Inrush Current Calc

[Seven-Delta-FortyOne]

3 motors, single phase, 1 HP.

Im sizing a battery backup for them.

Unlikely they will all start at the exact moment, but, if I size the system for that, I have LRA of roughly 48A per motor, 144A total, at 240V we have nearly 35K VA.

That seems excessively high for a relatively simple setup. I would never spec a 38kW genset for this installation, but the UPS manufacturers recommend 200% of inrush for all motors.

I haven’t done a ton of these, but I’m getting calls for them, so I guess I should learn.

 

[David Castor]

The issue is the ability to start ac motors via the inverter. The batteries probably won't be the limiting factor. Batteries can produce high short-term current, but not inverters. The main issue for sizing the batteries will be how long the loads are expected to run.

 

[Seven-Delta-FortyOne]

Correct. Both the inverter and the battery system need to be able to handle the peak demand.

Pretty easy to size batteries based on expected load. Sizing based on motor start loads gets a bit more complicated.

 

[MyCleveland]

Stagger your starting and you only need to account for the largest motor start + operating amps of the others.
Your UPS salesman should have advised you on this...find another UPS guy.

 

[Seven-Delta-FortyOne]

That was the idea I was leaning towards.

I could wire up relays and contractors with timers that would shut off power to the other motor loads for say two seconds when one starts.

Id have to put my thinking cap on to design that system.

Kind of hoping there is a UPS with priority load switching built in.

 

[winnie]

Or look at replacing the across the line motors with VFD fed motors that can ramp up without inrush.

-Jon

 

[Seven-Delta-FortyOne]

Talked to Generac tech support today.

Evidently, the PwrManager device that is an add on for the PwrCell system has relays, that you connect motor loads to.

It will keep all the motors from starting at once.

And, I got specs on the install, and it’s 3, 1/2HP motors, not 1HP.

So with a 50A breaker on the inverter, it looks like it will work.

 

[Dennis Alwon]

Is the actual power 240v or 208? Would you not multiply the voltage x 3 sq.rt for 3 phase

 

[Seven-Delta-FortyOne]

Single phase 240v. It’s residential.

 

[Dennis Alwon]

Single phase 240v. It’s residential.

I read the post 3 times and I still had 3 phase in my head. duh---sorry