Technical Points to Maintain Normal Operation of Permanent Magnet Motors

If a permanent magnet motor becomes demagnetized, the typical solution is to replace the entire motor, as the repair costs are extremely high. So how can we determine if a permanent magnet motor has become demagnetized? Let's explore further. 

 Initially, when the machine starts running, the current is normal. However, over time, the current begins to increase. If this continues for an extended period, it will eventually trigger an overload alarm on the variable frequency drive (VFD). 

•  First, we need to confirm that the air compressor manufacturer selected an appropriately sized VFD, and then check if any internal parameters of the VFD have been altered. 

•  If both of those checks are satisfactory, then we can evaluate the back EMF to diagnose demagnetization. To do this, we disconnect the motor from the machine head and perform a no-load test run, gradually increasing the frequency up to the rated value. At this point, the output voltage corresponds to the back EMF. If this measured back EMF value is more than 50V lower than the value printed on the motor nameplate, we can confirm that the motor has indeed become demagnetized. 
  

 1. Keep the permanent magnet motor working at rated current. 

Permanent magnet motors can become overloaded mainly due to excessive load torque, low voltage, or mechanical jamming of the driven equipment. If the overload condition persists for too long, the permanent magnet motor will draw a large amount of active power from the grid, leading to increased current and rising temperature. At high temperatures, the motor's insulation will degrade and the magnetic steel can become demagnetized. 

 Therefore, during operation of a permanent magnet motor, it is crucial to frequently inspect that the transmission components are running smoothly and reliably, that the couplings are properly concentric, and that the gear drives have sufficient flexibility. If any jamming is detected, the motor should be immediately stopped, the root cause investigated, and the issue resolved before resuming operation. 

 2. Always check whether the three-phase current of the permanent magnet motor is balanced 

For a three-phase AC permanent magnet motor, the current in any one phase must not deviate from the average of the other two phases by more than 10%. Adhering to this requirement ensures safe operation of the permanent magnet motor. If the current imbalance exceeds 10%, it indicates a fault condition in the motor that needs to be investigated and addressed. 

 3. Check the temperature of the permanent magnet motor. Always check whether there are any abnormal changes in the temperature of the bearings, stator, casing and other parts of the permanent magnet motor. 

Regularly inspect the bearings of a permanent magnet motor to check if they are overheating or lacking lubrication. If an abnormally high temperature rise is detected near the bearings, immediately stop the motor and perform an inspection. Examine the rolling elements and raceways for any cracks, scratches, or damage. Also, check for excessive clearance causing shaft wobble, or if the inner race is rotating on the shaft. 

The presence of any of these issues necessitates replacing the bearings before resuming operation. 

 4. Observe whether the permanent magnet motor has any vibration or loud noise 

If the permanent magnet motor vibrates, it will cause the load part connected to it to be non-concentric, causing the load of the permanent magnet motor to increase, causing overload operation and burning the motor. 

Therefore, when the permanent magnet motor is in operation, especially high-power permanent magnet motors, it is necessary to frequently check whether the anchor bolts, end covers, bearing glands, etc. are loose, whether the grounding device is reliable, and solve problems in a timely manner. 

 5. Keep permanent magnet motors clean

During operation, ensure that the area within at least 3 meters around the permanent magnet motor's air intake remains free from dust, water stains, and other debris. This precaution prevents the ingestion of contaminants into the motor, which could create a conductive path leading to short circuits, insulation damage, and potentially motor burnout. 

Consequently, it is crucial to maintain adequate insulation resistance and proper ventilation for cooling the permanent magnet motor. Only by providing these favorable operating conditions can the motor sustain safe and stable performance over extended run times.