With their unique triggering and measuring functions, Fluke ScopeMeter 190 Series II portable oscilloscopes are ideal for analyzing the voltage-frequency ratio of pulse-width-modulated variable speed drives.
Variable Speed Drives
Variable Speed Drives (VSDs) provide a convenient and affordable method for varying the speed of robust ac motors. When connected to the mains, the rotation speed of an ac motor is directly related to the mains frequency and the number of poles of the motor. Traditionally an external gearbox was the only way to work with different speeds. This all changed with the introduction of high-power semiconductors, which made it possible to build variable speed drives by electronically creating a supply voltage that operates at different frequencies. This has, however, created the need for new measurement capabilities, which are provided by the Fluke 190 Series II portable oscilloscope.
Pulse Width Modulation
AC drives using Pulse Width Modulation (PWM) have found their way into many applications such as fans, pumps and conveyor belts driven by squirrel- cage asynchronous ac motors. These motors are robust and require little maintenance since they have no brushes that would need regular replacement.
The basic structure of a variable speed drive includes an input rectifier converting
the mains voltage into a dc voltage that feeds the so called dc-bus. This dc voltage is then converted into a ‘variable frequency voltage’ using electronic switches. Since the speed of the motor can be adjusted with a simple potentiometer or control signal from an external source, these drives have become a popular replacement for gearboxes.
AC motors are designed for use with a rotating magnetic field of constant strength. The magnetic field is generated from the applied voltage, and its strength is proportional to the V/Hz ratio. Normally the motor is designed to work with the local mains voltage (230 V/400 V or 120 V /208 V) and mains frequency (50 Hz or 60 Hz). The nominal values are shown on the motor type plate When the motor is connected to a VSD, the drive unit changes the frequency of the output voltage, thereby changing the speed of the rotating magnetic field and thus the speed of the motor.
By: Fluke Corporation
The automation of more and more processes and operations in today’s factories and commercial buildings is helping to reduce energy consumption and increase safety and productivity as never before. However, automation has also added a large dose ofcomplexity for the technicians who maintain and troubleshoot the systems. Click here to learn how Fluke's CNX 3000 Wireless system can help.
By: Gates Corporation
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By: Gates Corporation
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By: ITT Corporation
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By: Flir Systems
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