[ausev] DIY controller?

[Chris Robison]

Charlesvsi at aol.com wrote:
> Basic functions off the top of my head:
>  1. sensors monitoring the relationship between accelerator pedal and  phase, 
> amplitude,etc. of current going into motor, while monitoring phase shift  of 
> back emf  that indicates how fast electric motor armature is going  around. 

In general when people talk about making their own controllers, they're 
talking about DC motor controllers for series-wound motors, which are 
vastly simpler in design, and don't require most of the (apparently 
inductive AC related?) considerations you've listed here. They don't do 
regenerative braking, don't require feedback from the motor beyond an 
RPM sensor for safety and user feedback, and in general consist of just 
a single MOSFET or IGBT switch. Charging and battery management 
functions are also not generally included in DC controllers.

So, the short list of necessary features for a DC controller:

A) Accept a throttle reading of some kind (0-5k pot is most common)
B) Translate that into a PWM duty cycle

Some things that are not necessary but are a Really Good Idea:
C) sense motor speed and cut power at or above a set value (many 
commercial controllers don't even do this)
D) sense controller internal temperature and cut back current in 
overheat condition
E) sense pack voltage and provide programmable Warn and Stop thresholds
F) provide programmable battery and motor current limits, and motor 
voltage limit
G) provide warning light (UI) outputs for the dash
H) provide a computer interface of some kind for programming

And then there are other issues, like your choice of switching device; 
do you go with MOSFETS which are cheaper, or IGBTs that are more 
efficient at EV power levels but can't normally be paralleled (forcing 
the use of larger more expensive devices)?  If your control electronics 
are powered via a 12V input, how will you maintain electrical isolation, 
and still provide enough power for your gate drivers (which can be 
significant, to overcome gate capacitance and keep your switching time 
short)?

It seems like a very simple thing on the surface, to make a DC motor 
controller. And to a certain extent it can be simple to make one that 
works, but making one that's safe, reliable, sufficiently powerful, 
efficient, and fails in a predictable and controllable fashion takes a 
bit more work.  Most people that I've talked to who have gone to the 
trouble of building their own will tell you that they ended up spending 
a LOT more money in the process, than if they'd bought one off the 
shelf.  Build one if you want to teach yourself about power electronics; 
it can be a great way to learn.  But don't do it to save money.

   --chris