Recently I had the need to assess the performance and determine some of the ratings of some low-voltage 240-v. primary mains transformers. I was somewhat surprised at the results obtained with two salvaged Japanese items. (In what follows I refer to one transformer, but the comments are applicable to both.)
I measured the N/L secondary voltage and noted that it had a manufacturer's label stating so many VA for the secondary winding. I connected the secondary to a variable resistive load and adjusted that resistance for an on-load current and voltage, the product of which approximately equalled the stated VA rating. The secondary voltage was about 5% down on the N/L voltage in this condition - as you would expect. At this point, I left the transformer alone, on the bench, to ascertain the transformer's temperature rise. You can only imagine my surprise when, half-an-hour later, I discovered that the core of the transformer was very, very hot!
Somewhat bewildered by this result, I did some investigations to discover how I could - simply - get an approximate determination of a transformer's actual VA rating. Briefly, for a 50 Hz supply freq. and for transformers in the range 20 VA to 500 VA and of the conventional E & I construction, the VA is principally determined by the core cross-sectional area where the windings are located. To a first level approximation, if this area = A sq. inches, then the VA = 26*A² volt-amps.
Using this simple (and approximate) equation, I tested half-a-dozen other transformers in a like manner: the results made sense. Also when I retrospectively 'downgraded' the stated VA of the two transformers that got very hot, they too now behaved 'normally' on a temperature 'soak test'. The 'new' VA was about half that as stated by the manufacturer!
O.K., so what's all this leading up to? What I am suggesting is that the very simple formula, VA = 26*A², is a useful guide to quickly and approximately assess the VA rating of low to medium-size transformers (and thus determine a suitable secondary current into a resistive load) when that VA is not stated - but can be estimated by a physical measurement.
Attached in an Excel spreadsheet, based on that formula, for quick and easy reference:
[attachment=6845]
The source for that formula was derived from:
[attachment=6846]
Note that that reference refers to a graph - which fails to be included in the above attachment (?). However, it is added here as an additional attachment:
[attachment=6852]
Another useful reference on this topic: re-using microwave oven transformers. [Note that he author seems to have a specific application in mind. This seems to be implied in his text, as opposed to being specifically stated: some sort of re-charging cct. for a perchlorate cell.]
► Please note that experimenting with microwave oven transformers can be very dangerous: they produce very high voltages that will kill you! ◄
[attachment=6848]
Again, a picture from that source is missing in the above - but it is presented below:
[attachment=6854]
The design of low-frequency transformers and their assessment is a very large subject and it can get quite complicated, but I hope others will find the above of some use: it is by no means intended to produce an accurate figure: only an approximation as a 'guide for applications'. Consequently, I am submitting this thread here for a 'peer review': your comments are most welcome.
Al. / Dec. 31, 2012 //
Post submit edit by Al., Dec. 31: two pictures added.
I measured the N/L secondary voltage and noted that it had a manufacturer's label stating so many VA for the secondary winding. I connected the secondary to a variable resistive load and adjusted that resistance for an on-load current and voltage, the product of which approximately equalled the stated VA rating. The secondary voltage was about 5% down on the N/L voltage in this condition - as you would expect. At this point, I left the transformer alone, on the bench, to ascertain the transformer's temperature rise. You can only imagine my surprise when, half-an-hour later, I discovered that the core of the transformer was very, very hot!
Somewhat bewildered by this result, I did some investigations to discover how I could - simply - get an approximate determination of a transformer's actual VA rating. Briefly, for a 50 Hz supply freq. and for transformers in the range 20 VA to 500 VA and of the conventional E & I construction, the VA is principally determined by the core cross-sectional area where the windings are located. To a first level approximation, if this area = A sq. inches, then the VA = 26*A² volt-amps.
Using this simple (and approximate) equation, I tested half-a-dozen other transformers in a like manner: the results made sense. Also when I retrospectively 'downgraded' the stated VA of the two transformers that got very hot, they too now behaved 'normally' on a temperature 'soak test'. The 'new' VA was about half that as stated by the manufacturer!
O.K., so what's all this leading up to? What I am suggesting is that the very simple formula, VA = 26*A², is a useful guide to quickly and approximately assess the VA rating of low to medium-size transformers (and thus determine a suitable secondary current into a resistive load) when that VA is not stated - but can be estimated by a physical measurement.
Attached in an Excel spreadsheet, based on that formula, for quick and easy reference:
[attachment=6845]
The source for that formula was derived from:
[attachment=6846]
Note that that reference refers to a graph - which fails to be included in the above attachment (?). However, it is added here as an additional attachment:
[attachment=6852]
Another useful reference on this topic: re-using microwave oven transformers. [Note that he author seems to have a specific application in mind. This seems to be implied in his text, as opposed to being specifically stated: some sort of re-charging cct. for a perchlorate cell.]
► Please note that experimenting with microwave oven transformers can be very dangerous: they produce very high voltages that will kill you! ◄
[attachment=6848]
Again, a picture from that source is missing in the above - but it is presented below:
[attachment=6854]
The design of low-frequency transformers and their assessment is a very large subject and it can get quite complicated, but I hope others will find the above of some use: it is by no means intended to produce an accurate figure: only an approximation as a 'guide for applications'. Consequently, I am submitting this thread here for a 'peer review': your comments are most welcome.
Al. / Dec. 31, 2012 //
Post submit edit by Al., Dec. 31: two pictures added.
