Inrush Current

There are very few 1 phase motor soft start devices because only a handful of 1 phase motors are suitable for use with softy starters, so the market for them is relatively small.

There are devices however called SCR Phase Angle Controllers typically used for heating, and they can be used to limit inrush current in transformers, it's done all the time. They also serve to provide variable output voltage, essentially replacing a variac. I don't know if it's worth using both, but I don't know why you are choosing a variac.

Another more simplistic way to limit magnetizing inrush on a transformer, if it isn't too big, is to use an NTC thermistor as a current limit device, then bypassing it with a relay or contactor once at full voltage.

JRaef.

Thanks for the info so far.

I need to control the inrush current going into a large 35Amp, 240Volt variac. The reason that I am using a variac (or rather two variacs) is one to control the voltage and one wired in series withn it to limit the current going into a 15kva distribution transformer. The whole set up is used for high voltage research. My problem is with the first variac. At turn-on the inrush current is about 6 times the limit of the MCB and so it knocks out.

Do you have a schematic of how to wire up a switch out device once the NTC has stopped the initial current spike? I understand that solid-state relays are best for this purpose. Any advice would be appreciated.

Paul.

No schematic (but what a great tool that would be for this forum...). But it isn't that difficult. Wire up a timer to the line power so that it comes on immedaitely when power is applied and a NO contact closes to another larger contactor (35A) coil after 1 second; the magnetizing inrush will be shrter than that. Then wire a Normally Open contact in parallel to the NTC thermistor with both feeding your variac primary.When power is applied, it goes to the timer coil AND through the thermistor to the variac and establishes the magnetic field, then 1 second later the timed contact closes, energizes the 35A contactor which shunts power around the thermistor.

Thanks for the much needed info. I will give the circuit you described a try; hopefully I will get it right.

Hello JRaef, where is answer to my question ?

Thanks for the reference,but i have question regarding to the following instruction:

http://www.ametherm.com/inrush-current/inrush-current-transformer-case.html

'Peak Inrush Current occurs in one cycle = 564 Amps as measured on the scope.'

Is this for the first cycle at switch on ?

'Practical Max Inrush Current occurs in 5 cycles = 188.0 Amps as measured on the scope.'

Is this for the max inrush current for the first 5 cycles of the switch on ?

If yes, why we got less current 188 amp and not 564 amp ?

Thanks for help.