Measuring insulation resistance and finding an interturn short circuit are two different diagnostic operations that are often mistaken for a single test. A megger assesses the insulation between the winding and the frame, between phases, and between electrical circuits of the machine, but it practically cannot reveal a short between neighboring turns of the same coil.
With an interturn defect, the winding can have very high resistance to frame and pass a standard insulation test with flying colors — so "the megger reads fine, but the motor overheats" is a perfectly real situation. Finding an interturn short calls for different methods: comparing phase resistances, measuring inductance, a surge test, phase-current analysis, thermal imaging.
Reliable diagnostics must answer two separate questions: does the winding have adequate insulation to frame and to the other phases, and is there a short inside the winding itself, between its turns?
Insulation resistance describes an insulation system’s ability to resist current flow between a conductor and other electrically separated parts. In a motor, insulation is checked winding-to-frame, phase-to-phase, stator-winding-to-rotor-winding, and across other electrical circuits.
A megger applies a DC test voltage and measures a very small leakage current made up of a capacitive component, an absorption component, a surface leakage component, and a component through any defective areas. Readings are often lower at first and climb gradually as the winding’s capacitance charges and polarization processes proceed in the insulation.
A megger cannot reliably reveal a short between neighboring turns, a wrong turn count, a shorted part of a coil, a weak interturn insulation that only breaks down under an impulse voltage, or asymmetry between parallel branches.
With an interturn short, both shorted turns stay insulated from the frame — no leakage path to ground appears, so the megger reading can stay high despite the defect being present.
An interturn short circuit is an electrical contact between turns of the same coil or winding that should normally be separated by insulation. Even a few shorted turns form a closed loop of very low resistance in which a large circulating current is induced — causing local overheating, damage to the insulation of neighboring turns, and a worsening defect.
The main causes are insulation aging, winding overheating, overload, frequent or prolonged starts, running at reduced voltage, phase imbalance, VFD supply, transient overvoltages, contamination, moisture, mechanical vibration, loosened slot wedges, or repair defects.
Possible symptoms include the motor heating up quickly, one phase drawing more current, humming, rising vibration, reduced power and lower starting torque. With one or two shorted turns, the change in a phase’s resistance can be very small, so an ordinary multimeter often misses the defect.
Fully disconnect the motor, lock out re-energizing, and verify there is no voltage present. Before connecting the megger, disconnect the power cable, the variable frequency drive, the soft starter, any capacitors and electronic sensors — a megger must never be connected to the output of a drive or other power electronics.
Inspect and clean the terminal box — a contaminated or damp box can significantly lower the result even when the winding itself is sound. If the three-phase winding has six leads brought out, it is best to remove the links and check each phase separately, to frame and phase-to-phase.
The test voltage is chosen based on the motor’s rated voltage, insulation class, condition and age, the manufacturer’s instructions, and the plant’s internal procedures. In practice, low-voltage motors are often tested with a 500 V DC megger, while higher voltage classes use 1000 V, 2500 V or other levels — the choice should never come from a single rule of thumb.
Sensors, encoders, thermistors and other low-voltage auxiliary circuits need much lower test voltages — an excessive voltage can damage weakened insulation.
The absorption ratio shows how insulation resistance changes over the first minute of the test — readings at 60 and 15 seconds, or 60 and 30 seconds, are compared. If the insulation is dry and shows strong polarization properties, resistance typically rises over time; a low ratio can point to moisture, contamination or aging, though the figure should be interpreted carefully.
The polarization index is the ratio of resistance at 10 minutes to resistance at 1 minute. It is more often used for large, high-voltage and critical machines — it helps assess moisture, contamination and the overall condition of the bulk insulation, but it is not a universal criterion for every modern insulation system.
A single measurement can be misleading — the trend is the most informative: results after manufacture, after repair, previous scheduled measurements, and the current value.
A low reading can be caused by moisture, contamination, carbon or metallic dust, oil, damaged leads, a dirty terminal box, a slot insulation breakdown, or thermal aging. Before concluding a rewind is needed, the source of the leakage must be localized.
