PQ Tech

LCF converts the distorted PWM waveform to sine waves so as to prevent overvoltage of motor terminal and dv/dt. Usually, a motor increases its capability in the condition of sine wave use. It is designed to prevent dielectric breakdown of motor and expand lifespan of motor.

A motor running with PWM inverter can increase its terminal peak voltage to the value almost two times higher than DC bus voltage.
It means that the terminal peak voltage of motor increases up to 1700V.
LCF prevents an insulator from being broken down by voltage transient so as to protect a motor and a cable.

A long-distance cable based motor running with PWM inverter can face its enhanced voltage amplification. In the case, a relevant factor is determined by motor surge impedance, cable surge impedance, and cable length. The voltage reflection coefficient R is referred to as the value calculated in the way of converting the reflection value generated by an impedance difference in a connection terminal to the ratio of input voltage to reflection voltage. In short, it is a figure representing a reflection value for input.

CRITICAL CABLE LENGTH

HP	R	Zm	Zc
25HP	0.9	1500 ohms	80 ohms
50HP	0.83	750 ohms	70 ohms
100HP	0.76	375 ohms	50 ohms
200HP	0.65	188 ohms	40 ohms
400HP	0.52	94 ohms	30 ohms

[Table 1. Typical reflection coefficient and surge impedance]

Zm: motor surge impedance, Zc: cable surge impedance
Motor terminal peak voltage = (1 + R) × DC bus voltage

LCF reduces a reflection coefficient with the total surge impedance and the reactor surge impedance of LCF.

The available distance of voltage amplification is called critical cable length which is determined with a voltage rise time and an electromagnetic pulse move speed .
CRITICAL CABLE LENGTH = (100 M/Μ SEC X VOLTAGE RISE TIME (Μ SEC))/2

A general speed is 100m/us. A voltage rise time means the time of DC bus voltage rising from 10% to 90%.
LCF delays the voltage rise time of motor, and therefore reduces dv/dt and expands a critical cable length.