Hub Motor Efficiency

[Deleted member 4366]

Selrahc1992 can't start his own threads, so this is to answer his question about whether a hub-motor will overheat dragging a high load through sand.

Hub motors have coils inside. At any fixed voltage, say 36v, the speed of the motor depends on how many turns of wire there are in the coils. Often, there are several versions of the same motor, so that they can spin at different speeds to compensate for different wheel sizes. If a motor is designated as 36v 201 rpm, that's how fast it can spin to at 36v. You can throw as much power as you like at it, but you can't make it go any faster; however, the speed also depends on voltage, so it will go proportionally faster at 48v, i.e. 266 rpm. 201 rpm is approximately 15 mph in a 26" wheel.

For the next bit, I'm only considering operating the motor at maximum power. The efficiency is different at different speeds. As a general rule, it's at maximum at about 3/4 of maximum RPM. Inefficiency becomes significant once the motor is spinning at less than 1/2 maximum RPM, where it's typically around 60%. By the time you go down to 1/4 maximum RPM, the efficiency goes down to something like 30%. It's important because 30% efficiency means that 30% of the power from your battery is making the bike move and 70% is being used to heat up the motor. With a typical 15A 36v controller, that means that your motor becomes a 400w electric fire.

Ideally, you would want your motor spinning at about 2/3 of its maximum speed, where it makes maximum power and is close to maximum efficiency. It won't overheat at that speed because it can shed the heat as fast as it gets it.

Now to answer the question. If you had a 180 rpm motor, it could go down to maybe 90 rpm before overheating becomes a problem, which is 6.7 mph, so as long as it has enough torque to drag the load above that speed, it would be OK. If the motor were 260 rpm (20mph), it would have to be able to keep above 10 mph.

Strangely, you can stop a motor overheating by giving it more current. If your motor struggles down to 5 mph on a steep hill, by giving it a few more amps, you can get the speed up to maybe 8 mph, which could double the efficiency from say 20% to 40%, so much less heating of the motor.

All these figures are just generalised examples for comparison.

 

[Emo Rider]

Selrahc1992 can't start his own threads, so this is to answer his question about whether a hub-motor will overheat dragging a high load through sand.

Hub motors have coils inside. At any fixed voltage, say 36v, the speed of the motor depends on how many turns of wire there are in the coils. Often, there are several versions of the same motor, so that they can spin at different speeds to compensate for different wheel sizes. If a motor is designated as 36v 201 rpm, that's how fast it can spin to at 36v. You can throw as much power as you like at it, but you can't make it go any faster; however, the speed also depends on voltage, so it will go proportionally faster at 48v, i.e. 266 rpm. 201 rpm is approximately 15 mph in a 26" wheel.

For the next bit, I'm only considering operating the motor at maximum power. The efficiency is different at different speeds. As a general rule, it's at maximum at about 3/4 of maximum RPM. Inefficiency becomes significant once the motor is spinning at less than 1/2 maximum RPM, where it's typically around 60%. By the time you go down to 1/4 maximum RPM, the efficiency goes down to something like 30%. It's important because 30% efficiency means that 30% of the power from your battery is making the bike move and 70% is being used to heat up the motor. With a typical 15A 36v controller, that means that your motor becomes a 400w electric fire.

Ideally, you would want your motor spinning at about 2/3 of its maximum speed, where it makes maximum power and is close to maximum efficiency. It won't overheat at that speed because it can shed the heat as fast as it gets it.

Now to answer the question. If you had a 180 rpm motor, it could go down to maybe 90 rpm before overheating becomes a problem, which is 6.7 mph, so as long as it has enough torque to drag the load above that speed, it would be OK. If the motor were 260 rpm (20mph), it would have to be able to keep above 10 mph.

Strangely, you can stop a motor overheating by giving it more current. If your motor struggles down to 5 mph on a steep hill, by giving it a few more amps, you can get the speed up to maybe 8 mph, which could double the efficiency from say 20% to 40%, so much less heating of the motor.

All these figures are just generalised examples for comparison.

Wow! So much to learn and so little time. Thank you for this, very informative.

 

[selrahc1992]

Selrahc1992 can't start his own threads, so this is to answer his question about whether a hub-motor will overheat dragging a high load through sand.

Hub motors have coils inside. At any fixed voltage, say 36v, the speed of the motor depends on how many turns of wire there are in the coils. Often, there are several versions of the same motor, so that they can spin at different speeds to compensate for different wheel sizes. If a motor is designated as 36v 201 rpm, that's how fast it can spin to at 36v. You can throw as much power as you like at it, but you can't make it go any faster; however, the speed also depends on voltage, so it will go proportionally faster at 48v, i.e. 266 rpm. 201 rpm is approximately 15 mph in a 26" wheel.

