Autotransformer Input MCBs Tripping On Apparently Normal Load

You have not mentioned the inrush current.

Please clarify the following

1) Is it tripping on load if so what is the load?

2)What is the secondary voltage setting

3) Is it tripping at the time of switching on?

Dear Sir,

I have already clarified in my query that the MCB is tripping on `NO-LOAD'. The secondary voltage setting is 205 VAC (for an input of 240 VAC). It is tripping at the time of `SWITCH ON' itself. (That too, it is happening about 80-90 % of the times. Once in a while, it does not trip).

Hope this clarifies your queries.

Look forward to your response.

Thanks for your interest.

Ravi Amdekar

circuit breakers generally only trip in the event of too high a current being drawn for a determined period of time. The first thing I would check is voltage drop on the feed side of the transformer upon power up. the transformer draws a load untill saturation is reached.

I agree entirely with your observation. However, we have even tried using a `D' curve type MCB (used normally for Motor operation and which has an inbuilt time-delay to handle abnormal inrush current much higher in magnitude than the rating of the MCB). However, the observations seem to defy all normal calculations and experience.

By the way, when you say that the voltage drop on the feed side of the transformer upon power-up needs tobe measured, are you referring to the `system-impedance' and therefore `fault-level'? Yes, this perhaps could be a very plausible explanation. We have however, not been able to find this out (the fault level that is). Could you perhaps elaborate on how this voltage drop can be measured (in view of the fact that the MCB is tripping instantaneously on power-up). Would it be necessary to hook up an oscilloscope to measure the instantaneous situation at power up before the MCB trips?

Thanks

Ravi Amdekar

Try using a storage oscilloscope, very carefully of course!!! You might get a handle on it when you see the trace.

I would send some Indian MCBs that you know work as there might be a split second of imbalance somewhere that they ignore better......

Let us know what the results are....

Hello,

Yes a scope should show the voltage drop very quickly. If you are still not able to get a good reading I suggest increasing the size of the MCB further untill you are able to get a good reading being cautious of fire of course.

I once had a similar situation with an auto transformer in the USA, I found the problem to be a large variable speed drive installed in a branch circuit. I installed an isolation transformer ahead of the drive and this corrected my transformer problem.

keep us informed

Hi Ravi,

big transformers take large magnetising currents when switched on, immediately comes to normal no load current ( if secondary is open circuited, ie load disconnected). You pl check this by replacing the 32 A MCB with a combination of 100A MCB and a 32 Amp HRC fuse in series. Also put a clamp meter (Analog type) at one of input leads. then switch on. If the clamp meter give a kick and come back to almost zero, without blowing the fuse means everything ok. Then tripping was due to magnetising curent and you replace MCB with a delayed tripping type in series with an HRC fuse. If fuse blows and / or shows abnormal or large current- means defect in the transformer winding or core. compare the test results with another healthy transformer.

regards

kallikadavil0

Apparently the MCB trips due to heavy inrush current.

solutions are

1) increase the rating of mcb to next higher rating with different curves

2) Redesign your Auto Transformer to have lesser Inrush current.