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Which Drive Is the Right One?
Tips on selecting between inverter, vector and servo drives.
By Design News Staff -- Design News, September 25, 2006
With inverter, vector and servo drive technologies available, it's difficult to decide which one is right for your application. So before you make a choice, here are some things you should know about these drives.
Inverter Drives
Inverter techniques are used to control induction motors. The drive uses either six-step techniques or synthesizes a sinusoidal waveform. The frequency of the generated waveform controls motor speed.
For standard AC induction motors, inverters provide speed regulation that is approximately 1.5 - 3 percent of base speed. The low-end of controllable speed starts about 10 percent of base speed. From there, constant torque can be provided to base speed, with constant horsepower to 1.5 times base speed. That's why you'll find inverters used in adjustable speed applications such as centrifugal fans, conveyors, pumps, mixers and variable torque loads.
Advantages of inverters include low initial cost, reliability, no tuning necessary and they run out of the box. Design specifications include peak overload capacity of 150–200 percent, controlled reversing, pre-set speeds and programmable I/O.
Vector Drives
Vector technology is used to control induction motors with an appropriate feedback device such as an encoder.
With feedback, vectors can provide full torque from zero rpm to base speed, and speed regulation approaching 0.1 percent of set speed is attainable. Constant horsepower is provided up to 2.5 times base speed. Because of these features, you'll see vectors in high performance adjustable speed applications such as machine tool spindles, test stands, extruders and hoisting applications.
Some vector designs include "line regen" capability. When stopping large inertia loads, the power generated by back driving the motor is returned to the incoming power line, back to the electric company. Since these units typically operate near unity power factor, they also offer energy savings.
Advantages of vectors include reliability, capability of rated torque to zero rpm, torque operating mode, good speed control and self-tuning. Design specifications include peak overload of 150–200 percent, control reversing and programmable features including programmable I/O.
Servo Drives
The trend with servo drives is toward brushless which use resolver or encoder feedback. The feedback selected depends upon application demands such as temperature, vibration, and accuracy.
These drives provide good low speed operation down to zero speed and higher speed range capabilities. Extreme accuracies are attainable using high line count encoders. You'll find servos used in applications like cutting, printing, labeling, packaging, food handling, robots and factory automation.
Advantages of brushless technology include higher torques in smaller packages, lighter weight (for the same HP motor), higher speed range capability, velocity or torque operating modes, faster acceleration and positioning. The control has 200 percent overload capacity, however the motor has 300–400+ percent overload capacity so oversize to make use of this extra capacity. Design specifications include rapid reversing, auto-tuning and programmable features.
Inverter, vector or servo drive technologies are all available covering a wide power range. Within each technology there are choices.
Ask an Expert
Find out which drive is right for your application. Go to http://rbi.ims.ca/4935-702.
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