MCSDK Modulation value

[AnthonyK]

I have read that in the MCSDK, there is an overmodulation technique that could exceed the linear region of the SVPWM. However, without it, I see in the WC Workbench, that the maximum modulation than can be achieved even without overmodulation is 100% so the modulation index will be equal to 1. Moreover, without overmodulation, I can see that I could attain a modulation index equal to 1 which is superior to the linear region which is 0.866. So what is the role of the overmodulation flag, and what is the difference without it?

Thank you

[Gael A]

 

Hello AnthonyK,

It seems you are mixing different topics. The Max Modulation Index is mostly useful for Shunt-Based current measurement. It is not directly linked to Overmodulation.

To put it simply, we use the Max Modulation Index to clamp the modulation whenever it would make one of the three pulses too short for sampling current. This is why MMI is always at 100% when using ICS for example, since the current can be sampled at any time, on the contrary of shunt-based measurement.

A 100% MMI simply means that the system is allowed to modulate the pulses up to their maximum capacity. Such MMI is indeed required for overmodulation, but having a 100% MMI does not necessarily means Overmodulation is used.

The structure of a PMSM motor makes the area of reachable voltages shaped like an hexagon. We make the choice to limit ourselves to the biggest circle that can fit that hexagon. This is the 0.866 limit you are talking about, this can be called Space Vector Modulation Index.

Overmodulation makes use of the full MMI to reach the corners of the hexagon, which creates distortions in voltages and currents, but reaches the full capabilities of the system.

You can find more information on how MMI is computed, and on how Overmodulation is made in our User Manual. Via WorkBench, you can find it by clicking on About -> Documentations -> Documentation -> in the newly open browser window, click on User Manual.

 

[AnthonyK]

Hello Gael,

 

Thank you for your reply.

But what can I see without overmodulation, that the modulation index can reach 1. I have calculated it using this formula : sqrt(Vd^2 + Vq^2)/Vdc/sqrt(3). And I have seen that the modulation index could be equal to 1 without activating the overmodulation flag.

[cedric H]

Hello @AnthonyK 

It is a matter of definition, MMI=1 means that we can reach the biggest circle that can fit the hexagon. 

Regards

Cedric