Issue
Can I use a uninterruptible power supply for controllers? UPS
Environment
site requiring constant power supply or unstable power from grid and need to take into account power supplies
Cause
Can I use the CX99-PSU-BATT for an ACX5720 controller?
If not, then what can I use if the customer wants a UPS for each
controller. Should I use a big UPS for the system and supply the controllers from it?
Resolution
The CX99-PSU-BATT is 70VA 120/240VAC and really designed for the CX9900 PSU. It is suggested to get one of our APC UPS and run the proper voltage transformer to the ACX5720.
The ACX 57xx series is rated with a 24VAC 90VA or 12-28VDC 50W on the power supply which I recommend due to devices which connect to this controller and require power. i.e. door readers
UPS or uninterruptible power supplies are available from APC by Schneider Electric.
Background
The power drawn by computing equipment is expressed in Watts or Volt-Amps (VA). The power in Watts is the real power drawn by the equipment. Volt-Amps is called the "apparent power" and is the product of the voltage applied to the equipment times the current drawn by the equipment. Both Watt and VA ratings have a use and purpose. The Watt rating determines the actual power purchased from the utility company and the heat loading generated by the equipment. The VA rating is used for sizing wiring and circuit breakers.
The VA and Watt ratings for some types of electrical loads, like incandescent light bulbs, are identical. However, for computer equipment the Watt and VA ratings can differ significantly, with the VA rating always being equal to or larger than the Watt rating. The ratio of the Watt to VA rating is called the "Power Factor" and is expressed either as a number (i.e. 0.7) or a percentage (i.e. 70%).
The power rating of the UPS
UPS have both Watt ratings and VA ratings. Neither the Watt nor the VA rating of a UPS may be exceeded. In most cases, UPS manufacturers only publish the VA rating of the UPS. However, it is a standard in the industry that the Watt rating is approximately 60% of the VA rating, this being the typical power factor of common loads. Therefore, it is safe to assume that the Watt rating of the UPS is 60% of the published VA rating.
How to avoid sizing errors
Using APC sizing guidelines or and APC Configuration can help avoid these problems, as the load power values are verified. Equipment nameplate ratings are often in VA, which makes it difficult to know the Watt ratings. If using equipment nameplate ratings for sizing, a user might configure a system which appears to be correctly sized based on VA ratings but actually exceeds the UPS Watt rating.
By sizing the VA rating of a load to be no greater than 60% of the VA rating of the UPS, it is impossible to exceed the Watt rating of the UPS. Therefore, unless you have high certainty of the Watt ratings of the loads, the safest approach is to keep the sum of the load nameplate ratings below 60% of the UPS VA rating. Note that this conservative sizing approach will typically give rise to an oversized UPS and a larger run time than expected. If optimization of the system and an accurate run time are required, use APC sizing and Worldwide Web configuration tools.
