Shunt soldering

[KeithH]

Yet another probably naïve question.
If I solder the shunt on my (no name) controller does it increase the torque throughout the assist range or just at "level" 5
I would like to keep the range offered at lowest assist but be able to cycle up a 10% hill which my current set up wont cope with even on max assist.

Conversion Cyclotricity MXUS front hub 36v. No label on the controller.

Thanks
Keith

 

[anotherkiwi]

What gearing do you have on the bike? The rear hub Mxus on level 5 has no problems with 10% climbs if you are in the right gear.

 

[danielrlee]

The increase in torque will be linear and will affect all assist levels.

 

[KeithH]

Thanks both.

ps Anotherkiwi Its a front hub with derailure gears trying to pull a total weight of 120 Kilos! It can just about manage it with a totally fresh battery.
I know I need to loose weight.

 

[Deleted member 4366]

No. It's a bit more complicated than that. It will increase torque at low speed, but not at high speed.

the motor has a maximum rpm without a load on it. That's because it acts like a generator as well as a motor. As soon as it starts to turn, it generates power (voltage) in proportion to its speed. The voltage is opposite to the battery voltage. Therefore the net voltage that powers the motor decreases as it speeds up until the net voltage is zero at maximum rpm, which us why it can't go any faster.

The current that is pushed through the motor is in proportional to the voltage, so it would be very high at zero rpm and zero at maximum rpm.

The controller limits the current to a maximum value according to its setting and what it can read through the shunt. Say it's 15A. That means that the maximum current you can get will be 15 amps until a certain rpm, at which point the current starts ramping down to zero.

By soldering the shunt, you increase the current where it's limited, but the ramp down point will be at lower rpm, though the current will be more than it was before between the two ramp-down points. after that it will be the same.

The ramp-down point depends on the controller's maximum current and the motor's maximum rpm, but it's often somewhere around 50% to 70% of the motor's max rpm.

 

[KeithH]

Thanks d8veh.
I think I understand.
Just to check my brain, do you mean that I will get more torque at a lower RPM whatever the assist level set on the LCD.
I think I'll give it a go, perhaps less than a quarter as you described a couple of years ago.

 

[Deleted member 4366]

Just to check my brain, do you mean that I will get more torque at a lower RPM whatever the assist level set on the LCD.
I think I'll give it a go, perhaps less than a quarter as you described a couple of years ago.

Yes. basically it improves hill-climbing a lot. The extra current gives a bit more speed on the hills. Extra speed means that the motor and you run more efficiently. It can be the case that by increasing the current, you use less battery, which doesn't seem to make sense, but it can be true.

As an example, a typical 250w hub-motor with a 15 amp controller running up a steep hill at 6 mph could have an efficiency around 30%, so only 5 amps is being used to make the bike go and 10 amps are wasted.

Increase the current to 20 amps, the speed goes up to 8 mph and the efficiency to 50%, so 10 amps are used to make the bike go and 10 amps are wasted, but you're going 33% faster, so spend 33% less time drawing that high current, which means that you got that 33% extra current for nothing and had an easier ride up the hill.

 

[danielrlee]

No. It's a bit more complicated than that. It will increase torque at low speed, but not at high speed.

I always thought is was a linear increase, i.e. a 20% increase in battery current yields a 20% increase in torque across all RPM's.

 

[Deleted member 4366]

I always thought is was a linear increase, i.e. a 20% increase in battery current yields a 20% increase in torque across all RPM's.

The current is limited by the motor at high speed, not the controller. You could use a 1000 amp controller, but you still can't push more than zero amps through the motor when it reaches its maximum speed, and 1 amp when it's a bit slower than its maximum speed.

Current flows according to Ohm's Law: I = V/R. The resistance of the motor is fixed. The net voltage depends on the speed of the motor, so the voltage might be 36v at zero RPM and zero at maximum rpm.

Have a play with this. Note the kink in the blue and the red lines, which is the point at which the current limit changes from the controller to the motor. It's a shame they don't show the battery current. If they did, it would be a horizontal line up to the kink, and then a straight ramp down to the maximum speed. You can see that by clicking different speeds and looking at the battery current in the middle table:

http://www.ebikes.ca/tools/simulator.html

 

[danielrlee]

The current is limited by the motor at high speed, not the controller. You could use a 1000 amp controller, but you still can't push more than zero amps through the motor when it reaches its maximum speed, and 1 amp when it's a bit slower than its maximum speed.

Current flows according to Ohm's Law: I = V/R. The resistance of the motor is fixed. The net voltage depends on the speed of the motor, so the voltage might be 36v at zero RPM and zero at maximum rpm.

Have a play with this. Note the kink in the blue and the red lines, which is the point at which the current limit changes from the controller to the motor. It's a shame they don't show the battery current. If they did, it would be a horizontal line up to the kink, and then a straight ramp down to the maximum speed. You can see that by clicking different speeds and looking at the battery current in the middle table:

http://www.ebikes.ca/tools/simulator.html

Thanks. Having played with the Grin motor simulator for hours previously, it's clear that I need to study it for a while longer. When I get the chance I'll have another go.

 

[Deleted member 4366]

Thanks. Having played with the Grin motor simulator for hours previously, it's clear that I need to study it for a while longer. When I get the chance I'll have another go.

It's really good to show how efficiency, motor speed, voltage and current affect each other. Before the simulator, I had problems getting it into my head.