What determines torque in a hub motor?
- Thread starter ibra2672
- Start date
larsb
1 MW
ibra2672 said:
These questions don't have single or simple answers. I think you should spend some time reading about electric motors before asking, it can't be summarised in one post. some starter material in the motor section of the forum:
https://endless-sphere.com/forums/viewtopic.php?f=30&t=240
For a given motor, the phase amps determine the torque, up until you reach the saturation point. Typically, you don't need to worry about saturation, as the motor will overheat and burn out if you get that high.
As larsb points out, it can get complicated, but it's still just the phase amps that determines the torque. Increasing the voltage may or may not give you more phase amps at low speed depending on your controller.
As larsb points out, it can get complicated, but it's still just the phase amps that determines the torque. Increasing the voltage may or may not give you more phase amps at low speed depending on your controller.
> phase amps that determines the torque
And super high phase amps requires a design targeted to handle it efficiently, aka avoiding waste heat
after that, the ability to shed waste heat effectively
If the battery and the controller can handle massive current, it is the motor - destructive heat level that becomes the limiting factor.
A temp sensor feeding a cut-off, or rollback limiting circuit (limp-home mode) is a good failsafe when testing
unless testing to destruction is the goal.
And super high phase amps requires a design targeted to handle it efficiently, aka avoiding waste heat
after that, the ability to shed waste heat effectively
If the battery and the controller can handle massive current, it is the motor - destructive heat level that becomes the limiting factor.
A temp sensor feeding a cut-off, or rollback limiting circuit (limp-home mode) is a good failsafe when testing
unless testing to destruction is the goal.
spinningmagnets
100 TW
As the previous posters covered the major points, torque is also affected by the diameter of the stator. The vast majority of the common hubmotors use a stator lamination that is 205mm in diameter, and that leaves just enough air between the hubmotor and the rim to use spokes in the conventional fashion.
The BionX D-series (D500?) was an extra-large diameter direct drive hubmotor, and it had a surprising amount of torque for a motor with a thin stator and a modest amount of copper mass. Of course it was very visible and hard to hide that it is an electric bike.
The BionX D-series (D500?) was an extra-large diameter direct drive hubmotor, and it had a surprising amount of torque for a motor with a thin stator and a modest amount of copper mass. Of course it was very visible and hard to hide that it is an electric bike.
DanGT86
100 kW
ibra2672 said:Thank you very much for the replies!
If I may ask, what are phase amps? Is it simply the current flowing in the phase wires of the controller? Or is it the maximum current that the controller can supply to the motor?
Phase amps is the current in amps going out to the motor.
The controller gets power in from the battery (voltage x current) and sends it out to the motor. Since the motor voltage is proportional to the RPM the controller can send out more phase amps to the motor than the amps its drawing from the battery.
Ignoring real world losses a basic example is something like:
1000watt ebike might be drawing 20 amps at 50v from the battery. But at half speed it can send that 1000watts to the motor as 40 "phase" amps at 25v.
The motor only cares about what is coming out of the controller. It has no idea what the current was from the battery to the controller.
Thx for the reply. There's one thing you said that I didn't quite understand. You said that " the controller can send out more phase amps to the motor than the amps its drawing from the battery." Shouldn't the amps that get sent to the motor via the controller be the same (ignoring any losses) as the amps drawn by the controller from the battery?
DanGT86
100 kW
The amps out to the motor will only be the same as the amps in when motor voltage is the same as the battery.
The main job of the controller is to vary the ratio of amps and volts.
When you accelerate you start off with very low voltage and very high amps. As the motor accelerates the amps taper down as the voltage and speed rise. This continues until you reach full speed where the voltage and current out are very close to the voltage and current in.
Obviously the watts out cant exceed the watts in. But power (watts) can be made up of high current at low voltage or low current at high voltage.
At zero volts you would need infinity amps which would blow up your controller. So the controller has a motor amps out limit that is preset. Generally people run a motor amps limit of 2-3x the max battery amps with a basic hub motor.
This is why slow speed high load situations burn up motors but high speed low current situations are fine.
