Consulting-Specifying Engineer - July 2008 - (Page 44)

The velocity of propagation depends upon the size and type of the cable between the VFD and the motor. For example, in a 1/0 1/C unshielded cable in a rigid galvanized steel conduit, the velocity is 333 ft/microsec. The rise time of the pulse depends upon the PWM carrier frequency. Typically it is in the range of 0.2 to 0.8 microsec. A second injurious effect of PWM pulses is the high-frequency ringing at the leading and trailing edge of each pulse (see Figure 5) due to the stray capacitances of the motor winding, which accelerates bearing wear. Finally, the high rate of rise of voltage stresses the insulation of the motor winding end turns to a greater extent than the other parts of the winding. This may cause partial discharge—a localized corona or arcing that degrades the winding insulation and eventually leads to insulation failure—at the end turns. The voltage appearing at the motor is not the same as the output voltage of the VFD. Figure 5 shows the doubling effect, which will occur when the length of the cable from the VFD to the motor is greater than the critical length. The figure also shows high-frequency ringing due to stray capacitances. Because the high rate of rise of the voltage causes additional stress on the winding insulation, many VFD manufacturers offer a dv/dt filter, which is essentially a shunt capacitor that slowly reduces the rate of rise. (The term “dv/dt” refers to a differential in voltage with respect to a differential in time.) An inverter duty motor is designed to withstand the additional harmonic heating and the increased insulation stress due to the PWM VFD. Bearing currents There are two mechanisms that cause circulating currents, accelerating wear of the motor bearings: electromagnetic-induced currents in the shaft and the bearings; and electrostatic-induced voltage between the bearing balls or rollers and the stationary surfaces. Both mechanisms are enhanced when the motor is powered by a VFD. There is a small electromagnetic-induced voltage in all motor shafts in the axial direction caused by the leakage of magnetic flux linking the rotor and the shaft outside the airgap. Because of the slight static and dynamic eccentricity of the shaft, the leakage flux induces a voltage that establishes a circulating current through the shaft, the two bearings, and the frame (see Figure 6). The circulating current is small and normally is not a concern. Its path is broken by insulating one of the two bearings, typically the non–drive-end bearing. However, if there are harmonics in the motor current, such as in the case of CSI VFDs, the circulating currents increase and can cause local heating in the bearing. Because most PWM drives provide a nearly sinusoidal current to the motor, they only slightly aggravate the circulating current problem. Input #220 at csemag.com/quickResponse 44 Consulting-Specifying Engineer • JULY 2008 http://www.sens-usa.com http://www.sens-usa.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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