- August 12, 2014
By Kip Larson, Littelfuse
This article discusses today‚Äôs motor protection relays and explain how they can be used not only to protect motors but also to simplify integration, diagnose problems, and provide control variables.
By Kip Larson, Littelfuse
The usual result of an overloaded motor is that its thermal overload opens and shuts it down — and that’s it. Because the overload does not communicate with the control system, there is no warning that the motor is getting hot, and no specific notice that it has stopped and why it stopped. Fortunately, there are motor protection relays that do considerably more. They monitor current and other parameters and report everything to a PLC, DCS or SCADA system via a standard network before a shutdown occurs. These intelligent products do a better job protecting the motor. This article will discuss today’s motor protection relays and explain how they can be used not only to protect motors but also to simplify integration, diagnose problems, and provide control variables. How a thermal overload works The traditional way to protect an electric motor from damage due to overloading is to install a thermal overload, also called a thermal overload relay. There are two kinds of these — bimetallic and eutectic, both activated by heating coils wired in series with the motor windings. A thermal overload is designed to approximate the thermal characteristics of the motor it is supposed to protect, but cannot do so very accurately for several reasons. For one thing, thermal overloads are affected by the ambient environment where they are located. This may lead to nuisance tripping in hot environments, and reduced protection in areas that are cooler than the actual motor they are protecting. After tripping, a thermal overload may cool down faster than the motor, allowing a restart while the motor is still too hot. This can damage the motor before the overload operates the second (or third) time. A thermal overload is basically dumb—as in voiceless. It can indicate a problem with the motor only by turning it off. Until that time it gives no clue that it (and the motor to which it is connected) is getting hot and will trip soon. A thermal overload is also dumb—as in brainless. It responds only to high currents passing through the motor windings, but cannot do anything about any of the other electrical problems that can affect a motor. As a crude device, it suffers from poor accuracy, and its timing to trip can vary widely. Clearly, proper protection of a motor requires something more. Enter the smart overload relay A smart overload relay is an effective and relatively inexpensive way to protect a motor against many more threats than simple overload. It responds to normal overloads just like a conventional motor overload, but it also responds to conditions that a conventional overload cannot, including undervoltage, phase loss, voltage imbalance, phase reversal, undercurrent (as from a pump running dry) and other problems. It communicates what it detects to a remote operator display or a control system, enabling operators to take action before there is a shutdown and resulting downtime. But what if there is a problem that does not result in overcurrent? What if current is too low, which can be caused by a pump running dry? By monitoring undercurrent, a smart relay can indicate this condition, which may be caused by pump mechanical wear, changes to the process, broken belts, loose couplings, or loss of flow for any reason. This “load loss” detection is useful in compressors, conveyors, or other loads driven by a motor. In contrast, a thermal overload will ignore an undercurrent condition, even though a lack of flow may be disrupting a process and putting equipment in danger. A smart overload relay can also detect a sudden increase in motor current that is less than an actual overload, but may warn the control system or operator about a problem with the process — for example too much material on a conveyor belt feeding a grinder. This information can be used as a control input to slow down the conveyor until the material has been digested. The relay can also detect the sudden increase in current of a stalled motor, or a current unbalance, and warn operators to take action long before a thermal overload would have reacted. Now the same information could be gathered by installing current and voltage sensors that communicate to the control system via a 4-20 mA loop, but that takes extra equipment (a set of sensors on each phase) and extra work: convert the signal to digital and then scale the reading using programming. The control engineer must do this for each phase. What’s more, because the dynamic range is small, the signal has poor resolution compared to the digital resolution of an electronic relay. Smart overload relays simplify integration in other ways. They are programmable, which means that the same model number can be purchased for multiple motor voltages and horsepowers, and each unit programmed for the particular motor it will be protecting. There is no need to buy a different protective relay for each, or to stock an assortment of heating coils (and risk the possibility of not having the right one when it’s needed). Smart overload relays make for better process management, as they make it possible to anticipate failures and shutdowns by keeping track of the condition of the motor, the machine and the process. They do this by monitoring and communicating the data that they gather on the motor’s voltage, current and power to a control system. This information can be provided to a control panel display, a human-machine interface, a programmable logic controller, a SCADA system, or a full distributed control system. Different network protocols are available such as Modbus RTU, Modbus TCP, DeviceNet or Profibus for easing integration. Summary Today’s motor protection relays provide better motor protection than old-style thermal overloads, and they can provide important information on the condition of equipment and of the process itself — information that can be used to prevent unnecessary shutdowns and even improve process operation — all for just a few hundred dollars per motor. That’s a pretty good deal. About the author Kip Larson is Director of Product Technology, Electrical Business Unit, Protection Relays, Littelfuse. He has more than 20 years of industrial electronics product design and application experience. He received a B.S. in Electrical Engineering from South Dakota School of Mines and Technology.Learn More
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