This is also why gearing down and increasing the voltage with wheel size or sprockets will make your motor run cooler for a given amount of power. The higher the rpm the less amps are required for a given amount if watts.
The main job of the controller is to vary the ratio of amps and volts.
When you accelerate you start off with very low voltage and very high amps. As the motor accelerates the amps taper down as the voltage and speed rise. This continues until you reach full speed where the voltage and current out are very close to the voltage and current in.
Obviously the watts out cant exceed the watts in. But power (watts) can be made up of high current at low voltage or low current at high voltage.
At zero volts you would need infinity amps which would blow up your controller. So the controller has a motor amps out limit that is preset. Generally people run a motor amps limit of 2-3x the max battery amps with a basic hub motor.
This is why slow speed high load situations burn up motors but high speed low current situations are fine.
This is also why gearing down and increasing the voltage with wheel size or sprockets will make your motor run cooler for a given amount of power. The higher the rpm the less amps are required for a given amount if watts.
larsb
1 MW
An example:
At lower speed full voltage is not applied to motor so phase current can be higher than battery current and still maintain energy balance.
Battery 10V at 10A is 100W input to controller
Phase voltage 5V and 20A is 100W controller output to motor (or some other combination)
This is a good representation figure to show this:
At lower speed full voltage is not applied to motor so phase current can be higher than battery current and still maintain energy balance.
Battery 10V at 10A is 100W input to controller
Phase voltage 5V and 20A is 100W controller output to motor (or some other combination)
This is a good representation figure to show this:
larsb
1 MW
Full voltage at zero rpm is the same as a short circuit through the motor. in my case (following ohms law) that's 72V/0.005ohm so 14400A of current if the battery could supply that.
Obviously it can't, motor, battery and controller would say bang. Voltage needs to be controlled, in my case to roughly max 0.005ohm*500A so only 2.5V max at zero rpm, at higher rpm it needs to counter the BEMF voltage so it needs to be increased.
It's not until high rpm that the full voltage is needed.
Obviously it can't, motor, battery and controller would say bang. Voltage needs to be controlled, in my case to roughly max 0.005ohm*500A so only 2.5V max at zero rpm, at higher rpm it needs to counter the BEMF voltage so it needs to be increased.
It's not until high rpm that the full voltage is needed.
Danishblunt
1 mW
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ibra2672 said:
As many have mentioned before its actually a very complex topic, but it can be dumbed down so that you get the jist of it.
These things have the highest impact on torque:
- amp provided by battery/controller
- Wheel size
- Type of motor
Now as people mentioned here the amp will direcly affect the torque of the motor. The more amp you get, the more torque it gets. Whats important to note here is that you can't just shove a 72v50AH battery + controller onto a hub that was designed to run 36v8AH, as there are parts in the actual hub such as cables, and magnets that can be saturated with power, that being said, turning a "250W" hub motor into a "750W" motor isnt impossible. Obviously the wheel size matters as well, smaller wheels cause less strain on the motor, making it easier for it to push you forward, this is why bikes with very small wheels have typically very good torque. The motor itself will also be quite important on what type of bike you want.
Depending on what power range you think you can either go for a mid drive or a geared hub motor for the best torque. The mid drive is good for a lower power draw, while the hub is a must for high power draw, the mid drive has some limitations which prevents it from being a good choice on higher power draw builds.
Chalo
100 TW
Danishblunt said:
Nope. Torque is independent of wheel size. The same torque does different things in different size wheels, but it's the same torque.
Nope. Torque equals force times radius. Changing the radius changes the force inversely, but doesn't affect torque.
larsb
1 MW
+1 on that.
F=m*a and t=F*l
a=F/m substitute F=t/l
a=t/(l*m)
the force at a smaller radius for a given torque will be higher so the acceleration F/m will be higher also.
F=m*a and t=F*l
a=F/m substitute F=t/l
a=t/(l*m)
the force at a smaller radius for a given torque will be higher so the acceleration F/m will be higher also.
Danishblunt
1 mW
- Joined
- May 3, 2022
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- 14
Chalo said:
In my brain I had torque = acceleration, had brainfart there, it was late :lol:
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