If a low reading is caused by moisture, drying can significantly improve the result — methods include a drying oven, a reduced-voltage current, an induction method, or hot air, with temperature and insulation resistance monitored throughout. If resistance does not recover after drying, an actual insulation defect, not just moisture, is likely.
Since a megger is poorly suited to this task, a combination of other methods is used.
For small resistances, the resistance of test leads and contacts can be comparable to the winding’s own resistance, so a four-wire (Kelvin) method is preferred, separating the current-supply loop from the voltage-sensing loop. Phases can only be compared at the same temperature, in the same scheme, and accounting for parallel branches — an interturn short usually lowers resistance slightly, while an open parallel branch, conversely, can raise it.
An interturn short changes not just resistance but the coil’s magnetic properties too — the effective turn count drops, inductance falls, and losses rise. An LCR meter or a dedicated analyzer can show a difference between phases even when resistances are nearly equal, but the rotor must be in the same position for every measurement.
A short high-voltage pulse is applied to the winding — its inductance and the tester’s capacitance form an oscillating circuit, and the instrument records the shape of the decaying wave. Three phases are compared to each other, matching coils are compared, or a current curve is compared to a reference. An interturn short changes the oscillation frequency, amplitude, decay rate and symmetry against the other phases — this is one of the most effective methods for finding interturn defects, but a wrongly chosen voltage can damage a weakened winding.
Another method is applying the same reduced AC voltage to each phase and comparing currents: a phase with an interturn short may draw more current and heat up faster. The voltage-drop method is useful for low-resistance windings, armatures and pole coils — the same test current is passed through them and the voltage drop is measured.
An interturn short can cause unequal phase currents, but a raised current in one phase can also result from voltage imbalance, a poor contact, rotor damage, or mechanical overload — so current asymmetry alone cannot be taken as proof of a stator defect. Current spectrum analysis can reveal electrical asymmetry and characteristic changes in harmonic content, and works best as part of a broader diagnostic.
Under a reduced test voltage, or during operation, a defective area may heat up faster — a thermal camera can pick out a locally hot coil or a bad joint, though the winding may be hidden by the frame and a short test may show only a small difference. For large machines, pole coils and armatures, induction sensors and search coils are also used to monitor magnetic flux.
| Test result | Likely cause | Next step |
|---|---|---|
| Low resistance on every phase to frame | Moisture or general contamination | Clean, dry, retest |
| Low resistance on one phase to frame | A local phase defect | Separate and localize |
| High resistance to frame, but unequal currents | Interturn defect, supply, or rotor | Resistances, inductances, a surge test |
| One phase’s resistance is lower | An interturn short or a connection error | Check inductance and run a surge test |
| One phase’s resistance is higher | An open branch or a poor contact | Check the connections |
| One phase’s inductance is lower | Shorted turns | Run a surge test |
| One phase draws more current | A winding defect or voltage imbalance | Measure voltages and winding parameters |
| Megger reading rises after drying | Moisture | Finish drying and retest |
| Resistance stays low after drying | Insulation damage | Disassembly and repair |
Usually not. It checks insulation to frame and between electrically separated circuits, not between adjacent turns of the same coil.
Possible causes include an interturn short, overload, rotor damage, incorrect supply, or a cooling problem.
A surge tester is the most informative. A milliohmmeter, an LCR meter, current analysis and thermal analysis are also used.
Only for a gross defect — one or a few shorted turns often produce no noticeable change on an ordinary multimeter.
No. The motor and cable must be disconnected from the power electronics as the equipment’s instructions require.
No. The cause can be moisture, contamination, the cable, or the terminal box.
No, not once an electrical contact has already formed between turns. Drying only helps with moisture.
EPR (Elektropromremont) performs comprehensive diagnostics, repair and testing of insulation systems in industrial electric machines.
The scope of work includes:
This material is for informational purposes. The values, diagnostic methods, scope of work and recommendations given here are general and do not replace the manufacturer’s technical documentation. The final decision for a specific machine is made from its own diagnostics and inspection, taking into account its type, power, design, duty, operating history and applicable standards.
We measure insulation resistance, search for an interturn short circuit with a surge test, and carry out a complete winding assessment with a test report.