Is it ok/safe to put both a resistive load (heating elements) and inductive loads (servo motor) on the same leg of a 240VAC transformer? I have some electrical knowledge but it has been a while since I've dealt with servos, and heaters. I sort of recall some caution about mixing both resistive and inductive loads. My heaters will be be controlled by zero crossing SSRs and temperature controller. The servos will be controlled by by a servo controller.

  • \$\begingroup\$ depends on load ratio . VAR load often needs XFMR to be derated 30% but with surge currents on motors, derated sometimes more, \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 15 '17 at 22:04

Mixing resistive loads with inductive loads should cause no special concern. It is important to remember that the total load current for the transformer is not the arithmetic sum of the inductive and resistive currents but the vector sum. Inductive loads are rarely purely inductive, so you need to understand the effect of power factor.

What might be of more concern is the harmonic content that can be expected with any electronically controlled load. That should not be an issue with a zero-crossing temperature controller. It might be an issue with the servo controllers depending on the design. The transformer would need to be derated or have a sufficient "K factor" to avoid overheating due to harmonic content. The harmonic content adds a third dimension of vector addition. The inductive and resistive content helps in a way. More resistive and inductive current means the harmonic current is a smaller percentage of the total.

With various types of electronic power conversion equipment on one transformer there is always a chance of one system generating noise that interferes with other systems. Quality equipment should be designed avoid excessive emissions and accept reasonable emissions from other equipment. However evaluating the equipment may be difficult.

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  • \$\begingroup\$ "It [harmonic content] might be an issue with the servo controllers depending on the design." Could you expand on "depending on the design"? I have two 1KW servo drives that run in velocity mode, simply turning two rollers at a constant speed. Two more servos of ~400W that only move occasionally (once prior to start of rollers) for positioning. \$\endgroup\$ – JoeChiphead May 17 '17 at 12:20
  • \$\begingroup\$ A servo controller is some type of some type of electronic power conversion device. All such devices are non-linear loads and thus have distorted input current. The harmonic spectrum pf the current depends on the circuit design and any built-in harmonic mitigation features. \$\endgroup\$ – Charles Cowie May 17 '17 at 21:59

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