Continuing my discussion of how to troubleshoot temperature control equipment, in this issue I will look at more potential problems and their possible fixes.
Problems With Electrical HeatingSuppose the temperature is well below setpoint, but heat remains off. Check the heater circuit for loss of line voltage, blown fuses, tripped breaker or heater open circuit. If the contactor coil is energized, check for failure to operate or burned out contacts. If the contactor coil is not energized, power could be failing to get to the controller's internal relay. Alternatively, that relay could be failing to close or have defective contacts.
Potential Trap. When the control relay is of a solid-state type - for example, a triac switch - you cannot check its continuity with a DC continuity meter. Triacs need an AC voltage and at least a 50 mA load to check their operation.
Another Potential Trap. Suppose you disconnect the contactor coil and use a voltmeter to check whether a controller's internal relay or triac is passing voltage through and blocking it when required. You probably will find a voltage there all the time. Why? A snubber (spark and contact erosion supressor) often is fitted internally across the triac or relay contacts. This can pass a tiny current - enough to show full line voltage on your meter when the relay or triac is open, but nowhere near enough to pull in a contactor coil. Even without the snubber, a triac switch could leak enough current to show on a modern high impedance voltmeter. A 120 V, 5 W filament lamp (night light) works well as a tester.
Another Potential Trap and Serious Hazard. In place of mercury or magnetic contactors for heater control, more processes are using solid-state relays. They come in two versions, designed for either low voltage logic, typically 10 VDC, or 120 VAC logic at the input terminals. The 120 VAC version from some manufacturers has an input impedance of some 120 kA. The result is that a controller, supposedly off, with a triac or relay output and a snubber fitted, will pass enough current to keep the solid-state relay switched on, overheating the process.
If you cannot convert your solid-state relay to low voltage DC logic, connect a 0.47 Kf, 400 V capacitor (not electrolytic) across the relay's 120 V input terminals. It will divert enough current to ensure that the controller can switch the relay off. Some solid-state relay manufacturers recognize this problem and make the input impedance low enough to defeat this hazard. The best solution is to use a solid-state relay designed for low voltage DC logic operation and a controller with a matching DC logic output.
Other Potential ProblemsTemperature Abnormally High, Heater Current On. Check for voltage on the contactor coil or solid-state relay logic input. If absent, the contactor is locked on or its contacts welded to a short circuit condition. Change it.
If coil voltage or logic input voltage is present, change the controller - its internal relay or logic signal is on when it should be off. Or, if you are using solid-stare relays, check for the potential trap and serious hazard previously described.
Temperature Indication Extremely High, Process Not Heating, Current Off. With this problem, the controller may show broken sensor message. The most probable cause is open circuit thermocouple or wiring. The resistance measured from the controller input terminals normally should be under 20 A. Disconnect the thermocouple wire from the controller and check continuity all the way back to the thermocouple itself. If the controller is good, it will indicate around room temperature when you connect a wire link or double alligator clip across its thermocouple terminals. If the thermocouple and wiring are good, change the controller.
Band, Cartridge and Clamped Heater Problems. Ensure clean heater mating surfaces, well clamped down (snugly inserted, in the case of cartridge heaters) for good heat transfer. Poor thermal contact or air gaps cause over-hot areas on the heater and premature local burnout. Use the largest contact area heater you can get into the space. The lower the watts per square inch, the longer the life. Tighten down routinely as thermal creep relaxes the contact pressure.
Heater connection wires are brittle and can break or disconnect during maintenance or heater changes. Corroded or dirty studs, nuts and washers spark, buzz and lose contact. Keep them tight and replace with bright new hardware before they get beyond repair.
No matter what the problem, the most important thing is to exercise self discipline. Change one thing at a time and wait. When troubleshooting, listen to everybody and believe nobody.