Motors & Drives Information

Alternately called speed controls, small variable speed drives (VSDs) typically range from 10 to 2000 watts in output power. The core of this lower power class is best represented by VSDs ranging from 75 to 750 watts, often described as the fractional horsepower segment of this important family. Applications can be found in many markets; some of the most popular uses include variable speed conveyors, multi-axis conveyors, bar feeders, fans, compressors, pumps, mixers, food processing equipment, automated ticket handling equipment, body scanning actuators, and centrifuges.

Regardless of the type of application, a VSD must control the speed of the motor over a wide speed range. It holds the motor's velocity signal within a specified range as a function of voltage, temperature, and load changes. Actual performance parameters include speed range and speed regulation. Other important performance parameters include motor torque, power levels, input current levels, and motor efficiency.

Two control strategies are used in industry: open-loop control and closedloop control. Open-loop control depends on the electric motor's internal regulation. Closed-loop control techniques measure the motor's speed and compare it to the desired speed values.

Most smaller VSDs in use today continue to employ brush DC motors for a majority of applications due to their lower unit cost and ease-of-use, but technology change is on the immediate horizon. Two different VSDs - brushless PM and AC induction driven VSDs - are competing technology replacements. Both provide performance enhancements and better long-term cost savings than equivalent brush DC VSDs.

At the beginning of the design process, users may select a brushless PM VSD (employing a brushless DC motor) because they need high performance capability and are willing to pay more to obtain it. AC induction VSDs, on the other hand, are selected for longer life requirements and acquisition pricing similar to brush DC VSDs.

# posted by Medical Information : 2:13 AM