Can an AC motor plugged into same wall outlet damage sensitive electronics ?  - Page 1
Forum section: electronics / general

author: electronic_guy
author_rank: Regular Contributor
timestamp: December 05, 2021, 12:47:56 am
content: Hi,Is it possible to cause damages to measuring equipment (oscilloscope, spectrum analyzer etc) and computers, if I plug in a device with high powered motor such an air conditioner or home fan to the same wall socket where I have plugged in my instruments and computers.Here I have a surge protector at the main distribution box of my home but there is no surge protector at this particular wall socket. And I'm not using a UPS for any of my instruments and computer.Thank you.

author: Faringdon
author_rank: Super Contributor
timestamp: December 05, 2021, 12:52:29 am
content: I would have thought only if its an old or poorly maintained, or damaged fan or aircon unit. Decent ones should have built in circuitry to stop them putting out spikes back to the mains.If you are worried you can always plug in some transient protectors...eg, a board containg MOVs.......or say, a HV capacitor board at the output of a FWB....these steps will have clamp any transients.But you can always make a "transient catcher".....use eg a 450V LR8 linear reg at the output of a FWB....it will blow at the slightest high mains transient.Maybe just set it up powering a led at 1mA or so......then see , over time, if it blows. Obviously dont put a filter in front of it.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 05, 2021, 02:53:47 am
content: Not much difference between a motor and transformer as far as switching transients go, they're both inductive loads.

author: james_s
author_rank: Super Contributor
timestamp: December 05, 2021, 04:26:33 am
content: Unlikely. I'm not even really sure what "sensitive electronics" would be, properly designed equipment should be able to tolerate voltage sags and spikes that occur in a typical household or office environment without any issues.

author: David Hess
author_rank: Super Contributor
timestamp: December 06, 2021, 12:28:10 am
content: Quote from: electronic_guy on December 05, 2021, 12:47:56 amIt should not result in damage with good power supplies, but I have seen it happen. In an industrial setting the usual solution is to use a separate dedicated circuit for heavy motor loads, but where that was not possible, an online UPS solved the problem.

author: floobydust
author_rank: Super Contributor
timestamp: December 06, 2021, 01:46:16 am
content: I have seen equipment damaged from janitors coming in and cleaning at night time, they plug their vacuum cleaner into any outlet. The universal motor and brushes arcing seemed to cause it. Took me a while to figure out what was going on. Also, I think power bars and surge protectors have the MOV's on the input side, not the outlet-side where the offender is plugged in.

author: james_s
author_rank: Super Contributor
timestamp: December 06, 2021, 02:40:05 am
content: It shouldn't really matter where the MOVs are, within reason. As long as there is a relatively low impedance path between it and the load, any transient that is there will be clamped.

author: floobydust
author_rank: Super Contributor
timestamp: December 06, 2021, 05:00:11 am
content: Many powerbars have differential and common-mode chokes, and MOV's on one or both sides. It depends on the price and brand.pic taken fromIsobar teardown and reviewscope traces look useless though.

author: james_s
author_rank: Super Contributor
timestamp: December 06, 2021, 06:13:59 am
content: I'm somewhat skeptical of the value of MOVs anyway, the clamping voltage is fairly high, and they fail silently, or they can fail such that they catch on fire. I have relatively clean power here and usually look for non-surge protecting power strips and I've removed the MOVs from most of my UPS's after seeing one of them melt down at work several years ago.

author: electronic_guy
author_rank: Regular Contributor
timestamp: December 07, 2021, 02:32:14 pm
content: Considering the risk associated with AC motors I rewired the power cables connected to the wall plugs to the separate RCCB in my home lab setup. I have installed separate earthing rod also for the lab taking some precuation since I wear an ESD wrist wrap with the ESD mat. The RCCB is powered from main distribution box where I have installed type 2 SPD. The AC motor (Air conditioner) is also powered from the same distribution box, but since any spike that comes due to AC motor now has to come to the ditribution box first, hence I assumed that SPD will discharge it before it goes to my lab setup RCCB to damage the instruments. I hope my reasoning is correct. I considered installing type 3 SPD with the lab setup RCCB, but I cannot find a one in local shops. Any ideas from you guys? Am I making a faulty assumption here ?

author: floobydust
author_rank: Super Contributor
timestamp: December 07, 2021, 08:04:38 pm
content: If it's a raw A/C compressor motor and 2-pole contactor, that can make the nastiest local transients. I find there is generator action due to the motor's remanence and kick back due to the mechanical load when it switches off, the spike can be huge and even make it past the distribution panel. I had some sites that were problematic and used aDranetzpower quality analyzers to figure that out.I found best is RC snubbers at the transient source, across the A/C's contactor contacts. MOV's would also work. It also gives longer life to the contactor (less arcing) and less stress for the motor's insulation, not dealing with a many kV spike.

author: rpiloverbd
author_rank: Regular Contributor
timestamp: December 08, 2021, 10:00:05 am
content: Depends on how well those 'sensitive' electronic devices were made.