EVLSERVO1 Over Current Protection (OCP)

[MotorHobbyist]

Hello everyone,

Based on the screenshots attached, I couldn't understand the formula obtained for the OCP feature of this inverter board.

3 phase currents are read through the shunt resistors, got offset, and amplified by the op-amp ICs. So far so good!

But PC1, PC2, and PC3 are tied together using 3 one kilo ohm resistors, then the common node is fed to the comparator pin of the MCU to do the OCP task.

I am wondering how the Equation 2 is obtained for implementing over current protection, and why it is like this. 

Why is the G.Rs.Ioc term divided by 3? 

If you could guide me, it'd be awesome, thanks.

[AScha.3]

Hi,

>Why is the G.Rs.Ioc term divided by 3? 

Because adding 3 signals together with simple 3 resistors, so "sum" is (a+b+c) /3 ;

if all "1" , sum is also 1; but if only one is "1", others zero, then sum is 1+0+0 /3 = 1/3 ;

and this is just cheaper than having 3 comparators , doing the same.

And its just error/overcurrent protection, so no need for a "precision" switching level, 

in "normal" use, it never should be active, only on a short it protects the mosfets from exploding.

ok ?

[MotorHobbyist]

*: Because adding 3 signals together with simple 3 resistors, so "sum" is (a+b+c) /3.

Yes, I see, but please refer to the screenshot attached. This is the formula for calculating the output of each op-amp network provided by ST.

Based on that, considering resistors values, PCx is equal to (Vref /2) + (G*Rsh*I).

So, (PC1+ PC2 + PC3) / 3 = ((3 * Vref /2) + (G*Rsh*(Ia+Ib+Ic))) / 3 = Vref /2 + (G*Rsh*(Ia+Ib+Ic)) /3

isn't this correct that at each specific instant, in FOC considering a balanced system: Ia+Ib+Ic = 0?

then, the sum would be Vref/2. 

Please correct me if I'm mistaken.

[AScha.3]

Yes, agree.  

But this Over Current Protection (OCP) is not for normal working state - here it should do just nothing.

It should react, if maybe a motor coil gets shorted or a short at connection - then one (!) current suddenly going through the roof - and this event should trigger ocp error and stop all outputs.

And your "Ia+Ib+Ic = 0" is at the center point of the 3 coils , currents relative to the coils pos or neg polarity,  but the current shunts are on the lo side ("gnd"), here always only positive current when power going to the motor,

so here you see the current going through the half-bridges and this is not "=0" , but always the sum of the currents going to the motor from the power supply !