UPS Inrush

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FaradayFF

Senior Member
Location
California
Hi,

Is there an inrush current on a standard UPS when it's first connected to power? Say we have a 5kVA, 3 PH., 208 V unit, will it draw 14 A per phase until the battery is charged, or is there an inrush and then the Amps settle to the steady state value until the internal battery is charged? I'm concerned that when the UPS is energized, it will draw too much energy in addition to supplying downstream panelboard causing the OCP device to trip.

Any thoughts would be appreciated.

Thanks,
FF
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Hi,

Is there an inrush current on a standard UPS when it's first connected to power? Say we have a 5kVA, 3 PH., 208 V unit, will it draw 14 A per phase until the battery is charged, or is there an inrush and then the Amps settle to the steady state value until the internal battery is charged? I'm concerned that when the UPS is energized, it will draw too much energy in addition to supplying downstream panelboard causing the OCP device to trip.

Any thoughts would be appreciated.

Thanks,
FF

There are two situations where a current could be drawn which is higher than the steady state load current.

The first is capacitor charging inrush if there is no current limiting circuit on the rectifier at the UPS input. This is not going to last more than a few cycles and as long as it is not high enough to hit the magnetic trip of the breaker there should be no problem.

The second is that the input power will have to both charge the UPS battery and deliver power to the load (via always on UPS or via bypass).
Most UPS designs charge the battery over and extended period (say 12-18 hours), which means that the battery charging current will be lower than the full load current by the ratio of the UPS hold time at full load to the recharge time (assuming constant current, which is not a great assumption.)
I would take a rule of thumb SWAG that the charging current for a "normal" UPS will be on the order of 1/6 of the full load current. Not likely to make the difference in thermal tripping of a conservatively sized input breaker.
 

ron

Senior Member
There are two situations where a current could be drawn which is higher than the steady state load current.

The first is capacitor charging inrush if there is no current limiting circuit on the rectifier at the UPS input. This is not going to last more than a few cycles and as long as it is not high enough to hit the magnetic trip of the breaker there should be no problem.

The second is that the input power will have to both charge the UPS battery and deliver power to the load (via always on UPS or via bypass).
Most UPS designs charge the battery over and extended period (say 12-18 hours), which means that the battery charging current will be lower than the full load current by the ratio of the UPS hold time at full load to the recharge time (assuming constant current, which is not a great assumption.)
I would take a rule of thumb SWAG that the charging current for a "normal" UPS will be on the order of 1/6 of the full load current. Not likely to make the difference in thermal tripping of a conservatively sized input breaker.

All good stuff. To add a little more color ....

The small UPS's like these are generally off-line type, meaning that they run in bypass with the utility power serving the load directly on a normal basis and charging the battery, sometimes with some power conditioning for the load depending on the model.
When power is bad or interrupted, the battery powers the load via an inverter.
Larger or better quality UPSs are double conversion, so during regular operation, the utility input has to charge the battery and overcome losses for the rectifier and inverter on its way to the load, and also power the load. There might also be an input filter with capacitors or transformer with inrush current to deal with.
 
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