What are Insulated Bearings? 

Insulated bearings are bearings that go through a spraying process wherein they are coated with a film that protects the bearing from erosive currents. This not only improves the service life of the bearing, but protects the motor from more severe consequences of bearing failure. 

Not all motors are vulnerable to these erosive currents, but those that are vulnerable will always require protection against them. If your motor is operated with a VSD (Variable Speed Drive), bearing protection is especially important as VSD motors produce PWM (Pulse Width Modulation). 

Pulse Width Modulation makes your motor prone to motor bearing failure. Insulated bearings – and better alternative solutions such as insulated end shields – are highly effective preventative measures against this failure afflicting your system.   

Why Bearings Need Protection  

The Danger of PWM 

As the VSD’s switching occurs, it induces eddy currents in the shaft that circulate within the motor, from the shaft, through the bearings, and to ground via the frame. The use of an insulated bearing unit can break the flow of the bearing current.

If a standard bearing is subjected to the effect in question, the insulation (lubrication grease) will generally be overcome by the induced voltage, past the lubrication’s breakdown voltage. Thus allowing the current to circulate through the bearings.

What is the effect of running current through the bearings? Bearing currents can create a large set of discharge currents which are able to damage the raceway of the bearing. Coupled with the high frequency of switching from the VSD, this will result in vibration in the motor shaft, noise, and added heat.

The general advice given by various manufacturers and industry groups is similar, to use insulated bearings, or other insulation and earthing systems on motors 280-frame and up.

VSDs are great for modulating motor speed according to your system’s needs, optimising efficiency. However, as the frequencies are modulated, so are the width of the electromagnetic pulses generated by the motor’s operation. PWM in inverter-drive systems creates surges in EMC, which can damage bearings. 

This damage occurs because PWM pulses are not clean square pulses, and every rise and fall of every pulse has an overshoot or transient over-voltage. Transient voltage, harmonic distortion and reflective waves all have the capacity to harm your bearings, and can all be negated with the correct protection. 

EMC Surges and Bearing Pitting   

EMC (Electromagnetic Current) modulation and surges often onset a chain reaction that ends in motor bearing failure.   

EMC surges propel circulating currents within the motor, which can track to a shaft and damage it, leading to pitting bearings. Pitting bearings then lead to overheating and inferior efficiency, with a shortening lifespan as the ultimate consequence. Motor failure is a common consequence of current-related bearing failure, and can be avoided through the use of insulated bearings, insulated endshields and earth brushes. 

Eliminate Electrical Corrosion 

Eddy currents are loops of electric current that flow within conductors, generating high heat and energy losses. Eddy currents result in limited operative efficiency and their heat is harmful to a motor’s lifespan.  

Eddy currents invade the electric motor’s magnetic field, as that current affects the bearings and causes pitting, and failure. Insulated bearings are immune to these electrical fields as they prevent the eddy currents from invading your motor’s shaft and magnetic field in the first place. 

Harmonic Noise 

Aside from the aforementioned side effects of overheating, poor efficiency and damaged lifespan as the result of pitted bearings, pitted bearings can also create unpleasant and regulation-breaching harmonic noise. 

Furthermore, high frequency electrical noises from the VSDs themselves can cause current discharge that results in premature motor failure. This is all the more reason to use insulated endshields or bearings, and protective earth conductor brushes where necessary. 

Electric Discharge Machining 

Motor voltage making its way into the motor shaft can penetrate the insulating effect of bearing lubrication film. When this occurs, Electric Discharge Machining (EDM) occurs within the bearing, causing premature wear and failure. 

This voltage can invade a motor by way of voltage levels overcoming the impedance of bearing oil film in large motors, capacitive discharge current where shafts are not earthed through driven machinery in small motors, and through shaft grounding current leaking into the stator frame in motors of any size. 

Why it Matters  

Hefty Cost Savings 

Bearing failure results in motor failure as the motor will be inoperable until the bearings are replaced or repaired. This downtime is costly, as are the potential failures of the motor’s other components and surrounding equipment if a failing motor continues to operate. Below are example costs associated with the failure of various motor sizes, and the cost of purchasing and installing the corresponding AEGIS insulated bearings that would have prevented bearing-related failure 

Compliance  

Motors with an IEC frame size of over 280 is regulated by Australian Standard. This is required by law. Although not legally mandated, your project consultant will usually specify motor bearing protection and grounding for motors with a nominal power of 30 kW or higher. 

Reliability 

Thanks to the protection against electrical corrosion and bearing pitting afforded by the use of insulated bearings, motors are able to weather the storm of currents that occur based on the Pulse Width Modulations created by a Variable Speed Drive. This protects not only motors, but compressors, circuit boards, capacitors and wires as well – all of which are sensitive to electrical issues – from the damages associated with system overload. 

Alternatives to Insulated Bearings: 

Insulated End Shields 

Insulated end shields serve the same purpose as insulated bearings, but offer several advantages in practicality and serviceability. 

Choosing an insulated NDE endshield rather than Insulated bearings is recommended as insulated NDE endshield protects the motor in the same way, with the added advantage of being to use a standard bearing, which is beneficial for easier replacement onsite as standard bearings are more readily available.

It is less common for factories to keep insulated bearings in stock when compared to insulated endshields, so opting for insulated bearings rather than insulated endshields often incurs additional and unnecessary costs and time management restrictions. 

For the shield to fully operate as a protective conductor, it should have at least 50% of the phase conductors’ conductance. If the shield does not have enough cross-section for that, a separate earth conductor is needed and the shield provides EMC and physical protection only. The shield high-frequency conductance should be at least 10% of that of the phase conductors. 

Protective Earth Conductors 

When the insulated end shield does not have 50% or more of the phase conduct’s conductance, it should be used with a separate protective earth conductor. This earth conductor is a brush that connects to the motor’s shaft and conducts the shaft’s current to the earth, preventing fluctuations in current flowing through the system. 

Exclusions 

The use of Insulated End Shields is not available for motors with frame sizes below IEC 225. For this reason, using insulated bearings rather than standard bearings on motors that fall within this size range is highly recommended for the avoidance of damage during voltage surges. 

For Your Project 

Whether you are in need of an insulated NDE endshield or insulated bearing for your electric motor, Masterflow proudly supplies WEG components. With a history in electric motor innovation dating back to 1961, WEG envisions itself as a global benchmark brand for electrical mechanics.  

Whether you are unsure about bearings, end shields, chillers or pumps – Masterflow’s expertise is at your fingertips. Enquire now to discover the right specifications for your next project.