Electrical Cancer affects Motor Reliability
Inverters typically use PWM (Pulse Width Modulation) to reform an approximate Sine wave enabling Electric Motors to run at Variable Frequency.
This reforming of the supply to the motor is made with IGBT’s (Insulated Gate Bipolar Transistors) that switch in the range of 4 to 18kHz.
The
characteristics of the cable often sets up a resonance amplifying
these switching voltages, which can be seen in the MHz range. (FFT
Spectra in RED) which attacks Motor Insulation.
These High Frequency currents can flow in Common Mode or Differential Mode.
The SKF TKED1 measures discharges of EMF (Electro Magnetic Frequency) in the MHz range, but is only an indicator
Very much like an Electric fence, the switching voltage pulses often amplified to twice line voltage can be grounded and then the TKED1 measures very low counts.
Effective grounding of the Drive, Cable Screen, Motor and Cable tray is essential.
The Drive must also provide a low impedance path back to the transformer star point.
3Phi Reliability experience suggests that 75% of bearing currents can be drained to earth with proper termination practice.
So Why is High Frequency Current so Deadly to Electric Motor Insulation?
The impedance of an electric motor has several components
R Ohms The resistance typically of the conductors.
XL Ohms The Reactance dominated by Inductance of the coils.
XC Ohms The capacitance which is complex but mostly Insulation related.
The Total Impedance is Z= sqrt R^2 +(XL – XC)^2
The resistance should be as low as possible to reduce Copper losses, which leaves the Coils XL & Insulation XC the major contributors to Impedance.
These two components are greatly affected by Frequency, the Reactance XL = 2Pi*frequency* L approaches infinity as the frequency increases. At the MHz range this current is stopped by the coil and is the cause of reflective voltages between the Drive and the Motor amplifying the attack.
XC component of the Electric Motor acts as a dead short for High Frequency in the MHz range.
XC=1/(2PI x Frequency x Capacitance)
Therefore the High Frequency Current tracks across Insulation in Cables, Sleeving, and Motor Insulation.
Insulation Resistance for this High Frequency Component rapidly approaches Zero as Frequency Increase.
I call this High Frequency
Current “Electrical Cancer”
If left this Attack from High Frequency Current will decay Electric Motor Insulation and eventual failure.
37% of Electric Motors are on VFD Variable Frequency Drives but account for 82% of Winding Defects
The Phase Angle test of the All TestPro MotorGenie is a useful instrument to detect this decay.
A number of methods aim to protect the Bearings by isolation or grounding through Insulated Bearings & Brushes.
While these methods are valid by far the most effective solution is to “STOP THE ATTACK AT THE SOURCE” being the drive.
This significantly lowers the Reflective Voltages seen at the Motor and Protects the Motor Insulation.
EMF Cores (Electromagnetic Frequency) provide Impedance at higher frequencies therefore burn these unwanted currents.
The impedance starts at approximately 40kHz therefore not affecting Power or Communications.
STOP THE ATTACK AT THE SOURCE, permanent low cost Solution.
