protection from Overvoltages - Page 1
Forum section: electronics / projects

author: alex2701801
author_rank: Newbie
timestamp: May 04, 2017, 05:00:48 am
content: To the experts,How to protect the water level controller circuit from over voltage(line surges due to switching of large induction motors)? please suggest any circuit/components for the purpose. Can MOVs serve the purpose,if then how to connect them?when i installed one in a factory, the step down transformer(230/(9-0-9v)) got burned after few days but secondary was safe. The same circuit is working fine in all other places(domestic homes and few other factories).I think it is due to overvoltages during on/off of large Induction motors.Thanks in advance

author: Jeroen3
author_rank: Super Contributor
timestamp: May 04, 2017, 06:38:50 am
content: Did you measure these overvoltages? You'll need long and high overvoltage conditions to fry a transformer.

author: Raj
author_rank: Frequent Contributor
timestamp: May 04, 2017, 10:24:38 am
content: Isolates the motors from controller using triac/scr and opto couplers

author: Ian.M
author_rank: Super Contributor
timestamp: May 04, 2017, 12:06:45 pm
content: As Jeroen points out, you need to take some measurements. To do it right you need to log the power quality (waveform, transients and DC offset) at the location in question for a week or so to determine a cost effective solution. It could be something as simple as a defective feed to your device with an intermittent connection that frequently arcs, that could be cured in 5 minutes by tightening a terminal screw.If the induction motor control system ever causes a DC imbalance on the mains supply, it wouldn't take very many cycles to flux-walk the 9-0-9V transformer into saturation causing its fine wire primary to fuse. If its power consumption is low enough, the addition of a 100uF 400/450V rated motor run capacitor in series with the feed to your transformer could fix it by blocking the DC. However I would expect problems with other transformers including the supply ones for the induction motors if that was the case, so DC imbalance is fairly unlikely.MOVs cannot help as they degrade with repetitive over-voltage transients, eventually failing low resistance and self-destructing. Transorbs and other TVS clamping diodes are better behaved and do not short if used within their repetitive transient energy limits.An appropriately rated ferroresonant transformer could be used to reduce harmonics and provide line regulation of your 220V AC supply. They are notoriously inefficient and impractical for high power levels but that shouldn't be a problem when all you need is power for a few sensors.

author: Jeroen3
author_rank: Super Contributor
timestamp: May 04, 2017, 01:04:30 pm
content: Quote from: Ian.M on May 04, 2017, 12:06:45 pmI've seen the occasional failure where a faston plug went loosy and caused pcb transformers to fail.Rent a Fluke Power Analyzer and put it there for a week.

author: alex2701801
author_rank: Newbie
timestamp: May 04, 2017, 02:02:49 pm
content: Getting a Fluke power analyser for rent in India is burdensome.

author: alex2701801
author_rank: Newbie
timestamp: May 04, 2017, 02:10:49 pm
content: no I havent.but i dont see any other reason by which the primary gets burned.also i have tried twice and same thing happened.

author: Ian.M
author_rank: Super Contributor
timestamp: May 04, 2017, 02:21:26 pm
content: Quote from: alex2701801 on May 04, 2017, 02:02:49 pmDoesn't have to be Fluke - any brand of power quality analyser with similar features to the Fluke ones would be suitable. You may have to contract the service from a specialist electrician, or try finding manufacturers or distributors of that sort of instrument and ask them if they know of any of their customers or anyone the can suggest in the rental business.

author: alex2701801
author_rank: Newbie
timestamp: May 04, 2017, 02:21:46 pm
content: Thank you for the valuable information.

author: alex2701801
author_rank: Newbie
timestamp: May 04, 2017, 02:39:40 pm
content: okay..

author: Seekonk
author_rank: Super Contributor
timestamp: May 04, 2017, 02:53:48 pm
content: The power system in some locations can be very suspect. Have you measured any voltages? Where did this transformer come from? It may not be rated well. I had a job in Japan and control modules were overheating and failing. They said they were rated for 50Hz, but the transformers were going into saturation and overheating. Had to use a buck trnsformer to lower line voltage a little. The actual upper voltage limit may be less than standard. Once a transformer saturates, it is just a resistor.

author: Zero999
author_rank: Super Contributor
timestamp: May 04, 2017, 06:26:46 pm
content: How about replacing the transformer with a switched mode power supply? Preferably one rated for 200V to 450V AC on the input. If you can't get one, then even a decent quality universal SMPS will be able to work from up to 280VAC and DC content on the mains won't be a problem. The output will need to be around 24V to 30V. This is of course, the circuit just rectifies the output and doesn't do anything like use the mains frequency as a clock signal.Failing that, what about using two transformers, with the primaries and secondary windings in series? It will be able to withstand twice the voltage (assuming the circuit on the secondary can) and double the DC content.

author: David Hess
author_rank: Super Contributor
timestamp: May 04, 2017, 06:45:07 pm
content: Quote from: alex2701801 on May 04, 2017, 05:00:48 amThe *transformer* failed? Wow.I like Ian.M's idea about using a capacitor in series on the line side to protect against DC imbalance.Active protection on the line side will be difficult and expensive because any active devices will have to withstand typical line conditions. Instead what I would try first is using two identical 230VAC input line transformers with the inputs in series to make a 460VAC transformer which when is derated by operating at 230VAC. The secondaries could be placed in parallel or series depending on the requirements.

author: Zero999
author_rank: Super Contributor
timestamp: May 04, 2017, 07:28:19 pm
content: Quote from: David Hess on May 04, 2017, 06:45:07 pmThe transformer failing doesn't surprise me much. Often small transformers are built down to a price, operating it even slightly above its rating for an extended period of time is likely to cause failure.QuoteI suggested that, but you've put it much better than I did.Quote from: Ian.M on May 04, 2017, 12:06:45 pmI wouldn't have thought an induction motor would result in DC on the mains, unless it was driven off an inverter and something went wrong with the rectifier.Although, connecting a large value capacitor in series with the transformer will block the DC, you need to be careful it doesn't cancel out the magnetising inductance, causing resonance, which would result in a high voltage appearing across both the transformer and capacitor, high current flowing and hopefully the thermal fuse blowing.

author: Ian.M
author_rank: Super Contributor
timestamp: May 04, 2017, 07:48:17 pm
content: Yes. Its vitally important to avoid being anywhere near resonance. The critical inductance for a 50Hz supply and a 100uF series capacitance is about 100mH. However even a small power transformer should have an order of magnitude higher primary inductance, (measure it - don't guess) and with any load on the secondary, will be fairly well damped.However as we both pointed out DC imbalance is rather unlikely unless there is a fault on the VFD - either one diode open in the bridge, or possibly a software fault if its capable of energy recovery and is used in an application that requires frequent motor braking, and if there was DC imbalance, I would expect other devices to be suffering as well.

author: Seekonk
author_rank: Super Contributor
timestamp: May 05, 2017, 02:57:45 pm
content: I've actually used that capacitor trick to RAISE the voltage. I can run 220V fans on 120V by putting a capacitor in series and operating near resonance. One time I did it put a capacitor in series with a transformer to limit load current. My estimate must have been off, transformer went up in smoke.