How To Prevent Motor Damage

A natural concern when operating a variable frequency drive system is the prevention of motor damage over time.

While VFDs can help to cut electricity costs by adjusting to match demand loads, the fluctuation of the electric current can have harmful effects on your system.

High frequency circulating currents produced by the variable frequency and voltage of these systems can result in motor bearing fluting, pitting, and frosting, lubrication breakdown, and electrical discharge machining (EDM). When preventing against motor damage and these unwanted effects, it is important to first understand what causes them.

Why Motors Fail

Electric motors fail for all kinds of reasons. Understanding the various causes and contributing factors will help you to troubleshoot and determine the appropriate solution for your system:

• Bad bearings – Bearing-related complications are the most common root cause of electric motor breakdown, resulting in more than half of all failures. Check for contamination, denting, scratching, or any kind of distortion, as this could lead to excessive endplay or vibration.
• Bearing currents – When a motor’s dielectric oil film breaks down, the voltage increases and begins surging through the lowest impedance path to the ground – the motor shaft. The high frequency current flows over the bearings, resulting in EDM.
• Inadequate grounding – Especially when dealing with the high frequencies of VFD systems, it’s crucial to provide a low impedance path for the current to flow to the ground without passing through the shaft. To some extent, bearing current is unavoidable — but preventing a high frequency flow will help to prevent the damage caused by EDM.
• High carrier frequency – While there is no magic number to determine the safest frequency range for your system, experts recommend adjusting it as low as possible without creating disruptive noise. Systems with higher carrier frequency will run quietly but will be more destructive to bearings over time.