Center-tapped transformer question

[tbsmith]

This is an old topic, but I have a very similar question.  I'm using a 120V in to 24V out center tapped transformer, and want to switch between using the full windings (24V) and only half the windings (12V).  My questions is when putting a load on just half the windings, do I have to derate the power I can get from it? 

Thanks,
Tom

[IanB]

This is an old topic, but I have a very similar question.  I'm using a 120V in to 24V out center tapped transformer, and want to switch between using the full windings (24V) and only half the windings (12V).  My questions is when putting a load on just half the windings, do I have to derate the power I can get from it? 

Yes, because at half the voltage you will be drawing twice the current and this will increase the copper losses in the winding. Since the wire will have been designed for the expected current at 24 V you should not exceed the rated secondary current.

[tbsmith]

Thanks for the quick reply.  Just to make sure I got it, let's say I have a transformer rated at 1000 watts, 24V out.  That gives about 41 amps.  If I only use half the windings, I can still draw 41 amps, but I'd be okay because I'm effectively only drawing half the power (12 x 41), or 500 watts.  Is that correct?

[IanB]

Yes, 41 A would be OK as that is the rated secondary current. If you only use half the secondary you might be able to take a little more than 41 A as long as you keep within the overall VA rating of the transformer. However, you would have to check the temperature and make sure the transformer does not get too hot.

[tbsmith]

Perfect, thanks!  I'm doing it this way for the same reason as the original poster.  I'm building a linear power supply and don't want to drop all that voltage in the pass transistors, but I want to maintain the ability to run it at high voltage so I'll add a high current switch to select secondaries (with no load).  I envision most of my high current needs will be at 12 volts.

Unfortunately, I pulled the transformer from surplus equipment and don't have any specs other than the power rating.  Is there a rule of thumb of what temperature not to exceed for big transformers?  Is the main concern melting the wire insulation?   

[SeanB]

Main concern is not to get the inner windings so hot that the insulation either degrades or becomes conductive. You typically will not want the inner windings to go above 70C, which typically means the core must be below 40C, which will apply to old insulation. If the insulation is modern it will be rated to typically 110C, and will be fine up to about 60C at the hottest point on the core surface.

Simple rule of thumb is if you can smell it when operating it is too hot, which is true for any unit that has been powered for a year or so. New ones will smell of varnish, but if it starts to get to being able to make your eyes water in close proximity it is too hot.

[bktemp]

The power rating of a transformer is mainly based on the temperature rise. Measuring the temperature is the most accurate way to determine the power rating.
If you use only half of the windings, it can take about 1.41x rated current for both windings to get the same losses.

[tbsmith]

The transformer is about 8-10 years old, so would that be "modern" enough to assume it would be good for 110C?  I have an IR thermometer and will bench test it, looking for a core surface not to exceed 60C.  I'll bring up the load gradually since there is a fair amount of thermal mass (or is that 110C/60C already taking the thermal mass into account?).

The rest of the supply can't handle more than 40 amps, so it is good to know I have some reserve (1.41).  Also this supply is for home hobby use, so it will never see continuous duty.