Higher watts advantages?

[Adwb]

High watts advantages?

I was looking at the official Bafang product catalog and noticed to my surprise that every motor model that offers 250/500/750 watts option list exactly the same max torque.
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What benefit therefore does buying a larger wattage motor give you?
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And What benefit do you gain by using 48 volt over 36 volts?

By changing the max amps that the control is set to via C5 for example from setting 10 to 04 what effect has this on battery power vs miles achieved ?

 

[Nealh]

Greater wattage means more power if one can supply that power by current, downside is battery needs to be able to deliver the extra current /power. Most 250w motors can handle 1kw if not abused.

With greater voltage one can use less current, less current is less heat.
48v /36v equals 33% more torque/power given the same current.

C5 depends on how you use the current/power via the five PAS levels.
Typically if one uses higher PAS levels then reducing C5 to 4 will use much less power output so battery use will be less demanding. Range should increase compared to having C5 at 10.

With C5 one should experiment to see how range is affected by changes in C5 value and PAS level use, the variation is quite marked.

 

[Adwb]

Greater wattage means more power if one can supply that power by current, downside is battery needs to be able to deliver the extra current /power. Most 250w motors can handle 1kw if not abused.

With greater voltage one can use less current, less current is less heat.
48v /36v equals 33% more torque/power given the same current.

C5 depends on how you use the current/power via the five PAS levels.
Typically if one uses higher PAS levels then reducing C5 to 4 will use much less power output so battery use will be less demanding. Range should increase compared to having C5 at 10.

With C5 one should experiment to see how range is affected by changes in C5 value and PAS level use, the variation is quite marked.

Thank you , Nealh, for the reply. So if I know my battery is 48 volt 17 amp hour capacity is there a formula to set the controller to a lower power setting with out sacrificing to much of the uphill grunt that having C5 at 10 gives.
if memory serves me right the controller is a 18 or 20 amp max output controller .

 

[saneagle]

Thank you , Nealh, for the reply. So if I know my battery is 48 volt 17 amp hour capacity is there a formula to set the controller to a lower power setting with out sacrificing to much of the uphill grunt that having C5 at 10 gives.
if memory serves me right the controller is a 18 or 20 amp max output controller .

If you have a KT controller, you don't need to turn down the current. The only logical reason to do it would be if your battery was cutting out because it couldn't supply the maximum of 20A that the controller would allow. If you want to use less current so that you can go further, select a lower pedal assist level on the LCD control panel.

 

[Nealh]

One doesn't need to sacfifice any up hill grunt to save battery power atbh it is here where one does need the extra oomph , the motor will be happier with more power up hill as long as one isn't riding too slowly.
Use the lowest PAS level one can manage with.
Typically this the the power one will get form each PAS level with a KT.
PAS 1 13% 17A controller = 2.21a.
PAS 2 20% ditto = 3.4a.
PAS 3 33% ditto = 5.61a.
PAS 4 50% ditto = 8.5a.
PAS 5 100% ditto = 17a.

 

[Nealh]

If one wants to reduce the C5 value then one needs to know which calcs the LCD uses. Older firmware LCD's decrease current by 0.5a for each lower value chosen up to 5a max , newer firmware LCD's use a % value and can decrease current by 50%.

If using PAS 4 or 5 , then decreasing current output levels will have negligable affect on range . The greatest range extender is use no PAS at all or limit power use to PAS 1 or occassionally PAS 2 when not ascending.

 

[thelarkbox]

Im still confused about hub motor max rpm and the impact of running a 36v motor with a 48v battery/controller?

Is the motor max rpm a constant regardless of the input voltage or does it also increase inline with the voltage supplied?

This seems like a good place to ask Cheers..
btw links to homework/recommended reading are welcome

 

[saneagle]

Im still confused about hub motor max rpm and the impact of running a 36v motor with a 48v battery/controller?

Is the motor max rpm a constant regardless of the input voltage or does it also increase inline with the voltage supplied?

This seems like a good place to ask Cheers..
btw links to homework/recommended reading are welcome

All electric motors have a constant called kV. It's a direct relationship between volts and rpm. When you change from 36v (10S) to 48v (13S), the motor will spin 30% faster. How fast it goes depends on the kV. A 36v that spins to 250 rpm has a kV of 250/36, so at 48v, it'll spin to 48x250/36 = 333rpm.

