Abnormal humming sound from Transformer

* Hum is mechanical and is caused by the alternating current shaking the steel laminations. Magnetostriction plays a roll in this. Generally, well designed and built transformers are solid enough so that there will be no audible hum. However, this can vary widely for individual units of the same design. Vacuum impregnation of resin varnish glues the whole core into one big block. If this is not complete, mechanical hum can result.

* If there is DC on the line, this can cause the core to saturate on one half cycle and cause hum. This can be caused by asymmetrical waveforms due to equipment with solid state switching devices. For a large transformer such as the one you have, this is unlikely to be a large enough effect, but you should check the waveform to rule it out.

I'm no engineer but I deal with rather large Pad mount utility transformers on a daily basis it seam to me that we have had this problem before and it was either a matter of Single phasing or(blown Bayonet fuse) or a low oil coolant level in the transformer caused by a bad seal IE. Bayonet fuse seal -over pressured blown and then the oil leaked out allowing the tranformer to hum . I could be quite wrong however this is simply a laymans view of your problem , Hope this was helpful for you

Kelly R. Bartholomew

To go along with the above. It is definitely a saturated core. 500kVa? The more material used in the transformer, the more room for mistakes, however small, they can add up quickly. Testing distribution transformers, I have found that the amount of HUM depends upon the quality of work done by production personnel. I'd make for certain that the 500kVa, is a 500kVa. I have seen multiple cases after having opened a transformer...that the 500kVa, was not...

With the saturated core, you are going to have to worry about considerable losses.

I noticed that the newer Power transformers for 10KVA upward and even smaller ones such as in a microwave hum a lot more than they used to, 30-50 years ago. I believe the reasons are; as follows:

Changes in lamination metallurgy allowing the use of smaller cores. The smaller laminations carry much more intense fields.

Changes in construction,.. rather than being bolted together with insulated (from the core) bolts (the old fashioned way). They are welded along the edges in several strips which provide paths for eddy currents.

When loaded the transformer becomes more of a resistive loan ( power factor of 1). so the forces in the core are more uniform, the eddy currents are in step with the magnetic fields and cancel. When unloaded the transformer is much more of an inductive load meaning the field strength is maximum when there is no current,... if a capacitor is used for phase shift such as for mercury lamps, operating off a single phase supply, this means that the two phases are much less evenly distributed so that the collapsing magnetic fields react more strongly to the eddy currents which are higher because of the different core material, and smaller size and the welds accross the laminations. Also the power waveforms of the AC (from the power company) may be more flattened on top, and bottom due to all the clipping from surge protectors which result in differential spikes being created by the unloaded, inductive transformers.

Lots of new high powered Transformers buzz like the Dickens,especially unloaded, I believe these are the reasons.

Geoffrey Reed

Not a hands on electrician, but checking on the internet under Electrical resonance transformer noise comes up with some answers that might help?

It is normal for a unloaded transformer to hum more than a loaded one. Check you load ( in amps) and the dB (hum) level you will notice a marked change in humming level when the load (amps) goes up.

Hope the data in this link will clear your doubt. http://www.federalpacific.com/literature/drytrans/10transformernoise.pdf

Perhaps it just doesn't know the words...