INTRODUCTION
6
Secondly, hipot testing is done as a quality control measure. Incipient failures in the
insulation of any portion of the product, whether due to workmanship, components or
materials are detected by the hipot test before the product is shipped out to cause
inconvenience, dissatisfaction and expense in the field.
The most often asked questions are, "Is hipot testing destructive?" and "Should I use AC
or DC for the hipot test?"
Today's modern, commercially available high voltage production line testing equipment is
generally not destructive. For most consumer product testing, testers have sufficient
sensitivity and response time that short circuit currents can be held to non-destructive
levels.
The question of AC or DC is best answered by the question, "What do the specs say?"
For the production hipot test, agency requirements almost invariably specify an AC test.
Generally, AC hipot testing is considered by many to be more stressful to the insulation
than DC hipot testing because of the periodic polarity reversal. Some believe AC testing
tends to accelerate breakdown due to material flaws. During use, products are more likely
to experience AC voltage transients than to experience DC voltage transients. Therefore,
AC hipot tests provide more realistic conditions than DC hipot tests.
The next most common question about hipot testing is, "How much voltage should I use?"
Again, "What do the specs say?" As a rule-of-thumb, many applications will require 1000
volts plus twice the normal operating voltage for one minute. Increasing the test voltage
by 20% usually allows the test time to be reduced to one second. Automotive products
will generally specify 500 volts.
Armatures are produced in both a "single insulated" and a "double insulated"
configuration. With single insulated armatures, the commutator and windings are
insulated from the iron stack and the shaft which, electrically speaking, are common.
Double insulated armatures additionally have the iron stack insulated from the shaft. This
provides "double insulation" between the current carrying components, the commutator
and the windings, and any exposed dead metal components, normally the shaft.
On single insulated armatures, the dielectric withstand test voltage is normally applied
between the commutator and the shaft.
Double insulated armatures, however, will normally have a dielectric withstand voltage
applied between the commutator and the iron stack and another dielectric withstand
voltage between the iron stack and the shaft. If these two voltages are applied
simultaneously and the voltage sources are properly phased, a consequential voltage equal
to their sum will be applied between the commutator and the shaft.
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