Is it safe to install a 24-pulse AFD next to a 1 MVAR capacitor for a 3.5 MW motor? 1

[Dvhez]

THD content in a 24-pulse rectifier is very low, however, those remaining high frequency harmonics (5th, 7th, 11th, etc) can be a problem at a resonance frequency. I saw this as design and distances are very short so I thought that it was a better idea to connect this capacitor at the primary side of the power transformer that will feed among other loads, this MCC with this motor, AFD and power bank.

 

[jraef]

What is the capacitor for anyway? It shouldn't be necessary for that motor on the VFD, because the VFD will correct the PF.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Dvhez]

It is used only to decrease the starting current. Then, the motor should be feed directly from the busbar. Maybe a soft starter would be a better approach.

 

[GroovyGuy]

hi DvHz
Do you have a simple one-line diagram of what you are trying to accomplish?
Is the vfd required only for starting the motor?
What is the nature of the load? (ie variable or constant torque?)
GG



"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[jraef]

The VFD will be able to provide maximum torque capability from the motor. The caps would contribute nothing but the possibility of resonance and possible damage to the BFD. If someone is recommending the caps and the VFD, they do not know what they are doing and should be banned from this project!!!


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Skogsgurra]

I would not be that harsh. The VFD is only for starting. The motor then runs DOL and does need the vars.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[edison123]

Start with a vfd and run DOL with the pfc cap. It's a good plan. Soft starters will not have same starting torque capability as a vfd.

Muthu

http://www.edison.co.in

 

[jraef]

But he said the cap was for starting current reduction, not PFC... that’s the part that makes no sense. The VFD will provide maximum Pu torque with minimum Pu current. The caps have no effect on the motor when the motor is connected to the VFD, whether it is used just for starting or continuously. The caps only add risk to causing damage to the VFD.

If the caps ARE only for PFC correction after the VFD is bypassed, then simply isolate the caps off line with a contactor until the VFD is bypassed and isolate the VFD off line while the caps are in the circuit. Most MV BFDs I know of will want a line isolation contactor anyway, so the only addition will be a contactor for the caps.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[FreddyNurk]

So, for a bystander to the discussion, what is the benefit of paying for a VFD and then switching over to DOL for most of the running requirement? Is there an efficiency saving to be made by running direct, or is there a different benefit that I'm not aware of?


EDMS Australia

 

[jraef]

FreddyNurk,
Yes, there is an efficiency penalty for running a VFD at full speed continuously, between 3 and 8% (depending on the technology) on most drives and on a 3.5MW motor, that's a lot of power loss. If you are varying the speed, this is the most efficient way to do it so the losses are irrelevant. But if NOT varying the speed, they loom large. The reason to use a VFD however is that it can (in theory) accelerate the load without ever exceeding the motor nameplate FLC, so long as the acceleration time is flexible. Not so with any other starting method so when you have a very limited source, a VFD is sometimes the only way you can start the motor.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden