Cover Story: What is your relay telling you?

Troubleshoot your motors by understanding your relays.

By Mark Pollock, Littelfuse Startco June 20, 2011

A major industrial company recently installed microprocessor-based motor-protection relays to replace the original electromechanical overloads on a number of motors. Wishing to take advantage of all the features of the new units, it enabled the current unbalance function—only to have the relays trip out almost immediately. What was going on?

After doing some investigating, the company found the cause: Pitted contacts on the motor’s starting contactor were reducing the current to one motor phase. Had the problem persisted it eventually would have caused overheating in the motor, potentially shortening its life. Not knowing that the phase currents were unbalanced might have led to needless examination of the motor, which was not at fault, or even a reduction in load to try to reduce the heating. The digital relay made it possible to find the true cause quickly.

Many features of digital motor-protection relays can be used to troubleshoot motor problems, but they may be unfamiliar to operators who are used to simple, traditional thermal motor overloads. This article covers the most common alarms an operator is likely to encounter, their most likely causes, and troubleshooting tips.

The basic protection for a motor, required by most electrical codes, is provided by a relay. Two types are common: an electromechanical device, such as a traditional “overload” relay, and a digital relay. An electromechanical relay is a single function device, but a digital relay typically protects against multiple threats such as overload, phase loss, overcurrent, etc.

A digital motor-protection relay is a more complex beast, but it provides a higher level of protection than the traditional electromechanical relay. Because one device takes all the measurements and can do very fast calculations, complicated parameters can be monitored and acted on intelligently. The digital motor-protection relay can communicate to a control system for monitoring and predictive maintenance, and it has alarms that give descriptive information that can be used to speed troubleshooting. As the prices of digital motor-protection relays have decreased in recent years, more people are using them, often on smaller horsepower motors than in the past. However, because users may not have much experience with these relays, they may not be familiar with the best ways to use them.

Thermal overload

When a relay displays this message about high motor temperature and shuts off the motor, look for changes that may have increased the load, such as a torn conveyor belt, stuck raw material, or failed bearing. Thermal overloads detected by digital relays are not really a result of high temperature. The cause is the motor current exceeding the normal motor current and service factor settings, which is then tracked by the thermal model programmed into the relay.

This model estimates the motor temperature based on the current. Some sophisticated relays use input from temperature sensors on the windings, but often only to “bump up” the thermal model if the calculated temperature is less than what is being measured by the sensor.

Too-frequent starts are another cause of overtemperature, and relays having dynamic thermal-overload capability will protect the motor. A motor built to NEMA standards is designed to provide two starts from cold without damage. To relate this to a thermal model, the motor uses approximately 50% of its available thermal capacity (I2t) with one start.

Therefore, if the motor is interrupted once or twice during a start, it will soon be in danger of damage and the relay should trip. “Thermal capacity” is based on motor specifications entered into the relay by the user and can be customized for fan-cooled motors that do not require the full 50% of the thermal capacity to complete a start.

The advantages of the dynamic thermal model are that it’s more accurate than winding temperature sensors alone, it’s not confined to discrete spots in motor windings, and it reacts faster to sudden changes. Also, because it tracks the motor temperature so accurately, it can keep the operator from restarting the motor when it’s still too hot. In contrast, electromechanical motor overloads cool faster than the motor, which may allow the operator to restart the motor at an unsafe temperature.

In addition, a digital motor-protection relay specifies the “time to reset” so the operator will know when the motor is cool enough to restart. This avoids the frustration of continually checking to see if a restart is permitted, and allows other duties to be performed in the meantime.


This message displays if motor current exceeds a set amount (less than starting current) while in Run mode. It directs operators to look for a problem with the load, rather than with the motor. The setpoint for this function (like all relay setpoints) can be password-protected, preventing operators or others from changing it.

With jam protection, the relay must be smart enough to know when the motor is in Startup mode, when it temporarily disables the jam protection. Without this ability, one must specify a time delay after which one assumes the motor has started. Jam protection can detect a mechanical jam in the motor (imagine a crowbar jamming the rotor) or a severe overload that has stopped the motor and will cause extreme damage in a short time.

By the way, some motor-protection relays have a Reduced Overcurrent mode that is separate from the Jam mode. It can be engaged to temporarily reduce the overcurrent setpoint when performing maintenance in a motor circuit when the motor is running. Because it has a fast response time, it may reduce arc-flash energy released during a short circuit.

Earth leakage current

If the relay reports earth leakage, also called a “ground fault,” the cause is likely to be a short in the windings or input wiring, or worn or melted insulation. When the motor is de-energized, check the resistance of the motor wiring. If that does not show a problem, then cautiously check the input wiring for a ground fault, being sure to wear the proper personal protection equipment.

Many plants are upgrading to resistance grounded systems to reduce the risk of arc-flash. Many traditional ground-fault relays were not designed for this type of system and assume large ground-fault currents. For example, the built-in ground fault detection on most variable frequency drives (VFDs) will not trip in a resistance ground system because the setpoint is not low enough. To detect ground faults in a resistance grounded system, newer digital motor-protection relays have the option to accept inputs from sensitive current transformers.

Another method of detecting a phase-to-ground fault is using an insulation monitoring relay. Traditional ground-fault relays detect a fault by measuring the current with a transformer, and some damage to the motor may occur before the relay trips.

An insulation monitoring relay prevents the motor from starting if it detects a ground fault, avoiding that potential damage. However, on a grounded system, the insulation monitor cannot be used while the motor is energized.

Other relay alarms

Electronic motor-protection relays have many alarms that can assist with troubleshooting. Here are some other common messages:

  • Current unbalance—An unbalance between the three phases. A typical cause is if you have a contactor and one of the contacts is pitted and reducing current on one phase.
  • Phase Loss, Current—One phase has zero current. The two remaining windings have to work harder and get hotter, which can cause damage quickly.
  • Phase Reversal, Current—This happens if a worker connects the wrong phase to a terminal.
  • Undercurrent—Indicates loss of load or a pump run dry.
  • Undervoltage or Overvoltage—If voltage is too low, it can damage the motor. If voltage is too high, it will stress the insulation.
  • Voltage Unbalance—Indicates one phase has a voltage different from the others, often caused by power supply or transformer issues.
  • Underfrequency or Overfrequency—The frequency of the input voltage (VA) is below/above the setpoint.
  • Failure to Accelerate and Underspeed—Uses input from a tachometer to provide information about the motor and load, such as knowing when the motor is rotating too slowly.
  • Motor Differential, Current—Uses additional current transformers to detect small current losses in the windings that could lead to ground faults.
  • Summary

    Plant managers can save troubleshooting time and better protect motors by using digital relays. As relay prices fall and as plants remain understaffed, managers have a good argument for upgrading older thermal-style overloads with modern devices.

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