Question

With reference to the condition monitoring, of electrical machinery:

(a) State TWO important parameters that may be recorded;

(b) Explain how the parameters are measured and what defects may be revealed.

Answer.

Condition Monitoring (another form of preventive maintenance) is that in which equipment is regularly monitored and tested.

When monitoring indicates that breakdown is imminent, the equipment is repaired or replaced and any other specified maintenance procedures carried out

Regular insulation testing and vibration testing are two forms of condition monitoring.

Insulation resistance – all electric equipment has insulation. The purpose is to keep electric current in the conductor and to prevent contact with live wires. The electrical resistance of insulation must be very high [mΩ] to prevent current leaking away from conductor. Insulation resistance is measured between

Conductor and earth,

Conductors.

The electrical insulation of electrical machines and cables is regularly measured using a standard 'Megger' type insulation tester.

Vibration measurement

All machinery installations have characteristic vibrations when in operation. Recordings of these vibration characteristics, taken when the machinery is in good condition and operating satisfactorily, provide a standard against which to judge the future condition of the machinery, to diagnose faults and to decide on the maintenance required.

Measurement of insulation resistance.

Preparing for the actual test.

Correctly preparing the equipment and insulation tester is crucial to your safety and the well being of your wiring and machinery. Adhere to the following four-step process before every test:

Take the equipment out of service.

Shut down the apparatus, open all switches, and de-energize the unit. Disconnect the equipment under test from all other equipment and circuits, including neutral and protective ground connections. Make sure you follow proper lock-out/tag-out procedures during this step.

Check what will be included in the test.

The more equipment included in a test, the lower the resistance reading. For this reason, it's very important to inspect the installation and understand exactly what you're including in the test. You don't want a true reading to be affected by additional equipment. However, if a complete installation with several pieces of equipment yields a high reading, it's safe to assume that each individual apparatus will yield an even higher reading. Consequently, sometimes separating components is unnecessary.

Discharge capacitance.

It's important to discharge capacitance before and after making an insulation resistance test. You should discharge about four times as long as the test voltage was applied during the test.

Check current leakage at switches.

Make sure readings won't be affected by leakage over and through switches, fuse boxes, or other connections. Such leakage can be detected by watching the level of resistance the moment the test leads are attached. Never perform an insulation test on an energized line or apparatus.

Interpreting test results.

Deciding what to do with the results of an insulation test can often be more complicated than actually conducting the test itself. Every piece of equipment has a general insulation “personality.” In other words, no two pieces of equipment may operate exactly the same, but if a machine is behaving in accordance with its normal tendencies, there's usually no cause for concern. However, a safe rule of thumb is to judge results against a 1 megohm per 1,000V ratio. Use the information shown in Table 2 on page 46 as a guideline for what to do with the various conditions you may discover during your testing.

It's extremely important that you consult the motor manufacturer's operating handbook for specific information and guidance as to whether a particular value measured between two points should be considered acceptable or questionable.

Insulation testing manufacturers can provide test equipment capable of providing you with accurate readings, but they have no way to determine if a particular measured value indicates that a piece of equipment meets its specifications for insulation integrity.

Continuity testing procedure.

Devices that can be used to perform continuity tests include multimeter which measure current

The following safety precautions are the MINIMUM for using a multimeter.

· De-energize and discharge the circuit completely before connecting or disconnecting a multimeter.

· Never apply power to the circuit while measuring resistance with a multimeter.

· Connect the multimeter in series with the circuit for current measurements, and in parallel for voltage measurements.

· Be certain the multimeter is switched to ac before attempting to measure ac circuits.

· Observe proper dc polarity when measuring dc.

· When you are finished with a multimeter, switch it to the OFF position, if available. If there is no OFF position, switch the multimeter to the highest ac voltage position.

· Always start with the highest voltage or current range.

· Select a final range that allows a reading near the middle of the scale.

· Adjust the "0 ohms" reading after changing resistance ranges and before making a resistance measurement.

· Be certain to read ac measurements on the ac scale of a multimeter.

· Observe the general safety precautions for electrical and electronic devices.

In case of 3phase motor and transformer all reading should be identical

If one reading is smaller than the other this could indicate the possibility of short circuit turns in that winding

Conversely a high continuity resistance value indicates a high resistance fault or an open circuit.