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Product : PM1000+Serial Number : 100008201004Accessories : No
Item Under Test : Switch mode power supplies, no PFC, just bridge rectifier into HT bulk capMeasured Values : measurement in question is PF (power factor)Question : Hello Supporting Engineer,
We are having a bit of a problem making sense of the PF measurement. Our setup is that the main's (240V) goes into a 1:1 isolation transformer, then a variac. Output of the variac serves as the input to the SMPS (230V ish). The SMPS is around 120W output. The PF we measured is always around 0.95 or higher, and the efficiency of the unit becomes very low..... like less than 50%. This is not a correct reading because with that low efficiency, the PSU will overheat and burnout (which it didn't). The SMPS of typical bridge-cap input stage would have PF of around 0.7ish.... so what have we done wrong to keep measuring 0.95 or higher?
Regards,
Ram
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Dear Ram,
The power factor on its own cannot burn out the power supply and does not effect the efficiency of the power supply. It just effects the proportion of the VA (Volts * Current) that is used as real power. How much power is going in and out from the supply? Have you measured the output at 120W during the test? The efficiency is then Output Power / Input Power.
Best regards,
Tim.
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Hi Tim,
Thanks for the reply. The concern here is NOT about efficiency of the PSU, but of the power factor measured and displayed. The apparent measured PF of 0.99+ to a SMPS with bridge+cap input is very unusual (no active or passive PFC). We believe the PF measurement from the PM1000+ (of 0.991 ish) is incorrect. To justify that, with the PF of 0.991, the input POWER is measured at over 200W, and output power is around 110W (E-load), this would give an efficiency of around 55%. And with that efficiency, the power losses (that could only be turned into heat dissipation) would overheat and burnt-out the PSU in question.
We want to find out WHY we are getting a PF of 0.991+ instead of a typical 0.7 ish in this kind of input stage (bridge+cap), is it the set up of our bench? Or is it some setting in the PM1000+ meter itself? And can you suggest some simple test for us to run?
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Dear Ram,
It is difficult for me to suggest anything that could be the cause without seeing the full setup details, but I can give you a few pointers. Firstly you can load the defaults on the PM1000+ to make sure an instrument setting is not causing the problem. You do this from the PM1000+ front panel by pressing the 'Menu' key, selecting 'User Configuration', and then 'Load Default'. Are you using an external shunt/module, or the internal shunt? There could be a phase issue with an external shunt system. Personally I would be more concerned with the efficiency power measurement. This seems to show a definite measurement error as the expected power factor can be effected by other circuit elements. For example, in theory at least, the isolation transformer could be adding a large leakage inductance to the power supply which has the effect of performing some power factor correction (by smoothing out the peaks into the rectifier).
Have you had the PM1000+ calibrated recently? It is possible that an error in the unit is causing the problem which calibration would show up. You could try measuring the power into a known load to check the PM1000+. Exact known loads are difficult to find, but a light bulb of known power would give a good indication. After reading this, if you are still unsure as to the cause, then let me know the answer to these questions and I will try to help further.
Best regards,
Tim.
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Hi Tim,
I have finally taken some photos of the display and measurement of the PM1000. How can I attach them over to you?
Do you know any company in Sydney Australia that can support this product..... calibration and repair?
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Hi Ram,
We only have service centres in the US and UK that can service this product at the moment. If it's calibration only you require, you are likely to find a good calibration laboratory in Australia, but they will not be able to adjust it. Normally adjustment is not required though. In terms of the photographs, please attach them to this thread by using the upload function visible when you click the reply button.
Best regards,
Tim.
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Hi Tim,
Sorry I didn't check my post for a while. Please find the photos attached.
In the photo of 'reading 2', the pf reading is 0.993. If check the waveform ''V&I' and 'harmonic content', I believe the pf should be a lot worst than that. Can you please advice?
Regards,
Ram
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Hi Tim,
Sorry I didn't check my post for a while. Please find the photos attached.
In the photo of 'reading 2', the pf reading is 0.993. If check the waveform ''V&I' and 'harmonic content', I believe the pf should be a lot worst than that. Can you please advice?
Regards,
Ram
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Hi Ram,
This does indeed look like the PF should be lower. The only thing I can think of is that the voltage input is distorted and effectively power factor correcting the system. Where are you getting your 230V supply from? What do the voltage harmonics look like? If the voltage harmonics also have high 3rd and 5th harmonic content then that could possibly explain it. If you would like some help with the calculations then please post the actual harmonic readings onto this thread (both voltage and current up to the 11th harmonic, magnitude and phase) and I will analyse it.
Best regards,
Tim.
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