Noise creation by Solid-State Relay controlling resistance heater 2

[PaulKraemer]

Hi,

I have an application in which I will be using a solid-state relay with a closed-loop temperature controller to regulate the power to a resistance heater. The voltage will be 208 VAC single phase and the heater resistance will be 7.2 ohms, so the expected current is 28.9 amps when the heater is powered. The temperature controller implements a PID loop that determines the appropriate power level (between 0-100%) to be applied to the heater based on the difference between setpoint and the measured temperature. The output to the heater (through the solid-state relay) is time-proportioned, with a default time base of one second. With this being the case, the SSR repeatedly turns ON and then OFF every second, with the ON time (fraction of a second) determined by the calculated power level.

I have used this control method many times over the years. I have always used individual wires (of the appropriate ampacity) for the heater power circuit. I have never used shielded cable. While I have avoided running these wires with circuits that I consider very sensitive, I have often run them in the same conduit as encoder cables, although my encoder cables are always shielded. In these conduits, I have also included wires used to switch solenoid valves and to bring signals from prox switches into PLC inputs. I have never run into problems that seemed to be noise related.

I am working on a new project in which I will be using a VFD. From what I've read, VFD's are known to create noise and it is generally recommended that one uses a high-quality shielded VFD rated cable so as to avoid noise related issues. This made me wonder how a heater control circuit like I've described above would compare to the power cable that connects a VFD to a motor in terms of how much noise it is likely to emit.

On my current project, I am just wondering if I should take more precaution in keeping my heater power wires separated from other circuits than I have in the past.

Any advice will be greatly appreciated.

Thanks in advance,
Paul

 

[waross]

The VFD will be doing the same thing, only several thousand times faster.
You will be switching a sine wave; the VFD will be switching a square wave.
If the SSR is using zero crossing turn-on, therewill be less issues.
Square wave switching or mid cycle switching has a steep wave front that tends to excite high frequency resonance.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[PaulKraemer]

Hi Waross,

Thank you for your explanation. The SSR I'll be using will be using a zero crossing turn on.

I'll be building a new control panel to replace one that has been in use in a machine control application for about 30 years. The main reason for doing this is just to end our reliance on components that are obsolete and that would be very difficult to replace should they fail. The SSR in the new control panel will be performing the same function as the SSR in the old control panel.

My plan is to detach the field wiring coming from the machine into the old control panel, replace the old control panel with the new one, and then reconnect the same field wiring to the new control panel (assuming we don't find any wires that are in questionable condition). The wires for the heater power circuit are individual wires without shielding, and they are routed with some other wires and cables, but not with anything I consider to be very sensitive. Being we don't seem to have any noise related issues with the old control panel, I don't think I'll go through the trouble of replacing the heater wiring with a shielded cable. I'll do this only if we find it to be necessary after testing.

I appreciate your help.

Best regards,
Paul

 

[jraef]

You really can’t compare VFD outputs with SSR outputs, the technologies are very different.

VFD outputs are high speed pulses of DC in a PWM scheme to emulate AC in an inductive load. In that process they are creating significant EMI and RFI that must be contained. Think of the output conductors of a VFD as being local broadcast antennae for a really bad radio station. The shielded cable or steel conduit is there to create a “Faraday cage” around those conductors to keep that crappy “music” from showing up on your other equipment. In addition, if there are other AC circuits in the same raceway, the VFD outputs will be operating at different frequencies from different origins, so none of the magnetic fields created around the conductors will be in synch with any others, therefore there is no cancellation effects to prevent mutual inductance and you will end up with added voltage signals on all of the cables. So again, the separate raceways and/or shielding prevents this.

SSRs that use zero-cross control are switching AC in bursts of complete sine waves, always switching at the zero cross point so as to AVOID switching transients and line notching. None of the issues described above will relate to the output conductors in this case.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[PaulKraemer]

Thank you Jraef for the great explanation - the difference is now very clear to me. I appreciate your help!

Best regards,
Paul

 

[che12345]

Mr PaulKraemer (Electrical)(OP)15 Apr 23 15:45
"....I am working on a new project in which I will be using a VFD. From what I've read, VFD's are known to create noise and it is generally recommended that one uses a high-quality shielded VFD rated cable so as to ...."

There are numerous learned advice posted by the readers. I look at it in a different angle.
Why change to VFD instead of SSRs that use zero-cross control are switching AC in bursts of complete sine waves, always switching at the zero cross point so as to AVOID switching transients and line notching?
For heater control, you only need to vary the voltage, frequency is not involved. I don't see any value added by using VFD, but with addition cost. If it is a marketing gimmick, then that is "commercial" not "engineering" with a different intention.
Che Kuan Yau (Singapore)

 

[jraef]

There are a few specific types of heating elements, such as quartz lamps, that exhibit very high positive temperature coefficient (PTC) of resistance in which the resistance, when cold, is very low, resulting in high initial current, high enough to do damage. Typically for these, you must use a phase angle controlled SSR or SCR controller, but that then introduces switching noise. The solution, when noise is a concern (such as on semiconductor manufacturing equipment), is to use a “hybrid” SCR controller that starts off as phase angle control in the beginning and then changes to variable time base zero cross once the element is at temperature. But even that has its limitations in controlling initial current and in a rare few instances, the solution has to be an inverter controlled source so that the long time current limiting capability of using PWM control can be taken advantage of. It’s somewhat specialized though, I’ve come across it only twice in the past 30+ years, I think it was on molybdenum bead heating elements (I can’t remember for sure any more).


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden