SVPWM Overmodulation

[derekcook]

Hello, 

I see that there are overmodulation states in the SVPWM module. I am having problems at 200V DC Bus holding 1155RPM. I am losing sync because it seems that I do not have enough voltage. It looks like at 200V DC bus I would only need 124Vac to drive the motor at 1155 RPM. 

I want to be sure that the SVPWM is overmodulating when the voltage gets low. How can I tell this? It looks like my modulation index is never going above 1. Will this go above 1? To 1.15? 

Is there any set up required to overmodulate?

[derekcook]

My SVPWM overmodulation does not seem to be working correctly - I keep going in and out of the overmodulation state 1 until I either stall or lose sync - below is a stall - see below I have placed the overmodulation states in debug 1 (scaled 0-5) and in debugs 2-4 I have the svpwm duty cycles (scaled 1 to -2). My DC Bus (DC Link scaled 0-600) is a little noisy due to having smaller filter caps than needed - I would like to overmodulate when my voltage drops below 215V on my DC bus. Could the noise cause problems with the overmodulation of SVPWM? I do have a software filtered DC Bus. Should I try to use this for SVPWM instead? How would I use my filtered DC Bus instead of the Raw DC Bus for SVPWM overmodulation?

[linestream-adam]

Overmodulation in KMS only supports region 1. KMS doesn't support region 2 overmodulation. So the modulation will only go up to 1.0.

In KMS, the SVPWM modulation will be calculated the same if it is in overmodulation or not. The biggest difference when KMS is in overmodulation is that the feedback block will start doing current reconstruction based on the SVPWM sector.

Bus voltage is used to overcome the back emf and to induce current in the phase resistors. So depending on the load on your motors it is entirely possible that you cannot sustain 1155RPM with a 200V bus.

When your motor is running at rated speed with rated voltage, it should be able to sustain rated torque (at rated current). What overmodulation does is that it will let you run faster than rated speed or with more than rated load. But you are limited by the voltage limit.

[derekcook]

Thanks Adam, 

Once you are in overmodulation though, you should stay in that state correct? I'm afraid maybe my DC Bus input or one of my settings is not allowing it to function correctly.My system is going in and out of that state when the voltage gets low.

My system is getting very close to the voltage limit - the system is designed to run all the way to the voltage limit at 3x rated load and then fault (on low voltage) or shut off before we lose sync. 

This is a quick analysis of what the system should be able to provide at 200V:

I did a fast analysis of the voltage drop in the resistor path to the motor and the converter should be able to provide around 124Vac ph-ph rms at 200Vdc using max modulation index. Therefore, it should be able to operate 140Nm at 1155 rpm, which would require around 115Vac ph-ph rms.

 

Instead, if we don’t apply common mode voltage in the modulation to get maximum modulation index we cannot provide the required 115 Vac ph-ph rms and we would need to reduce speed until around 1050 rpm (according to the calculation).

On my opinion, what I would check is if we are providing and defining properly the modulation index by trendering all modulation parameters and converter output.

[linestream-adam]

No, it is perfectly acceptable to toggle in and out of overmodulation. There is no penalty or issue with doing that.

If you assume a back emf of 101.3046 V, what speed does that translate to on your motor? 

[derekcook]

Hey Adam, the motor is rated at 200V at 2000RPM. 101.3046V should be around 1000RPM. Our system should be able to supply 124V at 140Nm at 1155 RPM at 200V DC Bus - Only 115V would be required to drive the motor at this load. However, we are losing sync at 200V DC Bus. It does not seem we have enough voltage. I am seeing the SVPWM going into the overmodulation states. I am also seeing 130Vrms on the scope at 200VDC Bus 140Nm of load on all three phases, so it seems to me overmodulation is working. Maybe our calculations are incorrect and we need more voltage.

[linestream-adam]

Derek,

I think you need more voltage. I don't think you accounted for the voltage drop required to establish the current in the motor phases. Both the back emf and the (phase current * phase resistance) contribute to the voltage required.

Another way to test this would be to turn off the fault / sync faults and see what speed your motor can achieve given the system constraints. This should show that KMS is working correctly and that you need additional voltage.

[derekcook]

Thanks for the help Adam. Currently I think the plan is to lower my speed as my voltage drops. That will resolve this issue as well.