Environmental Engineering Reference
In-Depth Information
Table 8.5 Thermaleze QS electrical properties
Wire type
18H APTz
18T APTz
18Q APTz
18H Tz QS
Insulation build
m
(m)
81
107
135
81
Dielectric strength
(kV at 25
C)
>
10
>
13
>
15
>
10
Corona inception voltage
at 25
C
570
656
720
570
Voltage endurance
(hours at 2 kV
rms
,90
C)
14.4
22.8
72.1
275.8
Pulse endurance index
1.0
3.4
10.2
>
120
exceeding 200
C. Tz QS was approved in 1997 and shown to have better than
acceptable chemical resistance to automotive fluids (gasoline, oil and freon).
Magnetek Motors and Controls offers a line of 'corona-free' inverter driven
motors in their E-Plus line that are designed to withstand 1,600 V spikes in appli-
cations rated 600 V or lower [6].
Additional losses occur in the copper conductors of electric machines due to eddy
currents and proximity effects [7,8]. As early as 1912, investigators made empirical
measurements of what was, and continues to be, referred to as stray load losses in
electric machines. These early investigators noted that even with very accurate mea-
surements of resistances, currents and voltages, the efficiency measurements did not
agree with measured electrical power input and mechanical power output, so these
additional losses were treated as stray losses in the machine. The definition and origin
of stray load losses continued to be puzzling well into the 1960s and even to this day,
although now we have a more fundamental understanding of these losses. The sources
of stray loss are considered to consist of flux pulsation in the core, tooth, slot leakage
and surface or tooth tips. A result of flux crossing the conductor slots, hence the
conductors transversely, is that the current distribution in the conductor is not uniform,
nor even in phase in different sections. Hence, the conductor resistance is higher by an
amount due to skin effect caused by transverse slot leakage flux and consequent eddy
currents in the central portions of the conductors. Giacoletto [9,10] has analysed the
issue of skin effect losses and, in particular, has shown that the voltage rise across the
conductor reaches 2.5 pu during the leading and falling edge transients for an inverter
drive operating at 10 kHz, with 5
m
s rise time of voltage as depicted in Figure 8.5. In his
derivation, Giacoletto uses a 1 A current source inverter driving the electric motor
when the 2.5 pu overvoltages are generated. His general observation was that a hollow
tubular conductor is more effective, on a mass basis, than a thin rectangular conductor
at low frequencies, but that at higher frequencies it becomes less effective.
In large turbo-generators for utility power (250, 600-1,000 MW rating), it is
customary to take precautions against stray load losses. In small machines the stray
load losses are noticeable and to some degree negligible. In very large machines the
second and third order effects contributing to stray load losses become signi-
ficant thermal design considerations. Armature winding stray load losses in large
