WHY a hot ZW?

[KRM]

WHY a hot ZW?

Got a question?

 I have two ZWs on my layout and one always gets warm after it has been on for a while. The other stays cool. Both seem to have plenty of power and if I switch the loads it does not change the one from getting warm and the other not. They both have new rollers and have been serviced.

I never noticed this until I pulled the one off because of the extreme heat caused by a stuck roller and had it serviced and the rollers replaced. It was only when I started using my back-up ZW did I find out that one does not get hot like the other one always has.

Any ideas on why??

[phillyreading]

Do they both have the same power rating? I have seen post war ZW's with either 250 or 275 watts, not sure if it matters. 

How were your ZW's doing before getting the rollers replaced? Did either one get warmer then the other one? Sometimes repair shops only do the service you request. If you mention about checking the ZW fully while being serviced most places will service it and charge a slight fee.

Lee 

[KRM]

phillyreading

Do they both have the same power rating? I have seen post war ZW's with either 250 or 275 watts, not sure if it matters. 

How were your ZW's doing before getting the rollers replaced? Did either one get warmer then the other one? Sometimes repair shops only do the service you request. If you mention about checking the ZW fully while being serviced most places will service it and charge a slight fee.

Lee 

 

One is from 1959 to 60 and the other is from 1965--to 66.

 The older one is the one that gets hot. They are both 275 Watt.

 The guy who did the work is a 82 year pro with transformers and I had them both done before I used them. Then the one stuck a roller after 6 years on the job and I took it to him and he found the stuck roller.

That one is the one that has always ran warm.

 

[lionelsoni]

 The difference may be in the core laminations, which in the hotter transformer may have lost or never had an adequate insulating coating.  These iron or steel plates are used instead of solid metal because, in addition to being magnetically permeable, they are also electrical conductors that would act as a short-circuited secondary winding.  Separating the metal into thin, insulated plates blocks that short-circuit current and keeps it from heating up the core, without inhibiting its magnetic properties.

The core insulation is usually a thin coating applied to the plates before they are stuffed into the copper windings.  It doesn't have to be perfect to do its job; but there's a limit to how much plate-to-plate electrical contact you can get away with.

It might be something else; but that's the only thing that comes to my mind.