Programming in the appropriate motor state is important to prevent unexpected production losses when network communication issues arise.
By Bill Martin, product manager, Electronic Overload Relays, Rockwell Automation
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 typically were hardwired to output relay cards or analog output cards on these early type of control systems.
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 doesn’t need to be located close to the motor starter,
- Less output cards,
- Greatly reduced motor control wiring,
- Easier troubleshooting from a maintenance perspective, and
- More diagnostic information available 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.
Address Network Issues
However, one scenario often overlooked is, “What happens if the communication network fails?”
If this question isn’t addressed before deploying the new networked-based motor control system, manufacturers and process companies could experience unexpected production losses.
A network issue can be caused by:
- Maintenance staff accidentally removing or cutting a network communication cable.
- A component in the network infrastructure failing (such as 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 “off,” “on,” or “hold last state.”
This allows some flexibility to make sure specific motors can remain running and 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 option to consider is 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.
Learn more about networked-based motor controls from Rockwell Automation.
The Journal From Rockwell Automation and Our PartnerNetwork? is published by Putman Media, Inc.