Capture drive system advantages while retaining reliability

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

By James W. Taylor

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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
Motors
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.

Table 1

Downstream (load side)

Upstream (line side) Power supply
Start motor
Stop motor
Deliver specified torque at specified RPM
Specified speed ramp up rate
Specified speed ramp down rate
Accelerate load from stop to operating speed
Adjust torque and speed on demand
Reduce power usage
Maintain power factor below limit
maintain total harmonic distortion below preset limit
Maintain minimum voltage at max load
Maintain voltage balance
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).

Table 2. Failure modes (partial list)
Function Functional failure Failure mode

Downstream (load side)

Start motor Motor won't turn Winding failure (stator)
    Insulation failure (stator
    Rotor failure
    Bearing seized
    Loss of Power
    VFD malfunction (start control)
Deliver specified torque at specified RPM Motor turns at wrong speed VFD malfunction (speed control)
    Load fault
Specified speed ramp up rate Motor ramps up at wrong rate VFD malfunction (ramp up)
    Winding failure (stator)
Accelarate load from stop to operating speed Load won't reach operating speed VFD malfunction (ramp up)
    Winding failure (stator
  Load reaches operating speed too slowly VFD malfunction (ramp up)
    Winding failure (stator)
    Bearing seized

Upstream (line side)

Maintain power factor below limit Power factor outside limits Power conditioning fault
Maintain THD below limit THD exceeds limits Power conditioning fault (THD)
    Distribution problem (THD)

At this point, don’t consider whether the failure is likely or has much impact. You’re just brainstorming for a complete list. Some functions can fail in more than one way. The table lists failures both upstream and downstream of the motor and drive. Some of these may be attributable to the supply transformer, but they still affect the motor-drive system.

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