Common Mode Chokes (CMC) For Motor/Generator Protection – Cool Blue

High Bearing Currents Degrade Motor/Generator Performance and Decrease MTBF

The Problem:

High bearing currents are damaging the motor bearing and raceway.  The damage then degrades bearing performance and ultimately shortens the life of the motor.  The raceway is damaged through destructive electric discharge machining (EDM) causing microscopic pitting of the raceway.  Continued exposure to the high bearing currents and shaft voltages softens the raceway metal creating fluting when the ball bearings roll over the pits.

All ball bearings and wear surfaces (raceway) have asperities no matter how smooth the finish is.  The problem is common to all devices with ball bearings and wear surfaces from motors to generators.  The ball bearings ride on a film of oil providing the lubrication isolating the bearings from the raceway (no ideal contact).  There is a voltage on the shaft of the motor/generator.  The shaft voltage is due to small dis-symmetries of the magnetic field in the air gap as well as the noise injected on the shaft from external sources such as electrostatic voltage.  Sources of electrostatic induced shaft voltage on the shaft are:

• Rotor charge accumulation from the mechanical systems such as belt driven couplings, ionized air passing over the rotor fan blades or high velocity air passing over the rotor blades in the turbine.
• External rotor sources such as the static exciter in a turbine generator.
• Electrostatic coupled shaft voltage from the PWM Inverter.

Under ideal conditions, high rotor shaft voltage will not charge due to a continuous contact between the bearings and the raceway thru the asperity contact.  However the oil establishes a circuit equivalent of a capacitor between the bearings and the raceway.  There is instantaneous discharge causing high currents when:

• The voltage exceeds the dielectric breakdown of the oil.
• An asperity point breaks through the oil film forcing contact between the bearing and the raceway.

The Solution:

The solution is to reduce or eliminate the electrostatic coupled voltage in the motor/generator/inverter system caused by the inverter or turbine drive.  The implementation calls for the minimization of the electrostatic voltage by placing a common mode choke on the drive lines of the inverter or turbine generator.

Cool Blue® cores made from nanocrystalline material NANOPERM® are used to reduce damaging motor bearing currents in modern high power inverter systems operating at high switching frequencies and turbine generator drives. In order to achieve an efficient reduction in these destructive effects, one or more Cool Blue® cores of suitable geometry have to be placed together over the connector cables in the DC-Link as well as at the inverter output.  In this configuration, the cores operate as a common mode choke.  The use of Cool Blue® cores not only significantly reduces the over voltage peaks (electrostatic induced shaft voltage) at the motor terminals, but also suppresses the asymmetrical EMI currents which are generated by the parasitic currents (magnetic field dis-symmetries) of the motor itself together with the motor cable.  Adding this common mode choke significantly increases the service life of the motor bearings and thus reduces maintenance costs and downtime.  This method is well suited to maintenance retrofit as well as new installs.