Electric Automation Forum
Forum » Variable frequency drive » VFD DBR and Active front end
Topics: VFD DBR and Active front end on Variable frequency drive
Start by
Ajith Nair
09-18-2013 08:10 AM

VFD DBR and Active front end

In a VFD...
1. When is DBR used ? and in which all application ?(Constant Torque/Variable Torque/..) and why ?

2.I am little confused about use of use of DBR & Regenerative concept ..so want to clarify that ?

3.When is Active front end suggested ? and is Active Front end available for Large drives like for 1800kw Motor ?
09-18-2013 10:18 AM
Top #2
Tony Cutner BSc CEng MIET
09-18-2013 10:18 AM
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.
09-18-2013 12:27 PM
Top #3
Sudipto Ghosh
09-18-2013 12:27 PM
DBR stans for Dynamic braking resistor.Regeneration will happen when the motor rotates a 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.
09-18-2013 02:41 PM
Top #4
Ajith Nair
09-18-2013 02:41 PM
Dear Sudipto ,

1.When is IGBT used in rectifier used ?

2.What is the advantage of that arrangement ?

3.Is Active frond end arrangement suitable for larger drives like above 1800kW ..... and above ?
09-18-2013 04:45 PM
Top #5
Sudipto Ghosh
09-18-2013 04:45 PM
IGBTs are used in the rectifier section of the drives?.Yes this is active front end technology.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 has very good immunity to input voltage fluctuations.

Ajith Just an advice.Please go through VFD 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 lot of marketing gimmicks in the world of VFD.Always be careful before selecting a VFD specially higher KW drives.
09-18-2013 07:27 PM
Top #6
Tony Cutner BSc CEng MIET
09-18-2013 07:27 PM
I'd just like add a bit of clarity- in the good old days thyristors were the "chopping " device for power circuit rectification. Modern design will use an IGBT. Unless you are designing the variable frequency drive at semiconductor level, this is a detail that does not cause you concern. An active front end will also use IGBT devices.
The harmonics, are generated as a result of chopping the sinusoidal wave, not as a result of using IGBTs or thyristor.
One disadvantaged of using a resistor for dynamic breaking - is how to get rid of the heat.
What are you doing using a 1800kW motor, and dynamic breaking ?
Reply to Thread