Motor and Variable Frequency Drive Validation

By Jonathan Susser | August 14, 2019

Original equipment manufacturers (OEMs) use electric motors to drive the machinery they produce, which can range from hand-held kitchen appliances to large industrial production equipment. While some OEMs build their own motors or purchase motor parts for specific fitting into their machines, most buy them from motor manufacturers. At the same time, if equipment operates at varying loads, requires synchronization with other motors or is a centrifugal load, OEMs will often include variable frequency drives (VFDs) with their motors.

Motor Complete Assembly

Even though motors and VFDs are extremely important for OEMs, some do not have a resident motor and drive engineer on staff to help select the best models or run tests to understand the performance of the products being considered. Instead, OEMs regularly rely on motor and VFD manufacturers for advice. While manufacturers are knowledgeable and provide solutions, they may not always take an independent view or consider options outside of their own offerings. Sometimes, a third party is needed that has the resources and time to conduct testing that validates motor and VFD performance, such as speed, torque, vibration, noise, operating temperature, reliability and efficiency.

Motor and VFD validation involves analyzing and testing the products an OEM is considering or currently uses. Here are some scenarios in which it might be pursued:

  • Replacing an existing motor or VFD with a less expensive one. OEMs may want to know whether potential new products perform as well as their existing motor or VFD, which can act as a baseline. Validation compares products and can identify differences that may impact machine performance and warranty periods.
  • Developing a new product. OEMs undergoing new product development likely would not have an existing motor or VFD to use as a baseline. Here, validation would involve comparing several motor or VFD manufacturers being considered.
  • Understanding warranty claims. OEMs may receive a spike in warranty claims regarding motor and VFD failures. Validation can help determine the cause of the issue.
  • Ensuring testing accuracy. Pump manufacturers in particular have new system efficiency requirements and are thus calibrating their lab testing equipment. Validation can help ensure the accuracy of the motors and VFDs used to operate these tests.

Consider the following options when validating and comparing motors and VFDs.

Inspection Build Analysis

When selecting a motor, it is best to disassemble samples and compare them to one another – even a motor you have used for years. Manufacturing defects or design differences can be easily pointed out, documented and even improved by the manufacturer before proceeding to select a motor.

Motor Stator

Performance Characterization

A set of sample motors should be tested to fully define torque over a given speed (speed/torque curves). Performance under design loads with continuous temperature monitoring is an excellent indicator of reliability, as heat kills motors (heat runs). If external conditions exist for the motor application, testing can be done in elevated or reduced ambient temperatures.

Example of a Speed/Torque Curve at 1000 RPM

Endurance Testing

Endurance tests should be based on your equipment’s operating characteristics and the duty cycle your motor will experience under the equipment’s full warranty period. Consider worst-case scenarios for your equipment. Once test parameters are defined, potential replacement motors should be placed in tests with known motors for comparison. If any motors fail during testing, conduct a post-endurance build analysis.

Post-Endurance Testing

Consider speed/torque curves and heat runs again to ensure performance has not degraded beyond an acceptable level. Compare the known (existing) motor and its potential replacements because you know the history of motor performance in your equipment.

Post-Endurance Build Analysis

During endurance testing, all motors tested will experience wear, and some may fail. It is valuable to tear down and inspect motors after testing to learn as much as possible about wear patterns and potential modes of failure. No endurance test can account for all of the potential faults a motor might see during operation. Knowing which components experience the most wear can lead to design improvements in the motor before you take shipment on your first order from a new motor or VFD manufacturer.

VFD Compatibility Testing

Validated product testing can be performed to ensure motor and VFD compatibility, including harmonics, rise time, dV/dT, VFD efficiency and system (motor and VFD) efficiency. Ideally, test conditions will replicate the speed and torque requirements of the motor application.

In-Process Inspection

An in-process inspection is beneficial to maintain quality with any manufacturer, but especially a new one with little prior history. An ongoing program for motor receiving inspection can maintain your reputation in the marketplace. Options to consider with this service range from in-house inspection by your own staff on the production floor to random sample testing.

These motor and VFD validation steps do not have to be followed in any particular order, but all can offer tremendous value in making sure you end up with the best products for your equipment.

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