Rockwell Automation 1336T Wiring and Grounding Guide, (PWM) AC Drives - Manual (Page 73)

Rockwell Automation Publication DRIVES-IN001M-EN-P - March 2014 73
Chapter
5
Reflected Wave
This chapter discusses the reflected wave phenomenon and its impact on drive
systems.
Description The inverter section of a drive does not produce sinusoidal voltage, but rather a
series of voltage pulses created from the DC bus. These pulses travel down the
motor cables to the motor. The pulses are then reflected back to the drive. The
reflection is dependent on the rise time of the drive output voltage, cable
characteristics, cable length, and motor impedance. If the voltage reflection is
combined with another subsequent pulse, peak voltages can be at a destructive
level. A single IGBT drive output can have reflected wave transient voltage
stresses of up to twice (2 pu, or per unit) the DC bus voltage between its own
output wires. Multiple drive output wires in a single conduit or wire tray further
increase output wire voltage stress between multi-drive output wires that are
touching. One drive can have a (+) 2 pu stress, while another drive can
simultaneously have a (-) 2 pu stress.
Effects On Wire Types Wires with dielectric constants greater than 4 cause the voltage stress to shift to
the air gap between the wires that are barely touching. This electric field can be
high enough to ionize the air surrounding the wire insulation and cause a partial
discharge mechanism (corona) to occur. The electric field distribution between
wires increases the possibility for corona and greater ozone production. This
ozone attacks the PVC insulation and produces carbon tracking, leading to the
possibility of insulation breakdown.
Based on field and internal testing, Rockwell Automation has determined
conductors manufactured with poly-vinyl chloride (PVC) wire insulation are
subject to a variety of manufacturing inconsistencies that can lead to premature
insulation degradation when used with IGBT drives. Flame-retardant heat-
resistant thermoplastic insulation is the type of insulation listed in the NEC code
for the THHN wire designation. This type of insulation is commonly referred to
as PVC. In addition to manufacturing inconsistencies, the physical properties of
the cable can change due to environment, installation, and operation that can also
lead to premature insulation degradation.