DC Drive with dynamic braking SCCR

[msshel0]

I'm trying to rate a control panel with a dc drive in it. The equipment uses a dynamic braking resistor to slow the load on the event of a power loss. On the load side of the DC drive, there is an armature contactor and 2 field contactors, all of which I'm assuming to have an SCCR of 5 kA. This immediately brings my panel rating down to 5kA. Can anyone point me in the direction of resource, or tell me how to handle SCCR for a DC load? Is it the same as an AC Load, you just have to find tested dc contactors?

 

[Ingenieur]

Is this a listed elevator driving machine control panel assy ?

 

[Jraef]

I'm trying to rate a control panel with a dc drive in it. The equipment uses a dynamic braking resistor to slow the load on the event of a power loss. On the load side of the DC drive, there is an armature contactor and 2 field contactors, all of which I'm assuming to have an SCCR of 5 kA. This immediately brings my panel rating down to 5kA. Can anyone point me in the direction of resource, or tell me how to handle SCCR for a DC load? Is it the same as an AC Load, you just have to find tested dc contactors?

Is it a DC source feeding the panel? If not, then the SCCR would be applicable to whatever is rectifying the incoming AC to provide the DC, i.e. your DC drive itself. After that, nothing else matters in the SCCR calculations, because you cannot create the DC energy without the AC. The braking energy, if that's what you are worried about, is only going into the resistor, not back into the line. The only time braking gets tricky with regard to SCCR is if the drive was line regenerative.

 

[Besoeker]

I'm trying to rate a control panel with a dc drive in it. The equipment uses a dynamic braking resistor to slow the load on the event of a power loss.

Something not right with that.
If you have a power loss won't all the contactors drop out including the field contactors? If so, dynamic braking would be ineffective.

 

[msshel0]

Is it a DC source feeding the panel? If not, then the SCCR would be applicable to whatever is rectifying the incoming AC to provide the DC, i.e. your DC drive itself. After that, nothing else matters in the SCCR calculations, because you cannot create the DC energy without the AC. The braking energy, if that's what you are worried about, is only going into the resistor, not back into the line. The only time braking gets tricky with regard to SCCR is if the drive was line regenerative.

No, the panel source is 480 Vac. I've attached a drawing of how we have designed our drive circuit. The drive is line regenerative. However, after looking at it more, we are trying to apply the following logic, can you tell me if this logic holds:

1) SCCR is by definition a function of symmetrical current.
2) Symmetrical Current is by definition an AC thing.
3) SCCR does not apply to a DC circuit.

 

[msshel0]

Is this a listed elevator driving machine control panel assy ?

We've dabble our toes in those waters before, so I can see why you'd think that, but no.

Something not right with that.
If you have a power loss won't all the contactors drop out including the field contactors? If so, dynamic braking would be ineffective.

One of the field contactors has normally closed power poles. When power is on, the contactor separates the field from the dynamic brake. On power loss, the contactor de-energizes and DB is allowed to affect the field. On power loss, the other field contactor is de-energized and isolates the field from the field exciter.

 

[Besoeker]

We've dabble our toes in those waters before, so I can see why you'd think that, but no.



One of the field contactors has normally closed power poles. When power is on, the contactor separates the field from the dynamic brake. On power loss, the contactor de-energizes and DB is allowed to affect the field. On power loss, the other field contactor is de-energized and isolates the field from the field exciter.

It's a differengement to what I've used in the past. We would normally leave the field excitation on at normal rated voltage. What you appear to have is the field energised from the armatute during the DB phase. This looks to me like a bit of a double whammy. Even with full field the braking effect drops off as the armature voltage reduces and doesn't pass as much current into the DBR. Here you have the field excitation fed from the decaying armature voltage further reducing the braking torque.

 

[Jraef]

No, the panel source is 480 Vac. I've attached a drawing of how we have designed our drive circuit. The drive is line regenerative. However, after looking at it more, we are trying to apply the following logic, can you tell me if this logic holds:

1) SCCR is by definition a function of symmetrical current.
2) Symmetrical Current is by definition an AC thing.
3) SCCR does not apply to a DC circuit.

No, not exactly.
1) Depends on how you define "a function of ...". The symmetrical current the unit is SUBJECTED to in an AC system is part of the oprocess of determining what SCCR you need, yes, but is not used in DETERMINING the SCCR. Think of SCCR as the "new and improved" version of what we used to call a "Withstand Rating". WHEN subjected to fault current inside of that unit, will the parts become shrapnel, and will that shrapnel cause collateral damage? That's all it means.
2) True, but irrelevant
3) Not true. SCCR is about SHORT CIRCUIT current, but the although the "symmetrical" does only apply to AC, the term "symmetrical" is not in the definition of SCCR, it's just interested in FAULT CURRENT, be it AC or DC. That's why I asked you if it was fed with AC or DC. If fed with DC, you will need to consider the SCCR of the device with regard to the available DC fault current. But as you are feeding it with AC, then only the AC fault current applies.

So bottom line, in your circuit, the SCCR of that system is the lowest of the fuses, the line reactor, the line contactor and the DC drive rating, unless they were specifically tested and listed in series already. The components on the DC side cannot be subjected to any amount of current in excess of what the rectifier can supply, so even if the components are 5kA, it's irrelevant and more importantly, irrelevant to the rating you must put on the label of the assembly.

It appears you are using an ABB DCS800 DC drive, those are UL listed with an SCCR of 65kA, most contactors are going to have been listed in series with fuses, so unless you picked a reactor that is not listed when protected by fuses, you should be good for 65kA overall.

 

[Ingenieur]

Fig 3 http://arlweb.msha.gov/S&HINFO/TECHRPT/HOIST/PAPER2.HTM

http://arlweb.msha.gov/S&HINFO/TECHRPT/HOIST/PAPER3.htm

 

[msshel0]

It appears you are using an ABB DCS800 DC drive, those are UL listed with an SCCR of 65kA, most contactors are going to have been listed in series with fuses, so unless you picked a reactor that is not listed when protected by fuses, you should be good for 65kA overall.

Thanks I really appreciate the help. For future reference it appears that line reactors are exempt from SCCR per UL508A SB4.2.1