Why Electric motors are inefficient at low revs?
- Thread starter avada
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liveforphysics
100 TW
Kiriakos GR said:
This is not at all how it works.
Then the problem is having the wrong motor, not whether motors in general are inefficient at low revs.avada said:
Use a motor that's made for the speed range you're going to use it at most, and use it there.
It'll still work at lower and higher speeds than those, but it'll be more efficient at the speeds you most need it to be.
Would you use an ICE/transmission designed for high speed racing in city traffic, and also expect that to be as efficient as one designed for city traffic?
major
10 kW
Arlo1 said:
Using the scientific definition of work, it is doing zero work and is zero percent efficient. You could just as easily use the parking brake and apply that 100 lb.ft. of torque to resist the force of gravity. Would the brake be doing work? No. Once motion stops and the brake is set, there is force but no displacement. Therefore there is no work, no energy, no power, and efficiency is meaningless like zero divided by zero.
speedmd
10 MW
Agree using the work equations, as motion approaches zero, so does the work. But we are still over simplifying the true real life energy picture here regarding motors and motion. Torque is real and a major factor in transportation /speed change terms the instant you overcome static loading. The real relevant power numbers I was told many years ago (at least that is what little I remember) was calculated by the area under the torque and power curves in some fashion. I remember a professor stating that you can not get a true picture of what a motor is capable of without looking at both. I wish I had paid closer attention.
avada
1 mW
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The problem is, as I described the range the motors need to work in is pretty broad. For an average car driver it includes some percentage highway travel and that's significant because of the high energy power output. And a lot of sub 50 km/h driving with acceleration/braking. This is significant because it constitutes most of the driving.amberwolf said:Then the problem is having the wrong motor, not whether motors in general are inefficient at low revs.
Use a motor that's made for the speed range you're going to use it at most, and use it there.
It'll still work at lower and higher speeds than those, but it'll be more efficient at the speeds you most need it to be.
Would you use an ICE/transmission designed for high speed racing in city traffic, and also expect that to be as efficient as one designed for city traffic?
Avada,
Are you just looking at graphs online without actual testing?? Bench racing so to speak.?
If you choose the right motor then this is not a problem.
Most of my riding is below 100km/h but my bike tops out at 142km/h I can do very efficient riding in stop and go and cruise on the highway. I never have to worry about the low speed efficiency its effeciant at any amount of movement.
As I type this I realize if you don't understand the graphs are at full torque then you will be misslead but if I need to cruise at say 10km/h which means I need uber pow torque and its actually using very low power which is efficient but not shown on the graphs.
Are you just looking at graphs online without actual testing?? Bench racing so to speak.?
If you choose the right motor then this is not a problem.
Most of my riding is below 100km/h but my bike tops out at 142km/h I can do very efficient riding in stop and go and cruise on the highway. I never have to worry about the low speed efficiency its effeciant at any amount of movement.
As I type this I realize if you don't understand the graphs are at full torque then you will be misslead but if I need to cruise at say 10km/h which means I need uber pow torque and its actually using very low power which is efficient but not shown on the graphs.
avada said:
Run a motor at max rpm with no load. Ouput efficiency is 0% (0 torque). Apply an increasing load. Efficiency will rise to a maximum. Now keep increasing the load. Efficiency begins to drop until the motor stalls. When the motor stalls you are back at 0 % output efficiency (0 speed)
major
10 kW
avada said:
It depends on the context. Back to your post #1, w.r.t. the first graph, load is torque. W.r.t. the second graph, load is power.
liveforphysics
100 TW
Kiriakos GR said:
No matter US or Europe, controllers make an effective and efficiency buck converter due to the inductance of the phase winding.
To send my motor a sustained 660A RMS phase current into a stalled motor, it only takes something like 7A off the battery.
One can use common sense and immediately realize controllers are extremely efficient devices. They lack the thermal mass and heat sinking to be anything else.
major
10 kW
flathill said:
Load on the motor has always been the torque opposing rotation as far as I know. That may vary with RPM in particular ways depending on what the physical load is, but when working with the graphs presented in post #1, load is defined as I said, isn't it?
avada
1 mW
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BTW, more closely to the topic:
Couldn't you simply maximize efficiency by increasing the number of poles to the maximum possible. That's essentially the same as going faster with a motor that has fewer poles. And since (by the diagrams) the most efficient area is near the max RPM it would it would improve efficiency all through the RPM range.
Couldn't you simply maximize efficiency by increasing the number of poles to the maximum possible. That's essentially the same as going faster with a motor that has fewer poles. And since (by the diagrams) the most efficient area is near the max RPM it would it would improve efficiency all through the RPM range.
speedmd
10 MW
Did you get a look at the windings end profile. 3:04 in the vid. Looks like / guessing they change the shape of the outer layers of the stack to allow / take advantage this shorter end profile.
To increase low rpm efficiency on a 9 phase motor by switching of 6 of them doesn't make a whole lot of sence to me,
looks to me like marketing b* to compete with the latest combustion engines that are able to turn off multiple cilinders
to increase efficiency at low power demands.
If there is any science to this I'm highly curious..
Kiriakos GR makes a point here, efficiency would be up by a couple of percents but the mayor thing that goes up is POWER!