For the next bit, I'm only considering operating the motor at maximum power. The efficiency is different at different speeds. As a general rule, it's at maximum at about 3/4 of maximum RPM. Inefficiency becomes significant once the motor is spinning at less than 1/2 maximum RPM, where it's typically around 60%. By the time you go down to 1/4 maximum RPM, the efficiency goes down to something like 30%. It's important because 30% efficiency means that 30% of the power from your battery is making the bike move and 70% is being used to heat up the motor. With a typical 15A 36v controller, that means that your motor becomes a 400w electric fire.

Ideally, you would want your motor spinning at about 2/3 of its maximum speed, where it makes maximum power and is close to maximum efficiency. It won't overheat at that speed because it can shed the heat as fast as it gets it.

Now to answer the question. If you had a 180 rpm motor, it could go down to maybe 90 rpm before overheating becomes a problem, which is 6.7 mph, so as long as it has enough torque to drag the load above that speed, it would be OK. If the motor were 260 rpm (20mph), it would have to be able to keep above 10 mph.

Strangely, you can stop a motor overheating by giving it more current. If your motor struggles down to 5 mph on a steep hill, by giving it a few more amps, you can get the speed up to maybe 8 mph, which could double the efficiency from say 20% to 40%, so much less heating of the motor.

All these figures are just generalised examples for comparison.

many thanks, even if not being able to start my own threads is deeply embarrassing

 

[tongxinpete]

Sadly it is very hard to find a 180 rpm motor at a good price. I measured a Q100 it = 249 rpm unloaded, A Q100H is said to be 225 unloaded. The BPM code 15 = 180 rpm unloaded, & the code 14 only slightly faster, but both are hard to find. A good guide for the smaller 36V motors is if there are stats then the rpm at 5A is the likely speed on the flat with a 200 Lb rider, this is from actual measurement. I am talking building into a 26'' wheel here. A cheap way round this is to build into a 24'' rear wheel. I did this with the Q100, performance not bad at all, if you have a disc brake well no probs, if you have rim brakes it is needed to make a caliper support a bit lower down the frame, as I did. Tonxgin (now Outrider) did a 180 rpm motor, I have one, it tops out at 14mph in a 26'' wheel I think an even slower one was available. If you have tough hills 180 rpm or a smaller rim do help. The Con61 / Ku63 controllers are supposed to be 14A peak but are often about 12.5 A - as was mine, I hacked it to 15A as the Q100 is said to slip at 17 or 18A
I feel that any hill you cannot climb at 6.5 mph well it is time to get off & push as the bike loses its balance (a brisk walking pace = 4mph) so yes the 180 rpm motor is the ideal single speed choice for tough hills & general use combined

 

[Deleted member 4366]

You could always run a 36v 270 rpm motor at 24v to get 180 rpm, then increase the current to compensate. It might be better with a 240 rpm 48v one with a 36v battery to get 180 rpm.

I found that the Q100H did 201 rpm at 36v. Did you check yours with a full battery, which would be more than 36v?

 

[cwah]

Sadly it is very hard to find a 180 rpm motor at a good price. I measured a Q100 it = 249 rpm unloaded, A Q100H is said to be 225 unloaded. The BPM code 15 = 180 rpm unloaded, & the code 14 only slightly faster, but both are hard to find. A good guide for the smaller 36V motors is if there are stats then the rpm at 5A is the likely speed on the flat with a 200 Lb rider, this is from actual measurement. I am talking building into a 26'' wheel here. A cheap way round this is to build into a 24'' rear wheel. I did this with the Q100, performance not bad at all, if you have a disc brake well no probs, if you have rim brakes it is needed to make a caliper support a bit lower down the frame, as I did. Tonxgin (now Outrider) did a 180 rpm motor, I have one, it tops out at 14mph in a 26'' wheel I think an even slower one was available. If you have tough hills 180 rpm or a smaller rim do help. The Con61 / Ku63 controllers are supposed to be 14A peak but are often about 12.5 A - as was mine, I hacked it to 15A as the Q100 is said to slip at 17 or 18A
I feel that any hill you cannot climb at 6.5 mph well it is time to get off & push as the bike loses its balance (a brisk walking pace = 4mph) so yes the 180 rpm motor is the ideal single speed choice for tough hills & general use combined

Not sure what your configuration is but...

bbs01 could be the right candidate as well as the xiondga.

