AMAZON multi-meters discounts AMAZON oscilloscope discounts
In many applications such as electrical heating systems, the heating element is controlled by a contactor that has only two states (on or off). The contactor is used as the control device instead of an analog controller because of the expense. Since the output of the process controller is analog (0-100%) and the heating contactor is on/off, the controller must be set up to provide the output signal to the contactor ' as a ratio of time on versus time off. This type of control is also called time proportioning.
The figure below shows an example of this type of control. From this figure notice that the coil of the heating contactor is connected to the controller, and the heating element is connected to the contacts of the heating contactor. Anytime the contactor coil is energized, full voltage is sent to the heating element and it provides 500 W of heat to the furnace. Anytime the contactor coil is de-energized, no voltage is sent to the heating element and no heat is produced.
The on/off type of control for the heating element can take advantage of the 0-100% analog output of the controller by selecting the ratio control feature of the controller. The ratio control feature uses ratio of on-time to off-time to get the effects of the analog controller. E.g., if the controller determined that the 10 seconds is generally used, which means that for an output of 50%, the heating contactor would be energized for 5 seconds and de-energized for 5 seconds. If the stroller determined the output should be 70%, the contactor would be energized 7 seconds and de-energized for 3 seconds.
The contactor will be continually turned on and off during each 10-second le as determined by the amount of output percentage. The heating effect from heating element is similar to that of an element controlled by the more expensive slog amplifier. This type of control is used extensively in the heating systems for slit presses.
Above: (a) Diagram of a 100-VAC heating contactor coil (HC1) connected to a controller and its contacts controlling a 208-VAC heating element. (b) The timing diagrams for a 50% duty cycle and a 70% duty cycle. Notice the on-time is 5 seconds and the off-time is 5 seconds for the 50% duty cycle, and the on-time is 7 seconds and the off-time is 3 seconds for the 70% duty cycle.
PREV: Proportional Band
NEXT: Types of System Response