Healthcare for a motor-drive system

Perform no test that isn’t cost-effective from the machine’s point of view.

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By James W. Taylor

PlantServices.com

You’ve spent a lot of time and money converting to an electronic drive system. The vendor has promised energy savings and increased production efficiency. You’ve carefully matched the supply, the drive, the motor, power conditioning and the process. The trick now is to capture the advantages of the system by maintaining its efficiency and reliability.

We know from studies by United Airlines and the U.S. Navy that nearly every complex machine fails in a random manner. The bathtub curve doesn’t really apply to a motor drive -- there’s no wear out period. Nor does it make sense to do preventive replacement or overhaul. That leaves you with predictive maintenance (PdM) or condition-based maintenance (CBM).

The basic premise of CBM is to detect a potential failure in its early stages so that we have time to take action to accomplish the remedial action in a planned manner. The idea is to manage the failures instead of letting them manage us.

Let’s design a CBM system for a typical motor-drive system. Instead of picking the technologies we want to use and then trying to apply them to our system, we’ll use a machine-centered healthcare approach.

Machine-centered approach
A machine-centered approach starts with the machine and, through a series of steps, helps you decide what tests to perform. It has six steps:

1. List the system functions.
2. List the possible failures to meet those functions.
3. Decide which failure modes are significant.
4. Decide how to get an early warning.
5. Select a suite of tests to detect those early warning signs.
6. Collect the test results at one decision point.

List the system functions: Identifying possible failures requires knowing what the machine is supposed to do -- its primary and secondary functions. For this example, we’ll examine a system from the power supply cables to the motor output shaft (Figure 1).

Limits of concern

Every component within this boundary is relevant to motor-drive system health.

At first glance, you might say that a motor-drive system’s primary function is to provide power to the shaft. In reality, its primary function is more complex than that. It must provide a minimum torque at a specific RPM. It may be required to accelerate a load from stop to operating speed, control speed ramp-up and ramp-down, and respond to signals from a control system. Other possible functions (Table 1) might be to maintain power factor, minimize source wave form distortion and maintain phase balance.

Don’t try to use this example without referring to your own system. You may have functions that aren’t listed and some of those listed might not be applicable to your system. Use this as a starting point.

List the possible failures to meet those functions: Once you’ve decided what the system’s function is, ask what can happen to prevent it from meeting that function. In the case of the motor-drive system, the answer might be the motor not starting, poor speed control, high total harmonic distortion (THD) reflected back into the power supply, or a number of other possible failures (Table 2).