Alternatively, plug in a 24v motor & controller to have lower speed

 

[tongxinpete]

Yes I was on a full SLA set, I should have said 36V nominal, as LVC for SLA is actually above 36v. (I tend to test things on SLA as I am not so worried about them)
I had another look at the Q100H curve & see that they plotted a constant 38V on their test - mine was a Q100 on bench test, not a Q100H.
Agreed that the 24v is an option but my batts & chargers were all 36v nominal.
If the Q100H is 201 rpm unloaded I guess this is where the claimed torque increase is from, or some of it. If I had not already a spare Q100 I would have bought a Q100H. The old bugbear of a hobby on a budget.
I have the Tongxin in its front wheel on it's own throttle & controller, this is cut in on tough hills, the bike really comes alive with both motors. If I was starting again it would be one bigger single rear motor

 

[Deleted member 4366]

The torque increase for the Q100H comes from its faster rotation speed than the Q100. The rotor spins faster and has a greater reduction ratio. I can't remember the actual numbers, but it was something like from 8:1 to 12:1. It should make more power too.

I wonder if you can take the gears off a Q100H and put them on a 201 rpm Q100 to get a 150 rpm motor? That would climb like the Xiongda then.

 

[selrahc1992]

Selrahc1992 can't start his own threads, so this is to answer his question about whether a hub-motor will overheat dragging a high load through sand.

Hub motors have coils inside. At any fixed voltage, say 36v, the speed of the motor depends on how many turns of wire there are in the coils. Often, there are several versions of the same motor, so that they can spin at different speeds to compensate for different wheel sizes. If a motor is designated as 36v 201 rpm, that's how fast it can spin to at 36v. You can throw as much power as you like at it, but you can't make it go any faster; however, the speed also depends on voltage, so it will go proportionally faster at 48v, i.e. 266 rpm. 201 rpm is approximately 15 mph in a 26" wheel.

For the next bit, I'm only considering operating the motor at maximum power. The efficiency is different at different speeds. As a general rule, it's at maximum at about 3/4 of maximum RPM. Inefficiency becomes significant once the motor is spinning at less than 1/2 maximum RPM, where it's typically around 60%. By the time you go down to 1/4 maximum RPM, the efficiency goes down to something like 30%. It's important because 30% efficiency means that 30% of the power from your battery is making the bike move and 70% is being used to heat up the motor. With a typical 15A 36v controller, that means that your motor becomes a 400w electric fire.

Ideally, you would want your motor spinning at about 2/3 of its maximum speed, where it makes maximum power and is close to maximum efficiency. It won't overheat at that speed because it can shed the heat as fast as it gets it.

Now to answer the question. If you had a 180 rpm motor, it could go down to maybe 90 rpm before overheating becomes a problem, which is 6.7 mph, so as long as it has enough torque to drag the load above that speed, it would be OK. If the motor were 260 rpm (20mph), it would have to be able to keep above 10 mph.

Strangely, you can stop a motor overheating by giving it more current. If your motor struggles down to 5 mph on a steep hill, by giving it a few more amps, you can get the speed up to maybe 8 mph, which could double the efficiency from say 20% to 40%, so much less heating of the motor.

All these figures are just generalised examples for comparison.

i'm planning to build a hub motored pedelec with a low range transfer - so to speak: hobbyking sells lipo relatoively cheaply - could one use a bpm 500w - say for argument sake 180rpm - use around 12S for lugging through sand on beaches around isle de oleronne at around 6mph - but then when one hits the tarmac and need to rapidly travel a longer distance (say thirty miles) between towns reconfigure the 4S lipo pack (16000mAH each) to give 16S (if a S12S controller would manage that) and so end up with a much higher speed motor - could one switch between the two in this way? many thanks

 

[Deleted member 4366]

You don't need to do that. A throttle has a similar effect. All the things I mentioned above were for full power situation. If you turn down the throttle, the speed reduces and the whole efficiency curve moves down with it. Let's say you have a 300rpm motor, which will become inefficient below 150 rpm (11mph). At half throttle, it becomes a 150 rpm motor, so the inefficiency band will be below 5.5 mph. Of course you get less power too, but that would be the same as if you reduced the battery voltage. These statements relate to a controller that uses speed control; however, the controllers often increase the power as you slow down, so it gets very complicated. Different controllersbhave different power algorithms.

Some of the nice new controllers like the S12S use current control. I'm still trying to figure out how that effects efficiency, but I think it's the same, so all you have to do is set a lower level on the LCD.

