transformer test gone wrong - Page 1
Forum section: electronics / beginners

author: littlebill
author_rank: Regular Contributor
timestamp: December 02, 2015, 06:25:49 pm
content: i was attempting a test of a transformermy goal was to test a T40-24F3 to convert voltage up to 600v.i put 120v through the secondary windings (24v), at which point i smoked my surge protector and had smoke pouring out of the transformer itself.there was no load on the transformer, thus i am very confused why it went up in smoke,updatei read this linkhttp://electronics.stackexchange.com/questions/21576/what-happens-if-i-connect-a-transformer-in-reverseso my assumption is the windings are not capable of isolating the 120v in the 24v windings, thus somehow shorted?

author: cjurczak
author_rank: Contributor
timestamp: December 02, 2015, 06:53:49 pm
content: You may have exceeded the insulation of the winding of the secondary. These are commonly only a vanish or enamel coating, with a couple layers of tape between primary/secondary/core. This usually works better when closer to the original specs of the transformer...

author: IanB
author_rank: Super Contributor
timestamp: December 02, 2015, 07:00:07 pm
content: If you apply the mains to the 24 V secondary of a transformer I suspect a very large current will flow. For instance, calculate the reactance of the secondary as a pure inductance, apply 120 V to it, and see what reactive current flows. I am surprised you didn't trip your circuit breaker.

author: TimFox
author_rank: Super Contributor
timestamp: December 02, 2015, 11:36:56 pm
content: An iron-core inductor (such as a transformer) can only support a finite AC voltage (regardless of load current) at a given frequency.Remember that V(t) = d (Flux)/dt and that the maximum flux (before saturation of the iron) equals the maximum B-field (a property of the ferrous alloy) times the effective area of the core. At a given frequency, the maximum d(Flux)/dt = 2 pi f x (Bmax) for a sine wave.Applying a large voltage (120 V) to a low-voltage (24 V) winding almost certainly exceeded the saturation voltage. Above that point, you only have copper wire on an air core in series with a saturated inductor, and the current will be huge.Many transformers operate rather close to that maximum voltage, to reduce the weight of the iron.(Do you think I learned that one the hard way or the easy way?)

author: littlebill
author_rank: Regular Contributor
timestamp: December 02, 2015, 11:41:23 pm
content: thanks for the answer guys.little help on locating this WITH the pigtails. this is all that is blown on the surge suppressor. 15A MDA ceramic. i really really need the pigtails

author: littlebill
author_rank: Regular Contributor
timestamp: December 03, 2015, 12:09:12 am
content: bought thishttp://www.ebay.com/itm/161207033261

author: szhighstar
author_rank: Contributor
timestamp: December 06, 2015, 08:43:10 am
content: I guess secondary winding maybe was short between wire and wire.

author: retrolefty
author_rank: Super Contributor
timestamp: December 06, 2015, 09:01:51 am
content: Quote from: TimFox on December 02, 2015, 11:36:56 pmWhich explains why aircraft AC frequently uses 400 Hz rather then 60 Hz. Weight saving in aircraft was and is very important.

author: matseng
author_rank: Frequent Contributor
timestamp: December 06, 2015, 09:24:20 am
content: Quote from: retrolefty on December 06, 2015, 09:01:51 amWhy does trains use 16 3/4Hz in Europe? It's a third of the 50 Hz mains....

author: Zero999
author_rank: Super Contributor
timestamp: December 06, 2015, 09:40:13 am
content: I doubt it was anything to do with the transformer's breakdown voltage.As many other people have stated, the 24V winding would've saturated, causing a large current to flow. If you'd increased the frequency, by a factor of the turns ratio, from 60Hz to 300Hz, then all would have been well.

author: rs20
author_rank: Super Contributor
timestamp: December 06, 2015, 09:41:51 am
content: Quote from: matseng on December 06, 2015, 09:24:20 amYou mean 16 2/3 Hz. And it's purely for historical reasons; if they could do it all over again they'd use the same frequency as everyone else."These drawbacks, plus the need for a separate supply infrastructure and the lack of any technical advantages with modern motors and controllers has limited the use of 16 2?3 Hz and 16.7 Hz beyond the original five countries. Most other countries electrified their railways at the utility frequency of 50/60 Hz." --Wikipedia

author: Seekonk
author_rank: Super Contributor
timestamp: December 06, 2015, 10:25:43 am
content: I did a double take when I saw 15 1/3Hz. The first US standard was 166 2/4Hz

author: Yansi
author_rank: Super Contributor
timestamp: December 06, 2015, 10:38:08 am
content: Quote from: matseng on December 06, 2015, 09:24:20 amThat's some Austrian garbage.. Nothing general for EU. Our trains (central Europe) use DC 3kV or 25kV AC 50Hz. (and yeah we have also dual system trains which accept both voltage systems.

author: HighVoltage
author_rank: Super Contributor
timestamp: December 06, 2015, 10:57:25 am
content: Train systems all over the world used to have a different much lower frequency to make it difficult to "steal" electricity.

author: SeanB
author_rank: Super Contributor
timestamp: December 06, 2015, 10:58:39 am
content: Low frequency was used simply because that was the maximum frequency that could be generated by the alternators of the day, higher would have resulted in them breaking from the forces in the rotor. They used a very large diameter rotor to get the magnetic field high enough to generate the required power, but so that eddy currents in the cast iron would be low. Stator used steel laminates, because that was where the AC was induced, though the iron also saw some of this field. As well the white metal plain bearings would not support a long life at high speed.50/60Hz alternators ( aside from some very specialised ones for radio transmission) had to await the development of reliable ball bearings and the associated lubrication technology for the bearings, to become cheap enough and capable of handling the load. That you now have engines that use plain bearings and run at 12k RPM is due to increased metallurgy of the bearings, from the original wiped tin, Babbitt metal and lead bearings.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 06, 2015, 11:13:40 am
content: Quote from: rs20 on December 06, 2015, 09:41:51 amOnly a few 16 2/3 lines remain,most of Europe is 50Hz.Oz is funny,Sydney & Melbourne use 1.5kV DC on their suburban commuter trains,Brisbane,Adelaide & Perth,use 50Hz 25 kV.

author: oldway
author_rank: Super Contributor
timestamp: December 06, 2015, 02:35:57 pm
content: Reason for 16Hz 2/3 is that, in this time, high power ac traction motors (brushed serial motors) could only work correctly (acceptable commutation) up to this frequency.Traction equipment was then very simple: an autotransformer 16Hz2/3 with different taps and serial AC brushed traction motors.

author: TimFox
author_rank: Super Contributor
timestamp: December 06, 2015, 02:48:33 pm
content: 16-2/3 Hz low frequency AC was chosen as a compromise between the use of "DC" motors (not liking high frequency) and weight of the control transformers needed to change speed. DC motors are easy to change speed, by changing the applied voltage.In the US, although 60 Hz is normal commercial power, the Pennsylvania Railroad electrification was done at 25 Hz for similar reasons.