Industrial Temperature Primer

Chapter 5.

Final Control Devices

Up to this point, we have briefly discussed temperature sensors, temperature transmitters, temperature controls, and temperature recorders and dataloggers. One more area deserves our attention in the discussion of temperature and the application of temperature instrumentation products. That is the area of final control devices.

To complete the loop in a closed loop temperature control system, you must have some device that takes the output from the temperature control device and converts it into heating or cooling production.

For electrical heating in industry, that device usually must carry a high current due to the amount of power needed to heat a large process. These processes cannot be controlled directly by the output from a temperature controller since that output is usually limited to a load of no more than 5 amps. The final control device can cover a range from the simplest electromechanical relay to mercury relays, to solid state relays and solid state SCR power controllers.

Electromechanical relays are the lowest cost of the final control devices. The problem with electromechanical relays is the ever switching output of a temperature controller results in a short lifetime to the relay's mechanical contacts. This results in frequent contact replacement which will quickly eat away any cost savings on the initial relay purchase. Mercury displacement relays offer an alternative over the electromechanical relays in that the contacts are hermetically sealed from air so that little, if any, spark is present when contacts are closed due to the displacement of the mercury. Mercury containing devices, however, have come under close scrutiny lately due to mercury being listed as a "Hazardous Material".

Replacing mercury relays with solid state relays is often the answer. Solid state relays offer the distinct advantage of having no moving parts to wear out. Solid state relays are selected based on their particular application. Not only do you need to know the load current requirement, you must also identify how the solid state relay is to be operated. Solid state relays are available for both AC and DC input triggering devices. The AC input solid state relay may be operated directly by the output relay of a temperature controller. The DC input solid state relay is usually operated by a DC input signal (open collector) of between 3 and 32 VDC. Any DC voltage in this range will cause the solid state relay to "close the circuit" to operate the heating device.

Electromechanical relays, mercury displacement relays, and solid state relays have one thing in common. They all switch the power to the heating load either full on or full off. Many applications that require extreme accuracy need what is called "true" proportioning control. True proportioning control requires what is typically called a "power controller". This device is operated by means of Silicon Controlled Rectifiers (SCR) which can be fired a number of ways to meet the requirements of specific applications. SCR Power Controllers normally take a proportional output from a temperature controller (normally 4-20 mADC) and convert it into a proportional control output to the heating device by means of either "Burst Firing", "Fixed Time Based Firing" , "Variable Time Based Firing", or "Phase Angle Firing".

Zero Crossover or Burst Firing provides a proportional output to the heating device by turning "on" for a number of cycles of the AC input (either 50 or 60 Hz) and then turning off for a number of cycles. The proportion of "on" to "off" cycles is dependant upon the command signal from the temperature controller. In other words, if the controller has a 4-20 mA output and the output is at 12 mA, then the power controller would be on for 30 cycles and off for 30 cycles on a 60 Hz power system.

With variable time based controls, the on and off times are still proportional to the command control signal, but the time base changes as a function of the demand. Using the example of the 12 mADC output you would have power on for one full cycle and then power off for one full cycle (50% Demand). If you had a 20% demand, the power would be on for one cycle and off for four cycles, etc. One of the biggest advantages with zero crossover control is that since the SCR fires at only the zero point of the 50 or 60 cycle sine wave, the signal is virtually immune to electrical interference.

Phase angle fired SCR power controllers offer the truest form of proportional control because the power passing through the SCR can be controlled. When the SCR is turned "on" it stays on until the polarity changes (the sine wave passes through the zero point). The turn on point, however, can take place at any point in the sine wave. Therefore, since the turn on point is not zero, but delayed inside the sine wave, then the actual amount of power allowed through the SCR is controlled. Although very accurate temperature control may be achieved with phase angle firing, since the turn on point may be at any point in the sine wave, the signal is susceptible to electrical noise in some applications.

A couple of available features make phase angle fired SCR power controllers even more attractive. Some electrical heaters, such as silicon carbide heaters, change resistance with temperature to such an extent that rapid temperature change will shorten their lifetime. A feature called "Soft Start" in a phase angle fired SCR allows the heaters to warm up slowly by delaying the on time of the SCR and gradually increasing the on time by using less delay in each cycle.

Another important available feature is "current limiting". Current limiting SCR power controllers have a current sensing transformer that will not allow more than a pre-established amount of current to pass through the SCR. This feature also lengthens the lifetime of many types of electrical heating elements. A note of warning here, however. If a short circuit in the heater occurs during the on time of a current limiting SCR controller, the high amount of current cannot be restrained from going through the SCR. For this reason, I2T fuses must be used in line with the SCRs so that they will blow at any time during the cycle.