Generator sizing for elevator inrush current

[Tainted]

I have an elevator fed by 3-phase 480V transformer secondary. Transformer is fed by 208V source

Generators need to be sized to handle inrush current.

I have an existing elevator motor with these specs:



How would I find the elevator inrush current to size the generator properly? Do I use NEC 430.7(B) or 430.251(B)?

 

[infinity]

Does this elevator have a VFD? What type of elevator? I've always assumed that since some motors ramp up that the inrush is much lower than across the line starting.

 

[Tainted]

Does this elevator have a VFD? What type of elevator? I've always assumed that since some motors ramp up that the inrush is much lower than across the line starting.

The elevator is a pully type elevator (not hydraulic) I saw a giant elevator controller enclosure so I would assume there was some kind of VFD in there I wish I took pictures.

I think maybe what I should do is monitor the load using a meter for a month to see the maximum spike.

Is it ok to assume 6x the current as worst case scenario or is that overkill and I should do 3x

 

[zbang]

Elevators tend to ramp-up the load, it's not across-the-line starting a loaded 20HP motor (although it probably is ATL starting the MG set, but that's with minimal load).

(I've seen at least a few hydro elevators which star-delta start the pump, probably pretty common.)

 

[JoeStillman]

The motor nameplate says it is Code Letter H. NEC table 430.7(B) tells us that the locked-rotor current kVA/HP will be between 6.3-7.09. So your starting kVA will be between 126 kVA and 142 kVA. At 460 V, that would be 158A-178A. Your generator will have a motor-starting kVA rating, typically somewhat higher than the nameplate kW.

 

[Tainted]

The motor nameplate says it is Code Letter H. NEC table 430.7(B) tells us that the locked-rotor current kVA/HP will be between 6.3-7.09. So your starting kVA will be between 126 kVA and 142 kVA. At 460 V, that would be 158A-178A. Your generator will have a motor-starting kVA rating, typically somewhat higher than the nameplate kW.

Is the generator nameplate usually rated at continuous load or starting load?

 

[JoeStillman]

Is the generator nameplate usually rated at continuous load or starting load?

The generator nameplate won't tell you the motor-starting kVA, but you can get it from the manufacturer. Most reps will run a sizing program for you, but they're not always motivated to minimize the engine. I like to run my own with Cummins Power Suite (https://powersuite.cummins.com/). Their results are pretty typical vs. Cat/Kohler/MTU.

 

[topgone]

Any generator will exhibit voltage dips depending on the available rotating inertia the system has prior to switching on a motor load and the load applied to the system. The typical voltage dip in the industrial sector is around 30 to 35% for about 1 to 4 seconds. Prolonged drop in the voltage may cause the magnetic starters to drop, hence a failed start. Caterpillar lists the percent voltage dip of its generator voltage as a function of the load applied on it.

 

[zbang]

Caterpillar lists the percent voltage dip of its generator voltage as a function of the load applied on it.

Well yes. the dip will be a function of the load, the prime mover, and the governor (some are faster to react than others). I'd consider a 30% dip as way out of line unless that motor is the only load on the generator.

(retracting part of my previous answer) Is that plate on an MG set? If so, it'll be starting unloaded but still have the locked-rotor current; have you asked TK?

(Also, if the machine is rated for 230/460, why is being fed through a transformer at 480? Could be that was the transformer available.)