Frequently Asked Questions about CoolBLUE®

What is 'CoolBLUE® Inductive Absorber'?

In the given configuration CoolBLUE® soft magnetic cores work as a single turn common mode suppression choke. They do not affect the symmetrical power currents but efficiently dampen the asymmetrical radio frequency noise currents. CoolBLUE® also suppresses the asymmetrical EMI currents which are generated by the parasitic currents of the motor itself together with the motor cable.

CoolBLUE® Nanocrystalline absorbers are superior to other soft magnetic materials like ferrites because of significant advantages concerning their magnetic properties. As a consequence only about 20% of ‘magnetic mass’ is required – i.e. a significantly reduced number of cores is suitable.


No maintenance required on the cores. Once installed, they last a lifetime, and can even be left on the system if a new motor or drive is installed.

Why install the CoolBLUE® cores close to the drive, instead of on the shaft where the discharge is occurring?

Very simply, CoolBLUE® cores ‘absorb’ the energy caused by the VFD’s at the source…before getting to the motor.

Do CoolBLUE® cores absorb all of the currents?


CoolBLUE® cores installed with suggested guidelines, absorb between 65-80% of the currents. At 65% and above absorption, the currents are no longer dangerous enough to cause damage to bearings, lubrication, raceways, and circulating currents.

How do you measure high frequency currents?

We recommend using a Rogowski coil around the cable power lines. Only around the phases, not ground or shielding. We do not recommend using shaft voltage probes in measuring. This is dangerous to the technician and equipment, and most times not accessible to the motor.

MH&W now sells a Rogowski coil specifically designed for high frequencies in VFD systems.

What can I expect to see if I connect the Rogowski coil to a scope?

With a dual trace scope, you will see the high speed switching of the VFD, and the high frequency noise associated with the switching. MH&W engineering can help with this as well.

High Frequencies in the MHz Range

High Frequencies in the MHz Range


Before CoolBLUE


After CoolBLUE

Can CoolBLUE® cores be used outside?


The CoolBLUE® cores are housed in a plastic case, and not typically affected by outdoor conditions.

I have motors that need to be washed down with high pressure water and chemicals. Will this affect operation?


The great thing about CoolBLUE® cores is that they are not affected by cleaning, because they are not on the motor. The cores are installed at the source, VFD’s, thereby not in the cleaning area.

Do I need to use ceramic type bearings with CoolBLUE®, especially in higher horsepower type motors?

Absolutely Not

In fact, we have many customers now totally switching over to CoolBLUE® only, and using standard steel bearings. Very simply, CoolBLUE® cores ‘absorb’ the energy caused by the VFD’s at the source…before getting to the motor, so there is no need for ceramic bearings…on either end of the shaft!

I have one of the best VFD/motor systems on the market. Do I need to worry about electrical discharge occurring?


Electrical discharge will take place, because the currents are caused by the high speed switching generated from the VFD’s.

What is bearing fluting and frosting?

Electrical fluting and frosting occurs when a current is passed through the motor bearing instead of a grounded source.

Fluting: A ‘washboard’ appearance along the raceway will indicate bearing fluting, usually with a white or grayish color:

cb-faq-2-1   cb-faq-2-2

Frosting (or pitting): Frosting will be similar to fluting, but will appear to be a solid white or greyish color along the raceway. Frosting generally happens long after general fluting has occurred.

Please explain electrical discharge.

Please see the following white paper that describes this in detail: Overcoming Motor Bearing Failures in AC Motor Drive Systems

Are there other solutions to discharge currents?


One method is to place a mechanical ring with fibers contacting the shaft. There are several manufacturers of such rings. In theory, the rings provide an alternate path for these discharges and divert current away from the motor bearings. However grease, dirt, corrosion, etc. on the shaft will cause the grounding rings to lose contact with the shaft. Maintenance of the mechanical fixture is advisable. Installation is a major endeavor.

There are literally hundreds of combinations of motors/motor frames to choose from, therefore, hundreds of products to choose from for rings. Installation of complete centering of the ring, gluing, or boring holes into the motor case, cannot be installed over shaft key, shaft preparation, and much more make installation difficult. And, circulating currents are still an issue.

With CoolBLUE, there are just a handful of products to fit the system, and circulating currents are absorbed as well.

Download the CoolBLUE Application Guide for a full description.

Can I measure bearing currents with my clamp on amp meter?

Typically No

The majority of clamp meters will not measure peak. We have to measure peak current spikes, and the majority of clamp amp meters show RMS, or average. This does not give you the full impact of the damaging currents. There are a few clamp amp meters showing peak, but they also need to read high frequency spikes from several 100kHz to several MHz.

Do I need to measure shaft voltage?


Measuring shaft discharge voltage is not advisable or needed. Voltage is only part of the equation. Ohm’s law tells us that voltage times current = power (watts). We are concerned with current. For example, 30 volts times 1 amp is 30 watts. However, 30 volts times .1 amp is only 3 watts. Also, the actual measuring of shaft discharge voltage is dangerous and many times not accessible.

The measuring of current with a Rogowski coil around the 3 power phases is the most accurate, safe, and reliable way of testing. This can be done easily without jeopardizing safety. Please contact MH&W at 201-891-8800, or for more information.

Do CoolBLUE inductive absorbers also absorb stray grounding currents?


CoolBLUE not only absorb the high frequency damaging bearing currents, but also reduces transient voltages, stray capacitive currents, and common mode currents before they reach the motor system. There are many applications where the stray capacitive currents are fed from the shaft to ground by shaft grounding devices, or no devices whatsoever. However, this just diverted the energy onto a ground path. These currents then lead back to the VFD or other electronic devices causing damage or random system shutdowns. The use of CoolBLUE is like this simple analogy…get rid of the virus before it gets to the patient.

I installed a shaft grounding device on my motor. The motor bearings lasted about 12 months, which is about 2x longer than without the device. What kind of longevity can I expect with CoolBLUE?

You can expect at least 5x with CoolBLUE.

This goes without saying proper lubrication, alignment, and other factors outside the realm of VFD induced currents. Absorbing the currents is the best way to provide longevity to your system. And, CoolBLUE never has to be replaced because it is not a mechanical device, prone to wear, such as a shaft brush.