Over Voltage protection of open circuit auto transformer - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: January 07, 2024, 09:43:06 pm
content: This is a follow up question to this post about an open circuit auto-transformer. I want to add some over voltage protection in the unlikely event that an open circuit occurs as shown below.Quote from: Andy Chee on December 30, 2023, 03:59:08 pmIn this particular application, the nominal voltage in the upper winding (above tap for load is 20V), and the lower winding is 220V (+/- say 5%). The transformer source is fused with a 15A medium speed fuse. If an open circuit occurs as shown with the red 'X' the load will see the full 240V. The load is rated 220V nominal with absolute upper limit of 240V. I would like to implement some sort of over-voltage protection, because if an open circuit occurs (as above), it will not be obvious (load will likely continue to function as normal until it fails from being run too close or over its limit.MOV or TVS does not appear to be very helpful as the 20V increase in voltage is not enough to blow the input fuse. Any ideas?A MOV or TVS will not be very effrIf the lower winding opens, a say 30V MOV/TVS will effectively become a short circuit (exponential portion of the curve approaching 1000A) or whatever the 30' of wiring from the circuit breaker box will deliver until the fuse at the transformer input opens

author: Ian.M
author_rank: Super Contributor
timestamp: January 07, 2024, 10:03:58 pm
content: Comes from:https://www.eevblog.com/forum/projects/autotransformer-open-circuit-over-voltage/and:https://www.eevblog.com/forum/projects/connecting-torodial-transformer-in-buck-boost-mode/Connect a low voltage AC relay with its coil across the upper winding and its contacts in series with the load. If the voltage drop goes away the load disconnects.Either use a 24V AC coil relay that you've checked is guaranteed to pull in sufficiently under 20V to have some margin, or a 12V AC coil one with a series resistor. As the coil current lags the voltage it wont be as simple as dropping 8V across the resistor.

author: Benta
author_rank: Super Contributor
timestamp: January 07, 2024, 10:16:45 pm
content: A safer option would be monitoring the current through the lower leg of the primary. Disregarding losses and other parasitics, this will either be the magnetizing current (= everything's OK) or zero (= open circuit).Plenty of ways to do this:An off-the-shelf current monitoring relay.A Reed tube with windings around it.A current transformer....

author: Ian.M
author_rank: Super Contributor
timestamp: January 07, 2024, 10:53:17 pm
content: Note that such a failure is vanishingly unlikely if the joint between the two windings is either wrapped and soldered or otherwise permanently spliced. However, it may be worth spending $10 on a relay to protect $10000 of equipment downstream.If the equipment downstream is very sensitive and valuable, it may be worth designing a more accurate overvoltage cut-out, (possibly of the crowbar type) to trip at say 235V RMS, as this will also protect against severe overvoltage on the incoming supply.

author: Benta
author_rank: Super Contributor
timestamp: January 07, 2024, 11:07:55 pm
content: Quote from: Ian.M on January 07, 2024, 10:53:17 pmI agree. But if the downstream equipment is valuable, why use an autotransformer? Something's wrong in this scenario.

author: Spar59
author_rank: Contributor
timestamp: January 08, 2024, 09:50:47 pm
content: If my understanding is correct and you are using it to reduce 240V to 220V I would not normally expect a 220V device to be unduly upset by 240V, much equipment these days uses switch mode power supplies that are ok with a wide range of input voltages, I certainly would not expect a current day manufacturer to produce separate designs for 220V & 240V.So I would first check the manual of the device(s) that you intend to feed to decide if protection is really necessary.If it is, there are dedicated AC voltage monitoring relays on the market intended for use in industrial control systems, often with adjustable set-points that would be able to differentiate between 220V & 240V.However note that a nominal 240V supply will drift either side of that, 240V possibly suggests a UK supply and the standard was 240V +/-6%, hence it could go as high as 254.4V without being out of statutory limits, pro-rata the 220V output of your transformer could go up to 233.2V, you will need to take this into account when setting your trip point to avoid nuisance trips. Obviously a 220V electricity network would also suffer drift.Assuming the 240:220V ratio means you wish to use equipment intended for the US in the UK then be aware that the frequency in the UK is 50Hz not 60Hz, this will affect the speed of induction motors which will run 16.6% slow and also result in increased magnetising current in conventional mains frequency iron-cored transformers which may cause them to overheat, the latter issue is not an problem if going the other way and using a 50Hz transformer on a 60Hz supply.