Hub Motor
- Thread starter mygrumpy
- Start date
No problem, it will work fine, but will only go at 2/3rds of the maximum unloaded motor speed possible at 36 volts. This may or may not affect maximum bike power assist speed to the same degree, as the motor may not have had enough power to run at maximum rpm on 36 volts anyway.
Torque will be relatively unaffected, as it is solely dependent on motor current, which is largely a function of the controller limit.
Jeremy
Torque will be relatively unaffected, as it is solely dependent on motor current, which is largely a function of the controller limit.
Jeremy
Max power will be reduced by about 50%, max torque by between 20% and 33% depending on controller current limit.
If you use the same controller then you would need to check low voltage cut-off as it may not run at all at 24v.
If you use the same controller then you would need to check low voltage cut-off as it may not run at all at 24v.
I'm afraid I don't agree with all of the above, John, although the controller cut off voltage is a good point.
There are two relevant motor constants that are important here, excluding any variation in controller capability.
The first is the motor kV, or rpm per volt, constant. This is primarily a function of the number of windings per motor pole and the total number of motor poles. Taking my present project motor as an example, it has a kV factor of about 50 under load, which means that it will spin at about 1200rpm at 24 volts, or 1800rpm at 36 volts. This has been verified, so my assertion that a motor will spin at about 2/3rds of the speed at 24 volts that it does at 36 volts is proven and correct.
Secondly, we have the motor torque constant. This is directly proportional to the motor current and is virtually independent of the applied voltage, as long as there is sufficient voltage available to overcome the winding resistance of the motor. Using the same example of my current project motor, this has a torque constant of about 1.03 lb ins per amp.
Motor current in use is limited by two factors, the increase in motor back emf with increasing rpm and the current limiting capability of the motor controller at low rpm. The practical limit is the motor controller current limit, as the need for maximum torque is most often at low speeds, when the motor rpm is below the point where it's back emf equals the supply voltage.
Motor winding resistance is rarely an effective current limit, as it is usually very low. In the case of the motor specs quoted above, the motor winding resistance is just a few milliohms, low enough to draw several hundred amps from a 24 volt supply when the motor is stalled, assuming the controller has no current limit.
Hope this is clear, and not too techie!
Jeremy
There are two relevant motor constants that are important here, excluding any variation in controller capability.
The first is the motor kV, or rpm per volt, constant. This is primarily a function of the number of windings per motor pole and the total number of motor poles. Taking my present project motor as an example, it has a kV factor of about 50 under load, which means that it will spin at about 1200rpm at 24 volts, or 1800rpm at 36 volts. This has been verified, so my assertion that a motor will spin at about 2/3rds of the speed at 24 volts that it does at 36 volts is proven and correct.
Secondly, we have the motor torque constant. This is directly proportional to the motor current and is virtually independent of the applied voltage, as long as there is sufficient voltage available to overcome the winding resistance of the motor. Using the same example of my current project motor, this has a torque constant of about 1.03 lb ins per amp.
Motor current in use is limited by two factors, the increase in motor back emf with increasing rpm and the current limiting capability of the motor controller at low rpm. The practical limit is the motor controller current limit, as the need for maximum torque is most often at low speeds, when the motor rpm is below the point where it's back emf equals the supply voltage.
Motor winding resistance is rarely an effective current limit, as it is usually very low. In the case of the motor specs quoted above, the motor winding resistance is just a few milliohms, low enough to draw several hundred amps from a 24 volt supply when the motor is stalled, assuming the controller has no current limit.
Hope this is clear, and not too techie!
Jeremy
I'm not sure what you disagree with but I guess it is the issue of reduced torque.
Not sure what you are getting at there, ohms law applies to all resistors, only an infinite resistance or zero voltage would fail to produce a current.
All the controllers I have come across limit the battery current rather than the motor current. The motor will of course be limited also, but this is not a constant, rather a function also of battery voltage. This is the reason for loss of (maximum) torque with a lower voltage battery.
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It is indeed the issue of motor torque reduction with voltage, as it's something that simply doesn't happen - motor torque is a function of the magnetising force generated by the windings, which is a simple function of current.
