To create a Closed Loop Temperature Control circuit you need at least 4 things, usually 5:
1) Heater
2) Power Switch
3) Controller
4) Sensor
5) Limit Controller with additional sensor (optional but highly recommended)
Heater: This already exists in your oven or furnace. Determine the power (voltage, wattage, amperage) and number of phases. If it is three phase, determine if you need to switch two legs or three, which can be noted by the number of fuses on the main power leading to the heater(s) in your current configuration (see related post).
Power Switch: From this power and heater info you can select the power switch. This could be an SSR (Solid State Relay), or a mechanical relay (see related post). We recommend SSRs in nearly all applications due to their higher speed and reliability. Mechanical relays are usually rated for only 100,000 cycles. The interface to the control is most commonly "switched DC.”
Controller: The main question with closed loop temperature control for an oven or furnace is, are you going to use "ramp and soak," i.e. do you want to have the oven change temperature automatically over time, or only run at a given temperature for a time and shut off? Essentially, choose capability vs. simplicity and thus lower cost. The next thing is unit size. Most industrial ovens have ¼ DIN controls because they are relatively big with large displays and buttons. This is important because you want your new controller to fit into the same hole as your old one. Other common sizes are 1/8th DIN and 1/16th DIN. If you would like an integrated multi-function process controller with a touch-screen, consider the Watlow F4T.
Sensor: For oven retrofits it is common to go with a cable sensor with a small sheath diameter and 18" of length, 6' of leads, and an ungrounded junction. The small diameter will ensure the sensor will have a fast enough response time and you will be able to locate it within the chamber where the other sensor is now. Most small diameter sensors, such as the Watlow 'MI' (Mineral Insulated), can be bent as needed to work within your existing system. 6' of leads should be enough to get back to the controller, and split leads will wire in directly. For ovens we typically recommend a J-type thermocouple (TC), however you can use a K-type or any suitable thermocouple.
Over-Temperature Safety Limit:
A Safety Limit is always a good idea. This would entail upgrading the control to one with the limit built-in; this feature can be add in nearly every case depending on your control supplier. When adding a Limit option, you need to select a Form A mechanical relay output to be associated with the limit function, and you will need a second sensor for the limit circuit. Sometimes you can use a dual element sensor but we recommend a separate redundant sensor. Lastly, you will need a "Definite Purpose Mechanical Contactor" that will turn on power to the heater circuit when the system is on and in a "safe" condition.
Other considerations:
Fuses - Typically we recommend semiconductor fuses to protect the control components within the heater circuit. In an oven retrofit situation, you probably have a fuse or circuit breaker set up already. If not, make sure you specify a fuse and fuse holder.
Serial Communications
Most modern controls have a variety of communication options as standard. For example, Watlow offers downloadable configuration software for all of their controllers. This makes setting up the control very easy, otherwise you will have to program the unit from the front panel; however, technical help is available if you get stuck. We recommend buying the USB to serial converter to use with the Watlow software. The cost will easily be recovered in time saved compared to manually setting up your system via the front panel.
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