I am a physicist by training but have been tasked with designing a heating controller box for a custom application that will be used in an industrial setting. I would like to make sure that I am following best practices before I follow through on assembling the control panel.

I want to simultaneously drive up to 4x 3 phase - 480v - 3kW heaters (configured as a 3-Wire delta) using an on-off cycle rather than actual PID to save switching lifetime and complexity. Based on my calculations the per phase load of each heater should be about 3.7A. I was planning to use contactors to switch each phase and I wanted to make sure that it would be kosher to drive all 4 contactor relays in parallel from the AC Relay output of my selected temperature controller.

The controller SL4848-RR is rated for 5A AC (240v) output and the contactors WDP25-3L-230 have a typical input power of 10VA@230V and inrush of ~50VA. This would put the peak power draw at <250 VA Peak to run the controller and and contactors off of my control transformer PH350MQMJ which is rated for 350VA at 240V. If I place the coils of the contactors in parallel I expect the total resistance to drop by a factor of 4 and the current to be shared equally across the coils of each contactor at the peak current of 200VA and running current of 40VA. Is this naive to assume? Should I explore a different configuration? I also assumed I don't need a flyback diode because the coils are AC driven, but this may also need to be revised.

For extra context:

I have selected 20A FAZ-B20-3 input breakers for each phase input and I will be placing supplementary breakers FAZ-B6-3 on the high side of each contactor with a per phase rating of 6A (3.7A/80% for thermal overhead and derating for worst case T of 50C). All breakers were selected with B type curves as these loads are 100% resistive. The control transformer will be powered on two of the input lines.

TIA and don't hesitate to ask for more information.

  • \$\begingroup\$ To what accuracy do you want to drive the temperature? What is the thermal load? \$\endgroup\$
    – Voltage Spike
    Sep 17, 2019 at 16:32

2 Answers 2


To reduce the risk of EMI pulse noise on contact wire inductance and abrupt current break and nuisance effects on uC, CPU , PC’s etc , I recommend you use ZVS turn on SSR’s and the Triacs have the benefit of zero current turn off.

Then you can gauge the loads within the current rating of the SSR. Heatsinks may be useful if approaching the limits.

E.g. https://www.omega.com/en-us/control-and-monitoring-devices/relays/solid-state-relays/p/SSRL240-660.

The closer the sensor is sunk to the heater, the shorter the on/off cycle time. Forced air can also affect this and too close may cause porpoising or motor-boating. With excessive rapid cycles or..not.

Use twisted pair for all wires and shielding for improved EMI reduction.


You would normally use just one contactor and connect the heaters in parallel.

A properly designed industrial temperature controller should have no trouble driving a 3-phase contactor. Lifetime will be somewhat limited because of the coil inductance causing contact erosion, and many contactors are going to be bad, even if you add snubbers or MOVs (the latter not recommended). I would consider using SSR drive output controller and a pilot SSR to drive the contactor or contactors.

Be careful to note their (temperature controller) rather severe limitation of requiring an ungrounded T/C junction, which can greatly impair control performance, depending on the thermal design.

Time-proportioning PID is "real" PID, by the way. You would normally have a cycle time of around 20-30 seconds, so you can predict contactor lifetime in advance. For this reason, time proportioning is usually inappropriate for IR heating, but works very well for most other kinds of thermal controls.

You can also try solid state contactors, but they are far more likely to fail randomly and tend to fail "on", which is sometimes undesirable.

  • \$\begingroup\$ Would you not recommend the additional control available by using the four heaters separately for a proportionate heating power? Although I suppose the question is under-specified in this case. \$\endgroup\$ Sep 17, 2019 at 18:49
  • \$\begingroup\$ @AndrewMorton If you have four zones and four temperature controllers, sure. Follow the manufacturer's recommendations for auto-tune in that situation. Or maybe you need a lot of range in the temperature setpoints. \$\endgroup\$ Sep 17, 2019 at 18:51
  • \$\begingroup\$ I don't think I worded that very well - I meant it more like heating one area with a power of 3/6/9/12 kW. \$\endgroup\$ Sep 17, 2019 at 18:54
  • \$\begingroup\$ You can do that with breakers too, if the control is manual. \$\endgroup\$ Sep 17, 2019 at 18:55

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