A/C Fan Startup Control


Room temperature can be controlled by using multiple A/C fans. The following is an example of using the UT55A Digital Indicating Controller with its ladder sequence function to control the operation of multiple A/C fans.

Customer Requirements

  • Run multiple A/C fans to lower the room temperature in the hot summer months.
  • Need to prevent all fans from running at the same time because the inrush current trips the breakers

Yokogawa's proposed solution

Figure 1. A/C fan startup control

Figure 1. A/C fan startup control

With 8 A/C fans

The UT55A can come with up to 8 alarm outputs that can control the fans. Ladder sequencing is build in, allowing you to use a timer or other such function to set up a time delay on each output.

* Ladder programs must be created on the LL50A Parameter Setting Software and loaded onto the UT55A.


  • Set the setting (SP) value to the temperature at which you wish the fan to turn on (e.g. 23°C).
  • Set alarm outputs 1 through 8 so that the 8 fans will turn ON when the room temperature reaches the SP value.
  • Set a delay (of 5 seconds, for example) between alarms so that all fans do not turn on at the same time.

User benefits

A single UT55A can automate startup of multiple A/C fans—lowers wiring and components costs and saves space (timers and compact PLC's no longer necessary).

Related Products & Solutions

  • LL50A

    The LL50A Parameters Setting Software is designed to build and set parameters, ladder programs, and the like of the UTAdvanced digital indicating controllers from a PC. The tuning and monitoring of ladder programs are possible during communication with the controllers.

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  • Mid-level UT55A/UT52A

    The UT55A and UT52A temperature controllers employ an easy-to-read, 14-segment large color LCD display, along with navigation keys, thus greatly increasing the monitoring and operating capabilities. A ladder sequence function is included as standard. The short depth of the controller helps save instrument panel space. The UT55A/UT52A also support open networks such as Ethernet communication.

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