Transformer Overheating Even Under No Load

[npalen]

I replaced a burned out 40 VA transformer on my sister in laws lawn sprinkler system. A bad (shorted) solenoid on one of the zone valves caused the failure. Replaced with a new 50 VA 120 VAC (24VAC output) This new transformer was getting too hot to lay a hand on (150-200F) even when under no load so replaced with a new exact same transformer and having same issue. The heat doesn't seem to change when under load.
Would appreciate some suggestions as to whether this kind of heat is normal on an open frame transformer.

 

[Industrial Toys]

No, it should not get that hot. Have you run it without anything connected to the secondary?

I've used hundred's of 24 volt, 40VA transformers for security cameras. Your sure it's getting 110 and not somehow 220?

 

[Redneck in training]

Assuming that the transformers are not defective. The power has DC component for some reason. Some other device is causing that. In example some dimmers or switched power supplies or UPS can cause such thing. If you use power analyzer it will show some distortion of the sine wave. Anytime the sine wave is asymmetrical there will be DC component that will burn the transformer because the winding has low impedance for DC current.

 

[npalen]

Haven't run it without anything connected to the secondary but can give that a try. I had an electrician verify the power-in which showed 125 VAC. It is a multi-input transformer but is wired for 120 VAC using the white and black wires as indicated on the device wiring diagram.

 

[SPYDERLK]

Haven't run it without anything connected to the secondary but can give that a try. I had an electrician verify the power-in which showed 125 VAC. It is a multi-input transformer but is wired for 120 VAC using the white and black wires as indicated on the device wiring diagram.

I am confused how it could be under no load with stuf connected to the secondary. :confused2:

 

[npalen]

What I mean by "under no load" is that everything on the secondary is switched off. By "anything connected to the secondary", I mean that no wires would be physically connected. I realize that shouldn't make any difference but I've been fooled too many times.
A good electrician could write books about what I don't know about "sparktricity".

 

[k0ua]

Step 1: remove all wiring from secondary.
Step 2: Measure open circuit voltage on secondary make sure it is what you expect.
Step 3: check temp on transformer after about an hour.
If is is much more than just warm.. You likely have a defective winding in this transformer.

Step 4: if transformer stayed relatively cool, then hook back up the load side wiring, using the AC current measuring function of your VOM to measure how much current is being drawn from the secondary.. You believe none, so if it is a significant amount, you have a fault in the load side wiring or devices.

 

[npalen]

Thanks. Will do.

 

[troutsqueezer]

Sounds to me like you still have something shorted in the load, another solenoid or something. If the xformer gets too hot the annealing on the windings will bubble and the wires will short together, then you will need another new one. I would do a DC resistance check across the load (secondary disconnected) with your voltmeter, look for a direct short.

 

[Redneck in training]

You can also measure voltage when nothing is connected and compare it with voltage when something is connected. If the voltage with wires connected is significantly lower the cable might be defective or there short somewhere. You can also use resistance feature of the meter and measure resistance of the wires normally connected to the secondary. You should measure high resistance (Greater than 500 kohms. Ideally greater than 1 megaohm). You can also measure resistance of the wires against ground. Low resistance would suggest damaged insulation. Normally one ground fault doesn't cause issues but it still should be fixed. Assuming there is a "controller" operation the solenoids check the pc board for contamination (in example flux residue) causing current leakage between traces.

 

[npalen]

OP here. It's a simple three zone system with a toggle switch for each zone and roughly 200 ft of wire to each zone including the common. I had checked the resistance through each circuit coming up with about 50 ohms on two circuits and about 30 ohms on the third which had the bad solenoid since replaced.
The solenoids are rated at about 0.4 amps inrush and about 0.25 amps holding current. (24VAC)
At this point I'm thinking it must be the nature of these transformers to run very hot since replacing a new one with another new one. I need to ask the supplier to talk to the manufacturer to see if that is the case.

Edit: Redneck in training suggested above that there could be a DC component in the primary causing the heat. Is there any way this can be verified with a VOM? I'm not aware of any DC devices in the vicinity.

 

[Redneck in training]

OP here. It's a simple three zone system with a toggle switch for each zone and roughly 200 ft of wire to each zone including the common. I had checked the resistance through each circuit coming up with about 50 ohms on two circuits and about 30 ohms on the third which had the bad solenoid since replaced.
The solenoids are rated at about 0.4 amps inrush and about 0.25 amps holding current. (24VAC)
At this point I'm thinking it must be the nature of these transformers to run very hot since replacing a new one with another new one. I need to ask the supplier to talk to the manufacturer to see if that is the case.

Edit: Redneck in training suggested above that there could be a DC component in the primary causing the heat. Is there any way this can be verified with a VOM? I'm not aware of any DC devices in the vicinity.

If the sine wave is asymmetrical then the difference in area under the positive and negative curve is in essence DC. There is a device called power analyzer the can show it. Only problem is that they usually cost many thousand dollars. Typical maximum temperature rise of the transformer is about 50F over ambient. Needless to say that some small transformers will run significantly hotter.

Here is a link to DC in transformer winding. Look in section 18. Transformers Part 2 - Beginners' Guide to Electronics

 

[SPYDERLK]

If the sine wave is asymmetrical then the difference in area under the positive and negative curve is in essence DC. There is a device called power analyzer the can show it. Only problem is that they usually cost many thousand dollars. Typical maximum temperature rise of the transformer is about 50F over ambient. Needless to say that some small transformers will run significantly hotter.

Here is a link to DC in transformer winding. Look in section 18. Transformers Part 2 - Beginners' Guide to Electronics

I would look for a non zero reading on the DC scale of my meter while measuring the AC. If the wave is symmetrical it should hold close to zero.
larry