Consulting-Specifying Engineer - July 2008 - (Page 46)

It is the electrostatic-induced voltage that has led to several bearing failures. The stray capacitances, which normally have negligible effect, come into play when high-frequency pulses are impressed upon the winding. Figure 7 shows the relevant capacitors and the voltage divider formed by these capacitances. A high-frequency voltage VB appears across the film of lubricant between the bearing balls and the stationary surface. The ratio VB/V is called the bearing voltage ratio (BVR). The BVR typically is 3% to 10%. If it is high enough, the voltage VB causes a dielectric breakdown of the lubricant film creating a current called the electric discharge machining, or EDM, current. The breakdown is in the form of a tiny arc between the rolling elements and the bearing race. The threshold voltage for the arcing to occur is approximately 5 to 30 V. The arcing causes bearing erosion at an accelerated rate, and surface irregularities (called fluting) appear in the bearing race. The irregularities become progressive until the bearing fails. The question is how to minimize the damage to the bearing. A low-PWM carrier frequency and suppression of the ringing helps. An output filter or a dv/dt filter reduces the rate of rise of voltage of each pulse and also reduces the capacitive-induced voltage in the shaft. Shielded cables between the VFD and the motor also assists in reducing the dv/dt. Insulating both the drive-end bearing and the NDE bearing reduces the BVR. Finally, a shaft grounding brush is an economical way of preventing the arcing in the bearing. How to ensure compatibility A few suggestions on how to engineer a successfully matched VFD-motor pair are: • Know the type of load and its torque-speed characteristic to correctly size the motor. Be aware of the heat dissipation problem in constant torque loads at low speeds. Specify the motor to have Class F insulation (239 F rise) but to have Class B temperature rise (194 F rise) at the rated full load. • Specify inverter-duty motors and reference NEMA MG-1 Section IV Part 31. • Estimate the length of the cable from the VFD to the motor and supply this information to the VFD vendor. • Specify an output filter and/or a dv/dt filter, particularly for medium-voltage drives. • Specify insulated motor bearings and a shaft grounding brush arrangement, particularly for medium-voltage drives. Some manufacturers offer an integrated VFD-motor package. This product transfers the responsibility of matching to the manufacturer. Use this product wherever it is possible to locate the VFD close to the motor. Peeran has more than 20 years of experience in the design of electrical distribution systems. For several years he was an adjunct professor at Northeastern University, Boston, and is a member of the CSE editorial advisory board. Input #222 at csemag.com/quickResponse 46 Consulting-Specifying Engineer • JULY 2008 http://www.uecorp.com http://www.uecorp.com http://csemag.com/quickResponse

Table of Contents Feed for the Digital Edition of Consulting-Specifying Engineer - July 2008

Consulting-Specifying Engineer - July 2008 Contents Viewpoint Letters News M/E Roundtable 40 Under 40 Using Demand-Based Reset Strategies VFDs and Motors: Making the Right Match Grounding and Bonding Practices for Hazardous Areas Codes & Standards Case Study New Products Equipment Lifecycles Advertiser Index Green Space

Consulting-Specifying Engineer - July 2008

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