I have a conveyor belt circuit that has been modified and the demand on the transformer has been increased from 1.5amps to around 3amp. Although protected by a circuit breaker at 2 amps the old transformer got too hot and shorted. What a mess. Anyhow I'm not an electrical engineer, but is there any danger to oversizing a transformer? Going to a 750VA from a 350VA will give me an output amp increase from 1.4A to 3.1A. Does this sound right?? I've searched Google for a quick answer but either it was over my head or dead ends. Thanks guys, Chris.

 

Oversize the transformer by 25% from the demand. It won't hurt to go bigger, but it's not necessary.

THEN, oversize the circuit breaker 20% from the transformer full-load amperage. Fun fact: most circuit breakers are only rated to guaranteed not tripping at 80% of their rated load. The magnetizing current is actually way higher than the full load rated current, but decays before the circuit breaker will trip (slow).

What voltages are we talking about?

 

480 to 240. I think I got lucky with choosing the 750VA, Thanks!

 

20% oversize on the primary side right?

 

Yes, primary side current.

 

Wait a minute... You have a load drawing over three amps and it was protected by a 2 amp breaker that didn't open at 50% overcurrent? You have a supply problem as well. I would chuck the breaker immediately for not doing its job. Looks OK on the VA ratings (if you didn't change the voltages of the primary side, remember as voltage drops current increases for a given load) IE if supply (primary) is 440vac @ 10 amps and you drop the primary to 220vac your current demand will jump to @ 20 amps to do the SAME job. This means you have to also do some calculations on the transformer and the load the transformer is supplying IF you dropped the supply voltage and went with a different wiring configuration on your motor and transformer design voltages (a 440vac transformer/motor to operate on a 220vac supply that originally was designed/wired to a 1.5amp load @ 440vac). A very quick guestimation (given the numbers posted if I am reading this correctly) would be originally a 440vac @ 1.5amp demand increased to @ 3amps same circuit, 2 amp breaker didn't trip, replacement went to 220vac same load (higher demand of 3 amp originally) would now be @ 6 amps. New transformer minimum (without 10, 15, or any other % increase safety factor) must handle this high current and of course the motor must be rated this higher current demand as a minimum. Now, the final problem that MUST be addressed. What size wiring is supplying all this upgrade and is it rated for the added current including the distance (wire length) to the end load? Its not as easy to just replace parts, a little more must be done or you run a risk of hurting someone or burning something else up.

 

Thanks! Let me clarify the voltage was not changed. 480 to 240 single phase. I was incorrect the secondary goes to a 3 amp circuit breaker, not 2 amp sorry. Using an amp clamp meter I am seeing about 2.3 amps. The wiring is 16 gauge stranded on the 240 side. 10 gauge 480 side with a breaker set to 7.5 amps.