We are going to purchase a 48kW electric drying oven.

As we know the AC motors have big starting current and a special starting equipment is needed - Star-delta, soft-starter, inverter and etc.

What is the situation with the simple resistors/heaters?

Is there a need for a special starting equipment? Or we can just connect them Direct-on-line. 16kW to each phase with a contactors?

Thank you in advance!

  • 1
    \$\begingroup\$ Hmm, if you need to ask this then I would strongly suggest to hire an expert for this. Someone with experience in this field. Do it wrong (how do you know the advice given here is good for your particular situation?) and things can blow up. Still, your choice but I know what I would do. Also: what does the datasheet say? \$\endgroup\$ Oct 3, 2017 at 7:21
  • \$\begingroup\$ More than strongly recommend an expert in your case \$\endgroup\$
    – Solar Mike
    Oct 3, 2017 at 8:18
  • \$\begingroup\$ How the oven's heater is going to work? Is it going to swich on/off during the industrial process (thermostat by)? or it will be on throughout the process? \$\endgroup\$
    – Hazem
    Oct 3, 2017 at 8:32
  • \$\begingroup\$ Do you have details on the oven? It will have some control electronics. Also multiple elements and fans. \$\endgroup\$
    – Jeroen3
    Oct 3, 2017 at 8:44
  • \$\begingroup\$ Yes, the heater is going to switch on/of during the process. The contactors will be controlled by a PLC \$\endgroup\$ Nov 1, 2017 at 13:14

1 Answer 1


Resistors/heaters are not simple.

The inrush to a heater depends on how hot it gets, and the temperature coefficient of the material.

For a resistor with a nominally zero tempco, so component resistors, or resistors made from specialist resistance alloys, there will be essentially no inrush.

For a filament lamp, the resistor runs hot, and the tungsten filament has a strong tempco. You can expect an inrush of up to 10x running current when they are first turned on.

There are other heater types between these two extremes. Single metals will tend to have a small tempco in the 0.5% to 1% per degree region, which will result in a tiny inrush, probably much less than 2x. Quartz tube radiant heaters do not run quite as hot as filament light bulbs, and tend to use an alloy rather than tungsten, so could be anywhere in the none to 10x inrush range.

You have to find out, for your proposed heaters, how hot the heater wire itself is going to run, and what material it's made from. Or just measure the inrush.

  • \$\begingroup\$ And its thermal time constant - whether it takes 10 ms or an hour to reach temperature. \$\endgroup\$ Oct 3, 2017 at 9:34
  • \$\begingroup\$ ... will alter the duration of the inrush pulse \$\endgroup\$
    – Neil_UK
    Oct 3, 2017 at 9:52

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