AC motor spark energy approximation - Page 1
Forum section: electronics / general

author: just4user
author_rank: Contributor
timestamp: January 26, 2025, 09:27:06 am
content: Hey fellas,I'm interested in suppressing the spark when turning off a single phase 220v AC motor. On its body, it mentions a current draw of 2.6A and a 0.25HP power output. (Very inefficient, I know...)I've put two unidirectional TVS diodes back to back in parallel with the switch.They're the 1.5KE400A (https://www.mouser.com/datasheet/2/240/Littelfuse_TVS-Diode_1.5KE-41903.pdf) diodes.My rationale was that 1.5KW is quite more than 220v x 2.6A so it should be good.However, on further thought, I remembered how two coils of different inductance can pass through the same amount of current, yet retain different amount of energies, and the TVS having a pulse-duration parameter.Now, I don't really know how I can calculate the spark's energy to know the right size for the TVS.Any ideas?!

author: amyk
author_rank: Super Contributor
timestamp: January 28, 2025, 03:30:10 am
content: You can measure the inductance of the windings to get a closer approximation.

author: just4user
author_rank: Contributor
timestamp: January 28, 2025, 06:20:46 am
content: Could it be approximated using the given parameters however? I don't have an inductance meter and setting up a circuit for that ain't easy.

author: Benta
author_rank: Super Contributor
timestamp: January 28, 2025, 10:01:47 pm
content: "Single phase" is ambiguous. Do you mean universal (brushed) or induction motor?

author: just4user
author_rank: Contributor
timestamp: June 12, 2025, 01:13:57 pm
content: Yayy... Finally figured it out!I noticed that there is also a pf (power-factor) mentioned on the motor. In my case it's ~0.7.Put my calculative hat on and after chaining a few formulae, got to this:E_max = P / (πf sin(2φ)) where φ = acos(pf).Putting in all the values, got 3.5 joules as the answer.The TVS can dissipate 1.5KW in the specified bursts mentioned in its data-sheet --> ~3 joules in ~3ms.Exceeding that slightly will probably derate it faster than expected, so I added another back-to-back pair to be safe.

author: Analog Kid
author_rank: Super Contributor
timestamp: June 12, 2025, 10:38:05 pm
content: Quote from: Benta on January 28, 2025, 10:01:47 pmI seriously doubt a quarter-horse motor will be universal; my assumption would be induction.

author: Gyro
author_rank: Super Contributor
timestamp: June 13, 2025, 09:29:32 am
content: Quote from: Analog Kid on June 12, 2025, 10:38:05 pmReally? 1/4HP is only 180W. I would say that most motors in that range are brushed universal. The Universal motors in hobby bench saws go into the multi kW range.

author: just4user
author_rank: Contributor
timestamp: June 13, 2025, 10:08:04 am
content: It's a two speed induction one & 2.6 (on slow) x 220 -> 570W

author: tom66
author_rank: Super Contributor
timestamp: June 13, 2025, 12:54:13 pm
content: Quote from: just4user on June 12, 2025, 01:13:57 pmI would not advise putting TVS devices in parallel. One will break down well before the other, and the other will dissipate almost no power during the event. The only benefit that gives you is that if one fails open, the other will take over, but the common failure mode for TVS devices is going short, not open.Placing lower voltage rated ones in series is one option (so two in series in one direction, two in the other), or better yet, a single larger TVS device that is bidirectional.I am not sure that a TVS device is the best device for this application in general, it is more typical to use an RCD or RC network to snub an arc.

author: mikerj
author_rank: Super Contributor
timestamp: June 13, 2025, 12:54:59 pm
content: Why not use a simple RC snubber? You can get them packaged as a two lead component, typically a 0.1uF capacitor in series with a 100 ohm resistor.e.g.https://cpc.farnell.com/ampohm-wound-products/fe-sp-hdr23-100-100/contact-suppressor-01uf-100r/dp/FT00715

author: just4user
author_rank: Contributor
timestamp: June 14, 2025, 10:44:16 am
content: On a side note, what you guys think would happen if the coil, switch, and everything else was in a vacuum, allowing no arc to form;Would the impedance of vacuum (~370ohm) come to play when electrons start to jump as the switch starts to open?

author: tom66
author_rank: Super Contributor
timestamp: June 14, 2025, 09:53:14 pm
content: Arcs can and do form in vacuums if the electric potential is high enough. Either the arc would occur there or the motor windings would themselves break down. You could model the motor as if it is a load of inductors in series with a load of capacitors parallel to the chassis of the motor (most likely earthed). If dI/dt is high, an arc will form -somewhere- because the voltage will rise high enough until something breaks down.

author: just4user
author_rank: Contributor
timestamp: June 15, 2025, 08:44:32 am
content: Arc is a plasma. Needs both positive and negative ionic clumps in its cloud. If those exist already, then you don't have a vacuum. Electron jumping is not plasma/arc.

author: mikerj
author_rank: Super Contributor
timestamp: June 15, 2025, 10:00:41 am
content: Quote from: just4user on June 15, 2025, 08:44:32 amhttps://en.wikipedia.org/wiki/Vacuum_arc

author: SteveThackery
author_rank: Super Contributor
timestamp: June 15, 2025, 11:31:00 am
content: Quote from: mikerj on June 15, 2025, 10:00:41 amThat's really interesting. So would it be true to say that a vacuum arc would be invisible to the eye, but one would probably see hotspots on the two electrodes?

author: just4user
author_rank: Contributor
timestamp: June 15, 2025, 08:16:53 pm
content: QuoteIf you check the link, you'll see that it has two main types, one is thermionic which needs high temperatures (not the case of our discussion), and the other one is cold emission, a quantum effect, that I simply referred to as 'jumping'. As far as I know, it's not self-reinforcing like the arcs or thermionic emission (requiring less voltage to maintain once established).But as Tom said, probably dielectric breakdown of windings will happen before it gets too sci-fi.

author: just4user
author_rank: Contributor
timestamp: July 28, 2025, 11:50:59 am
content: Hey guys,My fascinations on this matter never seem to endToday I encountered a bizarre phenomenon.This motor is connected to mains via a switch (rated for 10A & 220v) that has been working fine and is only 1 year old, indoors.There was a power outage today and after some time when it came back, I happened to be facing that switch and saw a big flash from the switch, like 10-30 times bigger than the usual flashes when I switch it on/off normally on slow (2.6A).Then I inspected things closer and noticed that the switch is melted shut and I can't turn it off anymore.I believe the motor has a start capacitor which may have caused an inrush current, but what I don't understand is1- the intensity of the inrush this time.2- why flash at all if the switch was already closed.Any ideas?!No other device was damaged so not sure if the mains voltage came back on so so high, be it brief, that caused this or some other weird thing was responsible.