Environmental Engineering Reference
In-Depth Information
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Figure 8.5 Voltage wave at machine terminal due to transmission line effects
peak transients on the machine can reach as high as 1,500 V
pk
and more. This is a
cause for concern and the reason that wire manufacturers have developed newer
grades of wire for inverter driven machines [5].
Figure 8.5 illustrates the corruption of the inverter PWM voltage waveform at
the M/G terminals due to transmission line effects and reflections.
Corona inception voltages of >800 V when the machine is hot are typical and
readily occur (Figure 8.5). Typical testing for corona inception voltages, CIV,
consists of a high voltage source and section of transmission line. A high voltage
pulse is applied to the line as discussed above and an oscilloscope is used to detect
the high frequency, 1-10 MHz ringing, characteristic of corona discharge. Dete-
cting the high frequency signature of corona gives the earliest indication of when
the CIV threshold is crossed because it is detectable even before audible noise
or ozone odour can be detected. The high voltage source potential is gradually
increased, discharged and motor terminal voltage monitored until the CIV thresh-
old is crossed.
Phelps Dodge magnet wire company has developed a line of Thermaleze
'quantum shield' (Tz QS) where the copper or aluminium wire is coated first with
polyester, followed by the quantum shield layer, and ending with a protective
coating of polyamideimide. Table 8.5 lists the properties of three conventional
insulation grades accounting for single build, double and triple build, plus a grade
of Thermaleze QS, 18H Tz QS single build insulation.
The pulse endurance index (PEI) is the ratio of endurance life of the test
samples to the endurance life of heavy MW-35 standard wire. The Tz QS insulation
system was tested on production inverter driven motors and shown to have oper-
ating life of 20- to 80-fold improvement over the heavy build APTz insulation
systems. With an insulation build of only 81 mm, the Tz QS wire passes manu-
facturing requirements of mandrel flexibility (30%, one time) and snap as well as
heat shock of 0.5 h at 220
C, 20% snap three times. It also has a thermal endurance