The original poster asked specifically about the effect of running a 36 volt motor on 24 volts and made no mention of the type of controller(s) being used. I gave an answer based on the effect on the motor, as that was the original question.
Controllers will usually current limit in some way, as both of us have noted clearly, but 10 amps at the motor is 10 amps at the motor, whether the supply voltage be 24V or 36V. All the motor torque depends on is the current through the windings, the number of windings, the number of motor poles, the diameter of the rotor/armature and the strength of the magnets/field winding magnetic field. The only relevant variable here is the motor current, as the motor is physically the same for both the 24V and 36V condition.
I still assert that a given motor will deliver the same torque at the same current, irrespective of the supply voltage.
Of course, different controllers may well produce different limiting effects, depending very much on the type. I deliberately didn't try to hypothesise as to what any given controller might do, as I have no idea of the type(s) being considered - the original question was about the effect on a motor. For example, it's perfectly possible to use a 24V controller of the same power rating as a 36V one to drive a 36V motor, in which case all that happens is that the motor has a lower maximum speed - motor torque remains the same.
The Tongxin controllers were rated like this at one time (although they now seem to have changed). The 36V controller was limited to 10 amps input current, the 24V controller was limited to 15 amps input current.
Jeremy
The original poster asked specifically about the effect of running a 36 volt motor on 24 volts and made no mention of the type of controller(s) being used. I gave an answer based on the effect on the motor, as that was the original question.
Controllers will usually current limit in some way, as both of us have noted clearly, but 10 amps at the motor is 10 amps at the motor, whether the supply voltage be 24V or 36V. All the motor torque depends on is the current through the windings, the number of windings, the number of motor poles, the diameter of the rotor/armature and the strength of the magnets/field winding magnetic field. The only relevant variable here is the motor current, as the motor is physically the same for both the 24V and 36V condition.
I still assert that a given motor will deliver the same torque at the same current, irrespective of the supply voltage.
Of course, different controllers may well produce different limiting effects, depending very much on the type. I deliberately didn't try to hypothesise as to what any given controller might do, as I have no idea of the type(s) being considered - the original question was about the effect on a motor. For example, it's perfectly possible to use a 24V controller of the same power rating as a 36V one to drive a 36V motor, in which case all that happens is that the motor has a lower maximum speed - motor torque remains the same.
The Tongxin controllers were rated like this at one time (although they now seem to have changed). The 36V controller was limited to 10 amps input current, the 24V controller was limited to 15 amps input current.
Jeremy
Yes, but as I said, controllers generally limit battery current and 10 amps from the battery is NOT 10 amps to the motor (via a PWM controller when it is limiting). This would be true for a resistive load, but not an inductive load like a motor.
This is supported by ebikes.ca:
"But when the controller is doing PWM, then the current through the motor is higher than the battery current by the inverse of the PWM duty cycle."
Have another play with their simulator and you should see what I am getting at.
John,
If the same, input current limited, controller was used at a reduced voltage (assuming that it didn't shut down from the LVC circuit) then I agree with you, as motor current would be affected by the voltage change, but that wasn't the question asked, was it? For what it's worth, I posted about the effective current/voltage transformation capability of a PWM controller a few days ago.
I assumed that the most obvious and practical way of using a 36V motor on 24V would be to utilise a 24V controller, in which case what I originally wrote is perfectly correct. I based this on the fact that many (perhaps most) 36V controller will shut down at around 28 to 29V, so simply won't work at 24V anyway.
Jeremy
If the same, input current limited, controller was used at a reduced voltage (assuming that it didn't shut down from the LVC circuit) then I agree with you, as motor current would be affected by the voltage change, but that wasn't the question asked, was it? For what it's worth, I posted about the effective current/voltage transformation capability of a PWM controller a few days ago.
I assumed that the most obvious and practical way of using a 36V motor on 24V would be to utilise a 24V controller, in which case what I originally wrote is perfectly correct. I based this on the fact that many (perhaps most) 36V controller will shut down at around 28 to 29V, so simply won't work at 24V anyway.
Jeremy
Of course if we take the controller (or limit) away altogether, max torque will be reduced from a reduced battery voltage. 
(assuming that the battery can maintain its voltage)
(assuming that the battery can maintain its voltage)As I've already posted, and as is clear from proven and accepted motor performance data, torque is a function of motor current, not voltage.
