How new testing approach matches real world conditions?

Electric motors are the key factor in many industrial processes and can account for up to 70% of the total energy consumed in an industrial plant and consume up to 46% of all generated electricity worldwide. Given their precarious nature for industrial processes, the cost of downtime associated with failed motors can be tens of thousands of dollars per hour. Ensuring that motors are efficient and operate reliably is one of the most important tasks that maintenance technicians and engineers face daily.

In many circumstances, energy efficiency can mean the difference between profitability and financial losses. And, since motors consume such a significant portion of energy in industry, they have become the main target for generating savings and conserving profitability.

Traditional motor testing methods

Calculating electric motor performance and effectiveness in traditional methods should be well distinct. However, the process can be inflated to set up and difficult to apply in working processes. To measure electric motor efficiency both the electric input power and mechanical output power must be established over a wide range of dynamic operating conditions. The traditional method of measuring motor performance first requires technicians to install the motor into a motor testbed. The testbed consists of the motor under test, attached to either a generator or dynamometer.

During testing the load is varied to determine the efficiency over a range of operating modes. The testbed system may seem straightforward but there are several essential disadvantages:

1. The motor must be removed from the service.
2. The motor load is not truly representative of the load the motor serves while in service.
3. During testing, the operation must be suspended (creating downtime) or a replacement motor must be temporarily installed.
4. Torque sensors are expensive and have a limited operating range, so several sensors may be needed to test different motors.
5. A motor testbed that can cover a wide range of motors is expensive and the users of this type of testbed are typically specialist motor repair or development organizations.
6. “Real-world” operating conditions are not taken into account.

Electric motor parameters

Electric motors are designed for specific kinds of applications depending on the load, and each motor has different characteristics. These characteristics are classified according to the National Electrical Manufacturers Association (NEMA) or International Electrotechnical Commission (IEC) 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 in accordance with either NEMA or IEC recommendations. The data on the nameplate can then be used to compare the requirements of the motor against the true operating use mode.

A new approach

The Fluke 438-II Power Quality and Motor Analyzer provide a modernized and cost-effective method for testing motor efficiency while eliminating the need for external mechanical sensors and costly downtime. The Fluke 438-II, based on the Fluke 430-II Series Power Quality and Energy Analyzers, has its full capability to measure power quality while also measuring mechanical parameters for direct-on-line electric motors. Using data from the motor nameplate (either NEMA or IEC data) coupled with three-phase power measurements, the 438-II calculates the real-time motor performance data including speed, torque, mechanical power and efficiency without the need for additional torque and speed sensors. Also, it directly calculates the motor de-rating factor in operating mode.

The data required by the Fluke 438-II 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, rated cos φ or power factor, rated service factor and motor design type from the NEMA or IEC classes.

How it works

The Fluke 438-II unit provides mechanical measurements (motor rotating speed, load, torque, and efficiency) by applying proprietary algorithms to electrical waveform signals. The algorithms combine a mixture of physics-based and data-driven models of an induction motor without requiring any of the pre-measurement testing typically needed to estimate motor model parameters, such as stator resistance. 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 is computed using torque times speed. The motor efficiency is computed by dividing the estimated mechanical power by the measured electric power. Fluke conducted extensive testing with instrumented motors driving dynamometers. Actual electric power, motor shaft torque and motor speed were measured and compared to the values reported by the 438-II to determine accuracy levels.

Summary

Taking critical motor efficiency measurements is 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 gives them the opportunity to trend motor performance over time, giving them a better picture of overall system health and performance. By trending performance, it becomes possible to see changes that may indicate forthcoming motor failures and allow replacement before failure. For best Fluke 438-II in the Philippines, check out our products at https://presidium.ph/product-category/products/fluke-industrial-group-tools/power-quality/.