According to the earlier (last year..) mentioned chart power output at 150 rpm is 7,8W now if there was selectable gear available with a ratio of 10 to 1 the motor
could run at 1500 rpm for the same vehicle speed but now capable of providing 387 watts of power!
In a perfect world one could hook the motor up to a CVT with infinite gear ratio's to let the motor run at optimum rpms providing almost 20kw from a dead start.
Just hooking it up to a simple gearing system/gearbox could make a big improvement in performance.
looks to me like marketing b* to compete with the latest combustion engines that are able to turn off multiple cilinders
to increase efficiency at low power demands.
If there is any science to this I'm highly curious..
Kiriakos GR said:
Kiriakos GR makes a point here, efficiency would be up by a couple of percents but the mayor thing that goes up is POWER!
According to the earlier (last year..) mentioned chart power output at 150 rpm is 7,8W now if there was selectable gear available with a ratio of 10 to 1 the motor
could run at 1500 rpm for the same vehicle speed but now capable of providing 387 watts of power!
In a perfect world one could hook the motor up to a CVT with infinite gear ratio's to let the motor run at optimum rpms providing almost 20kw from a dead start.
Just hooking it up to a simple gearing system/gearbox could make a big improvement in performance.
John in CR
100 TW
mrass said:
It could make sense if the motor isn't a permanent magnet motor by reducing iron losses of the unpowered sections. The video shows that the motor is a direct drive PMM outrunner, a pretty massive one, so it's not surprising that it's over-sized enough that 1/3 of the motor is sufficient for low speed power needs. The only way it could come out ahead is if the controllers themselves use more power than the increased copper losses of supplying all of the low speed torque needs using just 3 phases.
Yeah it looks like just a marketing gimmick. My 6 phase motor runs fine on just 3 phases, and it has more power running just 3 phases than the vast majority of ebikes, but the only way I'd ever run it on just one controller is if the other one failed.
crossbreak
1 MW
this is something that i thought too, but it seems there is no correlation of rpm and peak eff at some number of poles. if pole count has reached some number, then raising pole count wont have a significant impact on eff anymore for a given rpm and torque demand. for me it looks like this number is at ~50 poles for a 26" wheel. seems like the DD hub manufacturers do their job well.Miles said:
a practical example is the Bafang BPM vs MAC geared hub example: Both are designed equally, the BPM has 16 magnets (or 80 at the wheel with its 5:1 reduction), the MAC has 32 (or 160 at its built-in 5:1 reduction). still they are very close in terms of loss at low rpm for a given torque demand. the MAC seems to have a little advatage over the BPM, but this seems to come from a little bit bigger stator/little more copper (about 20%). This means the MAC may produce 20% more torque at low rpm for a given loss compared to the BPM.
The MAC has thinner gears to fit the bigger stator. this somewhat cancels out in practical use
the MAC has to use thin .35mm lamination to gain an equal eddy current loss as the BPM. but this is more expensive. So the BPM is much more cost effective with it's lower pole count and cheaper (=thicker) lamination.
Still it would indeed be interesting to see if there is even a slight difference between them. In practical use this is really hard to measure. maybe differences even come from other factors
The greatest advantage of increasing the pole count is the reduced material requirement for the rotor and stator yokes. It also has the potential for increasing the LCM. The disadvantages are the greater iron losses and proportionately higher stray flux.
newbiebiker
100 mW
Poor efficiency at low speed (say, that is from zero to 10 kmph), better i can pedal ebike initially, in NON zero rpm (atleast some speed) i switch ON controller and use throttle, now it require less torque to run bicycle , so little more efficiency ??
Both are true:
1) Motors are inefficient when the load is too low or too high
2) Motors are inefficient when the rpm is too low or too high
In both cases you hit zero percent efficiency at either end
for any given rpm>0
start with 0 load and you have 0% efficiency (do no work)
increase the load and the efficiency increases
keep increasing the load and you hit maximum efficiency
increase the load pass this point and efficiency declines
keep increasing the load and now you stall the motor and are back at 0% efficiency
for any given load >0 and <stall
start at 0 rpm and efficiency is 0%
increase the rpm and efficiency increases to a maximum while power output keeps increasing
increase the rpm and efficiency begins to drop while power output keeps increasing
keep increasing the rpm until you hit peak power (somewhere around 50% efficiency)
keep increasing the rpm and now more power input results in less output trending back towards 0% efficiency
1) Motors are inefficient when the load is too low or too high
2) Motors are inefficient when the rpm is too low or too high
In both cases you hit zero percent efficiency at either end
for any given rpm>0
start with 0 load and you have 0% efficiency (do no work)
increase the load and the efficiency increases
keep increasing the load and you hit maximum efficiency
increase the load pass this point and efficiency declines
keep increasing the load and now you stall the motor and are back at 0% efficiency
for any given load >0 and <stall
start at 0 rpm and efficiency is 0%
increase the rpm and efficiency increases to a maximum while power output keeps increasing
increase the rpm and efficiency begins to drop while power output keeps increasing
keep increasing the rpm until you hit peak power (somewhere around 50% efficiency)
keep increasing the rpm and now more power input results in less output trending back towards 0% efficiency
