Voltage Reflection Analysis

Introduction

The occurrence of bearing currents in an induction motor has been known for decades. Asymmetric flux distribution is the basis that causes bearing currents inside the induction motor [1]. It has been productively solved using modern motor design and manufacturing methods.

Fig.1 shows two level PWM inverter fed induction motor with long motor cable. In many new and retrofit industrial applications the PWM inverters and motors must be at separate locations thus requiring long motor cable, which contributes over voltage at the motor terminal due to voltage reflection phenomenon.

Figure 1: PWM inverter fed induction motor with long motor cable.

In PWM inverter with IGBT operate at switching frequencies of 2 to 20 kHz and rise time of 0.1µsec with

Voltage Reflection Analysis

Voltage reflection is a function of inverter output pulse rise time and the length of the motor cables, which behave as a transmission line for the inverter output pulses .The PWM pulses travels from inverter to motor terminal at approximately half the speed of light (150 to 200mt/µsec), if the pulse takes longer than 1/3rd of output pulse rise time, then a full reflection occurs at the motor terminals and the pulse amplitude will be approximately doubled in the presence of long motor cable [7]

Long cable lengths contribute to a ringing at the motor terminals due to the distributed nature of the cable leakage inductance and coupling capacitance (L-C), which result in overvoltage and further stress the motor insulation. The common-mode voltage at motor terminal with long cable also doubled [7].

Over voltage at the motor terminal depends on rise time of the PWM pulses, the cable length and the impedance mismatch between the power cable and motor. The impedance of smaller motors is dominated by winding inductance, thus in comparison with the low surge impedance of the cable the motor impedance is high [8].

Simulation