Is this APC Smart-UPS any good?

[Mat1926]

On 3/1/2018 at 3:54 PM, unevent said:

You can assume a 90% efficiency or it can be 80%.

 

So it should be 1000/0.8, i.e. 1250 W minimum? I'm not sure where can I find such UPS...

[Frank1940]

Please guys.  The rating of the PSU is not the determining factor in picking the size of the UPS,  it is the power draw of the system.  The fact that the PSU could deliver 1000W  should not really be a consideration is picking the size of the UPS!   

[pwm]

It's not the rating of the PSU, but the inner design of the PSU, that will affect how much inrush current the PSU might give when powering on. And really big PSU (if good quality) normally have better slow-start functionality than small PSU. A high inrush current is bad for the components in the PSU and bad for the components in the disks and motherboard.

 

So a 1000 W PSU doesn't mean the UPS needs to be able to supply 1200-1500 W. What matters way more in selecting the UPS is the startup power needed by the equipment connected to the PSU.

 

Next thing is that low-end UPS often directly cuts off the output on overload, while high-end UPS will continue to feed power but will current-limit the output to help with slow-starting the connected equipment.

[unevent]

On 3/3/2018 at 5:56 AM, pwm said:

It's not the rating of the PSU, but the inner design of the PSU, that will affect how much inrush current the PSU might give when powering on. And really big PSU (if good quality) normally have better slow-start functionality than small PSU. A high inrush current is bad for the components in the PSU and bad for the components in the disks and motherboard.

 

So a 1000 W PSU doesn't mean the UPS needs to be able to supply 1200-1500 W. What matters way more in selecting the UPS is the startup power needed by the equipment connected to the PSU.

 

Next thing is that low-end UPS often directly cuts off the output on overload, while high-end UPS will continue to feed power but will current-limit the output to help with slow-starting the connected equipment.

 

Late getting back to this as I was traveling.  To both Frank and pwm, the load on the supply is not the only determining factor and the power supply can draw significantly more current than just the load connected to it at times which is what causes the majority of the dropouts when switching to battery for certain active power factor correction power supplies and older UPS designs.  Also, a larger capacity supply will have a much larger initial cold current draw than a smaller supply independent of load due to the increase capacitance on the output stages for the various rails which is part of power supply design to maintain output ripple within design standards.  Higher quality designs may include inrush limiters, but most do not because of the cost.  A large capacity supply with a very small load is very inefficient and will have current draws that do not follow the load and will be greater with a UPS that does not produce a clean sine wave with good crest factor capability.  The APC paper is a guide, not Gospel, and is where I think things took a turn. The APC guide was simply meant as a means to steer the conversation and advice away from only factoring in the load on the supply and more toward the load on the supply and the supply itself.  No supply is 100% efficient and all have power factor correction which corrects for current and voltage phase (lag) errors.

addition...that said it comes down to what is the measured load and what is the PSU.  One of the early posts was to measure watts which is always where you want to start.  From that you factor in the PSU (efficiency, PFC type and any Google hits on UPS issues with it), how does the capacity compare to the measured load, how long you want the backup to last and how many and what type of other loads you want to place on it.  Models from Cyperpower and Tripp Lite are also available that work with modern PFC supplies and offer true or 'pure' sine wave output.  Avoid those that say 'PWM Sine Wave' or 'Stepped Approximation', etc.

Edited March 7, 2018 by unevent

[pwm]

14 hours ago, unevent said:

Also, a larger capacity supply will have a much larger initial cold current draw than a smaller supply independent of load due to the increase capacitance on the output stages for the various rails which is part of power supply design to maintain output ripple within design standards. 

A larger capacity supply may have a much larger initial cold current draw - the DC/DC converters normally have special logic to reduce the inrush current by implementing a slow-start functionality. And it really isn't much of an additional cost, because this logic is part of the DC/DC controller chip. It just doesn't start with full PWM modulation from cold start but instead ramps up the output voltage.

 

Another thing here is that the PFC isn't just about compensating phase. The main thing with PFC on a computer PSU is to make the consumed current sinus-shaped instead of spike-shaped. With an old PSU without PFC, the power supply will only draw current when the voltage reaches the maxima - it's only then that the rectified input voltage will be above the voltages of the input capacitors and be able to top up the capacitors with more energy. I measured an older Chieftec PSU and it had a crest factor of 10 compared to 1.41 for a pure sine wave. And this is why the EU has laws that only puny power consumers may be without PFC - the current spikes produces a huge amount of noise in the local cables but also flattens the top of the voltage curve introducing large amounts of overtones on the whole power grid.

 

Look at the first image on this PDF for an example what an older PSU without PFC looks like.

http://www.programmablepower.com/support/FAQs/DF_Crest_Factor.pdf

[unevent]

5 hours ago, pwm said:

Another thing here is that the PFC isn't just about compensating phase. The main thing with PFC on a computer PSU is to make the consumed current sinus-shaped instead of spike-shaped. With an old PSU without PFC, the power supply will only draw current when the voltage reaches the maxima - it's only then that the rectified input voltage will be above the voltages of the input capacitors and be able to top up the capacitors with more energy. I measured an older Chieftec PSU and it had a crest factor of 10 compared to 1.41 for a pure sine wave. And this is why the EU has laws that only puny power consumers may be without PFC - the current spikes produces a huge amount of noise in the local cables but also flattens the top of the voltage curve introducing large amounts of overtones on the whole power grid.

 

Look at the first image on this PDF for an example what an older PSU without PFC looks like.

http://www.programmablepower.com/support/FAQs/DF_Crest_Factor.pdf

 

EN/IEC61000-3-2 limits the amount of harmonic current allowed (reflected) back onto the mains (supplies greater than 75W).  It has slowly been adopted worldwide and is good reason why active PFC has become more prevalent and more efficient and also why it has become cheaper because most do not use lossy large inductors and expensive high voltage capacitors to perform the task rather performed with specialized silicon controller ICs.  PFC corrects the phase error caused by inductive loads where the current and voltage waveforms do not follow each other.  The more phase error the lower the power factor and the more harmonics result on the neutral that are reflected back into the mains supply which can affect other equipment.  Closer the PF is to 1 the less harmonics on the mains and the more sinusoidal the input remains.