Motor Torque question

[1boris]

If you have an Bafang bbso1 motor and you somehow doubled the rotor diameter and of course the motor house so the rotor fits inside.Othervise the motor stayed exactly the same.same speed windings,same gearing and controller.Would this doubled the torque of the motor?

 

[Deleted member 4366]

Interesting question. I think it would have the same torque because the controller would provide the same power and the gearing would be the same. If it made more torque with the same power, it would have to spin more slowly, but it's a crank motor, so torque at the back wheel depends on gearing. If the motor turned more slowly, the bike would go slower. You could achieve the same by using a smaller chainwheel.

 

[Danidl]

If you have an Bafang bbso1 motor and you somehow doubled the rotor diameter and of course the motor house so the rotor fits inside.Othervise the motor stayed exactly the same.same speed windings,same gearing and controller.Would this doubled the torque of the motor?

It cannot be the same. If the dimensions of the rotor change, the amounts of iron, area of windings , length of wire must all change. ..these are the parameters which actually create the torque in the rotor. At its most simplistic the larger rotor would be capable of more power.. size matters

 

[1boris]

It cannot be the same. If the dimensions of the rotor change, the amounts of iron, area of windings , length of wire must all change. ..these are the parameters which actually create the torque in the rotor. At its most simplistic the larger rotor would be capable of more power.. size matters

yes but If you change all those parameters and have the same controller current and gearing will it double the toque?

 

[Deleted member 4366]

If the dimensions of the rotor change, the amounts of iron, area of windings , length of wire must all change. ..these are the parameters which actually create the torque in the rotor.

Not so. I have a massive hub-motor sitting next to me right now. It's making no torque at all. The only thing that causes a motor to make torque is the electrons, and they're regulated by the controller.

 

[Danidl]

Not so. I have a massive hub-motor sitting next to me right now. It's making no torque at all. The only thing that causes a motor to make torque is the electrons, and they're regulated by the controller.

Can't argue with that...

 

[1boris]

Interesting question. I think it would have the same torque because the controller would provide the same power and the gearing would be the same. If it made more torque with the same power, it would have to spin more slowly, but it's a crank motor, so torque at the back wheel depends on gearing. If the motor turned more slowly, the bike would go slower. You could achieve the same by using a smaller chainwheel.

I had a feeling it wasnt that easy,but what if you take an 5T mac motor and give it 1:10 gearing.that would give it same speed as an Mac 10T right? but the windings would be thiker and it should be harder to overheat.And also the torque should be better as far as I understand.what do you think D8veh?

 

[Danidl]

yes but If you change all those parameters and have the same controller current and gearing will it double the toque?

. We don't have perpetual motion machines, so the effect will be that the bigger dimensioned electrical machine will rotate slower for the same electrical power input from your controller, but with a higher torque, so by the time you then put it through gears to speed up the chain speed you get the same mechanical power output. Mechanical Power = torque X revs X 2π.
Torque is only half of the power equation, the revolutions per second are equally important.

 

[1boris]

. We don't have perpetual motion machines, so the effect will be that the bigger dimensioned electrical machine will rotate slower for the same electrical power input from your controller, but with a higher torque, so by the time you then put it through gears to speed up the chain speed you get the same mechanical power output. Mechanical Power = torque X revs X 2π.
Torque is only half of the power equation, the revolutions per second are equally important.

So that means that the size of the motor has noting to say for the output of the bbs 02. It could be bigger or smaller as long as it can take the current it wil not change anything?

 

[Danidl]

So that means that the size of the motor has noting to say for the output of the bbs 02. It could be bigger or smaller as long as it can take the current it wil not change anything?

I am not sure what you are saying. A physically larger rotor , containing more material will be able to absorb more electrical power and create more mechanical power . But it is the voltage and amps coming from the controller which determines that electrical power and consequently the mechanical power. Other things being equal the bigger rotor will rotate slower for the same number of turns and magnetic field, but with a larger torque... So that changes everything.
As d8veh has indicated and I have said, using a gearbox or the number of teeth on chain wheels, one can convert power from high speed low torque to lower speed higher torque, but we cannot make power !!

 

[1boris]

I am not sure what you are saying. A physically larger rotor , containing more material will be able to absorb more electrical power and create more mechanical power . But it is the voltage and amps coming from the controller which determines that electrical power and consequently the mechanical power. Other things being equal the bigger rotor will rotate slower for the same number of turns and magnetic field, but with a larger torque... So that changes everything.
As d8veh has indicated and I have said, using a gearbox or the number of teeth on chain wheels, one can convert power from high speed low torque to lower speed higher torque, but we cannot make power !!

I understand,I didnt know that a bigger motor rotated slower for he same number of turns..Thanks for answering

 

[Deleted member 4366]

I had a feeling it wasnt that easy,but what if you take an 5T mac motor and give it 1:10 gearing.that would give it same speed as an Mac 10T right? but the windings would be thiker and it should be harder to overheat.And also the torque should be better as far as I understand.what do you think D8veh?

Good example. That's why the Q100 and Q128 motors are so good. They use high speed motors with larger reduction ratios. Normal hub motors are 5:1, but the Q100h is 12.6:1. In that case, they can produce more power, but you have to use a controller that can provide the extra power to make them spin at the high rpm.

 

[Danidl]

I understand,I didnt know that a bigger motor rotated slower for he same number of turns..Thanks for answering

You need to go back to an A level physics book, for a better explanation but the principle is that the force on the rotor is BINL B magnetic field, I the current in a wire at right angle to the field, N the number of such wires (the number of turns in the winding,) L the length of wire in that configuration. The torque produced will be that force multiplied by the diameter of the rotor. Torque will cause the rotor to rotate
The back EMF is determined by the magnetic field B , the number of turns N the speed of rotation and the area subtended by the wire... Which relates to L. Larger rotors have larger areas . Motors can only accerate up until the backemf equals the applied voltage.