6 replies to this topic

[AB2005]

Dear Friends;

I am working on checking of the quality of output wave form of different kind of available UPSs in local market. APC is a well known company which makes UPS. We also have there UPSs to protect m/c PCs. When i tested the wave form of there two UPS i astonished as i was asuming a "near sine wave" out put. I also tested other one which pictures of output wave form are as following. Please guide is this normal?

[AB2005]

Dear Marke,

I installed two energy meters one analoge and other digital at output of UPSs where a fan was connected to measure the output power. I found difference in the reading of both energy meters. For example, digital EM showed 155W and analoge showed 135W. Which one we should follow? If we connect the load with utility power, both EMs shows correct reading almost 123W.

[marke]

Hello AB2005

That does not surprise me, there is often errors with measuring equipment when the waveforms are non sinusoidal.
In terms of which one is correct, that is hard to say from here. You need to find out which is correct for a non sinusoidal waveform, or possibly both are incorrect.
A good digital will read correctly, and so will a good analogue, but if they are lower cost, they are probably compromised.
A good digital will have a very high sample rate and execute a larg number of samples and instantaneous volts times amps calculations which are then summed.
A lower cost unit will measure the current, and the voltage and the angle between them and multiply volts times amps time power factor, but this is only true for a sinusoidal voltage and current waveform.

One way to get an idea is to use a rotating disk power utility type meter and count the disk revolutions. This should be pretty accurate.

The waveforms abovew are much as I would expect.

Best regards,
Mark

[AB2005]

Thank you Marke,
I got the point and now working for arrangement of a Disc Type Energy Meter.

What do you say about measurment DC current of input side of UPS (battery? I also installed two meters, digital clamp on meter (very good quality HIOKI 3285) and analog and found a little bit diiference in their reading i.e. digital was measuring 10.5A and analog 9.1A at 24VDC. So which one should we follow?

[AB2005]

What do you say about measurment DC current of input side of UPS (battery)? I also installed two meters, digital clamp on meter (very good quality HIOKI 3285) and analog and found a little bit difference in their reading i.e. digital was measuring 10.5A and analog 9.1A at 24VDC. So which one should we follow?

Hello Marke,
Waiting for your comments on this point.

[AB2005]

Dear Mark,

After arrangement of a Disc type energy meter, I did again tests of 7 different brands UPSs and still faced problem in measurement of non-sinusoidal output energy of UPS. Here are results of test of two UPS.

Pedestal fan 150W checked on utility power. Voltage and current measured by a very good quality true RMS digital meter.
V = 218V
I = 0.685A
P1 = 131W (measured by a digital energy meter)
P2 = 140W (measured by a needle type analog energy meter)
P3 = 130W (measured by a disc type energy meter)

UPS 1 (square wave type) with pedestal fan 150W
Input power to UPS
V = 23.05A, I = 10A, P = 231W (V x I)
Output power of UPS to fan
V = 222V, I = 0.89A,
P1 = 162W (measured by a digital energy meter)
P2 = 160W (measured by a needle type analog energy meter)
P3 = 148W (measured by a disc type energy meter)


UPS 2 (near to sign wave type) with pedestal fan 150W
Input power to UPS
V = 11.91V, I = 16A, P = 191W (V x I)
Output power of UPS to fan
V = 201V, I = 0.735A,
P1 = 172W (measured by a digital energy meter)
P2 = 142W (measured by a needle type analog energy meter)
P3 = 133W (measured by a disc type energy meter)

Now I am still confused which output power should I use to calculate the efficiency.

[marke]

Hello AB2005

The problems that you are seeing are due to differences in metering.
I would rely on the rotating disk in the absense of something better.
Just shows how easy it is to get fooled by poor instrumentation!!

Best regards,
Mark.