modified sinewave: when can it be a problem? - Page 1
Forum section: electronics / projects

author: mcinque
author_rank: Unknown
timestamp: July 28, 2022, 08:20:59 pm
content: I have a doubt about uninterruptible power supplies and modified sinewave.When this kind of waveform can be a problem?I mean, I'm not talking aboutdisturbancesgiven by harmonics on maybe old CRTs or audio devices.I'm wondering when this waveform could get to the point of damaging the devices that use it, because of many harmonics or unpure sinewave.In some devices such as sleep apnea devices, it is reported that powering them with modified sinewave could damage them, so I wonder why. Perhaps it is that the motors or inductive loads are adversely affected by modified sinewave because of the heat that builds up, so the sleep apnea treatment devices being equipped with a motor to pump air, could be affected.Almost all devices with switching power supplies should have no problem. But are there other devices that can be adversely affected by this waveform? Maybe linear power supplies? Or lab instrumentation like SA, PSUs, bench DMMs?if so, how can the damage occur?Attached myuglymodified sinewave.

author: moffy
author_rank: Super Contributor
timestamp: July 29, 2022, 12:07:23 am
content: You seem to have a pretty good understanding already of what the problems are with the modified sine wave. I think you would have to take it on a case by case basis and I wouldn't rule out modern PSUs either because those fast incoming edges could look like noise or produce glitches. They really are nasty.

author: Circlotron
author_rank: Super Contributor
timestamp: July 29, 2022, 12:22:20 am
content: The fast rising edges make the X caps on an SMPS work harder than necessary and increase inverter losses too. I put together a special power cord with a bridge rectifier halfway along in a box for whenever I need to run my scope off a battery and inverter e.g. probing car electronics (while someone else is driving!) works well, and lowers the power drain from the 7Ah battery duct taped to the side of the inverter.

author: Terry Bites
author_rank: Super Contributor
timestamp: July 29, 2022, 12:34:35 am
content: The main issue outsdie of being trash generator, is that the RMS power is lower than hoped for- area under the curve stuff.Higher harmonics make your transformer or pumps/ fans run hot compared to a multlevel. The modified sine is just a special case of can't be assed to do things rightBecause there is less stress at each level, the muiltilevel dolution will be more reliable.All that noise is lost effciency so you have to go multi level strategiesThat said, a typical smps dosent care unless it has active pf correction. Even then I'm not sure it matters much.

author: TheMG
author_rank: Frequent Contributor
timestamp: July 29, 2022, 01:19:12 am
content: Also, it's problematic for anything that uses a capacitive dropper power supply. The capacitor will pass more current than normal, which can cause things to overheat and/or fail.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: July 29, 2022, 01:25:41 am
content: Quote from: moffy on July 29, 2022, 12:07:23 amFriends don't let friends use modified sine wave inverters.

author: moffy
author_rank: Super Contributor
timestamp: July 29, 2022, 04:44:39 am
content: Quote from: TheMG on July 29, 2022, 01:19:12 amThat is a good point as quite a few devices use the capacitive dropper for a standby supply.Quote from: Ed.Kloonk on July 29, 2022, 01:25:41 amWasn't endorsing just answering the question, but I agree you make a good point.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 29, 2022, 01:24:20 pm
content: How well a motor would run on modified sine depends on its inductive time constant. Prius motors are driven with sine waves at low speed and square waves at high speed since at a certain point, the inductance is enough to smooth out the current to close to a sine wave.

author: coromonadalix
author_rank: Super Contributor
timestamp: July 29, 2022, 03:03:41 pm
content: I have some Lenovo laptop adapters, they simply refuses to start with modified sine and square waves inverters , tossed the inverters for true sinewave, no problems since ...I know of some problems with home appliances (tv, fridge .. tools) with some inverters ...

author: james_s
author_rank: Super Contributor
timestamp: July 29, 2022, 04:08:59 pm
content: Back in the 90s my dad plugged a charger for his cordless drill into a modified sine wave inverter and it buzzed for a few seconds then smoked and failed. Lamps with dimmer switches don't work properly on them, iron power transformers and motors buzz.

