Variable Frequency Drives – VFD/AFD/VSD

To understand Electrically Induced Bearing Damage, a good understanding of VFD's (Variable Frequency Drives) must first be attained. When VFD's started to become popular (since the late 80's) one of the major issues was shaft current. For quite a while the problem was treated in the usual manner: insulating the motor ODE (Opposite Drive End) bearing, or adding a grounding brush on the motor drive end (DE) Driven end. The problem was that each of these proven solutions yielded mixed results. Many people disagreed on which solution was the best and many reported less-than-satisfactory results with either method. Higher switching frequencies of around 20 kHz caused more bearing problems than slower drive settings of around 5 kHz but there is no clear line above which problems are expected. The VFD works by rectifying AC (Alternating Current) to DC (Direct Current), and chopping the DC into positive and negative pulses to simulate an AC sine wave. Varying the DC pulse width simulates a variable AC sine wave and changes the frequency, thereby changing the motor speed. One problem with that is the common voltage (the voltage common to both input terminals of a device). When a 3-phase motor operates from a true sine wave, the common mode voltage is always zero, but with the VFD, the balance no longer exists.

Draw a vertical line (that's a point in time) at any point on the graph. Sum the voltages (above the horizontal axis is positive, below is negative), and the common mode voltage is always zero.

DC is either positive or negative, so at any point in time the three phases are either plus, plus, minus or plus, minus, minus. Common mode voltage is essentially line voltage, which, if not zero, as in the three-phase motor case, induces a current on the motor shaft. With the introduction of VFD's, shaft currents became a significant problem for motors, even ones much smaller than had previously experienced problems.