Generator and Motor Testing for Onshore Wind Turbines

Wind energy is a fast-growing sector where operational efficiency is crucial to profitability. Onshore wind turbines operating in harsh environments with constant exposure to temperature fluctuations, humidity, dust, vibration, and electrical surges rely on electrical and mechanical components working in harmony to generate power efficiently. One of the most critical parts of the wind turbine’s system is the generator and motor, which convert wind energy into electrical energy. Regular rotating machine testing is essential to prevent failures, reduce downtime and maximise energy output.

Without proactive testing and maintenance, wind turbine issues relating to the motor or generator, such as winding insulation failures, bearing wear, and electrical faults, can lead to costly repairs and prolonged downtime. The key reasons for conducting regular rotating machine tests include:

• Early fault detection to identify issues before they cause complete failure, allowing timely wind turbine repairs
• Increased reliability and safety to ensure continuous operation with minimal disruptions
• Cost savings through reducing expensive emergency repairs and revenue loss from downtime
• Extended wind turbine lifespan by helping to prevent premature wear and tear on critical components

Electrical components are particularly vulnerable – according to industry surveys, electrical system faults are the most frequent, taking an average of 1.6 days to resolve. While gearbox failures are more severe – typically causing 6.2 days of lost generation – they occur less frequently. This makes electrical system maintenance a top priority in wind turbine operation and maintenance (O&M) strategies.

The Role of Electrical Testing in Preventing Failures

Proactive rotating machine testing can help detect issues early, reducing downtime and wind turbine maintenance costs. Common failure modes in electric rotating machines include bearing failures, stator failures and rotor failures with other issues including contamination and electrical surges. Among these problems, winding failures can be particularly costly due to:

• Lost power generation and revenue
• High repair and replacement costs
• Crane/jack-up hire expenses
• Reputational damage
• The stress involved in urgently restoring operations
• Higher insurance premiums after costly unplanned repair work

Winding failures in wind turbine generators and motors are often the direct result of insulation degradation. The insulation system in a motor or generator is designed to protect windings from electrical, mechanical, thermal and environmental stresses. When this insulation weakens or breaks down, it can lead to catastrophic failures, including short circuits, phase-to-phase faults and turn-to-turn shorts.

Understanding Insulation Failures

The root causes of insulation degradation in onshore wind turbines and generators include over-voltage spikes caused by surges from switching currents, lightning strikes, and other sources. Thermal ageing is an issue exacerbated by higher operating temperatures reducing insulation life. Mechanical stress is another issue, as movement during startup, load changes, and harsh vibration damages insulation. Finally, contamination from moisture, dirt and chemicals can weaken winding integrity.

Many companies rely primarily on Insulation Resistance (IR) testing, which measures the resistance of the insulation between windings and ground, detecting moisture ingress, contamination and insulation degradation. For high voltage (HV) motors, they’ll add a Polarisation Index (PI) test to measure resistance over time. But is this enough for effective wind farm maintenance?

Recommended Motor Tests for Wind Turbines

For a full picture, a multi-function, computer-controlled test unit can conduct comprehensive wind turbine power performance testing, giving you insights for a complete picture of machine health. At Megger, we recommend the following low-voltage and high-voltage tests:

Low Voltage Tests (<12VDC)

• Winding Resistance Test – checks for imbalance between phases, and detects high-resistance connections, open circuits and welded shorts.
• Inductance and Impedance Test – evaluates the electrical characteristics of windings and identifies reversed connections, open circuits and welded shorts.
• Phase Angle Test – evaluates electrical symmetry in windings.

Insulation Resistance (IR) & Diagnostic Tests

• MegOhm Test (500VDC) – measures insulation resistance to ground.
• Polarization Index (PI) Test (500VDC) – diagnoses the ground wall insulation condition over time.

High Voltage Tests

• DC Step Voltage Test (2400VDC) – detects weak insulation between winding and earth, ensuring reliability. 
• Surge Test with Partial Discharge Detection (2400VDC) – simulates transient voltage spikes to identify turn-to-turn and phase-to-phase insulation weaknesses; essential for monitoring HV generators and motors.
• Hi-Pot (Withstand) Test – determines ground wall insulation strength under elevated voltage stress.
• Diode Test – ensures proper electrical performance of rectifiers in the system.
• Partial Discharge (PD) Detection – finds small electrical discharges that signal insulation degradation or breakdown.
   

Why Testing Above Line Voltage Matters

Test voltages are typically based on a 690V winding, following the industry rule of thumb:

• In-service machines: 2x operating voltage (Un) + 1000V
• New windings: higher voltage thresholds apply

Applying higher-than-operating voltages in a strict, controlled sequence is crucial for several reasons. It replicates real-world voltage spikes and detects early insulation breakdown, preventing unexpected wind turbine motor and generator failures, as well as ensuring compliance with insulation safety standards.

Final thoughts

Rotating machine testing is a critical part of onshore wind turbines’ maintenance. A proactive approach to wind turbine repairs can help onshore wind farm operators avoid costly failures, maximise uptime, and optimise energy production. By integrating advanced testing techniques into wind farm maintenance schedules, operators can significantly improve the reliability and efficiency of their onshore wind energy assets

As stated, relying solely on MegOhm/IR tests is not sufficient. Instead, a full suite of static and dynamic tests should be factored into the wind energy maintenance cost to help identify faults before they lead to costly failures involving time-intensive wind turbine repair work. By implementing these best practices, onshore wind farm operators can reduce unplanned downtime, extend equipment lifespan and improve reliability and profitability. Investing in advanced motor testing techniques is a worthwhile approach that can prevent major failures and revenue losses in the future.

Ready to assess the health of your onshore wind turbines? Contact us today.