Consulting-Specifying Engineer - July 2008 - (Page 43)

From a control point of view, conventional VFDs for pumps and fans provide open-loop control; the VFD supplies a certain voltage at a certain frequency, and the motor rotates at a speed determined by its characteristics. Speed adjustment is either manual or by a remote signal. In a flux-vector drive, there is a continuous feedback of the motor speed and torque to the VFD. The VFD adjusts the voltage and frequency to produce the required operation. The flux-vector drive gives the ac motor the same capabilities as the dc motor. The direction of rotation can be reversed without actually switching the phases. Earlier flux-vector drives needed feedback from a shaft position sensor. Modern drives are sensor-less drives relying on the current, voltage, and speed measurement to compute the torque. Voltage doubling, critical cable length, and dv/dt PWM drives produce an output voltage in the form of high-frequency rectangular pulses of the same magnitude. The frequency of the pulses (known as the PWM carrier frequency) is in the range of 4 to 16 kHz. The width of each pulse is modulated such that the instantaneous average value is a sine wave. Figure 4 shows a line-to-line output voltage of a PWM VFD. For illustration, only seven pulses in one half-cycle are shown. Actually, for a typical carrier frequency of 4 kHz, there would be 33 pulses in each half-cycle. The magnitude of each pulse is equal to the dc link voltage of the VFD (typically 648 V in 480-V 6-pulse drives). The pulses are rectangular with a rate of rise of approximately 5,000 V/microsec. Higher frequency in the range of 4 to 16 kHz makes the motor current more sinusoidal, reduces the noise in the motor, and reduces the switching losses in the VFD transistors, thus increasing the efficiency of the VFD. However, the high-frequency pulses create greater stress in the motor insulation. The train of pulses invading the motor causes three deleterious effects. First, each pulse nearly doubles its magnitude after it hits the motor winding because of the higher impedance of the motor winding as compared to that of the cable. There is a traveling wave phenomenon here. The pulses travel toward the motor at a velocity of approximately 330 ft/microsec (approximately one-third the speed of light). The peak voltage impressed upon the winding at the leading edge of the pulse depends upon the rate of rise of the voltage and the velocity of propagation. If the length of the cable is more than a certain length—called the “critical length”—then the voltage will be twice the magnitude of the pulse (the full doubling effect). The critical cable length is determined by the following equation: Lcritical = v*Tr/2, where v = velocity of propagation, ft/microsec Tr= rise time, microsec Input #221 at csemag.com/quickResponse Consulting-Specifying Engineer • JULY 2008 43 http://www.yaskawa.com http://www.yaskawa.com/E7 http://www.yaskawa.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

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.