Friday, April 13, 2007
Motor Technologies Compete for Space in Small Variable Speed Drives
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.
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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.
Emerging Pressure Points
Factory automation machines used in lower intermittent duty cycle applications will continue to use brush DC VSDs as their first choice; however, an increasing number of factory machine builders are looking at lower life cycle costs. Key elements for better life cycle costs include longer life expectancy, lower maintenance costs, better power efficiency, lower electromagnetic interference (EMI), and lower thermal stress. These elements provide the user with cost savings over a longer time period.
The rapid increase in the use of nonlinear power electronics found in today's DC and AC VSDs has significantly increased the level of EMI in industry. The impact of government regulations and industry standards (FCC, IEEE, and IEC) concerning EMI levels places a premium on lower EMI performance. As a result emphasis is now placed on implementing new circuitry dedicated to minimizing EMI and using electric motors with lower EMI signatures.
AC induction USDs
The popularity of smaller AC induction motors in lower cost and higher volume markets (appliances, vending machines, etc.) has fostered its use in VSDs in industrial and factory automation markets.
The AC induction motor is a brushless AC motor with demonstrated longer life and lower maintenance performance. Power electronics packages continue to drop in price as quantities begin to climb and the semiconductor industry's manufacturing processes shrink electronic device size and package more components on a smaller volume of silicon. Of the many types of VSDs used, the inverter driven three-phase VSD and the single-phase control VSD are currently the most popular.
Benefits and disadvantages of the brushless AC VSD versus the brush DC VSD can be seen in the following comparison. The 90-watt, single-phase ES01+VS1590A VSD and the 200-watt, three-phase BHF62AT VSD from Oriental Motor represent two different AC closed-loop control schemes. The ES01 AC VSD uses a pulse generator to control speed regulation over a 17.8 to 1 speed range. The larger three-phase BHF62AT AC VSD uses a sensorless vector closed-loop control with inverter drive. The. two AC VSDs show similar regulation capabilities to the 90-watt brush DC VSD using a closed-loop back EMF control scheme. The DC motor has a wider speed range and better power efficiency, but its major limitations are in life cycle costs and EMI performance. (The brush DC motor generates large amounts of EMI.) While the DC VSD has lower acquisition, installation, and operating costs, the unit life expectancy is much shorter (due primarily to motor brush life). The lower performance AC induction motors achieve the same acquisition costs as the brush DC VSD at a longer life expectancy and much lower EMI levels.
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.
Advertisement
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.
Emerging Pressure Points
Factory automation machines used in lower intermittent duty cycle applications will continue to use brush DC VSDs as their first choice; however, an increasing number of factory machine builders are looking at lower life cycle costs. Key elements for better life cycle costs include longer life expectancy, lower maintenance costs, better power efficiency, lower electromagnetic interference (EMI), and lower thermal stress. These elements provide the user with cost savings over a longer time period.
The rapid increase in the use of nonlinear power electronics found in today's DC and AC VSDs has significantly increased the level of EMI in industry. The impact of government regulations and industry standards (FCC, IEEE, and IEC) concerning EMI levels places a premium on lower EMI performance. As a result emphasis is now placed on implementing new circuitry dedicated to minimizing EMI and using electric motors with lower EMI signatures.
AC induction USDs
The popularity of smaller AC induction motors in lower cost and higher volume markets (appliances, vending machines, etc.) has fostered its use in VSDs in industrial and factory automation markets.
The AC induction motor is a brushless AC motor with demonstrated longer life and lower maintenance performance. Power electronics packages continue to drop in price as quantities begin to climb and the semiconductor industry's manufacturing processes shrink electronic device size and package more components on a smaller volume of silicon. Of the many types of VSDs used, the inverter driven three-phase VSD and the single-phase control VSD are currently the most popular.
Benefits and disadvantages of the brushless AC VSD versus the brush DC VSD can be seen in the following comparison. The 90-watt, single-phase ES01+VS1590A VSD and the 200-watt, three-phase BHF62AT VSD from Oriental Motor represent two different AC closed-loop control schemes. The ES01 AC VSD uses a pulse generator to control speed regulation over a 17.8 to 1 speed range. The larger three-phase BHF62AT AC VSD uses a sensorless vector closed-loop control with inverter drive. The. two AC VSDs show similar regulation capabilities to the 90-watt brush DC VSD using a closed-loop back EMF control scheme. The DC motor has a wider speed range and better power efficiency, but its major limitations are in life cycle costs and EMI performance. (The brush DC motor generates large amounts of EMI.) While the DC VSD has lower acquisition, installation, and operating costs, the unit life expectancy is much shorter (due primarily to motor brush life). The lower performance AC induction motors achieve the same acquisition costs as the brush DC VSD at a longer life expectancy and much lower EMI levels.
# posted by Medical Information : 4:59 AM
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