Flux vector drive dynamic braking resistor

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W@ttson

W@ttson

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The manufacturer has a limitation on the length of conductors the the dynamic braking resistors use to connect to the drives D.C. Bus. The smaller the drive the smaller the length can be. Why is there a limitation on the dynamic braking resistor conductors? I understand for the conductors on the output to the motor due to the reflective wave phenomenon. But why for the D.C. bus/dynamic brake?

Second question is, let's say you have two flux vector drives load sharing. You mount the DBR for drive 1 roughly 2 times as far as drive two. Does the change in electrical characteristics of the dynamic brake ckt change the performance/ load sharing of the drives?
 
W@ttson

W@ttson

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Yeah that's the question. I can do the Calc and upsize the conductor as much as I want. Why is the manual not letting me upsize as required? I feel like there has to be some sort of high frequency noise effect from the chopper circuit.

Ingenieur said:
probably more to do with the Z and conductor heating
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I

Ingenieur

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must have to do with L and the time constant

i = V(1-e^-(t / tc) where tc = L/R and R = braking resistor, L = cond L
the larger the tc the slower the rise time
the smaller the drive the smaller the conductor the higher the L
the smaller the drive the larger the R
on the smaller drives they note to twist the cond to minimize L

my guess they want rise time to ~100% to be < chopper freq to maximize power transfer?
and dissapate any stored energy? probably very little though

I'm sure one of the drive experts will chime in
 
Jraef

Jraef

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Yes, it’s an inductance issue as it relates to the rise time and how that factors into the fact that a DC braking circuit is different from the pseudo AC output to a motor. It’s not about the effects ON the conductors, it’s about the effects OF the conductors wrt the rise time, and how that affects the capabilities of the transistors.

Smaller drives are all now made with IPMs, Intelligent Power Modules, where the transistor mfrs have bundled all 7 transistors + their firing circuits into one potted package. That keeps the size and cost down, which is why Modern VFDs are so much smaller than 20 years ago, but drive mfrs have little choice in component selection in that the transistor you get for the braking chopper is the same as you get for the PWM AC outputs. In larger drives, we go back to having discrete power devices, so the brake chopper transistor can be selected separately from the main power transistors. What that means then is that they can be selected to be capable of dealing with the different complex aspects of being used as a chopper into a purely DC resistive circuit as opposed to being used in the AC output into an inductive load, where there is this big inductive filter called a motor.
 
W@ttson

W@ttson

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Jraef said:
Yes, it’s an inductance issue as it relates to the rise time and how that factors into the fact that a DC braking circuit is different from the pseudo AC output to a motor. It’s not about the effects ON the conductors, it’s about the effects OF the conductors wrt the rise time, and how that affects the capabilities of the transistors.

Smaller drives are all now made with IPMs, Intelligent Power Modules, where the transistor mfrs have bundled all 7 transistors + their firing circuits into one potted package. That keeps the size and cost down, which is why Modern VFDs are so much smaller than 20 years ago, but drive mfrs have little choice in component selection in that the transistor you get for the braking chopper is the same as you get for the PWM AC outputs. In larger drives, we go back to having discrete power devices, so the brake chopper transistor can be selected separately from the main power transistors. What that means then is that they can be selected to be capable of dealing with the different complex aspects of being used as a chopper into a purely DC resistive circuit as opposed to being used in the AC output into an inductive load, where there is this big inductive filter called a motor.
Click to expand...

very interesting. That even answers a question I had about the size! What about the effects on load sharing? If two drives with different length conductors to the DBR, will the difference in inductance have an effect on the load sharing?
 
Jraef

Jraef

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spraymax6 said:
very interesting. That even answers a question I had about the size! What about the effects on load sharing? If two drives with different length conductors to the DBR, will the difference in inductance have an effect on the load sharing?
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Yes. You need to also use diodes to make them share evenly if you are connecting the DC busses together. I take it you are using A-B drives? If so, there’s another really good document they provide that explains common bus applications. I’ll find it and post a link later today.
 
Jraef

Jraef

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Besoeker

Besoeker

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Jraef said:
Link to document on sharing the DC buses on drives and implementing dynamic braking with that.
http://literature.rockwellautomation.com/idc/groups/literature/documents/at/drives-at002_-en-p.pdf

Also, some of this in duplicated in documents from Bonitron, who sell stand-alone braking systems that can be used to tie drives together, and the diode sharing module mentioned in the Rockwell document is this one from Bonitron.
http://www.bonitron.com/PDFs/Brochures/CB_Diodes_111007_20141106.pdf.
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Yes, I can see how diodes would prevent other drives would be prevented from dumping current into the IGBT chopper of another drives. Prevention. I just don't see any sharing mechanism.
 
W@ttson

W@ttson

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Jraef said:
Yes. You need to also use diodes to make them share evenly if you are connecting the DC busses together. I take it you are using A-B drives? If so, there’s another really good document they provide that explains common bus applications. I’ll find it and post a link later today.
Click to expand...

Yes, using AB drives but not coupled together to the same DC bus. Will have the drives talk to each other through their communication protocol to make sure they are doing the same amount of work. In this case would it still be an issue? And I am talking 50' difference in length or so.
 
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