The thing to remember is that the motor terminal voltage directly relates to rpm, as it's almost wholly a function of the motor back emf (which is directly proportional to motor rpm), rather than resistive voltage drop in the windings.
When a motor is loaded to a speed below is unloaded speed for the applied voltage, then it will draw a current that is directly proportional to the torque required to maintain that speed. In the ultimate case of the motor being stalled, the current, and hence torque, can be extremely high. For my project motor, the ME0709, the stall current can easily exceed 1000 amps, although the normal maximum operating current is very much lower.
A look through any motor spec sheet will clearly show the relationship between applied voltage and speed and current drawn and torque. Attached is a a set of typical PM motor graphs that clearly illustrate these characteristics. The horizontal axis is the motor torque; note that it is directly proportional to motor current.
Jeremy
The thing to remember is that the motor terminal voltage directly relates to rpm, as it's almost wholly a function of the motor back emf (which is directly proportional to motor rpm), rather than resistive voltage drop in the windings.
When a motor is loaded to a speed below is unloaded speed for the applied voltage, then it will draw a current that is directly proportional to the torque required to maintain that speed. In the ultimate case of the motor being stalled, the current, and hence torque, can be extremely high. For my project motor, the ME0709, the stall current can easily exceed 1000 amps, although the normal maximum operating current is very much lower.
A look through any motor spec sheet will clearly show the relationship between applied voltage and speed and current drawn and torque. Attached is a a set of typical PM motor graphs that clearly illustrate these characteristics. The horizontal axis is the motor torque; note that it is directly proportional to motor current.
Jeremy
Jeremy,
Surely you are not disagreeing with my last post. Voltage and current are not independant. In a stalled motor, current through the windings is directly proportional to the voltage applied.
John
Surely you are not disagreeing with my last post. Voltage and current are not independant. In a stalled motor, current through the windings is directly proportional to the voltage applied.
John
I'm not really disagreeing with you, John, but it's just that motors aren't a simple resistive load, so the relationship between voltage and current is not so straightforward as implied - it depends very largely on the back emf that is proportional to motor rpm; the effect of motor winding resistance is quite small.
It's quite possible to have a motor that remains at a constant speed, so has a constant voltage across it's terminals, yet draws a current that may vary from a very low value to a very high one, depending on torque load.
Motors don't really have a rated voltage in practice, even though a notional number may be printed on a data plate. The motor I've used as an example illustrates this well, as the same motor has a set of performance curves for voltages from 24V to 72V (I only copied the 24V and 36V curves, due to the file size limit on here). The important characteristics are the rpm/volt constant (the voltage constant), the amp/Nm constant (the torque constant), the maximum permissible winding current and the power dissipation capability.
If you look at the graphs in the attachment to the last post you can see that motor rpm remains fairly constant at the applied voltage - it drops by perhaps 10-15% from no load to maximum load, at a constant applied voltage. Current, on the other hand, is virtually independent of the applied voltage - the current plot on the graphs shows the current going from a very low value at no load to a very high value at maximum load.
Jeremy
It's quite possible to have a motor that remains at a constant speed, so has a constant voltage across it's terminals, yet draws a current that may vary from a very low value to a very high one, depending on torque load.
Motors don't really have a rated voltage in practice, even though a notional number may be printed on a data plate. The motor I've used as an example illustrates this well, as the same motor has a set of performance curves for voltages from 24V to 72V (I only copied the 24V and 36V curves, due to the file size limit on here). The important characteristics are the rpm/volt constant (the voltage constant), the amp/Nm constant (the torque constant), the maximum permissible winding current and the power dissipation capability.
If you look at the graphs in the attachment to the last post you can see that motor rpm remains fairly constant at the applied voltage - it drops by perhaps 10-15% from no load to maximum load, at a constant applied voltage. Current, on the other hand, is virtually independent of the applied voltage - the current plot on the graphs shows the current going from a very low value at no load to a very high value at maximum load.
Jeremy
Jeremy,
Please, please read my posts carefully! I keep them short (apart from this one!) and choose my words carefully.
).
