UPS sizing for compressors 1

[Power0020]

A 0.8 kW compressor is to be supplied by an online UPS, the LRC is about 6 x In, i.e. 27.3 A @ 220 V.

Strictly speaking is to use the LRC to size the UPS, will be a 6 kVA unit to ensure that no overload/excessive VD occurs during starting.

Does the inverter allow any overload for a short time?

if so, less than 6 kVA UPS may be sufficient in this case, not sure about how to calculate the VD in this case as the internal impedance of the UPS isn't known.

Any ideas?

 

[MikeHalloran]

Put a big air receiver on the compressor discharge?


Mike Halloran
Pembroke Pines, FL, USA

 

[Power0020]

the compressor supplies a dentist set,

 

[MikeHalloran]

Why would a dentist keep drilling when the lights go out?


Mike Halloran
Pembroke Pines, FL, USA

 

[ScottyUK]

Air receiver, transfer switch, generator. Keeps air and lights on.

If this is a 1-phase compressor it will almost certainly be a cap start, cap run type to meet the starting torque requirements of the load. Capacitors on a UPS output are not a smart idea. Find another way.

 

[Power0020]

I agree ScottyUK, how to get rid of capacitor problem on a UPS? I know switching isn't favorable for caps, if the UPS has a very smooth output, is it still an issue?

 

[ScottyUK]

3-phase UPS and a replacement motor for the compressor? Even then the UPS is going to have a tough time with a heavy inductive load being switched in and out, particularly given the cyclic operation of most compressors.

 

[sibeen]

I'm not so sure about small UPS systems, but the larger beasts could handle short term overloads out to about 150% before they began to current limit.

 

[MikeHalloran]

The sort of motor found on small compressors, given current limiting, will fail to actually start rotating. Instead it will bounce against the air that it can't quite compress enough to get past TDC, and bounce back toward BDC, and repeat. Its cooling fan, oscillating, will be ineffective. The motor will overheat, and smoke. ... for a whole weekend. Remediating the smell of the smoke will require specialists. The dentist will have to clean _everything_, and will be unable to accept patients for several days. The dentist will sue your ass off.


Mike Halloran
Pembroke Pines, FL, USA

 

[JayMaechtlen]

how about a battery powered DC compressor to run when AC compressor can't?
Maybe with a suitable compressed air reservoir to carry the load during switch-over.


Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[davidbeach]

Motors and UPS don't go together well, at all. That's why critical functions in power plants have AC motors on normal station service and DC motors on the station batteries.

Scotty is absolutely correct, large enough receiver tank, generator and transfer switch is the only practical way to go.

 

[Parchie]

The sort of motor found on small compressors, given current limiting, will fail to actually start rotating.

AFAIK, compressors that I have dealt with in the past don't start loaded up. They're equipped with loader solenoids that are timed to come live when the motor shall have reached top speed. Or am I missing a lot?

 

[MikeHalloran]

Unloader valves are not commonly fitted to small compressors until there is an incident that demonstrates their value to an MBA.



Mike Halloran
Pembroke Pines, FL, USA