Environmental Engineering Reference
In-Depth Information
Electric motor current is synthesized from the dc line voltage through a
modulation process. The inverter is essentially a class D amplifier controlled to
modulation depths necessary to create the fundamental component at the output. A
rule of thumb for sinusoidal ac drives is that the bus capacitors must be rated for
60% of the M/G phase current. For example, if the hybrid propulsion system M/G
is rated 200 A
rms
, then the ripple capacitor bank must be capable of sinking
120 A
rms
of ripple current at the inverter switching frequency,
f
s
. Since
f
s
ranges
from 5 kHz to over 20 kHz in production traction inverters, the capacitor bank must
be sized to sink this much current continuously and remain within its thermal
constraints.
Electrolytic bus capacitors with organic electrolytes are restricted to operation
at 85
C or less. It is true that aluminium electrolytics have temperature ratings
of 105
C to as high as 125
C, but these are not continuous ratings. Polymer
multilayer (PML) type capacitors
1
are stable over temperature, resilient under
thermal shock, stable over mechanical stress such as mounting stress, and
have ultra-low ESR. It is this term 'equivalent series resistance' (ESR) that dis-
tinguishes a bus capacitor for ripple current bypassing from a dc link hold-up
capacitor for energy storage, such as in an uninterruptible power supply. The ESR
of a capacitor is a strong function of operating temperature and frequency of the
ripple current.
The dc link capacitor ESR model consists of a bulk capacitance component
(capacitance of the etched foil area,
A
, and electrolyte gap,
d
, where
C
=
e
A/d
), the
dielectric loss capacitance modelled as a capacitor value in parallel with a resis-
tance, and the series combination of electrolyte and foil resistances. Figure 4.24 is
the ESR model currently in use by researchers to characterize losses in the inverter
dc link capacitor bank [30].
C
b
R
ated capacitance
C
d
Dielectric loss
components
R
d
R
e
Resistance of electrolyte
R
f
Resistance of foil + current
collectors
Figure 4.24 Dc link capacitor ESR model
1
ITW Paktron, www.paktron.com, manufacturer of polymer multilayer PML capacitors for use in
power electronic converters and inverters. PML style capacitors outperform ceramic, MLC style.
