Maximum change in current that a transformer is rated for.

[rotmgbetter]

Hi. If i have a transformer rated at lets say 100 KVA and the secondary winding is at 240 volts, this means I can pull around 400 something amps from the secondary side. My question is, if I am using a purely resistive load, what is the maximum I can switch on and off without damaging the transformer? Can I instantly turn off a device that is purely resistive and pulling lets say 200, 300, 400 amps? I can't find any details on this online. I just know that during inrush, the device pulls much more power than the rated value. When the device is suddenly turned off is it possible the primary wires will send a big voltage spike which can potentially damage downstream devices.

 

[jaggedben]

... When the device is suddenly turned off is it possible the primary wires will send a big voltage spike which can potentially damage downstream devices.

Generally speaking... No. That's pretty fanciful.

I suppose theoretically it's possible that if you had a huge load causing terrible voltage drop, and a source that was trying to raise the voltage back to an appropriate range, that you could see a dangerous voltage spike when the load turned off. But that would be a very unusal failure of voltage regulation by the utility (or other source), and doesn't need to be considered for transformer sizing under the NEC.

A listed transformer should be able to handle its own inrush current, as long as it's energized from a direction approved by the manufacturer.

 

[winnie]

A purely resistive load doesn't have any inrush. Loads such as incandescent lamps are not quite purely resistive because the resistance changes with time.

Transformer inrush occurs when the primary is switched, not when secondary loads are switched.

In general there is no specification for rate of secondary load change on small transformers.

All electrical devices have 'parasitic' features which differentiate them from ideal. Transformer parasitic inductance or transformer acoustic resonances, inductance and capacitance of wires, etc.v if you have an extremely unusual load, it might trigger a problem with these parasitic features. Detailed circuit simulation might be needed if you are introducing an unusual load into a sensitive environment.

Jon

 

[rotmgbetter]

A purely resistive load doesn't have any inrush. Loads such as incandescent lamps are not quite purely resistive because the resistance changes with time.

Transformer inrush occurs when the primary is switched, not when secondary loads are switched.

In general there is no specification for rate of secondary load change on small transformers.

All electrical devices have 'parasitic' features which differentiate them from ideal. Transformer parasitic inductance or transformer acoustic resonances, inductance and capacitance of wires, etc.v if you have an extremely unusual load, it might trigger a problem with these parasitic features. Detailed circuit simulation might be needed if you are introducing an unusual load into a sensitive environment.

Jon

Even though an incandescent is not purely resistive, this only affects the current and voltage when turning it on as resistance is lower when the filament is cold. There is no inductive or capacitive component so you will not have voltage spikes when turning it off right?

 

[winnie]

Incandescent lamps and the wires feeding them have very little inductance. The inductance is negligible for any practical proper installation, but there has to be some.

If we were talking an ordinary installation I'd say you could ignore the inductance...but you seem to be asking about something non-standard and need to actually calculate the parasitics to determine if you can ignore them.

Jon