The kV comes from the back emf. When any motor turns with or without power, it becomes a generator. The faster it spins, the more volts it generates. The voltage generated is in the opposite direction of any battery voltage powering it, so it has the effect of reducing the applied voltage. At a certain speed (max rpm), the generated voltage is the same as the battery voltage, so the net voltage is zero and the motor no longer has any power to turn, which is why all motors have a max speed and the power ramps down as you approach it.

The controller fires pulses of power to the motor. The pulses have a width and a height. The height is the voltage (36v or 48v) and the width is the time the pulse lasts in microseconds. The energy provided in each pulse is proportional to the width times the height of the pulse, so there is 30% more energy in each 48v pulse compared with a 36v one, which manifests itself as more torque.

The current is controlled by the width and frequency of the pulses. The controller is not very sophisticated. The battery is connected directly to the motor, but is blocked by MOSFETs. The MOSFETS are nothing more than gates that open and close, so every pulse is the height of the battery voltage and the power/current is controlled by how long the MOSFET is open and closed.

Motors have an efficiency curve. They have low efficiency at low rpm and highest efficiency at about 75% of max rpm. If you increase the voltage from 36v to 48v, the rpm for best efficiency increases to 75% of the new max rpm, so it's not a good idea to increase the voltage if you only ride slowly. Generally, going from 36v to 48v doesn't make a big difference on efficiency, but going from 24v to 48v would or 36v to 72v.

There's no point in increasing the voltage of a crank-drive bike because it would make the cranks spin faster and reduce the motor's efficiency at your normal comfortable pedal speed. Instead, you should increase the current, unless you have a background in BMX and like to spin at 120rpm.

 

[thelarkbox]

Cheers @saneagle, i will probably need a few read throughs before it starts to gel, ive lurked on the peripheries of rc and model engineering communities a little not delving deeper than admiration for the most part tho. so other the odd servo/stepper motor ebiking is my first real exposure to bldc motors..

 

[saneagle]

Cheers @saneagle, i will probably need a few read throughs before it starts to gel, ive lurked on the peripheries of rc and model engineering communities a little not delving deeper than admiration for the most part tho. so other the odd servo/stepper motor ebiking is my first real exposure to bldc motors..

It's worth getting to understand all that stuff. It's actually very simple when you get it. The biggest barrier is that people think itmore complicated than it is. Take it one paragraph at a time. If there's anything you don't understand, just ask.

 

[Ghost1951]

Wouldn't it be great if we could extract that back emf with some diodes and use it. I've said before that when I was 11 or 12 years old and we dd something about motors, dynamos and back emf, in physics I had the great idea of a perpetual motion machine. Turns out most kids have the same idea. Imagine getting a way of using that back emf.

EP0932248A1 - Method of harnessing a back-emf, and apparatus used in performing the method. - Google Patents

A method of achieving high efficiency of energy usage includes passing current through an inductor, causing the current to be repeatedly interrupted, thereby generating a back emf in the inductor and thereafter harnessing the back emf so generated to supply energy to an energy receiving or...

patents.google.com

.

 

[saneagle]

Wouldn't it be great if we could extract that back emf with some diodes and use it. I've said before that when I was 11 or 12 years old and we dd something about motors, dynamos and back emf, in physics I had the great idea of a perpetual motion machine. Turns out most kids have the same idea. Imagine getting a way of using that back emf.

EP0932248A1 - Method of harnessing a back-emf, and apparatus used in performing the method. - Google Patents

A method of achieving high efficiency of energy usage includes passing current through an inductor, causing the current to be repeatedly interrupted, thereby generating a back emf in the inductor and thereafter harnessing the back emf so generated to supply energy to an energy receiving or...

patents.google.com

.

All you have to do is turn the motor. It's a generator. You can put duct tape on your spokes to make the wheel like a fan, then park your bike outside upside down to get free electricity. It can charge itself up if you use the right controller. Don't forget that the wheel needs to turn backwards to drive the motor if it's a geared motor with a clutch.

 

[Ghost1951]

That looks hard to make. This guy is running a light bulb with a nail, some wire a speaker and a magnet.... Fantastic. I'm calling my energy provider right now to cancel my supply contract. I have nails, wire, speaker and magnet. That should power my house easily.......

