How to Control an Electric Motor When the Communication Network Fails

Controlling Electric Motors When the Network Fails

When automation control systems were first introduced in the 1980s, manufacturers used a motor control strategy that involved multiple programmable logic controllers or distributed control systems to directly control contactor coils, soft starters and variable frequency drives. 

These devices were typically hardwired to output relay cards or analog output cards on these early type of control systems. 

Today, these legacy control systems are starting to age, so manufacturers and process companies are replacing their old hardwired motor controls with networked based motor controls. A centralized control system can issue start and stop commands via a communication network. 

Advantages of networked based motor controls include:

  • The automation control system does not need to be located close to the motor starter
  • Not as many output cards are needed
  • Motor control wiring can be greatly reduced
  • Troubleshooting is easier from a maintenance perspective
  • More diagnostic information can be reported back to machine and process operators

From a system integration and system maintenance perspective, a networked-based motor control system sounds ideal for process applications or manufacturing applications that use a lot of electric motors.  As manufacturers and process companies upgrade their motor control systems to networked based motor control system, they are realizing the benefits of this new motor control methodology.

However, one scenario that they often overlook is, “What happens if the communication network fails?” 

If this question is not addressed before the new networked based motor control system is deployed, manufacturers and process companies could experience unexpected production losses.

A network issue can be caused by:

  • Maintenance staff accidentally remove or cut a network communication cable
  • Component fails in the network infrastructure (i.e.  a network switch for an Ethernet network)
  • Adding a device with the same network address of another device already on the network
  • Electrical noise from the motor control technology interfering with network communications

Most networked-based motor control systems will go to an “off” state when network communications are disrupted, but is that the right motor state for a specific process or application?

When choosing a network-based motor control system, make sure that the motor control system can be programmed to go “off,” “on,” or “hold last state.” 

This allows some flexibility to make sure that specific motors can remain running to not ruin a process or batch if the network is disrupted.  

Some networked-based motor control systems will hold the last motor state for a specified period of time until a secondary control system takes control; otherwise, the motor will go to a programmed state (“off” or “on”) after this specified time period until an automation control system establishes communications again. 

Another feature to consider is being able to override the state of the electric motor manually at the electrical control panel. Some networked-based motor control devices allow local/remote (hands/auto) selector switches to be wired into the motor control system, or they have an integrated or panel mounted human machine interface to allow a manual override. 

Finally, some networked-based motor control devices have programmable control logic. This provides the maximum flexibility in which an electric motor could be programmed to finish its process and gracefully shut off when the communication network is disrupted instead of ruining the batch.

If you are considering upgrading your motor control system to realize the benefits of using network based motor controls, choose the appropriate intelligent components that have the features to execute your plan if the communication network fails. 

If you are interested in learning more about the latest in networked-based motor controls, visit our web site.

Bill Martin
Posted October 17, 2016 By Bill Martin, Product Manager, Electronic Overload Relays, Rockwell Automation
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