Electrical Testing
Typical Testing Can Leave You Testy

Typical Testing Can Leave You Testy

Your preventive maintenance plan must be specific and well thought out.

Have you ever had one of those days where things just seem to pile up on you? After a couple hours into it, you feel irritable. Take a look at what kinds of things pile up on you. If you’re in plant maintenance, most of those have to do with equipment breakdown. What makes it especially grating is you know for a fact the Preventive Maintenance procedures (PMs) have been done as scheduled. Your department has done its job to prevent breakdowns. Or has it?

Think of what’s in the typical PM procedure and what happens to any data taken during that procedure. Let’s pick one as an example.

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Suppose it’s the drive motor of a palletizer or a similar production bottleneck. When that equipment goes down, production managers immediately experience a 40 point increase in their blood pressure. And they want you to share their pain. You’re supposed to keep this equipment running. How did you let that motor fail?

The PM doesn’t have much in the way of Predictive Maintenance (PdM). It calls for the following tasks:

  • Grease the bearings.
  • Check voltages on each phase.
  • Check insulation resistance of windings, if you can shut the motor off.

Look at this first task. Motor lubrication is not as simple as many people think. It typically gets done wrong, causing more harm than good. Over lubrication is the primary mode of maintenance failure here, but there are other modes too.

The PM needs to establish the exact method (e.g., turn motor off, open grease drain) of lubrication, exact lubricant to be used, and exact quantity to be used. But what do you know about the condition of the lubricant before starting? When you replace the old lubricant, you also remove the information it contained.

Step one of this process should involve taking thermographic readings of the motor bearings. This provides important information about the condition of the existing lubricant. For example, if the bearings are hot the lubrication interval is probably too long. Maybe the interval is fine for ideal conditions, but it exceeds the manufacturer’s recommended interval for the environment where the motor is installed.

Be careful not to discard information as part of performing the PMs. For any equipment, think about what “As Found” information might be present. Then capture it and trend it, so you can harvest actionable PdM information from PM work.

What about checking voltages on each phase? That’s good, but what other electrical data can you collect while you’re taking measurements? For example, you’ll probably make those voltage checks at the motor starter or the motor’s variable speed drive (probably opening an enclosure). Why not collect temperature data while you’re there?

Maybe at the time the PM is performed, the voltages on each phase are close enough that voltage imbalance is acceptable (e.g., under 2%). But if a connection is starting to go bad, the increasing impedance across it means that soon voltage imbalance will pass the threshold for what a motor should be subjected to. Before the next PM, the motor burns windings and fails. This was entirely predictable, but you didn’t predict it.

Insulation resistance tests are always a good idea, but if the motor must run all the time then this test isn’t available to you. So if this test is in your PM, it just won’t get done. The reason for conducting this test on motor windings is so you can spot deterioration of the insulation. This test doesn’t tell you much of value, unless you establish baseline data and conduct the test at regular intervals.

For this motor, the “if you can shut the motor off” clause undermines the whole process. Maybe you can shut the motor off at shift change. If you can coordinate maintenance for that very narrow window of time, great. If the motor doesn’t need to be run during shift change, consider installing an automatic insulation resistance testing system on that motor. This has many advantages, and is worth looking into.

Whether you can get insulation resistance readings or not, taking thermal readings will give you additional predictive insight into what’s going on with those motor windings. The key is to take the readings exactly the same way every time, so you can trend the data. Thermal camera software helps make this process easy. If you’re not doing this step, you may well be ignoring the motor’s cries for help.

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