The only logical way to get what you want, i.e. high torque and low speed for sand and high speed and low torque for the road is with a crank drive motor.

 

[trex]

I think efficiency v RPM is more or less fixed. For a 250RPM motor like the xiongda XD, best efficiency (82%) is around 205-210 RPM, around the typical 80% of max RPM. If you run the motor at half that, 105 RPM, efficiency would drop down to around 50%, more or less at any current, the operating mode of the controller will have little effect. That shows that when climbing at 8mph, a typical bike fitted with SWX would have an efficiency of 50% while the XD can switch over to slow mode and would run at 82% efficiency.

 

[selrahc1992]

You don't need to do that. A throttle has a similar effect. All the things I mentioned above were for full power situation. If you turn down the throttle, the speed reduces and the whole efficiency curve moves down with it. Let's say you have a 300rpm motor, which will become inefficient below 150 rpm (11mph). At half throttle, it becomes a 150 rpm motor, so the inefficiency band will be below 5.5 mph. Of course you get less power too, but that would be the same as if you reduced the battery voltage. These statements relate to a controller that uses speed control; however, the controllers often increase the power as you slow down, so it gets very complicated. Different controllersbhave different power algorithms.

Some of the nice new controllers like the S12S use current control. I'm still trying to figure out how that effects efficiency, but I think it's the same, so all you have to do is set a lower level on the LCD.

The only logical way to get what you want, i.e. high torque and low speed for sand and high speed and low torque for the road is with a crank drive motor.

many thanks, without triggering any sectarian conlict, a thing that - hypothetically - appealed about hub motos is possible simplicity - separating physical an electrica input - dragging a 100kg trailer though thick sand with a crank drive creates to my mind a whole set of strains on a drive system that fees boudn to end in tears. I dont mean to hijack another thread, but money also comes into it - does ayone kno o a supplier of TCM crank drives - from an experince on a friends biek i'd say their not poerful but light, reliable and just about torquey enough

 

[james@justebikes.co.uk]

The only logical way to get what you want, i.e. high torque and low speed for sand and high speed and low torque for the road is with a crank drive motor.

...Or buy a Sparta RXS with bomb-proof silent 50Nm motor?

 

[Deleted member 4366]

Nice try, but that couldn't hold a light to a 48v code 14 BPM at 25 amps, which is what I'd use. It's an interesting bike though. I wouldn't mind a go on one. Will you be taking a demonstrator to any shows this year?

 

[trex]

Nah... not the DD Sparta ion motor, no match for a Bosch bike. It says 'up to 50NM' on their website. Show us a motor performance chart. What sort of efficiency at 50NM?

 

[james@justebikes.co.uk]

When did you try one?

 

[james@justebikes.co.uk]

Nice try, but that couldn't hold a light to a 48v code 14 BPM at 25 amps, which is what I'd use. It's an interesting bike though. I wouldn't mind a go on one. Will you be taking a demonstrator to any shows this year?

Yes we will, Rotwilds too, will keep you posted.

 

[james@justebikes.co.uk]

Nah... not the DD Sparta ion motor, no match for a Bosch bike. It says 'up to 50NM' on their website. Show us a motor performance chart. What sort of efficiency at 50NM?

Bosch! ... the most unreliable drive system I've come across in a decade.

 

[flecc]

.
Fight, Fight, Fight!
.

 

[tongxinpete]

Having looked again at the Q100H Curve I see that the 201 rpm is at about 5A (about 200W nom 36v) so I guess this is why they class it as a 201 rpm motor. I also see the controller they tested is about 12A max giving 20 Nm.
(I would guess projecting the graph that it is about 24 Nm @ 15A.
Interestingly with the Con61 controller connecting the 12 mph max limit wire does not reduce the 15A draw, so I guess it limits frequency pulses. This would support the idea that a part open throttle moves down the revs without much loss of torque. This is not so on the Ku 60 generic self learning controller though
A 180 rpm motor is not essential, about 200 would be quite OK. Dave having ridden both the Q100 & Q100H is there a noticeable improvement in torque for the Q100H over the 100?
The motor I would like to sample is the Q128H - maybe the bugs of the old Q128 are gone. Speed estimated at 215 rpm @ 5A, Good torque.
I have run an old crank drive bike & found them to be very maintenance intensive, personally I would rather put the money into a few more batt amps & a bit beefier single speed gearmotor. We have a good network of dedicated cycle paths, shared with walkers, & sometimes horse riders, thus I think the 25 kmh limit about right. The extra amps are only needed on hills, most of the time they would not be used.