It's a pity that we are not able to have a constructive discussion about this. I don't want to keep defending what I say, all of which has been factually correct. I guess that you believe that you have a near complete understanding of DC motor theory. From what you have written, I believe that your understanding is far better than most on this forum but that you are learning as you go.
I believe that my understanding is better than yours (although also not complete). I don't post all the technical details because I think that most people would not follow them, I just post conclusions which I think might be useful. My depth of understanding may therefore not come through in my posts.
You probably disagree with this, and that is OK. I really don't mean any offence and hope that none is taken.
I just hope that you, or anyone else, looks back at my posts on this thread and finds them helpful.
Cheers, John
Please, please read my posts carefully! I keep them short (apart from this one!) and choose my words carefully.
I said "In a stalled motor". Then, it is a resistive load, nothing else was implied. (For complete accuracy, there is still an inductive component, but for the purposes of this calculation, that can be ignored).
Strictly speaking, sorry but this is not true. If it is, then the text books will need to be rewritten (the only exception would be superconducting armature windings, that would be great if we could afford them in our bike motors
).Those graphs only show the bottom end of the torque range, they don't extend to anywhere near stall torque.
It's a pity that we are not able to have a constructive discussion about this. I don't want to keep defending what I say, all of which has been factually correct. I guess that you believe that you have a near complete understanding of DC motor theory. From what you have written, I believe that your understanding is far better than most on this forum but that you are learning as you go.
I believe that my understanding is better than yours (although also not complete). I don't post all the technical details because I think that most people would not follow them, I just post conclusions which I think might be useful. My depth of understanding may therefore not come through in my posts.
You probably disagree with this, and that is OK. I really don't mean any offence and hope that none is taken.
I just hope that you, or anyone else, looks back at my posts on this thread and finds them helpful.
Cheers, John
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John,
I think the issue here is about real world, versus theoretical, performance.
For a well-designed motor, run within it's normal safe operating envelope, resistive losses should be very small, perhaps around 10 to 15%.
I have tended to simplify things by not emphasising the resistive loss element, as that isn't that important in terms of limiting practical working torque - maybe that was wrong.
As for the comment about those graphs showing the bottom end, in fact they show the whole safe working range of that motor. The stall current is many times the maximum current shown, but that's the case for any well-designed motor.
Any decent motor should be designed to have the very lowest DC resistance possible, for best efficiency. I've just looked at the specs for my 350 watt brushless model aircraft electric motor, it has a winding resistance of just 90 mOhms, for example, so even though it's maximum operating current is about 25 amps, it's stall current would be up around 150 amps at the normal controller operating voltage.
I don't have any figures for other bike motors, maybe they are much worse as you suggest, although the efficiency graphs available imply that they are similar. If they are as efficient as the motors I've played with, then there shouldn't be any really significant current reduction, and hence torque reduction, with a 33% supply voltage reduction, provided that the controller doesn't limit.
My real point was that a given current through a motor, at a given motor rpm, will give a particular torque, which is virtually independent of the supply voltage to the controller.
Jeremy
I think the issue here is about real world, versus theoretical, performance.
For a well-designed motor, run within it's normal safe operating envelope, resistive losses should be very small, perhaps around 10 to 15%.
I have tended to simplify things by not emphasising the resistive loss element, as that isn't that important in terms of limiting practical working torque - maybe that was wrong.
As for the comment about those graphs showing the bottom end, in fact they show the whole safe working range of that motor. The stall current is many times the maximum current shown, but that's the case for any well-designed motor.
Any decent motor should be designed to have the very lowest DC resistance possible, for best efficiency. I've just looked at the specs for my 350 watt brushless model aircraft electric motor, it has a winding resistance of just 90 mOhms, for example, so even though it's maximum operating current is about 25 amps, it's stall current would be up around 150 amps at the normal controller operating voltage.
I don't have any figures for other bike motors, maybe they are much worse as you suggest, although the efficiency graphs available imply that they are similar. If they are as efficient as the motors I've played with, then there shouldn't be any really significant current reduction, and hence torque reduction, with a 33% supply voltage reduction, provided that the controller doesn't limit.
My real point was that a given current through a motor, at a given motor rpm, will give a particular torque, which is virtually independent of the supply voltage to the controller.
Jeremy
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