After installing sensors in the Turnigy 80-100 motor I needed a high current sensored BLDC controller. Since I’ve decided to use a 12 S LiPo battery the maximum voltage of the newly charged battery is 50,4 V with a nominal voltage of 44,4 V. Most high power e-bike controllers are designed to operate on >72 V and are quite large.
When i find the time I will build my own controller but for now, I want to modify a small 48 V 350 W controller, that I bought for $25 from eBay, into something that is a lot more powerful. The key to increase power handling capability is to decrease the heat losses under high power. As a side effect, more of the energy in the battery will be used to move the bike and less to heat the controller.
the modification is done in a couple of steps described below.
The controller originally contained six STP75NF75 MOSFET which can handle a voltage of 75 V and (according to the datasheet) 75 A. The typical resistance when turned on is 10 mΩ which is quite high. Realistically I think six of these is capable of handling ~15 A continuously with decent cooling. I’m not even sure if they are genuine and 48 V 350 W will be ~7 A so the original controller isn’t really pushing them.
Instead i will use six IRFB3006 which can handle 60 V and up to 195 A (again, according to the datasheet). The silicon could actually handle up to 270 A but the wire bonds between the silicon and the case limits this to 195 A. The typical on-resistnance on these are 2 mΩ, five times lower than the original FETs! Another popular transistor to use when modding e-bike controllers is the IRFB4110 which is capable of handling 100 V but not as much current as the IRFB3006.
Beef up the PCB traces and wiring
The original high current PCB traces of the controller had some extra solder on them to increase the current capabilities. To increase this even further i added 3×1.5 mm copper wire to these traces. Compared to copper, solder is a pretty bad conductor so this will decrease losses and heating under high currents considerably.
There was one problem with this, copper and PCB laminate have different Coefficients of Linear Thermal Expansion, a view from the side reveals that the board got a little curved when soldering. I hope this doesn’t break anything.
The wimpy phase and battery wires on the original controller is replaced with 6 mm² wire instead to handle he increased current. And a lot of the special function wires on the controller are removed. I only need the throttle and brake inputs.
Modify the current shunt
When I ordered this controller I was pretty sure that it were based on the Infineon XC846 ship as most china-made e-bike controllers are. These controllers can have the current limit and many other properties changed in software by connecting your computer ti the controller. Instead this controller is based on a STM8 microcontroller, maybe this is programmable but I haven’t found any information on how.
Instead of programming I can increase the current limit by decreasing the resistance of the current shunt. The processor measures the voltage drop across a short bit of wire with a known resistance to determine how much current the motor uses. If I for example decrease the resistance of this wire to half, the current will be twice of what the processor thinks.
Today I recorded two videos running the motor. The first one is just running the motor in sensorless mode. I actually got it running once in sensored mode but as soon as I started adjusting the sensor angle the controller fell back into sensorless mode. The throttle in this video is a tired 10k trimpot hence the uneven throttle signal.
I also made a small load test just holding the motor. In this video the motor is run on the lowest speed possible in sensorless mode. The battery used in this clip had a voltage of 46 V. Since the currentmeter maxes out at 5,5 A the load power I created is somewhere around 250 W