What constitutes a control system? Each control type and power-switching device has advantages and disadvantages, including accuracy, useful life, heat generation, derating, electrical noise and heater life. An electric heater consists of a coiled nichrome wire surrounded by compressed magnesium oxide. Long cycle times on power-switching devices produce temperature changes in the nichrome wire. If the wire is constantly expanding and contracting, it eventually will break due to thermal stress. Also, most power-switching devices must be derated. As ambient temperature rises, the current-carrying capabilities of power-switching devices decrease.
Every process requires controls. Among those used on your line are temperature and power controls. Here's a look at a few commonly used.
Temperature Control TypesBulb-and-Capillary Thermostats. Relatively easy to operate, a bulb-and-capillary thermostat simply requires the user to set the dial to the required temperature. As the temperature increases, fluid in the capillary expands. The fluid pushes against the bellows and turns the switch off near the desired temperature. As the fluid cools, the pressure on the bellows reduces and the switch turns on, providing a control voltage to the coil of either an electromechanical contactor or mercury-displacement relay (MDR). This is a slow process, which reduces system accuracy.
Electronic PID Temperature Controls. A PID control accepts a thermocouple or RTD input from the process and provides a control voltage to an electromechanical contactor or MDR. Some-what complicated to program, most modern temperature controls have an autotune feature that enables the control to program itself to match process specifications. It also can be purchased with an AC or DC output, which provides the gate signal to turn on and off solid-state relays, or a 4 to 20 mA signal to control burst-fired SCRs.
Power-Switching DevicesElectromechanical Contactors. With this device, control voltage from the temperature control pulls in a spring-loaded coil that brings the contacts together, allowing voltage to flow to the heater. These contactors should not be cycled faster than once every 30 sec. Due to carbon build-up on the contacts and wear on the coils, they have a limited life. Accuracy is poor due to the cycle time. Electrical noise is generated on the line and heater life is short.
Mercury-Displacement Relays. MDRs operate much like an electromechanical contactor. The devices use encapsulated mercury as the contact media. They have a 5 sec cycle time for fast response and slightly better control. The enclosed coil generates heat up to approximately 23 W per three-pole contactor. Use the manufacturer's derating curves for higher-than-normal ambient temperatures.
Many plants do not allow mercury in their facilities, which limits MDR usage. Electrical noise is generated, but heater and relay life are longer than an electromechanical contactor.
Solid-State Relays. Competitive with MDRs, solid-state relays have a cycle time of 1 sec and provide good control accuracy. They generate heat at 1.5 W per A switched per leg. Remember to take this into consideration when using them in nonvented electrical enclosures. Use the manufacturer's derating curves for higher than normal ambient temperatures. Little or no electrical noise is generated and heater and relay life are long.
Burst-Fired SCRs. The cycle time for a burst-fired SCR can be as fast as 16 ms. SCRs provide tight temperature control but also generate heat. Therefore, higher current units must be fan cooled. This must be taken into account when designing and sizing an electrical enclosure. Little or no electrical noise is generated and heater and SCR life are long.
As a general rule of thumb, if you have a noncirculating application and tight control is not required, you can use thermostats and contactors or MDRs. If, however, you have a flowing media, a PID control with solid-state relays or SCRs will be required to provide the type of control necessary for your application.