ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
ShoutChat Box
Recent Posts
Where is Everyone?
by gfretwell - 04/09/21 01:08 AM
HI gang, long time since I was here...
by gfretwell - 04/09/21 01:02 AM
Lock-down Thread
by gfretwell - 04/06/21 09:30 PM
New in the Gallery:
Facebook follies, bad wiring
FPE in Germany pt.2
Who's Online Now
0 registered members (), 5 guests, and 22 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Some transformers work fine when powered from sec #215683 07/01/15 12:57 PM
Joined: Jul 2004
Posts: 783
L
LarryC Offline OP
Member
Why do some transformers work just fine when powered from the secondary and other don't?

I was using a small (500VA) 240 to 120 step down transformer as a step up for a small load, and I could only get 200V when powering the 120V winding with 120V.

Tools for Electricians:
Re: Some transformers work fine when powered from sec [Re: LarryC] #215684 07/01/15 01:56 PM
Joined: May 2005
Posts: 984
G
ghost307 Offline
Member
The turns ratio is calculated to account for the losses inherent in the transformation.
Instead of adding more power to compensate for the 240 to 120 losses...you may be deducting twice that value when going from 120 to 240.

The effect is much less noticeable in large transformers so you may not notice it in those cases.


Ghost307
Re: Some transformers work fine when powered from sec [Re: LarryC] #215685 07/02/15 12:45 AM
Joined: Nov 2005
Posts: 824
J
JoeTestingEngr Offline
Member
Larry, are you saying that the unloaded voltage was only 200VAC? Did you verify it by driving the primary at 240VAC? I've driven quite a few backwards and everything followed the turns ratio.

Re: Some transformers work fine when powered from sec [Re: LarryC] #215686 07/02/15 03:55 AM
Joined: Jul 2004
Posts: 9,575
G
gfretwell Offline
Member
I wonder if it was really 208-120.


Greg Fretwell
Re: Some transformers work fine when powered from sec [Re: gfretwell] #215692 07/02/15 06:13 PM
Joined: Jul 2004
Posts: 783
L
LarryC Offline OP
Member
It really is a 240 to 120 transformer. I checked it with 240 Input and I got 120 Out. I was told this specific transformer is a cheap design. Low power, 50/60 Hz operation.

I have been told that there is now a new(?) rating on other larger dry transformers stating that it is suitable for reverse operation.

I am no longer at that facility to get manufacturer and model numbers.

Re: Some transformers work fine when powered from sec [Re: LarryC] #215699 07/02/15 11:37 PM
Joined: Jun 2004
Posts: 613
S
shortcircuit Offline
Member
450.11(B) of the 2014 NEC has language about feeding the secondary of a transformer.

Winchester Speedpark is up there in NH. Nice place...had lots of fun there.

Re: Some transformers work fine when powered from sec [Re: shortcircuit] #215703 07/03/15 11:41 PM
Joined: Jul 2004
Posts: 783
L
LarryC Offline OP
Member
Just down the road a couple of miles. Some nights we can just barely hear the cars.

Re: Some transformers work fine when powered from sec [Re: LarryC] #221167 02/09/21 01:06 AM
Joined: Jan 2021
Posts: 6
quickspark Offline
New Member
The transformer supply will be definitely damaged (when there is no protection scheme) due to nearly zero resistance of the primary winding. Moreover, it is obvious that you cannot see any voltage signal in the secondary! So take the advice of licensed electrical contractors and use transformers in your AC.

But the beauty of transformers is that they allow us to have more than just one winding in either the primary or secondary side. Transformers which have more than one winding are known commonly as Multiple Winding Transformers.
Note that transformers are only used for AC systems because of the phenomena of mutual induction between the coils. We recommend you review the functionality of a transformer again and get the gist of why we use transformers in AC power systems.

Transformers, on the other hand, are able to convert levels of AC voltage and current at very high efficiencies with no moving parts, making possible the widespread distribution and use of electric power we take for granted.
In all fairness, it should be noted that motor supply sets have not necessarily been obsoleted by transformers for all applications.


Featured:

2020 National Electrical Code
2020 National Electrical
Code (NEC)

* * * * * * *

2020 Master Electrician Exam Preparation Combos
2020 NEC Electrician
Exam Prep Combos:
Master / Journeyman

 

Member Spotlight
AllClear
AllClear
Belmond Iowa US
Posts: 44
Joined: August 2005
Show All Member Profiles 
Top Posters(30 Days)
BigB 1
Popular Topics(Views)
277,597 Are you busy
211,258 Re: Forum
198,287 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.7.3