How to Determine If a Permanent Magnet Motor in Your Air Compressor Has Lost Magnetism

Permanent magnet variable frequency screw air compressors are widely recognized for their high efficiency, energy savings, and stable pressure output. However, not all permanent magnet motors are created equal. Poor quality components or improper configuration can lead to demagnetization, which often results in costly motor replacements.

So how can you determine if your permanent magnet motor is demagnetized? Let’s explore the signs, causes, and effective prevention strategies.

● Abnormal Current During Operation

If the compressor starts up normally but the operating current increases significantly over time—eventually triggering a frequency inverter overload alarm—this could indicate demagnetization. First, verify that the frequency inverter is correctly selected and its parameters haven’t been tampered with. If everything appears normal, check the back electromotive force (back EMF):

• Disconnect the airend from the motor
• Run the motor in no-load mode up to rated frequency
• Measure the output voltage (this is the back EMF)
• If the measured back EMF is more than 50V below the rated value on the motor’s nameplate, demagnetization is likely

● Continuous Overcurrent Alarms

A demagnetized motor usually runs with current well above the rated level. If overcurrent only occurs at very low or very high speeds, or only occasionally, demagnetization may not be the cause.

● Misdiagnosed Selection Issues

Demagnetization takes time—sometimes months or even over a year. If overcurrent occurs shortly after installation, it’s more likely due to incorrect motor sizing rather than actual demagnetization.

• Overheating due to fan failure
• Lack of thermal protection systems
• High ambient temperatures
• Poor motor design

✅ Select the Right Motor Power

Demagnetization is strongly linked to excessive operating temperatures, often caused by motor overload. Choosing a motor with the appropriate power rating is key. It’s recommended to size the motor with about 20% extra capacity based on actual load requirements.

✅ Avoid Heavy and Frequent Starts

For squirrel-cage asynchronous start permanent magnet motors, avoid heavy-load direct starts et frequent startups. During asynchronous startup, fluctuating torque can cause magnetic stress, accelerating demagnetization.

✅ Improve Motor Design

• Increase Permanent Magnet Thickness: A thicker magnet can resist demagnetization forces caused by armature reactions and radial magnetic flux.
• Add Rotor Ventilation Channels: Designing internal cooling ducts into the rotor helps lower magnet temperature, reduce rotor temperature rise, and improve efficiency.

Detecting and preventing demagnetization in permanent magnet motors is crucial to the reliability and long-term operation of your air compressor system. Regular current checks, proper motor selection, and design improvements can significantly reduce the risk of motor failure.