A Method for Compensating Wave Reflections in Transmission Lines


This technology is a method to eliminate voltage overshoot in cables used to connect AC electric motors and pulse width modulation (PWM) inverters. As a function of cable length and voltage rise time, voltage overshoot occurs when high frequency currents reflect between the motor and source ends of a cable. University of Illinois researchers have devised a compensator that shapes the output of the PWM inverter in order to eliminate these reflections; the method only requires knowledge of the transmission line characteristic impedance and propagation delay. This technique extends the life of motor insulation and protects voltage-sensitive devices.

Voltage overshoot occurs when high frequency currents reflect between the motor and source ends of an electrical cable. Reflective waves of current can build up, causing motor insulation failure as well as damage to voltage-sensitive devices. While there have been techniques developed to compensate for voltage overshoot, many are complex, need load characteristics, and often require the use of bulky and expensive equipment.

The University of Illinois technique provides a mathematically exact solution that modifies the output of the PWM inverter in order to eliminate wave reflection. This technique is designed to eliminate voltage overshoot in the cable that connects alternating current (AC) electric motors to pulse width modulation (PWM) inverters that use insulated gate bipolar transistors (IGBT). IGBT motor drive cables are particularly susceptible to voltage overshoot due to their extremely fast switching speeds. The compensator, which attaches to either the source or motor end of a cable, is a filter that uses an appropriate linear combination of voltages and currents to transform the transmission line into a pure delay transfer. This prevents wave reflection and thus voltage overshoot. Users do not need to know the motor or drives characteristics; however, they do need to know the transmission line impedance and the propagation delay in order to use this device.

This invention improves on existing solutions to voltage overshoot by providing an exact solution rather than approximation. 


The range of applications that use AC motors is vast, examples include:

  • Heavy Industrial Machines or Manufacture: Heavy industries including automotive, materials handling, mining operations, plastic and rubber production, ceramics, textile, and utilities.
  • Workshops: Metalworking, printing and woodworking shops.
  • Chemical Industries: Chemical refining, pharmaceutical production, and plastic fabrication.


  • Cost-Effective: Eliminates damaging wave reflection, prolonging the usefulness of motors and voltage-sensitive devices.
  • Low-Cost and Small: No large or expensive equipment is needed to implement this technology.
  • Self-Adapting: Modifies waveform as transmission cable properties change over time.
  • Versatile: Can be implemented at either the motor or source side of a cable, allowing users to select the side that is easier to maintain.