Reactor Capability 1

[Bronco425]

Hello,

I'm trying to figure out if a reactor - for which I have not been given the ratings - can handle a specific level of current during intermittent duty.

The reactor is wound (in air) with 9 AWG solid coppper wire with an air core. I see that the max. amp rating for 9 AWG copper wire in air is 64 A.

The "ON" time is 750ms and the total cycle time is 18s; The current is 265 A.


The calculation I used is as follows:

Ith = sqrt[(267^2 * 750ms)/18s]

= 45 A

With the Ith = 45 A being less than the 64 A, I believe the reactor is able to handle the 265 A at the above duty cycle.


Are these the correct calculations? Any input would helpful, thanks.

 

[magoo2]

I get 54 A not 45 A, so it will still be OK since it is less than 64 A. Note: I used 265 A in the calculation.

 

[Bronco425]

Yes, typo -- the total cycle time was supposed to be 25s...thank you much.

 

[pie314]

You CANNOT use the 64 amp rating when the wire is used in an Air Core reactor. Air core reactors can have high eddy currents. Also the wires are blanketed with insulation. Usually the current density in air core reactors is 2000 amps per square inch or less. 64 amps looks very high to me

 

[bacon4life]

As Pie suggests, assuming a single wire in free air is not reasonable. As a sanity check you could use NEC table B310.1 and table 310.11. Approximating table B310.1 for #9 gives a 90C ampacity of ~40 amps in a multiconductor cable. Assuming at least 43 turns results in derating by 50% to 20 amps per table B310.11. Hopefully someone else can comment on the magnitude of the additional eddy current losses.

 

[scottf]

Are you sure there aren't paralleled winding layers. Unless there is only a single layer of winding, it's probably difficult ascertain the exact winding configuration visually.

 

[LiteYear]

Another consideration from my experience with wound resistors:

Heat transfer to air is quite slow. Applying more than continuously rated current to air cooled wire needs to be done either for a very short duration or at not much more than rated current. Your on time of 750ms is relatively long and your over current is relatively high. In that case, the duty is irrelevant - the wire can heat significantly just during a single on time. And given the I^2 relationship, your over current represents significantly more heat. I don't think there's any definitive calculation to be made, but on a hunch I think you're well outside the comfort zone. There are localised heating effects to be concerned about - sections of the wire that wont cool as well - and they'll be the first to fail. 750ms is plenty long enough for one of these sections to get very hot.

In my case I specified a 7A (continuous) 2500W resistor for a application with rare and momentary currents of 14A. It blew @ 14A after well less than a second.