ACS Drive in Torque Mode - Torque Oscillation?

[Total_newbie]

Hello,
I am posting this question (regarding drive control) on this PLC forum because I see that there has been discussion regarding this issue (with a lot of expert input) on forum in past.

I am working on control of ABB Drives (ACS 800, ACS880) on dynos. For some applications, we are operating drives under torque, rather than speed, control. (Drives are operating in this regard under Direct Torque Control). Recently, upon installing a higher speed torque sensor on drive shaft, we observed that we are getting a fairly constant (independent of RPM) torque oscillation - which in some cases can be ~10% of our target. (See attachment. Blue is measured torque (Nm). X-axis is time (sec)). This torque oscillation shows as ~76Hz (we are sampling at torque at 1000Hz).
a) Frequency of oscillation is independent of drive RPM
b) Magnitude of oscillation decreases as I decrease P-gain of drive. (I can't decrease it too much - or we don't get to targets quickly or smoothly enough)
c) Magnitude of oscillation decreases if I decrease switching frequency (on ACS 880) from 4.5 kHz to 1.5 KHz.
d) Oscillation is not coming from load. Even if there is very little load on shaft - we still see this.
e) We see it in pretty similar manner on both ACS800 and ACS880 drive.
f) Oscillation increases in magnitude when drive is accelerating/decelerating, or torque demand (which I am calling for from drive) is changing. But it is there even when I am calling for constant torque.

I am trying to understand both where this oscillation is originating from, and how I can reduce it. I see a lot of discussion about "torque ripple", as a consequence of DTC torque control. Is this what we are seeing? (I could not get any clear answer from drive tech support). Would this ripple show up as a constant frequency (independent of drive RPM?). Any ideas as to what we can do to reduce it - as it significantly affects our operational goals.

Thanks

 

[g.mccormick]

Are the torque values coming from an in-line torque flange?
What is the dyno? Is this a rigid mounted AC motor with a torque flange or is this a trunion mounted AC motor with a reaction arm load cell?
What is the speed doing?
What is attached to the dyno?
Is there a combustion engine?

 

[Total_newbie]

Yes - torque is coming from in-line torque sensor on shaft.
This is brake-testing dyno, with rigid A/C motor.
While drive torque is applied - dyno is decelerating. I cannot see whether there is any oscillation in speed - as we don't have same resolution to pick up any oscillation at 76 Hz. Drive torque may be adding to or reducing overall torque coming from brake. Torque oscillation though appears even when brake is not applied - and torque is just coming from drive.