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Variable frequency drive Constant Torque/Variable Torque

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 with a mechanical engineer, get them to show you curves and explain.

DBR stands for Dynamic braking resistor. Regeneration will happen when the motor rotates a speed higher than the speed which corresponds to the frequency setpoint ie.. the rotor speed is more than the speed of the rotating magnetic field.
Regeneration feeds back energy to the drive which results in DC bus overvoltage. To prevent the drive from tripping due to DC bus overvoltage the DBRs are used. The regenerative energy is discharged in the resistor as heat.

Regenerative Breaking - we used to have VFD on a vehicle rolling road. So when the car is travelling faster than 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 variable frequency 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.

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.
In active front end technology the rectifier is basically an inverter with IGBTs.
The main advantage are:
1) Low current THD <5 %
2) It is basically a four quadrant rectifier .Referring my last post please note that you will not require a DBR with AFE. The increase in voltage of DC Bus due to regeneration can be fed back to the input AC supply in the form of energy. So you don't require a DBR.
3) AFE drives have very good immunity to input voltage fluctuations.

Just an advice. Please go through variable frequency drive literatures (available in plenty) to have a good understanding of the different VFD technologies.
Selection of VFD requires proper understanding of the VFDs and the overall electrical system. There are lots of marketing gimmicks in the world of VFD. Always be careful before selecting a VFD specially higher KW drives.

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 ac drive. I think Siemens have bought the company since.

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