SURT 6000Kva + Step Down transformer to Transfer Switch - Different voltages?

[BillP]

I have a pair of SURT 6Kva UPS units, each with the same model and type od APC step down transformer.
I use an APC ATS (Auto Transfer Switch) across each transformer to provide redundant power to my 110v devices with a single power cord.

In the measurement section of the ATS, I see that the input voltages on each side wander around slightly, but always are about 5 volts off from each other.
Here is what I see:

Source Measurements
	Source A		Source B
Input Frequency:	60	Hz	60	Hz
Input Voltage:	124	VAC	121	VAC

So, I looked in the UPS  status pages on each UPS and I see less than 1v deviation between the output voltages of each.
Like so...

UPS-A

Power
Input Voltage:	243.3 VAC
Output Voltage:	209.4 VAC
Input Frequency:	60.02 Hz

UPS-B

Power
Input Voltage:	244.8 VAC
Output Voltage:	208.2 VAC
Input Frequency:	60.02 Hz

Why would the voltage be so different at the ATS measurements page?
Can I adjust this or "tune" it via some interface or by making some internal adjustment?

[BillP · Accepted solution]

Hi Lenny - 

I see that your ATS is 120v and your UPS is inputting nominal 240v and outputting nominal 208v. Ideally in this situation you would be outputting 240v and use a 240v to 120v stepdown (I didnt see the model of your transformer listed so I'm not sure if you have a model that could do this or not.)

A 240v to 120v stepdown outputs 50% of the input. So if you feed it 240v, it outputs 120v. If you fed it 246v it would output 123v.

A 208v to 120v stepdown outputs roughly 0.577 of the input. If you feed it 208v, it outputs 120v. If you feed it 212v, it outputs 122v

The output of your SURT is typically within 3% of what it is set for, which in your case is 208v. This is going to fluctuate some, and the output of your transformer will always be a set % of the UPS output. The ATS uses a different chipset to measure voltage than the UPS does, and there is always a slight lag between measurement and reporting. You may find that they are not always 100% in agreement, but they will be close.the output is always going to be well within CBEMA standards for voltage regulation in IT environments. From your numbers it appears that the calibration on the ATS reporting may be off slightly. You could double check it with a quality voltmeter if it is critical to your application. 

Hope this helps,

Liam