Why the buzzing with simulated sine wave UPS - damage to equipment? - Page 1
Forum section: electronics / general

author: Kuro
author_rank: Regular Contributor
timestamp: February 05, 2020, 01:34:43 pm
content: Please note: I'm still very much a beginner to electronics, but when it comes to 230V stuff, I consider myself on the bottom rung, I don't feel safe messing with it yet.Recently, we've been having a lot of power outages here. To safeguard my data I bought a cheapish UPS to make sure the home servers (a small HP one and a couple of Intel NUC with attached DAS cases) shut down properly. The UPS is an Eaton Ellipse Eco, offline model. It outputs a simulated sine wave in battery mode that looks like a mangled square wave - I found some screenshots here:https://www.hardwareinsights.com/database-of-ups-output-waveforms/5/#EatonWhenever the UPS now switches to battery mode, the PSUs of the NUCs (which are just laptop brick PSUs start to buzz loudly. The NUCs and HP server do all work though. I did some research (yeah, should have done that before buying the UPS) and it turns out everything in the EU over 75W now has active PFC power supplies. And these do not like simulated sine wave power. The common advice is to buy a pure sine wave UPS but these cost an arm and a leg (unless you buy one from the Chinese marketplace, not a reputable brand like APC or Eaton).Can someone explain why the buzzing, and would it hurt the attached devices to keep using this UPS? I mean, they are set up to shut down if the power goes down more than 10 seconds, so they're just running for about a minute on that power each time there's a power outage, which happens maybe once a week. How much damage if any does the simulated sine wave power do?

author: Pizzalover
author_rank: Newbie
timestamp: February 06, 2020, 11:51:00 am
content: Hi,The buzzing you hear is made by components that are working outside the design parameters.In the long run it might damage something.Try finding a used online-UPS that you can refurbish and put new batteries in.I have done exactly that a few times with good success rate.Good luck.Regards,Thor

author: Kuro
author_rank: Regular Contributor
timestamp: February 06, 2020, 03:06:51 pm
content: Does it have to be online (these use a lot of power themselves) or is sine wave output enough?Also, which components do you think are affected, caps, transformer...?

author: Pizzalover
author_rank: Newbie
timestamp: February 06, 2020, 04:24:11 pm
content: I'm a beginner with electronics, so I'm not qualified to answer that question, but perhaps some of the more experienced members of this forum can elaborate?Regarding off-line or on-line UPS, most of the reasonable priced UPS's are off-line models due to cost. Here locally an on-line UPS costs from €1200 and upwards, not including shipping (which is expensive due to the weight).One of my previous jobs was server-room design and building, including HVAC and power requirements. We used APC, Kipor and Piller units, ranging from 3000VA up to 300000VA. All were of course on-line types with line filtering, etc.In my experience, many SMB's get rid of a lot of equipment that still have useful life if the right hands pick it up. Look around, perhaps someone knows of a smaller company just gone bankrupt, or similar?I've tried a "true modified sine-wave" UPS, and it fried the psu for my wife's MacBook Pro. Didn't happen immediately, but after a few times it suddenly just died and let the magic smoke out. On the oscilloscope you can usually see the sawtooth shape of the "modified sine-wave" and sometimes the uneven frequency because of it on lesser capable units.

author: David Hess
author_rank: Super Contributor
timestamp: February 07, 2020, 04:49:07 am
content: Quote from: Kuro on February 05, 2020, 01:34:43 pmThe significance of the "modified sine" wave-shape is that it has the same RMS and peak-to-peak values as a sine wave. So incandescent lamps and motors see 120/240 volts RMS and capacitor input power supplies, including PFC ones, see 170 or 340 volts peak.QuotePFC input power supplies should not care if they are well designed.QuoteI buy mine used and refurbished.QuoteFor linear power supplies, the buzzing comes from the transformer because of the harmonic currents. The same applies to motors. I do not know what is going on with off-line switching converters to make them noisy and unreliable with modified sine power; it should not happen.QuoteIt should not do any damage to a well designed power supply but of course most power supplies are not well designed.Quote from: Kuro on February 06, 2020, 03:06:51 pmNo, standby UPSes with true sine wave outputs are available so it is not absolutely neccessary to get an online UPS. Online UPSes usually phase lock to the incoming sine wave power so that there is no discontinuity when bypassing the inverter if that becomes necessary like during an overload and this requires a sine wave output from the inverter.

author: Kuro
author_rank: Regular Contributor
timestamp: February 08, 2020, 06:01:48 pm
content: David, thanks for chiming in with such valuable input!Considering that a lot of power supplies are not well designed, can you say anything about how they can be damaged by simulated sine wave power? That is, do they blow capacitors, shortcircuit etc., and how long do you think it would take to make them fail? Also, when they fail, any chance they could damage the device electronics powered by the power supply?

author: David Hess
author_rank: Super Contributor
timestamp: February 09, 2020, 04:16:09 am
content: Quote from: Kuro on February 08, 2020, 06:01:48 pmI suspect the problem is with active PFC input stages which were only tested with sine wave sources and operate poorly with a square wave source.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 09, 2020, 05:15:57 am
content: Buzzing isn't by itself indicative of wear or damage. The harmonic content of the waveform is converted to mechanical motion through electro- and magneto-striction of capacitors and inductors. Primarily EMI filter capacitors, and PFC inductors where applicable (which as you note is probably most things). These components aren't particularly good loudspeakers, but the higher frequency components still emit alright, enough to be heard.PFC perhaps might not respond fast enough to quite deliver on its promise (i.e., the power factor for the square wave may be somewhat less than 1). Or it may outright confuse the controller. But it depends on the controller.So, unfortunately it's hard to tell. On the upside, most things either fail immediately due to excess voltage or current, or fail eventually due to overheating. That it works at all, likely puts it in the second category, and you can test (at least in principle, and with some care) the temperature of components to see if they're much different from the normal mains condition.I might even worry more about the inverter itself than the loads, as the capacitance of the filter, and the dynamic load of the SMPS, may not play as nicely with it. Hmm, I haven't actually looked in an inverter like that in a while, don't know if they have protection or compensation circuits to deal with that. SMPS are definitely a common load, and it would be a silly oversight not to provide for them in the design. (I can speak for myself, at least: the last time I put together an inverter, I used a current limited output stage, so that it simply cranks merrily away into a capacitor, or a heavy or shorted load, as if nothing is wrong at all. Don't know if they're doing that in commercial designs though.)Tim

author: Messtechniker
author_rank: Frequent Contributor
timestamp: February 09, 2020, 08:23:16 am
content: How I hate this "modified sine wave" advertising.Its never a sine wave, but always a sawtooth insteadand thus of no use in connection with equipmentspecified for true sine wave power input.