The recommended fuse rating for a motor and for a transformer is
3times the power divided by the supply voltage.
Your 900VA running on 115Vac would require approximately >=23.5A fuse to start up reliably.
You may find that you have to use a B or C rated 25A breaker to get it to start up.
Alternatively fit a soft start using either suitably rated Power Thermistors or Power Resistors.
3times the power divided by the supply voltage.
Your 900VA running on 115Vac would require approximately >=23.5A fuse to start up reliably.
You may find that you have to use a B or C rated 25A breaker to get it to start up.
Alternatively fit a soft start using either suitably rated Power Thermistors or Power Resistors.
Hi,
the transformer will be pulling a lot higher than 24Apk at start up.
Have a look at the time vs current of a ClassC breaker.
The transient that can pass for short periods is many times the rated breaker value.
At some point in the starting sequence your transformer is taking the breaker outside the ClassC curve. It might be at the very first instant of 1us after closing the switch , or it might be after 1ms after closing the switch.
More likely it is an accumulation of [i^2t] that opens the breaker.
The same happens with a fuse.
What you try to do is keep the current during the first few millisecond under some form of control so that the transient current limits for the chosen breaker an/or chosen fuse are not exceeded.
Soft start is designed to specifically control the starting currents.
the transformer will be pulling a lot higher than 24Apk at start up.
Have a look at the time vs current of a ClassC breaker.
The transient that can pass for short periods is many times the rated breaker value.
At some point in the starting sequence your transformer is taking the breaker outside the ClassC curve. It might be at the very first instant of 1us after closing the switch , or it might be after 1ms after closing the switch.
More likely it is an accumulation of [i^2t] that opens the breaker.
The same happens with a fuse.
What you try to do is keep the current during the first few millisecond under some form of control so that the transient current limits for the chosen breaker an/or chosen fuse are not exceeded.
Soft start is designed to specifically control the starting currents.
It is strange that it trips every time, check there is not some other issue, Random trips are more characteristic of inrush current. 900VA is not a large transformer, 2 Kva transformers do not cause inrush current trips on the 240V domestic supply over here. (C curve breakers)
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.