Hello:
Is anyone building their own motor controllers? I am not able to find much art on the subject and am simply curious. What is it about the commercial units that make them so expensive? Technology or volume?
I am curious because it seems so simple to connect the batteries to the motor using a PWM circuit and the proper FET’s.
Thanks for any insight.
Ty
Some people are.
http://www.diyelectriccar.com/forums/
the technical forum has two long threads on PWM controllers.
I am a novice at ac systems, but even to me it does not look too bad.
sure, at low voltages and currents Its just a fet with a PWM signal… but that all goes out the window when you start introducing more than 1 Fet and making sure you drive them all at the same time (copper path to each fet is roughly the same inpedance). Also, as you add FETs, you increase current, and the eddie currents and transients (from turning the fet on and off at high currents) cause spikes in the circuit…
It is simple for low V/A but it gets tricky when you try and get higher than 60V and higher than 100A. They’re expensive due to packaging, quality of parts (so they all gate at the same time), voltage level of parts (some FETs in there are 200V parts), the quantity of parts (to get the currents you need), the FET driver, programming of the microcontroller to handle battery current/voltage and motor current/voltage, intarface (Rs232/USB etc), bussbar, the development of the board so you reduce ringing and shield the parts to reduce the EMI… etc…etc…etc.
There is a post in the conversions and diy about it. If you look you can find a lot of information about it. It just depends on how quick and dirty you want to go.
Why are they expencive? Because there’s no competition. The companies charge whatever they want. In this country “reasonable profit” is unheard of. Everyone wants to die with 3 Ferraris in the garage. What these companies don’t understand is their kicking themselfs in the ass overcharging for controllers. If the price was more reasonable they would be selling more. Make profit in quantity instead of trying to pay 2 months rent off one controller. That’s ok… Things are about to change. 
[QUOTE=lazzer408;3390]Why are they expencive? Because there’s no competition. The companies charge whatever they want. In this country “reasonable profit” is unheard of. Everyone wants to die with 3 Ferraris in the garage. What these companies don’t understand is their kicking themselfs in the ass overcharging for controllers. If the price was more reasonable they would be selling more. Make profit in quantity instead of trying to pay 2 months rent off one controller. That’s ok… Things are about to change. [/QUOTE]
Your not taking into account most companies that make them arent very big and there is r&d, as well as other operating costs. They shouldn’t starve so they can sell your cheap rear a controller. Some people don’t think enough to relize sure it may cost 500 bucks to make but then there is the 3 years it took to design it and all the loans they racked up, all the operating costs of a company like insurance, etc. So that 1000 dollar controller might only cost 500 to make but at the end of the day they don’t come close getting 500 in profits.
I plan on making my own motor controller, the only thing that bugs me is that it wont have regenerative braking. But for being a go cart and not a street legal vehicle, I can live with it.
The schematic I’m using is over at http://www.solorb.com/elect/solarcirc/pwm1 It seems like the easiest one that I can reasonably build. I haven’t looked into this eddie current loss yet, but I know that some trains in europe use eddie current as their brakes 
The schemat is set up to run at 400Hz, but under my scope it’s reading 500Hz so give or take a bit. I’m hoping I can get my cart up soon.