 

[Adwb]

If one wants to reduce the C5 value then one needs to know which calcs the LCD uses. Older firmware LCD's decrease current by 0.5a for each lower value chosen up to 5a max , newer firmware LCD's use a % value and can decrease current by 50%.

If using PAS 4 or 5 , then decreasing current output levels will have negligable affect on range . The greatest range extender is use no PAS at all or limit power use to PAS 1 or occassionally PAS 2 when not ascending.

Thanks I have tried hard to only use pas 1 or 2 and reduced the chain wheel by 4 teeth to get a better hill and pull away with pedals, but either of my batteries go flat long before my wife‘s one do , she gets far more range on a identical etrike.
we have swopped bikes and batteries around but makes no difference.the only difference is hers still has has the original 11 amp max Lishui controller and mine got changed to kt 18 amp.

 

[saneagle]

Thanks I have tried hard to only use pas 1 or 2 and reduced the chain wheel by 4 teeth to get a better hill and pull away with pedals, but either of my batteries go flat long before my wife‘s one do , she gets far more range on a identical etrike.
we have swopped bikes and batteries around but makes no difference.the only difference is hers still has has the original 11 amp max Lishui controller and mine got changed to kt 18 amp.

Mass makes a big difference too, so if you're heavier than her, that's the reason. Twice as heavy requires twice as much power.

 

[Nealh]

Thanks I have tried hard to only use pas 1 or 2 and reduced the chain wheel by 4 teeth to get a better hill and pull away with pedals, but either of my batteries go flat long before my wife‘s one do , she gets far more range on a identical etrike.
we have swopped bikes and batteries around but makes no difference.the only difference is hers still has has the original 11 amp max Lishui controller and mine got changed to kt 18 amp.

Current can and does make a difference but that doesn't explain the range difference between your wifes battery or your battery, typically if using only PAS1/2 then one is using a third less current then the wife so there must be other factors in play. The Lishui is likely speed controlled vs your current control, the speed control will put out max current until near the cut off speed of each PAS level.
Is your wife a fitter rider then you or maybe she is far lighter then you , in which case either can make a difference.
Are the batteries identical ?
What about the motors used ?
Who follows who ?

 

[Adwb]

Current can and does make a difference but that doesn't explain the range difference between your wifes battery or your battery, typically if using only PAS1/2 then one is using a third less current then the wife so there must be other factors in play. The Lishui is likely speed controlled vs your current control, the speed control will put out max current until near the cut off speed of each PAS level.
Is your wife a fitter rider then you or maybe she is far lighter then you , in which case either can make a difference.
Are the batteries identical ?
What about the motors used ?
Who follows who ?

Yes identical batteries
motors identical
I tend to follow her
i try to match her cadence at lowest pas 1/2
I wonder if I have made some screw up when setting up on the lcd 3 display?

 

[RollingChunder]

Yes identical batteries
motors identical
I tend to follow her
i try to match her cadence at lowest pas 1/2
I wonder if I have made some screw up when setting up on the lcd 3 display?

Any bodged soldering in your set up? I noticed my voltage sag reduce slightly when I cut out gobs of solder and replaced with crimp connections.

 

[Adwb]

Any bodged soldering in your set up? I noticed my voltage sag reduce slightly when I cut out gobs of solder and replaced with crimp connections.

Nope all plugs
l

 

[Ghost1951]

Thanks I have tried hard to only use pas 1 or 2 and reduced the chain wheel by 4 teeth to get a better hill and pull away with pedals, but either of my batteries go flat long before my wife‘s one do , she gets far more range on a identical etrike.
we have swopped bikes and batteries around but makes no difference.the only difference is hers still has has the original 11 amp max Lishui controller and mine got changed to kt 18 amp.

I might have missed it, but I don't think you have addressed saneagle's point about the likely weight difference between you and your wife. Also, the range difference might be part due to the fact that your KT set to limit consumption to 18 amps will more rapidly drain your battery if you used that extra over your wife's trike's 11 amps. The range difference is likely due to those two factors. Most likely, your wife is a fair bit lighter than you are which saneagle pointed out earlier. If she is, let's say - 25% lighter, her range on electric power will be that much more than yours if you are both putting in equal effort. Add to that possibility, the extra capacity of the KT controller to pull more amps and the problem is solved.... That's what I'm guessing anyway.