power. This determines how easily it will go up hills and how heavy a rider it can accommodate........

The controller may supply the electrical power but its the motor that converts this electrical power into mechanical power and its this mechanical power that counts.
For example, a larger motor (e.g Bafang BPM) may be more efficient than a smaller (e.g mxus Xf07) even with similar controllers when climbing a hill as the smaller motor get bogged down and becomes more inefficient with much of the electrical power lost in heat.
I have seen this myself when I initially changed from an xf07 (250w) to a xf15 (350 -500W) before I upgraded the controller.
I have tried to model this below (comparing the xf07 with the BPM) with the simulator and there seems to be a huge difference in efficiency with the same controller (15A) and 250 rpm motors.(48% versus 66%) and the BPM supplying over 33% more mechanical power for the same electrical input.
So in my view, even changing the motor alone can also give a power boost on hills as the stronger motor is more efficient under heavy load.
If more speed is required, then of course changing to a higher rpm may also be desirable.

Motor Simulator - Tools

Our ebike motor simulator allows you to easily simulate the different performance characteristics of different ebike setups - with a wide selection of hub motors modeled, and the ability to add custom batteries and controllers and set a wide variety of vehicle parameters you'll be able to see...

ebikes.ca

 

[saneagle]

The controller may supply the electrical power but its the motor that converts this electrical power into mechanical power and its this mechanical power that counts.
For example, a larger motor (e.g Bafang BPM) may be more efficient than a smaller (e.g mxus Xf07) even with similar controllers when climbing a hill as the smaller motor get bogged down and becomes more inefficient with much of the electrical power lost in heat.
I have seen this myself when I initially changed from an xf07 (250w) to a xf15 (350 -500W) before I upgraded the controller.
I have tried to model this below (comparing the xf07 with the BPM) with the simulator and there seems to be a huge difference in efficiency with the same controller (15A) and 250 rpm motors.(48% versus 66%) and the BPM supplying over 33% more mechanical power for the same electrical input.
So in my view, even changing the motor alone can also give a power boost on hills as the stronger motor is more efficient under heavy load.
If more speed is required, then of course changing to a higher rpm may also be desirable.

Motor Simulator - Tools

Our ebike motor simulator allows you to easily simulate the different performance characteristics of different ebike setups - with a wide selection of hub motors modeled, and the ability to add custom batteries and controllers and set a wide variety of vehicle parameters you'll be able to see...

ebikes.ca

What you're describing sounds more like the effect of a lower winding speed on the BPM. If it were the same winding speed (max motor speed) as the other, it would have bogged down too, since it would have the same climbing power. If you ran it with higher current, of course it would go up the hill faster

With the above in mind, you can reduce the amount of battery used up a hill by increasing the current. That's counter-intuitive. The higher current gives more power, so more speed, the gain in efficiency from the higher speed is more than the extra power from the higher current. That only works when the motor originally didn't have enough power to reach around 40% of its max speed or less.

 

[Sturmey]

What you're describing sounds more like the effect of a lower winding speed on the BPM. If it were the same winding speed (max motor speed) as the other, it would have bogged down too, since it would have the same climbing power. If you ran it with higher current, of course it would go up the hill faster

With the above in mind, you can reduce the amount of battery used up a hill by increasing the current. That's counter-intuitive. The higher current gives more power, so more speed, the gain in efficiency from the higher speed is more than the extra power from the higher current. That only works when the motor originally didn't have enough power to reach around 40% of its max speed or less.

I tried it myself on a local 10% hill. The bike had a Kt Lcd3 and 15A controller. On swapping out the xf07 for the xf15, I gained a noticeable increase in climbing ability for the same power as shown on LCD3. I put this down to the increased efficiency of the larger motor under heavy load. Mxus own graphs show a huge deterioration in efficiency (50 to15%) above 30nm for the smaller xf07 motor. Climbing a 10% hill for me drives the motor 'over the cliff' graph wise so to speak as the motor is approaching its limit. The xf15 (65nm rated) has more copper and a bigger stator as well as slightly larger reduction gearing due to its increased size and weight, so I think its reasonable to assume that the larger motor is much more efficient under heavy load/torque. The simulator also models this. I did afterwords upgrade the controller to 22 amps but I did note that the change of motor alone gives a noticeable increase in mechanical power which I think can only be accounted for because the bigger motor does not suffer the same decrease in efficiency as the smaller motor whose load/torque is reaching towards its limit.
Anyhow, its something you could try yourself some day.

 

[Peter.Bridge]

I tried it myself on a local 10% hill. The bike had a Kt Lcd3 and 15A controller. On swapping out the xf07 for the xf15, I gained a noticeable increase in climbing ability for the same power as shown on LCD3. I put this down to the increased efficiency of the larger motor under heavy load. Mxus own graphs show a huge deterioration in efficiency (50 to15%) above 30nm for the smaller xf07 motor. Climbing a 10% hill for me drives the motor 'over the cliff' graph wise so to speak as the motor is approaching its limit. The xf15 (65nm rated) has more copper and a bigger stator as well as slightly larger reduction gearing due to its increased size and weight, so I think its reasonable to assume that the larger motor is much more efficient under heavy load/torque. The simulator also models this. I did afterwords upgrade the controller to 22 amps but I did note that the change of motor alone gives a noticeable increase in mechanical power which I think can only be accounted for because the bigger motor does not suffer the same decrease in efficiency as the smaller motor whose load/torque is reaching towards its limit.
Anyhow, its something you could try yourself some day.

