Large 480V heaters vs motors

[Rson]

I know with larger motors you do not want them to start with a contactor above 30HP or so because they will 'sag' the power in the plant and the inrush current can be insane. We typically put anything at or above 30HP on a VFD or Soft Start unit.

Is there any such worry about a large 480V heating element? They are resistive loads, so there should be no worry about inrush. Is there a certain point you would want to switch to an SCR controller even if you want 100% of the heat all the time?

A 60kW heater, for example, would pull just over 72A at 480VAC. Is a simple contactor still a suitable way to turn it on and off, or is there merit in controlling this with an SCR instead?

 

[parky]

Don't really think it matters, although contactors being mechanical SCR's can be more reliable with the exception of shorts causing damage. If these are not being switched at any great frequency then contactors are a better idea, an SCR can go short especially on a short i.e. somebody drops a spanner across the heater, so providing it is fused correctly either is good.

 

[Tom Jenkins]

In my mind, the only reason an SCR would be preferred would be if the switching frequency was high. If rapid and frequent on/off cycles are employed with mechanical contactors they would beat themselves to death in short order.

The downside to using SCRs for a sustained resistive load is the heat that must be dissipated. The SCR should be fitted with a heat sink, and that heat would go into the panel. You can use a bypass contactor that pulls in after the SCR has switched on, but then I see little point in the SCR in the first place.

 

[I_Automation]

I almost always use mechanical contactors unless I want to vary the power to the heater. EDIT: for high amp I use mechanical mercury coontactors.


Because of the possibility of frequent cycling I always put a ant-chatter control.



When the input comes on it stays on for at least 10 seconds and when it turns off it stays off for at least 10 seconds.


I also do the same for solenoid valves to fill or empty a tank in case the float is bouncing on an unstable liquid surface.

 

[BOWILLY]

The heater is a resistive load. It won't affect power factor. Ramping the heater to set point from rest will prolong the life of the heater.