By putting 208v on a 200v primary/24v sec. transformer (250VA) I get 25v @ ~10 amps.  I put a bridge rectifier on the output and get roughly 22V, which is fine. If my input voltage is 50Hz rather than 60Hz, is the power output reduced any?
 
In addition to several LED's, my main load is (4) 24VDC solenoid valves with a 1.3A rating on them, with varying duty cycles....for testing, maybe 50%.  

I'm guessing my 120 "lumps" being reduced to 100 'lumps' would not be significant enough as seen by the valves to make a difference, but I've got a ghost in the machine...

I noticed that the failure occurred when it was cooler outside, which I figured meant the valve had to work harder to 'warm up', in addition to the hydraulic fluid being cooler, therefore slightly more viscous (although our ME thinks the viscosity is not changing much-not that big of an ambient temp difference....10-15 deg. F)

Lastly, I was thinking maybe after the initial firing of the valves, they may have built up a field that had to be overcome (back EMF) on subsequent firings.  

All this considered, why would there be NO problem 95% of the time??
 

Just as a wild suggestion, we may be better able to help with your failure if you tell us what failed.
Viscosity may be a problem with some AC powered valves but should not affect DC valves.

Bill
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"Why not the best?"
Jimmy Carter

the 'failure' was that the valves would not shift...I failed to mention that the transformer I replaced with the one mentioned, had a 240v primary (20v @ sec. I think), and had about 17v on the output of the rectifier...the problem did not occur with the new one, however the temperature at that time was warm.  The original transformer was what was working 95% of the time...sorry about that....

Running a 60 Hz transformer at 50 Hz will probably give you saturation problems.

Benta.
 

Do you have a capacitor on the output of the bridge rectifier?  I suspect from the voltage it should be a much larger value.  200V and 208 at best is the real problem.  Chances are the line voltage has a lot of drops.  I had one customer that put a 30A linear regulated supply to operate only one relay to prevent shutdown from voltage sags.

I do not have a cap on the rectifier....a regulated power supply was considered, however, deemed unnecessary as the new transformer seemed to get rid of the failures.

 

You should use a capacitor on the rectifier output.  Any transformer you use should be rated 50/60Hz or you are likely to smoke the transformer as benta mentioned.

Also with a transformer output of 24V you should have an unloaded voltage of about (24V x 1.41)- 2V = 31VDC

If you are seeing values like 17V,(once you account for incorrect input voltages), you have rectifier or transformer issues.

Keith Cress
kcress - http://www.flaminsystems.com

Your transformer which is suitable for (say) 200V at 60Hz is only suitable for (50/60)*200 or about 167V at 50Hz. If you are connecting it to a 200V supply then the iron core will saturate and the output voltage will not be a sinusoid. The core will get hot and the primary current will rise very rapidly as you enter saturation. On a 50Hz supply you either need a different transformer, or you could use a 240V / 60Hz tap at 200V / 50Hz.
  

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If we learn from our mistakes I'm getting a great education!
 

We had a legacy product that a customer returned for analysis.  It worked just fine in the test jig. I noticed it was an overseas customer running at 50 Hz.  At that frequency it just buzzed and failed to close the relay. It was a bad cap in the power supply.

The lesson here was that at 50 Hz, the mechanical speed was faster than the pulsed supply voltage.  Some cap is needed.  A 470uF 50V Is likely large enough to solve the problem. Your transformer is likely rated 240V and is not saturating yet.  Adding a capacitor will likely solve your problems.

I just found out that the transformer was rated for 50/60Hz. Just out of curiosity, how would they construct a transformer so?

A 50 Hz transformer will work on 60 Hz at the same voltage.
For example, transformer may be rated at 100V 50Hz/ 120V 60Hz, or the same transformer may be rated at 100V 50/60Hz.  

Bill
--------------------
"Why not the best?"
Jimmy Carter

Just add a little more iron.  Of course, every manufacturer tries to be as cheap as possible. I was working at a nuclear plant and they asked me to look at the alarm system.  Half the alarm modules had failed and the plant hadn't even been fueled yet.  The modules were rated 120V 50/60 and they were running them at 50 Hz.  In the center of the cluster temps were reaching 90C. If I remember the transformers saturated at 106V and just turned into resistors. I dropped line voltage to 100V and replaced the zener with a 7808.  Not a failure after that.