Is it bad if a transformer makes sudden buzz sound at statup ?  - Page 1
Forum section: electronics / general

author: electronic_guy
author_rank: Regular Contributor
timestamp: February 01, 2022, 02:56:35 pm
content: Hi,I have bought a transformer to reduce 230v to 110v. It makes a sudden buzz sound every time when I power up the transformer and then the buzz sound quickly fades into small hum sound. Is there anything bad about this transformer ? Should I be concerned about the safety of myself and equipment connected to it when using this ?Thank you.

author: bdunham7
author_rank: Super Contributor
timestamp: February 01, 2022, 03:30:50 pm
content: With or without a load on it?It probably is not an issue, but if if does it without any load, that's weird.

author: electronic_guy
author_rank: Regular Contributor
timestamp: February 01, 2022, 03:39:54 pm
content: It's actually without a load. Just turning ON, nothing connected to the plugs in front.

author: Gyro
author_rank: Super Contributor
timestamp: February 01, 2022, 05:15:32 pm
content: That's the sound of the transformer saturating and pulling a high current for the first few cycles. It normally happens when the switch closes near a voltage zero-crossing and there is no reverse magnetization from the previous cycle to prevent the magnetic flux from reaching saturation level.It is not normally harmful if it just happens randomly with mechanical switching, but is the reason why a transformer should never be switched by a zero cross switching solid state relay, where it would happen on every occasion.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 01, 2022, 06:17:57 pm
content: Quote from: bdunham7 on February 01, 2022, 03:30:50 pmI think weirdest is you saying it's weird, because it's completely normal. It's almost surprising if you haven't heard it.Maybe you have only dealt with transformers with generous iron dimensioning (a lot of margin for saturation), or small enough transformers you just don't hear it.But I assure you, it's completely normal. This is because the transformer "remembers" the state of magnetization at the last power-down, so during the next power-up, chances are high that the magnetic flux starts to build up on top of that already high flux, resulting in saturation. This causes increased inrush current, but it only lasts for a short while, as the magnetic flux returns into the middle (around zero, varying in both directions).Now of course, the design could still be marginal, but you can't guess that from the initial sound. The best indicator of marginal design is excess temperature or smell. With no load, it shouldn't heat up to actually hot temperatures. Some heating is inevitable, optimization of price vs. no-load power dissipation is a manufacturing decision.

author: BrokenYugo
author_rank: Super Contributor
timestamp: February 01, 2022, 08:30:08 pm
content: The bigger issue is these converter boxes, like most bottom dollar power electronics from China, are generally rated and fused about double what they should be and constructed twice as sloppily as any safe mains handling device should be built.I wouldn't plug one of these in without a tear down and inspection, and then don't load it anywhere near the nameplate rating.

author: bdunham7
author_rank: Super Contributor
timestamp: February 01, 2022, 10:23:47 pm
content: Quote from: Siwastaja on February 01, 2022, 06:17:57 pmI would expect that effect to have dissipated before the end of the third half-cycle, and indeed the only turn-on noise I've heard from an unladen transformer is just a 'thump' sound. Are you saying the imbalance can persist longer than that?

author: Siwastaja
author_rank: Super Contributor
timestamp: February 02, 2022, 09:28:28 am
content: The OP just said "quickly", nothing about the number of cycles. I don't know how many cycles it takes (typically, or in worst case) to restore the balance around zero Tesla. My hunch is, significantly more than 1 half-cycle, based on my own auditory observations. Note that acoustical resonances might make the sound longer (the transformer and casing acting like a gong).The important takeaway is that the magnetic remanence is a real thing, which causes saturation, and the sound. This alone isn't worrying. Of course, the transformer may or may not be underdimensioned. Excessive no-load heating is a telltale sign of undersized iron and significant saturation even after the initial thump. That should not happen.

author: jonpaul
author_rank: Super Contributor
timestamp: February 02, 2022, 11:22:13 am
content: Well designed and fab transfomers do not do that.Indication of poor design, flux density too high, or sloppy assembly, loose laminations, poor or no varnish. etc.The Chinese transformers are poorly made and you can expect such problems.Use a high quality unit and it will be solved.We use old General Radio, Stacor, Variac and Superior Electric for lab variable transformers.We use Signal Transformer for isolation, etc.Jon

author: Gyro
author_rank: Super Contributor
timestamp: February 02, 2022, 12:24:40 pm
content: The problem is also exacerbated by mechanically resonant steel chassis (not a magnetic effect), particularly with single bolt mounting toroidal transformers. These also tend to have a higher inrush current too.

author: Faringdon
author_rank: Super Contributor
timestamp: February 05, 2022, 09:20:23 pm
content: I worked in one place where switching in the transformer woudl often trip the breaker to the factory....it just depended on thr flux level that was left in the core when the contactor was switched.

author: AaronD
author_rank: Frequent Contributor
timestamp: February 05, 2022, 11:38:42 pm
content: Would it help to have a "startup load", like a power resistor, incandescent light bulb, or something else that can handle a weird waveform for the first few cycles, after which you unplug it and carry on like normal? The idea is to provide some load current to counteract the line current, and keep it away from saturation.Transformers are unintuitive in that the worst case for saturation is completely UN-loaded. It's common on sound installation and similar forums to see a cheapie distributed audio installation that blew the dumpster-special amp when enough speakers were DIS-connected. Likewise for tube/valve guitar amps that have a transformer output: no speaker or dummy load = dead power tube. In both cases, the transformer saturation became an intermittent short circuit at high voltage. Transistor amps that are *designed* to drive a transformer, have special protection for that; but anything else, and certainly a tube amp with a strong pressure to use as few of those expensive parts as possible, doesn't.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 06, 2022, 03:27:12 pm
content: Adding load won't help. Well it does very slightly, because now there is a bit of resistive voltage drop on the primary, reducing the flux respectively. But this effect is very small. Thus, magnetization current is almost constant regardless of load.What you can do is to add resistance in series with the primary, called inrush limiting. Classic solutions like an NTC, or a power resistor/light bulb bypassed with a switch or relay, do work.Light bulb works the "wrong way", increasing in resistance when it heats up, though, so it needs bypassing by a relay.Large transformers requiring slow and large fuse sizes is a very usual problem and not indicative of poor quality transformers. The remanence effect can be very large, it makes little sense to limit the available flux density to half just to keep inrush current down, because this means the transformer will be much larger and more expensive. Some amount of saturation, and some inrush current, is noticeable in nearly every transformer.

author: radiolistener
author_rank: Super Contributor
timestamp: February 07, 2022, 04:38:07 pm
content: This is a usual behavior for a powerful transformer startup. If that boom noise bothers you, you can turn on slowly with help of autotransformer, just slowly add voltage to the normal to avoid high current pulse.

author: IanB
author_rank: Super Contributor
timestamp: February 07, 2022, 04:49:37 pm
content: See the video below. All of it is interesting, but skip forward to 3:40 if you want to hear the fun part. Notice how the camera shakes.

author: electronic_guy
author_rank: Regular Contributor
timestamp: February 26, 2022, 06:00:26 am
content: Thank you all for the replies. It really helps when things are explained in detail this way.That video reminds me of terminator sounds in the movie. I'm glad to have my transformer now.