author: langwadt
author_rank: Super Contributor
timestamp: July 29, 2022, 06:01:38 pm
content: Quote from: NiHaoMike on July 29, 2022, 01:24:20 pmmore likely a square wave is just the best it can do since the current slewrate is limited by voltage/inductance

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 30, 2022, 01:06:19 pm
content: Quote from: langwadt on July 29, 2022, 06:01:38 pmIt goes to a square wave well before the inverter reaches its maximum voltage.http://techno-fandom.org/~hobbit/cars/ginv/VH.htmlQuote

author: CaptDon
author_rank: Super Contributor
timestamp: July 30, 2022, 08:06:53 pm
content: Magnetic fluorescent lamp ballasts work very poorly on modified sinewave inverters. I tried a 4 stage L/C lowpass filter scheme to try to recover a bit of a sinewave shape. It was not very successful. On the inverter side of things the spikes got huge and the output of the filter really didn't look much better than just running the inverter direct. I used 130L10 MOV's at each stage, L1-N, L2-N and L1-L2. What I really ended up with as a nice transient suppressor!!

author: Seekonk
author_rank: Super Contributor
timestamp: July 31, 2022, 07:28:20 pm
content: I run my refrigerator off MSW and that is the only thing which is powered. The inverter is old based on a TL494 and I have often thought about making a soft start for it by modulating the dead time pin. All motors would probably run better with a longer dead time. One issue is that many MSW inverters modify the dead time instead of the high voltage to get average power the same. I'm sure motors don't like having a longer constant voltage on time at startup. I've collected broken MSW inverters and some of them have had the H bridge removed. I run my scope and wall warts off 140V DC. Have one MSW inverter which I run solar panels directly into the H bridge to power my washer with no battery. I have a stack of pure sine wave inverters and haven't used one in years. The rise times of the H bridge are generally too slow to make real nasty spikes.

author: Circlotron
author_rank: Super Contributor
timestamp: July 31, 2022, 10:29:04 pm
content: From what I understand, most refrigerators have a two phase induction motor, the second phase is shifted 90 deg by an external capacitor. At full load and when fed with a sine wave this works very well, but when fed with a MSW the second winding would be powered with some awful kind of wave. I expect the power consumption of the motor would go way up and maybe not very powerful either. Maybe some could give real life experience with one of these “capacitor run” motors?

author: Seekonk
author_rank: Super Contributor
timestamp: August 01, 2022, 06:01:13 pm
content: No, many refrigerators do not have run cap or even a start capacitor. The second winding is just connected to the first thru a thermistor and that goes to high resistance in about 5 seconds. After 2 minutes the refrigerator settles down to about 80W. I could make something very elaborate, but why at that power. Fractional single phase HP AC motors have poor efficiency and many fridges are going to inverter drives. They wouldn't care about MSW. There has been interest elsewhere on soft starters and I think I might do a microprocessor version to get the 120A inverter draw down to about 50A at 12V.

author: TheMG
author_rank: Frequent Contributor
timestamp: August 01, 2022, 09:02:43 pm
content: Quote from: coromonadalix on July 29, 2022, 03:03:41 pmThe power supplies probably have active PFC. Some active PFC chips/designs don't handle the "modified sine wave" well or at all.

author: james_s
author_rank: Super Contributor
timestamp: August 02, 2022, 02:51:47 am
content: Quote from: Seekonk on August 01, 2022, 06:01:13 pmThat's true here in North America, but may or may not be true where he is in Australia. Every air conditioner I've seen except for the modern inverter drive systems uses a PSC motor which works as he describes, with a capacitor to create the phase shift.

author: bdunham7
author_rank: Super Contributor
timestamp: August 02, 2022, 03:05:54 am
content: I have a rather ordinary refrigerator and a chest freezer in my garage and I can run them together on a cheapo Harbor Freight 750W MSW inverter without any trouble, even on startup or extended periods. The same inverter can't even run a shop vac because of the starting current. So the small ~100-200W sealed refrigeration compressors are a pretty benign and tolerant sort of load.