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Application Notes
9.7 Exiting Air Temperature
A simple formula can be used to predict the exiting air temperature of the cell:
Tair,out = Tair,in + 1.75 x Watts / CFM
For example, consider the 140A cell operating at rated current with a 45°C ambient:
Tair,in = 45 °C (given)
CFM = 180 (minimum allowable air flow)
Watts = 1160 (from losses chart)
Tair,out = 45°C + 1.75 x 1160 watts / 180 CFM = 56.3°C
Typically the exhaust air temperature of the cell is approximately the inlet air temperature of the drive's fan. If the
cell's air out temperature exceeds the fan's maximum air temperature, the cell must be de-rated or a higher
temperature rating fan must be used.
9.8 Capacitors
9.8.1
Flammability
Aluminum electrolytic capacitors contain materials which can catch fire and support combustion when contacted by
flames. Flammable parts include plastic parts, insulating sleeves, paper, and the electrolytes. Most capacitors will
pass the needle-flame test requirements of UL 94V-O and not support combustion to the requirements of Category B
or C. In rare cases the capacitor may self-ignite from heavy overload or capacitor defect. Hydrogen in the capacitor
can ignite if sparking occurs during capacitor failure.
9.8.2
Shelf Life and Reforming
9
Capacitors should not be stored in an ambient less than 85°C and above -55°C. It is recommended
that capacitors be stored at room temperature in a dry environment free of halogen gasses like
chlorine and fluorine that can corrode aluminum. Keep out of direct sunlight.
.
When an aluminum electrolytic capacitor is stored under no load conditions for a long period of time, its leakage
current tends to increase. If the storage temperature is high, the leakage current will increase substantially. This is
due to a drop in the withstand voltage of the dielectric caused by the reaction of the anode oxide layer with the
electrolyte. At this point the capacitor should not be subjected to full voltage until the leakage current returns to its
initial level, which can be done by reforming the electrolyte. Reforming can be done by applying a voltage that is
slowly increased to maximum over a period of several hours, with current limited to rated leakage current.
If capacitors have been out of service for more than 6 months, they must be reformed.
The capacitors can be reformed with the cell in or out of the drive. If reformed out of the drive, then one cell must be
formed at a time. If reformed in the drive, then all the cells can be formed at the same time.
Follow all safety guidelines. The chassis of the Power cell is electrically hot. Isolate the Power cell when performed
outside the VFD. The chassis can be grounded if the cell is connected to an input isolation transformer.
Reforming Recommendations
It is advisable to use a three-phase variable voltage source capable of producing up to the 10% high line rating of the
cell (750V x 1.1 = 825Vrms). This can be generated with a 480V Variac connected to a step up transformer. If not
available, at least a 480V variac should be used, although this will only partially reform the capacitors and could be
damaged when rated voltage is applied.
9-14
MicroHarmony Cell Sizes 40 - 260A Manual
s
19001467: Version 1.0
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