Electric motors are a critical component in many industrial processes, and can account for up to 70 percent of the total energy consumed in an industrial plant, and can consume up to 46 percent of all generated electricity worldwide. Given their prominence in industrial processes, the cost of downtime associated with failed motors can be tens of thousands of dollars per hour.
So ensuring that motors are efficient and operate reliably is one of the most important tasks maintenance technicians and engineers face. Indeed, the efficient use of electricity in many circumstances can mean the difference between profitability and financial losses. And, since motors consume such a significant portion of energy in industry, they have become prime target for generating savings and maintaining profitability.
Traditional Method
The traditional method for measuring electric motor performance and efficiency can be costly to setup and difficult to apply in working processes. In many cases, motor performance checks even require a complete system shutdown, which can result in costly downtime. The traditional method of measuring motor performance requires technicians to install the motor into a motor test bed. The test bed consists of the motor under test, mounted to either a generator or dynamometer.
Electric motors are designed for specific kinds of applications - depending on the load, and as such each motor has different characteristics. These characteristics are classified according to NEMA (National Electrical Manufacturers Association) or IEC (International Electrotechnical Commission) standards and have a direct effect on the operation and efficiency of the motor.
Each motor has a nameplate that details key motor operating parameters and efficiency information. The data on the name plate can then be used to compare the requirements of the motor against the true operating use mode. For example, when comparing these values you may learn that a motor is exceeding its expected speed or torque specification, in which case the motor’s life may be shortened or premature failure may occur.
Other effects such as voltage or current unbalance and harmonics associated with poor power quality may also decrease motor performance. If any of these conditions exist the motor must be “derated”—that is the expected performance of the motor must be reduced—which could result in a disruption to the process if not enough mechanical power is produced. The derating is calculated according to the NEMA standard.
A Potential New Approach
Another method, using a recently developed test instrument from Fluke, could offer the ability to measure power quality while also measuring mechanical parameters for direct-on-line electric motors. Using data from the motor name plate and coupled with three-phase power measurements, the motor analyzer calculates the real-time motor performance data without additional torque and speed sensors. The motor analyzer also directly calculates the motor derating factor in operating mode.
The data required by the motor analyzer to perform these measurements is entered by the technician or engineer and includes the rated power in kW or HP, rated voltage and current, the rated frequency, power factor, rated service factor and motor design type from the NEMA or IEC classes.
The testing unit provides mechanical measurements by applying proprietary algorithms to electrical waveform signals. The algorithms combine a mixture of physics-based and data-driven models of an induction motor. Motor speed can be estimated from the rotor slot harmonics present in the current waveforms.
Motor shaft torque can be related to induction motor voltages, currents and slip by well-known but complex physical relations. Electric power is measured using the input current and voltage waveforms. Upon obtaining torque and speed estimates, the mechanical power (or load) is computed by multiplying torque by speed. The motor efficiency is computed by dividing the estimated mechanical power by the measured electric power.
While the traditional methods for measuring electric motor performance and efficiency are well defined, they are not necessarily widely implemented. This is in large part to the cost of downtime associated with taking motors, and sometimes entire systems, offline for testing purposes.
Taking critical motor efficiency measurements could be simplified by eliminating the need for external torque and separate speed sensors, making it possible to analyze the performance of most industrial motor-driven processes while they are still in service. This gives technicians the ability to decrease downtime, and trend motor performance over time, giving them a better picture of overall system health and performance.
Frank Healy is the marketing manager for Fluke power quality products.
