Understand Motor/System Baselines

Want to get the most from your electric motors? Think of St. Louis-based EASA (Electrical Apparatus Service Association,  easa.com) as a treasure trove of practical information and its members as a “go to” source for help with specific applications. Consider this insight on motor/system baselines.

According to EASA’s technical experts, changes in motor/system vibration readings provide the best early warning of developing problems in a motor or system component. Other parameters to monitor may include operating temperature of critical components, mechanical tolerances, and overall system performance, including outputs such as flow rate, tonnage, and volume.

Motor-specific baselines incorporate records of electrical, mechanical, and vibration tests performed when units are placed in operation or before they’re put in storage. Ideally, baselines would be obtained for all new, repaired, and in situ motors, but this may not be practical for some applications. These baselines typically include some or all of the following:

Load current, speed, and terminal voltage

Changes in these parameters usually indicate that a vital system component is damaged or about to fail. Other electrical tests may include insulation resistance, lead-to-lead resistance at a known temperature, no-load current, no-load voltage, and starting characteristics.

QUICK TIP: Some changes in the current and speed may be normal, depending on the type of load.

Motor current signature analysis (MCSA)

This test diagnoses squirrel cage rotor problems, e.g., broken bars or an uneven air gap. It’s more accurate if a baseline is established early in the motor’s life.

Mechanical tests

These normally consist of measuring shaft runout (TIR) and checking for a soft foot.

Vibration

Although overall vibration readings can be used as baseline data, Fast Fourier Transform (FFT) spectra in all three planes at each bearing housing are preferred (see “Vibration Analysis” on page 22). Shaft proximity probes can be used to determine sleeve bearing motor baselines.

Infrared thermography

This tool can detect changes in the operating temperature of critical motor components, especially bearings.

New-motor baselines

Comparing factory terminal winding resistance and no-load amps with data taken under load can be useful when monitoring the condition of a new motor or troubleshooting system problems. Factory baselines are often available from the manufacturer or its website. The accuracy of factory data depends on how it was obtained, but it’s usually sufficient for field use.

Baseline data for a newly installed motor could reveal an error, e.g., misconnection for an incorrect voltage, and prevent a premature motor failure. Rather than simply “bumping” a motor for rotation before coupling it to the load, operate it long enough to measure the line current for all three phases, as well as the voltage and vibration levels.

QUICK TIP: Comparing the baselines of a failed motor and its replacement could reveal application- or process-related weaknesses in the system.

Repaired motor baselines

Service centers usually provide no-load and/or full-load (when stipulated) test data for repaired motors, including voltage, current, and vibration spectra. Comparing these results with historical baselines and those obtained on site when the motor is returned to service may confirm the quality of the repair or possibly reveal underlying system problems. For example, increased vibration levels in on-site tests might indicate a deteriorating motor base or a problem with the driven equipment rather than a balancing issue with the motor.

With newly repaired motors that have been in operation for many years, baseline comparisons are invaluable in root-cause failure analysis and may even expose consequential damage from certain kinds of failures, e.g., a broken shaft. To correctly identify cause and effect and prevent recurrences, always investigate equipment failure at the system level.

For details on using motor/system baselines, as well as expert advice on a wide range of other motor-related issues, download Getting the Most from Your Electric Motors, or contact a local EASA service center.