Today’s critical manufacturing operations that involve components such as control valves, pumps, fans, heaters, or conveyor systems rely on fast and precise switching. Upon start-up, the electric motors used in these applications can draw up to 6x the full load current used during normal operation. The larger the motor is, the more inrush or demand it places on the electric utility’s distribution system when it starts.

In addition to kilowatt-hour (kWh) meters that measure total energy consumption, utilities typically use a demand meter to measure a commercial customer’s power usage rate.

Power factor is the ratio of the real power flowing to the load over the apparent power in the circuit and is usually expressed as a decimal between 0 and 1.0. The ideal power factor is 1.0 or “Unity.” When a commercial customer’s power factor goes below 0.95, utilities often add a surcharge to the customer’s bill. The meters are most affected by the large inrush currents that electric motors draw when they start. Consequently, motor loads are a prime cause of inductive or lagging power factors.

Most plants have numerous motors in place. If all of these motors started simultaneously, an enormous amount of energy would be demanded upon start-up. To prevent this excessive energy draw, most plants use a sequential motor start-up process.

During a manufacturing process, the cycling of start-ups and stops can be random. Eventually, however, it is possible that two or more large horsepower motors will start at or very near the same time. When that happens, the large inrush currents will increase and draw more energy demand. This can lead to higher energy expenses for that month, even if that level only existed for a moment.

Other problems that can lead to increased current draw include:

• Clogged filters in a pump application
• Excessive product loading on conveyors
• A mechanical bearing starting to seize due to contamination, wear, or end-of-life condition

These issues can exist for long periods of time without stopping the motor or alerting the plant manager that there is a problem. This can lead to significant energy waste, decreased efficiency, and higher utility costs, all without the utility customer’s knowledge.

Measuring current

When analyzing the motor load’s physical variables, the motor current I for low and medium loads hardly changes. The magnetic saturation makes this effect especially noticeable for small horsepower motors. The current only significantly increases in the maximum load range. Classic motor-protection relays and motor-protection circuit breakers use current-dependent bimetallic, eutectic alloy, or solid-state overload relays to evaluate this range. This means that they can protect the drive motor from an overload condition once sufficient current has passed through them.

The curve of power factor cos φ manifests an almost opposite characteristic, changing the most in the motor’s lower load range. The power factor only marginally changes if the motor power increases. With this characteristic, power factor cos φ is suited to detect load changes when the motor is close to no-load operation, and in turn, protects drive elements against underload conditions. Both the power factor and the motor current are significantly influenced by voltage fluctuations, which can cause them to supply inaccurate values.

In practice, most applications use electric motors with more capacity than necessary. These oversized motors offer a few advantages, including longer lifespan for the motor bearings, a power reserve, and a smaller replacement motor inventory. However, these motors present a significant disadvantage. Because the motor has a lower load, it does not use its full load range. Major changes in the motor current no longer lie in the typical load range, so overload protection is inefficient and more difficult to detect.

Independent of this effect, evaluation of standard overload relay response is not effective when it comes to protecting plants and systems given that they respond slowly when the current increases. This is because an overload relay must permit normal start-up of an electric motor with an inrush current of between 5x and 7x the rated current.

For 7x the rated current, a Class 10 trip curve with a comparatively fast tripping characteristic requires approximately nine seconds before it trips the motor. The extent of damage to the connected mechanical system can range from increased wear to complete destruction, depending on the particular application and system involved. To protect more sensitive systems, a faster response speed than normally expected from an overload relay is absolutely critical.

Electronic motor management

Using an electronic motor management module in conjunction with a Programmable Logic Controller (PLC) is one solution to this problem. The energy-monitoring device senses the motor current and then communicates with the PLC to prevent two or more motors from starting simultaneously. This minimizes energy demand and can significantly reduce the plant’s energy bill.

Electronic motor management modules constantly monitor three currents, voltages, and phase angles every 6.6 milliseconds to determine a motor-driven system’s actual power consumption. An entire system, which includes a motor driving a specific load (pumps, actuating drives, fans, and machine tools), can be monitored for proper functioning, contamination, and wear.

The modules can measure currents up to 16 A via integrated internal current transformers. Other modules can measure much higher currents through external current transformers. Note that the module itself does not actually perform the load switching; rather, it controls any contactor rated for that particular motor load.

These devices can detect electric current usage through inputs from onboard or external current transformers. By programming alarm thresholds, users can minimize or eliminate downtime. The module can signal an alert that a process motor is drawing excessive current, which could lead to an overload condition. Fast response is necessary to protect a system’s operations. This advance knowledge helps prevent downtime.

An electronic motor management module such as Phoenix Contact’s CONTACTRON motor manager uses the power curve’s linear trend to detect critical load states. Only active power (P) has an almost linear characteristic that is independent of the motor load. The calculation formula already includes the voltage influence, so it is not considered an external disturbance variable.

With its linear characteristic, active power reliably detects all load states and infers motor torque, allowing the module to reveal overloads, underloads, and all critical states. Figure 1 demonstrates two such cases in a pump application.

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Using a motor manager that is completely programmable is especially valuable in this situation. The programmable warnings allow fast response times when the loads reach critical levels.

Monitoring the power factor

Some motor management devices can be used in stand-alone applications as part of an energy data acquisition center. The plant manager can keep track of energy consumption data to ensure that the system is energy efficient.

For example, the module could be programmed with a day counter limit of 20 kWh (see Figure 2). When the power usage reaches that level, an output will send a warning to alert the plant manager.

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A configurable electronic motor management module thus improves motor reliability by making it easy to query the motor’s load levels and analyze the active power curves of the different levels. Ultimately, this not only prevents costly downtime, but also helps the plant decrease its energy bill.

Mark Buckley is a lead product marketing specialist at Phoenix Contact.

Greg Dixson is director of industrial electronics at Phoenix Contact.

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