Insulation testing with the Fluke 1587 megohmmeter

1. Overview

When debugging an AC drive system, insulation tests must be performed on the main circuit and control circuit of the inverter and motor to ensure safe operation of the system. The Fluke 1587 multi-purpose insulation meter (Figure 1-1) not only performs fast and accurate measurements on the main circuit, but also provides the "resistance range" function to perform insulation tests on the control circuit.

II. Test insulation

1) The insulation measurement of the inverter main circuit (taking VACON NXP product as an example) should follow the following steps:

● Disconnect all the connections between the inverter and the outside world and ensure that the circuit under test is not connected to any power supply. Then short-circuit the input L1, L2, L3 and the output terminals U, V, W together, as shown in Figure 2-1.

● Insert the test probe into the '+, -' input terminals, and then connect the other end of '-' to the 'ground point' as shown in Figure 2-2.

● Select the 'Insulation test' function on the Fluke1587 megohmmeter.

● Select the appropriate test voltage (500Vdc) through the 'Range' button.

● Connect the probe to the circuit to be tested, and the meter will automatically detect whether the circuit is powered. If the voltage in the circuit exceeds 30Vac or dc, the test is automatically prohibited.

● Press and hold the 'Insulation test' key and read the impedance value displayed on the meter. It may take a few seconds for the reading to stabilize. The higher the impedance value, the better. At this time, the reading is 550 megohms, which can be judged that the insulation is good. As shown in Figure 2-3.

● Keep the probe on the test point and release the 'Insulation test' button. The circuit

under test will begin to discharge through the meter until it is finished. 2) When measuring the insulation of the inverter control circuit (taking VACON NXP products as an example), it should be noted that the continuity test must be carried out using the 'resistance range' of the Fluke1587 meter strap. Generally, the test value is >= 1 megohm, indicating good insulation.

3) When testing the insulation of the motor, the three-phase winding wires U, V, and W should be tested relative to the ground wire. Generally, the measured value is >= 5 megohm, indicating good insulation.

III. Conclusion

The Fluke-1587 Insulation Test Multimeter integrates a digital insulation tester and a full-featured digital multimeter in a single compact handheld instrument, providing maximum versatility for AC drive system commissioning, fault detection and preventive maintenance.

References:
[1] VACON Drives Co., Ltd. NX User manual. Finland.
[2] Fluke Electrical and Electronic Test Tools Sample. (end)