Long Cable Applications - Siemens 6SR41 series Product User Manual

7.5

Long Cable Applications

When a step of voltage from a drive is applied to one end of a transmission line, it causes a
traveling wave to be propagated toward the opposite (or motor) end of the transmission line.
When the traveling wave reaches the motor end of the cable, the motor leakage reactance is
so high that the line behaves as if it were open-circuited. A reflection wave begins to travel
back toward the drive end. The total voltage at any point is the sum of all waves present, in
this case the forward wave and the reflected wave. The reflection at an open circuit is such
that the reflected voltage wave has the same polarity as the forward voltage wave. At the
motor end, the reflected wave appears at the same instant as the forward wave arrives, so
that the effective voltage step is doubled. At other points, there is a delay from the time that
the forward wave passes until the reflected wave passes, so that two separate steps appear,
each equal to the original.
When the reflected wave reaches the drive end of the cable, the drive impedance is so low
that the line behaves as if it were short-circuited. A second reflection wave begins to travel
back toward the motor end. The reflection at a short-circuit is such that the second reflected
voltage wave has the opposite polarity as the first reflected voltage wave.
If the drive output remains static long enough after each step, then these reflected waves will
bounce back and forth, losing energy at each reflection, until the voltage stabilizes. In this
case, the worst step size imposed on the motor will be almost twice the step size from the
drive, while the number of steps per second will be multiplied by the number of significant
reflections per step.
However, a much worse scenario can occur if the drive produces another step before the
waves from the previous step have decayed. The absolute worst case occurs when the next
step from the drive coincides with the arrival of the first reflection back at the drive, and when
the next step has the opposite polarity to the previous step. This happens most often when
the propagation delay of the cable is equal to ¼ cycle at the effective switching frequency,
which Siemens LD A defines as the critical length. In this case, the reflected waves from
successive steps reinforce each other, and the worst step size on the motor can become
many times higher than the step size from the drive.
The same effect will occur when the cable length is an odd multiple of the critical length, but
it is less severe because the waves are reflected several times before they are reinforced.
For a Perfect Harmony Drive, the critical length is a function of the number of power cells per
phase and the effective switching frequency of the drive. If the cable length exceeds a critical
length (refer to Section 4.4), an output filter is included with the drive.
Product User Manual
Operating Instructions, Version AE 12/2009, A5E01454341C
Application Specific Features
7.5 Long Cable Applications
161