Electrical machines are critical components in Kenya’s power generation, manufacturing, and industrial sectors. Testing these machines ensures their reliability, efficiency, and safety during operation. This chapter addresses the essential practices for performing electrical machine testing, focusing on safety, proper use of disassembling tools, and key electrical tests such as turns ratio, winding resistance, voltage, and current measurements. Mastery of these aspects safeguards personnel and equipment while providing accurate diagnostics for maintenance and troubleshooting.

4.1 Test equipment set up

A test is only as good as the way the instruments are prepared and connected. A meter set on the wrong range, a lead with a broken wire inside or a test point that is not clean can give a reading that looks right and is wrong, and a wrong reading leads to a wrong decision about the machine. Setting up the test equipment means choosing the instruments for the tests to be done, checking them, arranging the test area and connecting the instruments to the machine safely. This topic describes these steps. The parameters that are measured are covered under test parameters, and the tests themselves under types of tests.

Choosing the instruments

The instrument is chosen for the quantity to be measured and the value to be expected.

Example of range selection: a motor is rated at 415 V and 22 A. The voltmeter chosen must have a range above 415 V, so a 600 V range is suitable and a 200 V range is not. A clamp meter with a 40 A range would show the full-load current of 22 A with good resolution, while a 400 A range would give a poor reading. An instrument is never used near the top of a range that is too small, because an unexpected rise in value can damage it.

Preparing the instruments

1. Inspect the case, the leads and the probes for cracks, bare wire and loose plugs.
2. Check that the instrument is in calibration by its label, and that the battery is charged.
3. Test the meter on a known source or a known resistor to see that it reads correctly.
4. Test the leads for continuity by touching the probe tips together on the ohm range. A reading above a few tenths of an ohm means a faulty lead or a dirty plug.
5. For the insulation tester, perform its own self-check, as the maker describes.
6. Zero or null the instrument where its type requires it, and note the lead resistance when measuring small resistances.

Preparing the machine and the test area

• The machine is isolated from the supply, locked and tagged, and proved dead before any connection is made.
• Windings are discharged by shorting them to earth after an insulation test, so that no charge remains.
• The terminal box is opened, the links are removed so that each winding is tested alone, and the terminals are cleaned with a dry cloth.
• The machine and the instruments are placed on a dry, clean bench or floor with enough lighting.
• The test area is marked and unauthorised persons are kept out. Signs warn of a test in progress.
• Earthing of the frame is checked before the machine is energised for any test with a supply.

Connecting the instruments

1. Select the function and the highest range of the instrument before connecting.
2. Connect the leads to the machine first and the supply or the instrument source afterwards.
3. Connect voltmeters in parallel, and ammeters or clamp meters round one conductor of the circuit only.
4. For an insulation test, connect one lead to the winding and the other to the frame, then raise the test voltage after the leads are secure.
5. Reduce the range only when the reading is known to be small.
6. After the test, switch the instrument off, disconnect the leads in the reverse order and return the machine links.

Setting up for tests with a supply

Where a test is made with the machine energised, such as a no-load run, the supply is taken through a protective device, the frame is earthed, the guards are fitted, the shaft is free to turn and the emergency stop is within reach. The person controlling the supply is in touch with the person at the machine.

A record is made of each instrument used, its serial number and its calibration date, so that the test results can be traced to a checked instrument.

Chapter Summary

This chapter focused on the essential procedures and precautions involved in performing electrical machine testing. It began with an emphasis on safety measures that protect both personnel and equipment during testing operations. The chapter then detailed the various tools and equipment required for disassembling electrical machines, highlighting the importance of using the correct instruments. Recording machine data was covered extensively, including critical winding parameters such as turns ratio, winding resistance, voltage, and current, which are vital for accurate diagnostics. Further, the process of disassembling rotating machine parts was explained, ensuring careful handling to avoid damage. Important winding tests such as continuity, insulation resistance, voltage testing, and current balance were presented to assess machine condition comprehensively. Finally, the chapter addressed housekeeping practices with a focus on maintaining general cleanliness to ensure a safe and efficient work environment.