A typical variable torque application, would be a centrifugal pump.
A typical constant torque application, would be a conveyor, and there are positive displacement pumps that are also constant torque. Have a talk wit a mechanical engineer, get them to show you curves and explain.
Regenerative Breaking- we used to have VFD
on a vehicle rolling road. So when the car is travelling faster then the VFD, the VFD generate back into the power supply - causing a break effect. If you had a large mass- large inertia that you want to stop quickly, you need to break the load- you can do that with regenerative breaking. Otherwise, disconnecting the drive, will mean your load just freely rotates, and that can mean it will take 30 minute to come to a stop for a large inertia.
I don't know what DBR stand for.
Active Front end- I first came across this term with ABB. It is all to do with how to mitigate harmonics from VFDs. You can use phase shift transformers, but with modern electronics, you can use a opposite phase current to counter act the harmonics generated from the VFD. So the overall impact on the network is small.
I like them personally, as they can often absorb some of the background harmonics already existing , which is a good thing.
For large drives, you need to speak with supplier to configure your machine correctly. There are many options., but yes active front ends are available. But there are other solutions; ASI Robicon use a current driven VFD , so harmonics are lessened in the first place, so an active front end is not the right terminology. It is a different solution. I used a 10MW version of that type of drive,. I think Siemens have bought the company since.