Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 182)

182 Rockwell Automation Publication 750-RM002B-EN-P - September 2013
Chapter 3 Diagnostics and Protection
The Terminator
Is it possible to match the surge impedance of the motor to the cable? There is a
device called the terminator that does this, shown in the figure below. It is an RC
network at the motor that matches the load surge impedance to the cable.
Figure 20 shows the surge voltages when using the terminator. The overshoot is
very low, with no ringing to speak of. Due to losses, this device is good for cable
lengths up to 600 ft, and for carrier frequencies less than or equal to 4 kHz.
However, its key advantage is that this one device works well for any motor in the
range from 0.5 to 500hp because it does not have to handle the motor current,
being a parallel device.
Line Reactor
What if we go the other way, matching the surge impedance of the cable to the
motor? There are several products available that do this. They all consist of the
addition of a line reactor at the output of the drive. See the figure below. A 3%
line reactor by itself also reduces the dV/dt, but a big disadvantage is that it
reduces the voltage available to the motor by 3%. This is useful for cables up to
about 600 ft. A better device is what we call a “reflected wave reduction” device
where the line reactor is reduced to about 0.2%, and a resistor is placed in parallel
with each of the reactors. This reduces the dV/dt and has a voltage drop of only
0.2% instead of 3%. It can be used with cables up to about 1200 ft.