1A no load transformer current  - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: December 27, 2023, 09:50:20 pm
content: This transformer 6" x 6" x 4.5", looks to be about 500-1000VA? based on size has 120V primary and 108V C.T. Secondary, two diodes feeding a big capacitor.With no DC load it's drawing 1A @ 120V, seems excessive current draw wise?Diodes are tested okay.

author: Psi
author_rank: Super Contributor
timestamp: December 27, 2023, 09:54:23 pm
content: Yeah, 120W idle is excessive.Is it getting really hot? it should be if its putting out 120W of heat.

author: pickle9000
author_rank: Super Contributor
timestamp: December 27, 2023, 10:06:11 pm
content: Was the secondary disconnected from the bridge?

author: Benta
author_rank: Super Contributor
timestamp: December 27, 2023, 10:10:59 pm
content: It's not drawing 120 W.It's drawing 120 VA, which is something completely different.For a 500...1000 VA transformer it seems a bit high. but there are tradeoffs on transformer design depending on application.It should be no more than hand-warm without load, if warmer you may have shorts in the primary (unlikely, but can happen).

author: ArdWar
author_rank: Frequent Contributor
timestamp: December 28, 2023, 12:19:46 am
content: If nothing are hot (or even warm) to the touch then it's fine.Actual 120W will get things to start literally burning already.

author: Jester
author_rank: Frequent Contributor
timestamp: December 28, 2023, 05:23:02 am
content: Quote from: Benta on December 27, 2023, 10:10:59 pmThe 1A measured was with a clamp on.I connected some better meters, the line dropped a bit, no load just the diodes feeding a 4700uF Cap : 118V. 0.92A. PF=0.55, 60W, 91VARWarm eventually gets to about 50 deg CThank you Benta

author: Jester
author_rank: Frequent Contributor
timestamp: December 28, 2023, 01:11:36 pm
content: Quote from: ArdWar on December 28, 2023, 12:19:46 amIt's actually dissipating 60W (no load) not sure why you would think it would burn if it was dissipating 120W (it's not), this is a fairly hefty transformer. It does get good and warm about 55C when loaded for a long time.

author: Jester
author_rank: Frequent Contributor
timestamp: December 28, 2023, 01:17:52 pm
content: Quote from: pickle9000 on December 27, 2023, 10:06:11 pmDisconnecting the bridge and 4700uF cap drops the current slightly (about 100mA)

author: SeanB
author_rank: Super Contributor
timestamp: December 28, 2023, 01:36:43 pm
content: Pretty normal for that age of transformer, they do run warm, but if you look at power factor it will show the power factor at no load being almost purely inductive, probably around 0.8 to 0.9 PF leading. You could place a capacitor in parallel with the supply, probably around 8uF 450V motor run or lighting power factor correction capacitor, and improve the no load current, but this will also mean higher inrush current at turn on, though current drawn from the line will be lower overall, though you will still have the current in the primary being the same.

author: MarkT
author_rank: Frequent Contributor
timestamp: December 31, 2023, 02:39:25 pm
content: Quote from: SeanB on December 28, 2023, 01:36:43 pmI think you mean 0.1 to 0.2 power factor. 1.0 means purely resistive.

author: jonpaul
author_rank: Super Contributor
timestamp: December 31, 2023, 03:06:47 pm
content: check for loose lams, corrosion causing a gapj

author: johansen
author_rank: Super Contributor
timestamp: January 01, 2024, 12:32:18 am
content: that core looks like its big enough, the primary could be wound with 10 gauge wire at a 1.2T flux density which implies the idle no load losses should not be as high as 60 watts. the 1 amp of idle current is reasonable. but a 20% power factor which is reasonable puts the losses around 20 watts.2KVA load would be reasonable depending on how much copper is in there. also the center tapped rectifier load is horrible. a 2kva transformer can only produce around 1KW dc from a CT secondary. if you cut the secondary and parallel the two windings and use a FWB you can pull about 20% more power out of the transformer for the same losses. only offset is doubling the rectifier losses, but anything above 10volts dc output, it's usually worth it.how much does it weigh? my spread sheet puts it around 14KG but i don't know the dimensions for sure.