Mains transformer core is earthed? - Page 1
Forum section: electronics / testgear

author: Faringdon
author_rank: Super Contributor
timestamp: July 24, 2024, 07:31:12 am
content: Hi,Is the core of this isolation transformer earthed?https://www.screwfix.com/p/carroll-meynell-3000va-intermittent-isolation-transformer-230v-230v-blue/320HV?kpid=320HV&cm_mmc=Google-_-Datafeed-_-Electrical%20and%20Lighting?kpid=KINASEKPID&cm_mmc=Google-_-TOKEN1-_-TOKEN2&gad_source=1&gclid=EAIaIQobChMIurKz6pO_hwMVZo9QBh2u4gyPEAQYASABEgJZOvD_BwE&gclsrc=aw.dshttps://www.carroll-meynell.com/product/isolation-transformer-230-230v-3kva-cm3000-230-euro/Earth goes into it...do you know what that earth connects to?& Why is it so noisy when its switched ON, even with its secondary open?(ie noisy electrically to other equipment plugged in on the same phase, making it malfunction)

author: techneut
author_rank: Regular Contributor
timestamp: July 24, 2024, 07:01:27 pm
content: My guess is that there is a shield between primary and secundary windings that is earthed. I have a 2A isolation transformer and you can realy hear the dumph when it is powered on. The magnatising current with it's mgnetic field is my guess.

author: Faringdon
author_rank: Super Contributor
timestamp: July 26, 2024, 05:51:38 am
content: Thanks, i was thinking the same, but the magnetising current isnt that much since the inductance is so high?I am thinking its capacitively putting a big common mode transient into earth, since the earthed shield is highly "strayly" capacitively coupled to the primary.But not sure the exact reason.

author: garrettm
author_rank: Frequent Contributor
timestamp: July 26, 2024, 07:49:42 am
content: Quote from: Faringdon on July 24, 2024, 07:31:12 amI doubt this is a capacitive effect here. At 50 to 60 Hz a few nanofarads of stray capacitance to earth ground would act as a very high impedance. I suspect you have a shorted turn or high leakage to earth ground from a damaged primary coil. You could disconnect the earth ground and see what that changes. You could also try measuring the current passed to earth ground with a current clamp, or use a power meter to see how much real power is being consumed by the unloaded transformer (should be mostly reactive).I suspect the transformer has a bad primary winding. It's rare, but it happens.

author: AVGresponding
author_rank: Super Contributor
timestamp: July 26, 2024, 09:30:43 am
content: If it's used rather than new it's probably more common than you think, as the gorillas on sites usually take the intermittent rating as the continuous one and cook the things

author: nali
author_rank: Frequent Contributor
timestamp: July 28, 2024, 02:06:07 pm
content: Quote from: Faringdon on July 24, 2024, 07:31:12 amJust undo the 4 screws holding the top on - all will be revealed!(And, no I'm not going to tell you - get your lazy arse in gear and look for youself!)

author: adam4521
author_rank: Regular Contributor
timestamp: July 28, 2024, 03:53:26 pm
content: At the moment of switch on, this system is not responding to ‘steady state’ ac equations. There’s no magnetic flux, no back emf, and you connect a step voltage. The dv/dt and the di/dt is very high. There will be considerable interaction between the supply impedance and the transformer, so transient effects fully expected- expect an instantaneous current in excess of 100A followed by oscillations to reach the steady state. I expect the transformer is behaving normally.

author: Faringdon
author_rank: Super Contributor
timestamp: July 28, 2024, 04:16:07 pm
content: QuoteThanks, thats a great idea....but the project has been completed now...with all targets met as by the organisation's requirement, so i wouldnt be at liberty to go taking apart their equipment for some post project research.