Medium-Voltage Motor Testing: What Is Measured and Why

Short answer

A medium-voltage motor is tested for insulation to ground (insulation resistance and polarization index per IEEE 43), insulation between turns (comparative surge test), winding continuity and balance (winding resistance) and, in higher-voltage machines, insulation aging with tan delta and partial discharge (IEC 60034-27). Together they show whether the motor can start safely and how its insulation is changing.

What is tested in a medium-voltage motor

A medium-voltage motor drives pumps, compressors, fans and mills that are often the heart of the process. A stator winding failure takes it out for weeks and almost always starts in the insulation: to ground, between phases or between turns. Electrical tests look for that deterioration before it ends in a short circuit.

In some countries these motors are called high-voltage motors, because they exceed 1 kV. The idea is the same: induction or synchronous machines with a form-wound coil winding, fed from switchgear with its motor protection.

Tests are grouped as static, with the motor stopped, disconnected and isolated from the supply cable, and dynamic, with the motor running. This guide focuses on the static ones, which decide whether a motor can start again.

Insulation resistance and polarization index (IEEE 43)

This is the basic test. DC voltage is applied between the winding and the frame, and the resistance is measured over time. The polarization index (PI) divides the 10-minute reading by the 1-minute reading. In dry, clean insulation the resistance rises because the absorption current decays.

IEEE 43, the recommended practice for rotating machines, defines the test voltage according to the rated winding voltage, and the acceptance minimums:

Rated winding voltage	DC test voltage (IEEE 43)
Below 1000 V	500 V
1000 to 2500 V	500 to 1000 V
2501 to 5000 V	1000 to 2500 V
5001 to 12,000 V	2500 to 5000 V
Above 12,000 V	5000 to 10,000 V

For form-wound windings made after 1970, the recommended minimum resistance at 1 minute, corrected to 40 °C, is 100 MΩ. The minimum PI is 2.0 for class B insulation and above. IEC 60034-27-4 covers the same measurement with its own procedure and recommended minimum values.

What it detects: moisture, conductive dirt, oil or dust contamination, and solid ground faults. What it does not detect: weaknesses between turns, or the thermal aging of insulation that is still dry.

Winding resistance

The resistance of each phase is measured with direct current and a four-wire method, as in the winding resistance test of transformers. In a healthy motor the three phases should be almost equal. A difference points to loose coil-end connections or cracked brazed joints, shorted turns or a conductor with broken strands. Values are corrected for temperature before they are compared with the factory values.

Some motor circuit analysis instruments add AC measurements of inductance and impedance per phase. The imbalance between phases helps find turn-to-turn problems and, in squirrel-cage motors, signs of damaged bars when measured at several rotor positions.

Comparative surge test

With direct current the voltage divides between the winding and the frame, not between the turns. The surge test applies fast-front pulses that do stress the turn-to-turn insulation, and compares the oscillation waveform of each phase with the others. If a turn gives way, the waveform of that phase changes in frequency or amplitude.

IEEE 522 is the guide for testing the turn insulation of form-wound coils, including machines in service as a preventive maintenance test. It suggests the test levels. IEC 60034-15 sets the impulse withstand voltage levels for form-wound coils; its 2025 edition aims to harmonize them with IEEE 522. It is the only test on this list that evaluates the turns directly, where many failures begin.

Tan delta and partial discharge in the stator

In higher-voltage motors, mica and resin insulation ages through temperature, vibration and internal discharges. Two tests evaluate it:

Tan delta (dissipation factor) and its increase with voltage (tip-up). It is measured at several AC voltages. Insulation with internal voids or delamination shows a tan δ that rises with voltage. IEEE 286 is the recommended practice for power factor tip-up of stator coil insulation, and IEC 60034-27-3 covers dissipation factor measurement on windings with conductive slot coating from 6 kV up. More detail in the guide on power factor and tan delta.
Partial discharge. It detects small discharges in voids, at the slot exit or between coil ends. IEC 60034-27-1 covers off-line measurement and IEC 60034-27-2 covers on-line measurement, with the motor in service. Its value is in the trend of one machine more than in a single number.

These tests apply mainly to motors of 6 kV and above and to critical machines. In small medium-voltage motors with little slot insulation the contribution is smaller.

When to test a motor

Moment	Why
Receipt and commissioning	Baseline; confirms that transport and installation did not damage the winding
Scheduled maintenance	Insulation trend; decides cleaning, drying or rewinding in good time
Before starting after a long shutdown	A cold, damp motor can fail at the first start
After a trip	If the 87M motor differential or a ground fault operated, it confirms whether the winding is damaged
After a rewind or repair	Checks the workshop’s work before returning to service

The sequence matters. Low-energy tests, such as insulation resistance and winding resistance, come first. Higher-stress tests, such as surge or applied voltage, are done only if the first ones came out well. IEEE 1415 is the guide for maintenance testing and failure analysis of induction machines and orders these tests by objective.

A single test says little. The value is in comparing the three phases, against the factory and against the previous measurement. It is also in reading the results together with the motor protections. Many trips at start-up are not from the winding but from the settings, as explained in the guide on overcurrent trips on start-up.

What about low-voltage motors?

The logic is the same, with two differences. The insulation test voltage is lower, because it is chosen according to the rated voltage of the machine. And in small motors the polarization index is less conclusive than the trend of the same motor. The tests that do change, such as those for motors fed by a drive, are in electrical tests on low-voltage three-phase motors.

Equipment for this test

These tests are done with equipment from recognized manufacturers; these guides explain what each one measures. In the quote we recommend the test and the equipment your case needs.

Motor testing with Tiza Group

Our team tests medium-voltage motors in industrial and generation plants in Colombia, or elsewhere on request. We do it at commissioning, during maintenance and after a trip, and interpret the results together with the motor’s history and its protections. See the scope in medium-voltage motor testing. Write to us to schedule the tests.

References

IEEE Std 43-2013, Recommended Practice for Testing Insulation Resistance of Electric Machinery.
IEEE Std 286, Recommended Practice for Measurement of Power Factor Tip-Up of Electric Machinery Stator Coil Insulation.
IEEE Std 522-2023, Guide for Testing Turn Insulation of Form-Wound Stator Coils for Alternating-Current Electric Machines.
IEEE Std 1415, Guide for Induction Machinery Maintenance Testing and Failure Analysis.
IEC 60034-15, Rotating electrical machines – Part 15: Impulse voltage withstand levels of form-wound stator coils for rotating a.c. machines.
IEC 60034-27-1, -27-2, -27-3 and -27-4: off-line and on-line partial discharge, dissipation factor and insulation resistance of rotating machine windings.

Frequently asked questions

What polarization index should a medium-voltage motor have?

IEEE 43 recommends a minimum polarization index of 2.0 for thermal class B insulation and above, which covers almost all modern medium-voltage motors, and 1.5 for class A. If the 1-minute resistance is very high, on the order of 5000 MΩ or more, the PI may not be meaningful.

What is the difference between the insulation test and the surge test on a motor?

Insulation resistance evaluates the insulation between the winding and the frame. The comparative surge test applies fast pulses that spread the voltage across the turns and reveal weaknesses in the turn-to-turn insulation, which the DC test does not see.

What are static and dynamic motor tests?

Static tests are done with the motor stopped and disconnected: insulation, windings, surge, tan δ. Dynamic tests are done with the motor running and measure currents, voltages, vibration or on-line partial discharge to see how it behaves under load.