Transformers feeding different sources with primarys in parallel.


Thread Starter

Jared Hankovsky.

Multiple transformers being powered up simultaneously is blowing fuses on the primary side. When powered up one at a time the fuses hold fine.

What causes this typically and how can it be resolved?

Each transformer feeds its own system. 1 lights and 2 controls different control voltages. The unit has to be capable of surviving a power failure and power being restored with all systems being powered at the same time.
Can you clarify? Is there one fuse/breaker supplying many parallel transformers, or are there individually fused transformers in parallel?

In general I'd try putting a current probe (with sub millisecond response) attached to an oscilloscope to see what your inrush is on each leg of the circuit. You'll probably find large capacitive loads (Power supplies) or motor starting up is causing large inrush. The capacitive inrush can be huge. There are ways to combat this, but first do some tests to see what is causing you problems.

consideration of in-rush current is critical part of the electrical design process when dealing with transformers.

you might consider leaving the transformers energized continuously and switching the secondary loads.

Bob Peterson

It is possible that your electrical system can't deliver enough current for the inrush so the voltage drops requiring more current for a longer time when multiple transformers are powered up simultaneously.

Without knowing more about your set up and what part of the world you are in, it is hard to advise you very much.

If you are in the US take a close look at NEC table 450.3(B) for the maximum setting or rating of the primary protection. Keep in mind that this only applies to the transformer primary protection and not to protection of the primary conductors.

They also make fuses (FNQ-R is one IIRC) that have a slightly different trip characteristic that is made for transformers.

Jared Hankovsky... Please provide the following information:

a) Transformer capacities.

b) Primary and secondary voltages.

c) % voltage-impedances.

d) Primary breaker type, and rating.

e) Protection Device(s), Type (digital or Stand-alone) and trip settings!

Regards, Phil Corso