Why motor drives use input and output line reactors

An electrical motor drive refers to the system that controls the speed, torque, and direction of an electric motor. The primary purpose of motor drives is to regulate the performance of electric motors in various applications, such as industrial machinery, robotics, electric vehicles, and HVAC systems. Motor drives are essential for achieving precise control and efficiency in motor-driven systems.

Regulating the electric power sent to an electric motor is a task performed by high-speed switching transistors inside a motor drive, modulating the pulse-width of a high-frequency square wave to the motor. The high-speed switching happening inside of a motor “drive” circuit results in the drive drawing current from the AC power source as high-frequency pulses rather than as sinusoidal waves.

Line reactors work by presenting a greater series impedance to high-frequency harmonic currents than to low-frequency fundamental currents, following the inductive reactance formula XL= 2πfL. The greater the frequency (f) of current, the greater the inductive reactance (XL) and therefore the greater the attenuation of that current through that conductor. As one might expect, line reactors cannot preventharmonic distortion in the AC power system, but they do a great deal to mitigate the ill effects of harmonics produced by a motor drive.

Line reactors may also be used on the outputof an AC motor drive to filter harmonics from the motor itself. Like transformers, AC induction motors suffer greater core losses when exposed to harmonic currents, causing the motor to heat up more than it would if powered by AC power of one pure frequency:

Further Reading/reference:

  1. Application of line reactors to AC and DC motors
  2. Download complete PDF article on Motor Drive Features and Application of Line Reactors Handbook PDF Download
  3. DC injection braking
  4. Dynamic braking of AC Induction Motors
  5. Regenerative braking of AC Induction Motors
  6. Braking using Plugging

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