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By Sheila Kennedy
The goal of a variable-speed drive (VSD) is to modulate motor speed to reduce power demand and generate energy savings. Drive options constantly change , and how you match motor and drive affects system efficiency. Understanding the VSD’s effect on speed and resonant frequencies can reduce a motor’s life-cycle cost.
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Transforming technology: “The drives are going in two directions. On the one hand, there are simple drives with fewer I/O, analog inputs and one output while, on the other hand, there are the fully-featured drives,” says Bill Colton, Baldor’s Los Angeles district manager, who has watched VSD technology evolve for 27 years. “Another trend is that drive types are gradually melding into a common product.
Inverters are doing sensorless vector, vector drives doing inverter and brushless servo applications, and so forth.”
Baldor’s H2 inverter drive offers open-loop speed control in V/Hz mode for multiple motors, as well as encoderless vector for higher performance — all in one package. The V/Hz mode is for applications that require adjustable speed operation and good speed regulation, such as pumps, fans and general machinery applications. V/Hz is preferred for operating multiple motors simultaneously from a single motor control.
The encoderless vector mode provides tight control of speed (e.g., for grinders) and current (e.g., for winders) using open-loop control. With fast microprocessors and three-phase current feedback, encoderless vector drives can provide full torque down to 20 RPM on most four-pole inverter-duty motors. You can use a full vector version for full torque down to 0 RPM.
To help match the motor and drive, Baldor developed a Matched Performance Curve using lab data from Baldor motors and controls. The curve illustrates the continuous and intermittent motor torque available at various speeds, and the motor’s safe operating envelope below and above its base speed.
However, VSDs can make it more difficult to conduct PdM and vibration monitoring, which require knowing motor speed. VSDs operate at many speeds, and detecting the motor’s instantaneous speed can be tricky. Variances also make it difficult to compare one set of ratings to another.
Another problem is the wear and tear on motors that operate at or near the resonant frequency of their supporting structures. “Quite often, people don’t give this any consideration whatsoever when installing VSDs, which can increase the risk of equipment failure and make it more difficult to monitor vibration,” says Douglas Smithman, president of EMP Engineering Services.
He recalled one steel mill with 112 motors and VSDs. If the mill operated the production line at a resonant speed, motor bounce would be on the order of 40 mils — almost like a jackhammer. This damaged motors, bearings and couplings.
Identify the system’s resonance points and avoid them. “Do modal testing to know what your resonant frequencies are,” Smithman suggests. “If you must operate at resonance, take steps to reduce the effects — otherwise the equipment will fail.”
Modal testing involves using an electromagnetic shaker or impulse hammer to apply a force that is measured by an accelerometer. The Modal Shop has recently introduced a new modal shaker that weighs less than 30 pounds and is capable of providing 100 lbf (440 N) of peak force excitation. It’s suitable for most structural modal analysis applications, including single and multiple inputs (SIMO and MIMO) using random, burst random, sine dwell or chirp excitation signals.
The SpeedVue 430 Laser Speed Sensor by CSI (a division of Emerson) addresses the speed measurement problem. The sensor automatically identifies shaft speed when used with specified CSI Machinery Health Analyzers. SpeedVue’s Class III laser and advanced signal processing can capture rotational speed from distances of 30 ft (without reflective tape) or 100 ft (with reflective tape). When the laser beam illuminates an exposed rotor, the analyzer automatically displays machine speed. In most cases, SpeedVue doesn’t require markers and can work on dirty and highly polished shafts.
E-mail Contributing Editor Sheila Kennedy, managing director of Additive Communications, at Sheila@addcomm.com.
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