How To Adjust the Maximum Amps of a Controller

I was visiting a controller factory in China seeing about getting some controllers made. It was a small factory with few people working there.
At the end of discussing different aspects of controllers I asked how they adjusted the maximum amps of a controller. I was thinking it was internally controlled by the resistance of the mosfets. The owner of the factory pointed to the thick piece of wire on the circuit board.............

The torque of a hub motor is controlled by the maximum amps that can be supplied to the motor by the controller. (compare with the speed of a motor is controlled by the voltage its run at). The more amps you have the more torque you have, better for hill climbing and quick starts and for heavy loads. The down side is you will have less range from your batteries.

By adjusting the maximum amps of a controller its very likely the controllers mosfets will get hotter than normal, also the motor will get hotter than normal. So the temperature of the controller box and the motor should be checked when going up a long steep hill or any time when the motor is under load for a period of time. It may well be the controller you adjust to get more torque will be fine with higher amps going through it but the temperature should be noted just in case the controller is getting to hot. Also the motor will most likely get warmer and it should be checked that it doesn't get too hot also.

The mosfets in controllers can handle up to about 150degrees celcius, so the box can get quite warm, it wont get to 150degrees as the heat has to travel some distance to get to the outside but it can get quite warm without damaging the mosfets.. The aluminium box of a controller acts as a heatsink taking heat away from the mosfets to the outside air. Before doing this experiment make sure you have plenty of heat transfer compound at the rear of the mosfets where they are in contact with the aluminium case. Some controllers will have an aluminium block that the mosfets are in contact with. It also acts to absorb the heat the mosfets create. An adequate amount of heat transfer compound ( a white paste available from electronics suppliers in small tubes and quite cheap). If you are concerned the mosfets will be damaged by the extra heat of having extra amps you can always add an additional block of aluminium in thermal contact with the mosfets, the bigger the block of aluminium the better its protection to the mosfets. But not all aluminium has the same thermal properties so best to use some aluminium that you know is designed to be used as a heatsink.

If a mosfet was used without any heatsinking at all it would fail very quickly (as I found out some time ago!), heatsinking is 100% necessary when using mosfets.

How  to adjust max amps:

1. Identify the BUZZBAR ( correct name is a SHUNT) wire in the controller. ( will redo pics one day so that the wire is called shunt, but buzzbar will do for time being)
The buzzbar will be quite easy to recognise, if there are two buzzbars, one small and one large, then its the larger one you need to adjust, I have no idea what the small buzzbar is for (note in the diagram below there are two buzzbars next to each other, the larger one is the one which controls maximum amps). Its the resistance of this the buzzbar which controls the maximum amps the controller can deliver too the motor.

Below are a series of pictures just of one controller showing the buzzbar

   

2. Small Increase in Max Amps
I suggest that first you make just a small increase in the maximum amps of the controller. This can be done by adding solder to the two ends of the buzzbar where they join onto the circuit board. Adding a small amount of solder lowers the resistance of the buzzbar and increases the max amps of the controller. On the pictures below i've already added solder to the ends of the buzzbar. Usually there is only a relatively small amount of solder at each end of it.

 
 
Note: if your using lithium or nimh batteries and you want to decrease the maximum amps of your controller to suit the batteries maximum current rating you can remove solder from the buzzbar where it connects to the circuit board to increase the resistance and hence decrease max amps the controller can supply to the motor.

   3.  Larger increases in maximum amps

To get a larger increase in max amps I solder an extra piece of wire across the buzzbar. The type of wire is not critical but just something that can handle say 15 or 20amps.

  

Below shows where I've connected an extra piece of wire at one end of the buzzbar ( the extra piece of wire still has some insulation on one end which will be removed before I solder it to the other end of the buzzbar).
It also has to be connected to the other end of the buzzbar and then the whole wire should be covered in solder so that it makes good contact all along the buzzbar (probably not really necessary thats just the way I've been doing it).

The two pictures below show the buzzbar after the additional piece of wire has been added and soldered to the buzzbar.



So now instead of their being a thin piece of wire with high resistance there is a thick wire (really two wires) soldered all along it, and it has a lower resistance, so the amps go up.................alot!

Make sure your controller and motor are ok with regard to how hot they get, keep a close eye on them initially, you might have to reduce the amount of solder if they get too hot.

4. A Fancy Schmancy Set Up

I haven't tried this but just had an idea while talking to a customer about increasing amps. You could put a switch on the handlebars to a wire which is parallel to the buzz bar. Fairly high amp (low resistance) wire would be needed (probably around 20amp current rating). You could then convert the controller to high amp mode only when needed on steepest hills or other situations where needed. Otherwise you might find your using up alot of amps un-necessarily by accelerating too hard or not pedalling at all (come on you've got to pedal a bit!! thats what bikes are for!)



How to Test what is the maximum amps of your controller:
To measure the maximum amps your motor can use ( or looking at it another way: the maximum amps your controller can provide to the motor). Basically you need to measure the amps coming out of your batteries with an ammeter or multimeter and then you need to apply a load to the motor while its running at full power. You can either just go for a ride and see your max amps on the ammeter puting the motor under load will get you to max amps (a steep hill). Or you can simply run the wheel flat out off the ground and then apply some pressure to the tyre, simply letting it drop slowly onto the ground so that the pressure against the wheel increases (wheel slows down ) as its lowered further onto the ground.
Interestingly max. amps doesn't occur when the motor almost stalls, it occurs at a certain rpm at a certain load.
Also for some strange reason I dont understand I seem to get slightly different results from going for a ride and testing on the ground while bike is stationary, it could be the extra load of a person sitting on the bike causing it, not sure.

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