Transformer Load Imbalance
A MIDWEST Thermographer found an interesting problem with a 208 volt, 3 phase, 75 kva dry type general purpose transformer. The Infrared Scan showed the enclosure of a Square D transformer was much warmer on one side compared to the middle and other side. It was an older transformer and had been in service for many years. An enclosure must re-emit enough heat from the air cooled transformer and the difference in heat from one area of the transformer to another must be substantial enough for the Thermographer to see the difference in heat pattern. But when you’ve scanned 1000s of these dry type transformers over 15 years, you have a pretty good idea of what’s normal or not. A load check revealed one phase was at 220 amps, another at about 115 amps, and the third phase 90 amps. They definitely had a large load imbalance on their old transformer. The maintenance man said they had been having nuisance tripping on the output breaker, but when he checked the load, there was less than 100 amps, 50% load. Unfortunately he only checked the load on one phase and got unlucky enough to pick the lightest loaded phase. But it was also very easy for our Thermographer to see the load imbalance on the circuit breakers on the load and line side of the transformer. It is amazing how tough these old air cooled transformers are. 10 kva to 500 kva, general purpose dry type transformers are found everywhere. Some are real dinosaurs, beyond old, but not obsolete because they are still running. Some over 70 years. A couple other things about these really old dry type transformers. They were so overbuilt that they can be very forgiving of overloading and they can be really loud.