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DIGITAL MOTOR CONTROL

Digital control allows more efficient motor control with variable speed and sensor-less control. The term sensor-less control means that there is no position/ velocity sensor on the motor shaft, so the rotor position/velocity is calculated from measured current and voltage.

The sensor-less control provides a cost-effective and reliable solution that eliminates the position/velocity sensor, sensor wiring, sensor power supply and increases reliability. Still, there are applications where higher cost of sensors is not as important as higher position resolution. The most common speed/position sensors are:

• Tachogenerators
• Hall sensors
• Encoders
• Re-solvers

Applications requiring the motor to operate with a required speed (pumps, fans, compressors, etc.) are speed controlled. In variable frequency drives, motor speed is typically proportional to frequency. The actual motor speed is maintained by a speed controller to reference speed command. Speed control offers low dynamic performance. For high dynamic and stability performance, speed control with inner current loop (cascade control) is required. The majority of variable speed drives are controlled by cascade control. Most complex drives (servos, industrial robots, linear motors) require additional position control.

Applications requiring the motor to operate with a specified torque regardless of speed (hand tools, electric power steering, traction, vehicles, etc.) employ torque control.

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