3-Pole NC Contactor to comply with 620.91(A) (Elevator with regen drive)

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derekk

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Hello.

I am trying to find a product and I'm hoping it exists somewhere, although I've not had luck finding anything yet. We have a 50KW UPS which is an EM power source for an elevator. The only loads on the UPS are elevator related. The motor is regenerative with no way to disconnect the regenerative function while on emergency (some elevators do have this ability). So, to comply with 620.91 we need to add a load to absorb the regenerative power. In addition, we need to protect the UPS as it's internal circuitry is not designed to handle regenerative power on the output. We are installing an ATS so that in normal mode, the UPS output is isolated from the elevator. There is sufficient load in the building to always absorb the energy from the motor. When on EM, the ATS transfers and we have a N.O. Definite Purpose Contactor which then closes and brings a bank of resistors into the circuit (to absorb the regen power). We're using the EM transfer contact on the ATS to control this relay. It seemed like a good design to me but then I wondered about one possible situation. There is no signal from the UPS to the elevator to let it know that the EM source is running low. It's very unlikely but still possible that the elevator could be moving at the exact moment that the UPS runs out of power. If this happens, then the contactor will open and the resistors will no longer be in the circuit and there will be nothing to protect the UPS or act as breaking resistors for the motor. I can easily solve this by changing the contactor to Normally Closed and connecting it to the normal power relay on the ATS. Then the resistors would be in the circuit under EM power or if neither EM nor utility power were present. I thought this would be an easy thing to do, but I just cannot find a contactor with greater than 30A contacts. We need a 3-pole, NC, with 50A (or larger) contacts. I've checked several mfg's and we can add (2) aux poles at NC but no more. Lighting contactors have interchangeable poles (NO or NC), but the ones I can find only allow the contacts to be NO/NC up to 30A (above this they are fixed at NO). Theoretically we could use 2 contactors with auxiliary poles, but this seems like a product that could, and should exist. I don't think there is any technical reason why an NC contactor could not be built at higher ampacities.

Does anyone know of a product or think of anything I'm missing? Thanks.
 

Jraef

Moderator, OTD
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Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
The motor is regenerative with no way to disconnect the regenerative function while on emergency (some elevators do have this ability).
The only way I know of that this could be true is if the elevator has a VFD, or the motor is DC. If it's DC, you only need 2 poles, that's why lots of suppliers make DC rated contactors with 2NO 2NC contacts. If it has a VFD, you can get a Line Regen unit designed (and I think certified for) elevator regen.

Seems to me you need a little more clarity on exactly what you have.
 

derekk

Member
Hello Jraef, seems we're both in SF. Nice day we're having here.

If this was anything other than an elevator, we'd have a lot more input and ability to do something with the motor or drive itself. However, if you know the elevator companies well, you know that anything beyond the AC disconnect is entirely their domain. The MFG of this particular unit is Otis and we have fully researched this with them (there is no way to stop the regen function). Schindler, for example, has a specific input that allows their controller to disconnect the regenerative "feature" of their drive when on EM power. Otis units do not have this functionality. NEC 620.91(A) is pretty specific in that means have to be provided to absorb the regen power and we've accomplished that. I'm just worried about a 1 in 1000 chance happening and I could eliminate any possibility of damage to the UPS if I could only get a 3-pole N.C. contactor with 50A or greater poles. It very well may not exist but I just thought that someone may know of another application where a large load needs to be directly controlled in this manner and could suggest something. If not, we can install two contactors with 2-pole NC Auxiliary poles to achieve this result, but it's really an inelegant solution.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Lemme see, Nice day in SF.

That would be 64 degrees, overcast, slight drizzle and 15-20 MPH winds.

Am I close?:D
No. It's 68 right now, skies are almost clear, 5MPH breeze, supposed to be 81 in a few hours.

People here call that "hot" though... nobody has AC in their houses in SF (proper).

I actually live east of SF where it's warmer though, I just worked in SF when I originally set up my profile. I happen to be in SF working right now however, I only get here about once per month, today's the day and I picked a good one it seems.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Hello Jraef, seems we're both in SF. Nice day we're having here.

If this was anything other than an elevator, we'd have a lot more input and ability to do something with the motor or drive itself. However, if you know the elevator companies well, you know that anything beyond the AC disconnect is entirely their domain. The MFG of this particular unit is Otis and we have fully researched this with them (there is no way to stop the regen function). Schindler, for example, has a specific input that allows their controller to disconnect the regenerative "feature" of their drive when on EM power. Otis units do not have this functionality. NEC 620.91(A) is pretty specific in that means have to be provided to absorb the regen power and we've accomplished that. I'm just worried about a 1 in 1000 chance happening and I could eliminate any possibility of damage to the UPS if I could only get a 3-pole N.C. contactor with 50A or greater poles. It very well may not exist but I just thought that someone may know of another application where a large load needs to be directly controlled in this manner and could suggest something. If not, we can install two contactors with 2-pole NC Auxiliary poles to achieve this result, but it's really an inelegant solution.
So what is it you are planning to use as your load to absorb this regen energy? A Load Bank? If so you could do that with 2 NC poles anyway. Leave one pole connected permanently, hold the 2P NC contactor open whenever you have power. When power fails, AND the UPS dies, the contactor closes, putting the other 2 poles of the load bank on line. The NEC does not say you have to control all 3 phases.
 

jumper

Senior Member
No. It's 68 right now, skies are almost clear, 5MPH breeze, supposed to be 81 in a few hours.

People here call that "hot" though... nobody has AC in their houses in SF (proper).

I actually live east of SF where it's warmer though, I just worked in SF when I originally set up my profile. I happen to be in SF working right now however, I only get here about once per month, today's the day and I picked a good one it seems.

I grew up in the Sunset district, Ocean Beach. 81 would have been a bloody heat wave.
 

derekk

Member
I suppose we could rearrange the load bank to run off two poles. No reason is needs to be balanced. Thank you for the suggestion. It was 77 today in my part of the City.
 
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