View attachment 59759

Very interesting, thanks.

Isn't this very much speed dependent? I thought the smaller motors were very inefficient at low speeds under high loads, but the larger motors are better at coping with it.

What sort of speeds were you getting on your test hills with the xf07 vs xf15 for the same controller current ?

My rule of thumb is not to go below 7-8 mph on hills for more that a few minutes

I wondered if higher current would let you go faster and make the motor run in it's efficient speed band ?

 

[saneagle]

Very interesting, thanks.

Isn't this very much speed dependent? I thought the smaller motors were very inefficient at low speeds under high loads, but the larger motors are better at coping with it.

What sort of speeds were you getting on your test hills with the xf07 vs xf15 for the same controller current ?

My rule of thumb is not to go below 7-8 mph on hills for more that a few minutes

I wondered if higher current would let you go faster and make the motor run in it's efficient speed band ?

You can go as low speed as you like if you have a slow enough motor. When you choose a hub-motor motor, you should choose one with a winding speed that suits your circumstances rather than one that comes with a kit or is generally recommended by people that might have different circumstances to you.

A code 16 Bafang BPM will drag you up a cliff. A code 6 one will get you to the shops at 30 mph. You can't do both with either of them.

 

[Sturmey]

What sort of speeds were you getting on your test hills with the xf07 vs xf15 for the same controller current ?

I have the 20A controller on the xf07 and a 22A on the xf15, so I I reduced the 22 amp controller to 20 amps for the test, (setting C5 to 9). I rechecked the figures today. Originally, the xf07 climbed the steepest part of the hill at 11km/hr with the 15 amps controller. With the 20 amps controller, the xf07 climbs the hill at 14 Km/hr.
The xf15 climbs the same hill at 17 km/hr with the 22 amp controller reduced to 20 amps.
Its also noticeable that the xf15 runs cooler.

 

[Peter.Bridge]

I have the 20A controller on the xf07 and a 22A on the xf15, so I I reduced the 22 amp controller to 20 amps for the test, (setting C5 to 9). I rechecked the figures today. Originally, the xf07 climbed the steepest part of the hill at 11km/hr with the 15 amps controller. With the 20 amps controller, the xf07 climbs the hill at 14 Km/hr.
The xf15 climbs the same hill at 17 km/hr with the 22 amp controller reduced to 20 amps.
Its also noticeable that the xf15 runs cooler.

That's interesting - were they both at 36v ? and presumably the XF15 has a higher no load max speed ? XF15 kits with 15A 36V/48V KT controllers and displays are available at PSWpower - seem very reasonable £7.09 shipping

 

[saneagle]

That's interesting - were they both at 36v ? and presumably the XF15 has a higher no load max speed ? XF15 kits with 15A 36V/48V KT controllers and displays are available at PSWpower - seem very reasonable £7.09 shipping

View attachment 59775

Logically, if the XF15 climbs faster with the same battery, it's because it has a lower maximum no-load speed. That's the only way it can be more efficient in converting the same battery current. A motor can't give any more power than another one because of its size or rating. The only variables is the efficiency at the speed it's going, which depends on the max no-load speed that's a result of the number of turns in the coils and the internal gearing ratio. There is also the external factor of wheel size, which affects the speed and efficiency the same as the motor's internal gearing.

The XF15 can handle more power than the XF07. If you put too much current into the XF07 it gets seems to get saturated somehow. Some years ago, I had some around 260 rpm XF7Cs 260 and found no noticeable power difference between running with the controller set to 18A or 22A, but it was warmer at 22A. When I used the 22A controller with a 230 rpm Bafang BPM, the difference was very noticeable. I contacted my friends at MXUS, who said that there were different versions of the XF07, depending on the market they sold them to. Some have lower grade magnets. I tanslate that as the cheap ones from Chinese sellers have crap magnets, and the ones with the stronger magnets go to OEMs.

 

[Peter.Bridge]

A motor can't give any more power than another one because of its size

Here - the xf07 has a no load max speed of 30km/h and the BPM a no load max speed of 39 km/h. However up a 10% hill the xf07 only generates 323w and reaches 13.8 kph compared to the BPM that generated 391w and reaches 15.8 kph for the same input voltage / current from the controller. The BPM shows 70.6% efficiency vs 58.4% for the xf07 - this is how I understood it worked - bigger motors are more efficient at lower revs (going up a steep hill)

 

[saneagle]

View attachment 59786

Here - the xf07 has a no load max speed of 30km/h and the BPM a no load max speed of 39 km/h. However up a 10% hill the xf07 only generates 323w and reaches 13.8 kph compared to the BPM that generated 391w and reaches 15.8 kph for the same input voltage / current from the controller. The BPM shows 70.6% efficiency vs 58.4% for the xf07 - this is how I understood it worked - bigger motors are more efficient at lower revs (going up a steep hill)

Look at the MXUS efficiency curve. It has a big dip just before maximum efficiency. That's not normal. If it had the same shape as the BPM, it would have made more power. I would have wanted that one tested again, if that's what they did, otherwise they need to sort out their prediction algorithms unless someone can explain why it should be like that.

Also, look at the torque. How can the MXUS make 35% more torque than the BPM. I don't believe that it can by that much.