# why do controllers and ac motors need to match so well?



## few2many (Jun 23, 2009)

I understand the dc controller, i understand the ac controller is kind of a dc controller(power section) with a vfd inverter section for 3 phase, what im trying to figure out is why some people have said the motor and controller have to be perfectly matched? So the ac or dc controller has to be rated above the motor and intended use. Why cant I just use v/hz map, a little v boost at the start? and be careful not push the accelerator too far beyond the motor rpm? 
i am working with a free motor, so id like to use it, but wont loose too much sleep if i have to sell on ebay or just recycle it


----------



## major (Apr 4, 2008)

few2many said:


> what im trying to figure out is why some people have said the motor and controller have to be perfectly matched?


Hi few2,

The DC motor controller is basically a buck converter which uses the motor as the load inductor. The output current and voltage are fairly simply related and relatively simple to stabilize even when various load reactance is encountered.

The AC drive must synthesize a 3 phase sinusoidal wave form. It also uses the motor windings as output filters. A simple v/h drive can work over a range of different motor loads fairly well if it is of appropriate size and the loads can be driven with reasonable accel ramps. But when you get into high overload (like vehicle mass acceleration) and the need for torque control (vs v/f with ramped accels), then the control becomes complex. This involves multiple mathematical transforms and multiple control loops for voltage and current regulation in two planes. All this is quite dependent on the load reactance. Hence the need for "tuning".

Regards,

major


----------



## few2many (Jun 23, 2009)

Thanks major. Seems to depend a lot on the vehicle vs motor stats. So, vector or torque control reads a hall effect or encoder to control frq and v to the motor? Or rather current.


----------



## major (Apr 4, 2008)

few2many said:


> Thanks major. Seems to depend a lot on the vehicle vs motor stats. So, vector or torque control reads a hall effect or encoder to control frq and v to the motor? Or rather current.


Yes and yes and yes.

Hi few2,

You might review this thread. http://www.diyelectriccar.com/forum...ng-my-own-17319p4.html?highlight=shunt+vector 

Regards,

major


----------



## few2many (Jun 23, 2009)

Thanks, I missed that one in my searches. Info on this is pretty sparse on most the net.


----------

