The function of a frequency converter drive is to regulate motor speed to reduce power demand and save energy. The selection of frequency converters often changes, and the matching of the motor and frequency converter affects system efficiency. Understanding the impact of frequency converters on speed and vibration frequency can reduce the life-cycle cost of the motor.
“Due to technological innovation, frequency converters have developed in two main directions. On the one hand, simple frequency converters have fewer input/output interfaces, analog inputs, and one output; on the other hand, fully functional frequency converters exist,” said Bill Colton, Los Angeles regional manager for Baldor, who has been following the evolution of variable speed drive technology for 27 years. “Another trend is that frequency converter types are gradually converging into a common product category. For example, inverters can be used for sensorless vector applications, vector frequency converters can be used for inverters and refresh-free servo applications, and so on.”
Baldor's hydrogen inverter frequency converters provide open-loop speed control in voltage/frequency mode for multiple motors, as well as high-performance, code-free vector control—all packaged together. Voltage/frequency mode is suitable for systems requiring variable speed operation and good speed control, such as pumps, blowers, and general machinery applications, and is particularly well-suited for controlling multiple motors simultaneously from a single motor.
Employing open-loop control, the vector mode, requiring no coding, precisely controls the speed of grinding machines and the current of winding machines. With the aid of a high-speed microprocessor and three-phase current feedback, the coding-free vector inverter can effectively reduce torque to 20 rpm on most four-terminal variable frequency motors. You can also use full vector mode to reduce torque to 0 rpm.
To match the motor and frequency converter, Baldor developed corresponding performance curves using experimental data from its internal engines and controllers. These curves reveal the torque of the motor during continuous and intermittent operation at different speeds, as well as the fluctuation of the motor's safe operating envelope around the speed baseline.
Measuring velocity and vibration
However, variable speed inverters make product data management and vibration monitoring more difficult because both require knowledge of the motor speed. Variable speed inverters can operate at many speeds and are capable of sensitively detecting instantaneous speed. Due to various variables, comparisons between different speed levels are difficult.
Another issue is wear and tear when the motor operates at a frequency close to the resonant frequency of the mounting frame. "Although this increases the risk of equipment failure and makes vibration monitoring more difficult, people often overlook this issue whenever they install a variable speed drive," said Douglas Smithman, president of an electrical discharge machining (EDM) engineering services company.
Douglas Smithman recalls a steel mill with 112 motors and variable speed drives. If the steel mill's production line were operating at resonant speeds, the electric motors would vibrate almost like a hand drill, about 40 milliinches.
Identify and avoid system resonance points. “Do modal testing to determine the resonant frequencies,” Smithman advises. “If you have to operate at a resonance point, try to minimize its impact, otherwise the equipment will malfunction.”
Modal testing involves generating a force measurable by a velocimeter using an electromagnetic oscillator or impact hammer. Modal Shop recently introduced a new modal oscillator weighing less than 30 pounds that delivers a peak excitation of 100 pounds of force (440 Newtons). It is suitable for most modal analysis applications, including single-input and multi-input methods, with excitation signals that are random, transient bursts, sinusoidal mode-closing time signals, or linearly modulated pulse signals.
The SpeedVue 430 laser speed sensor, manufactured by CSI, a division of Emerson, solves the speed measurement problem. With the assistance of a designated CSI machine health analyzer, the sensor automatically detects shaft speed. The SpeedVue's third-stage laser and advanced signal processing technology enable it to capture rotational speeds from reflective areas as far as 30 feet (non-reflective) or 100 feet (reflective). The analyzer automatically displays the machine speed when the laser beam hits the bare rotor. In most cases, the SpeedVue requires no recorder and can operate on dirty, highly polished shafts.
