Variac Inrush Current?

The simplest way is to install a by pass switch in parallel with the resistor. I would not think you would need a schematic to do that.

So once the variac is has gone beyond the initial inrush stage (say after 2 seconds), the resistor can then just be switched out of the circuit and there will be no effect on the magnetization and thus current drawn into the variac. Is this correct?

Thanks.

Manfred,

You might also want to consider using a special surge-limiting thermistor. I've used one of these parts to replace a resistor/delayed-relay type of surge limiter on a large transformer. I don't know how many companies make these types of devices, I used a part from a manufacturer called "Ametherm"

LINK

These devices are Negative Temparature Coefficient thermistors; they work by presenting a series resistance to the load, and as they heat up, this resistance drops. The nice about these is they are always in-circuit, they do not have to be bypassed after the inrush. The only down side is they do waste a bit of power in use.

Tom D.

quote "So once the variac is has gone beyond the initial inrush stage (say after 2 seconds), the resistor can then just be switched out of the circuit and there will be no effect on the magnetization and thus current drawn into the variac. Is this correct?

The resistor stays connected but the bypass device shorts it out. The switch can be anything from an on-off switch to a timed relay contact. I depends on the current.

i kinda like the thermistor idea.

Rate the protection correctly inrush current shouldn't be a problem.

From your original post the primary supply is under rated.

Simply use a choke and not a resistor. This will limit inrush current and it can stay in the circuit.

Elroy

Not a good idea it will interfere with the correct operation of the Variac. A shorting switch across the resistor is best as I recommended in the OPs previous soft start relay post.

The resister does not have to carry large amounts of current as it will be in circuit for a few seconds so long as it is rated for the expected averaged current.

Instead use NTC thermister. Like used in TV set using Picture tube for Degaussing in series with Degaussing coil.

Hi All,

After some success with a number of resistors in series with the variac and thermistors I need further advice please. I installed a 90 Amp soft-start relay (variac is rated at 45 Amps); this worked well until last week. The MCB has started tripping again. So, I have decided to put a large resistance load in series with the variac input (before the soft-start, but I don't think that it matters if it goes before or after it). Given that the resistor will only be in the circuit for a few cycles of the AC:

1. What power handling is needed?

2. How do I calculate the required resistance needed to limit the inrush current to a particular value in a variac and what would be the suggested power handling capability of that resistor?

3. What is the best way of switching the resistor out of the circuit?

You should be able to simply increase the acceleration time value on the soft start. That will decrease the amount of inrush current with out the need of adding resistors.