How to Dry a Moisture-Damaged Three-Phase Asynchronous Motor

How to Handle Moisture-Damaged Three-Phase Asynchronous Motors

1. Drying Methods for Small-Capacity Three-Phase Asynchronous Motors

Small-capacity motors are relatively easy to disassemble and inspect. Depending on the working environment, they can either be dried on-site or taken to a maintenance area for treatment. The following are two commonly used drying methods:

Small low-voltage asynchronous motors are typically suitable for external heating. This method is simple and uses radiation, convection, or conduction from an external heat source to remove moisture. The two common approaches include:

●Drying with light bulbs or infrared lamps / ovens

●Drying with hot air blowers

When using bulbs or halogen lamps, avoid placing them too close to the windings to prevent overheating. Use properly shielded safety lamps. When using an oven, the temperature must not exceed 100°C.

2. Drying Methods for Medium and Large Three-Phase Asynchronous Motors

For larger motors, the following methods are commonly used:

Method 1: Current Drying Method

This method works by applying a low-voltage current to the stator windings while the rotor is locked. The heat generated by internal losses is used to dry the motor. This allows both stator and rotor to heat up, resulting in faster drying. It is especially suitable for large-capacity low and high voltage motors.

⚠️ Note: Calculate the maximum current allowed per phase during locked rotor operation — it should not exceed 50–60% of the rated current. Based on this, you can choose the appropriate voltage level for drying.

Method 2: Core Loss Drying Method

This method involves winding an excitation coil around the stator core and feeding it with AC power. This creates magnetic flux in the stator core, and the eddy current losses generate heat to dry the motor.

Additional Precautions:

1）For motors with removable rotors, drying should be done in clean air, and all parts must be cleaned before drying begins.

2）During drying, the winding temperature must remain below the insulation class limit. To be safe, keep it at least 10°C lower than the maximum permitted. The optimal winding temperature is typically 70–80°C.

3）For motors with severely damp windings, do not use DC current drying, as DC may cause electrolysis.

4）When using current or core loss methods, ensure the motor housing is properly grounded to prevent electric shock. Similarly, when using bulbs or halogen lamps, ensure electrical safety.

5）For sealed motors being dried without rotor removal, open the terminal box or inspection holes slightly to allow moisture to escape. For small low-voltage motors, slightly open the end covers for ventilation.

6）During drying, measure and record the temperature and insulation resistance of the windings regularly. In the initial stage, record every 15–30 minutes; later, record every 1–2 hours. This helps prevent localized overheating that could damage the motor.

In fact, there are many practical drying methods for moisture-damaged motors. The appropriate method should be chosen based on on-site conditions. Regardless of the method, always ensure the motor temperature stays within safe limits, and avoid causing new insulation damage. Pay special attention to equipment and personal safety.

In humid and rainy regions, especially in southern climates, motors must be properly protected against moisture. It is also important to select the right type of motor for damp environments and to establish preventive anti-moisture measures to ensure reliable and safe motor operation.