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on the central rotary switch. Timer t1 is set using the up and
down arrow buttons and OK confirms the setting.
The ‘DD’ test requires the instrument to measure the
discharge current 1 minute after the removal of the test
voltage, which is greater than the primary time constant of the
discharge. On completion of the test, the instrument uses this
measurement along with the test voltage and calculated
capacitance to produce a figure of merit indicating the quality
of the insulation.
DD = I
1min
/(V x C)
where
I
1min
is the discharge current in mA one minute after
removal of the test voltage
V
in Volts and
C
is the capacitance
in Farads.
DD results can identify excess discharge currents that arise
when a layer of multi-layer insulation is damaged or
contaminated, a condition that will be missed by both the IR
and PI tests. Discharge current will be higher, for a given value
of voltage and capacitance, if an internal layer is damaged.
The time constant of this individual layer will mismatch the
other layers, giving rise to a higher value of current than for
insulation that is ‘good’ in this respect.
Homogenous insulation will have a DD value of 0, while good
multi-layer insulation will have a value up to 2. The following
table is a guide to DD test results:
Insulation Condition
DD result
Bad
> 7
Poor
4 - 7
Questionable
2 - 4
Good
< 2
Homogenous
0
Step Voltage Test
The SV test is a controlled overvoltage test that can be applied
to stator and rotor windings on synchronous and
asynchronous AC motors and the armature and field windings
on DC motors. It is advisable to perform a PI test before an SV
test to determine if the insulation is suitable for overvoltage
testing. If a PI test was performed to verify the winding’s
suitability for over voltage testing, the winding must be
completely discharged before the overvoltage test is
performed.
The SV test is based on the principle that an ideal insulator will
produce identical readings at all voltages, while an insulator
which is being over stressed, will show lower insulation values
at higher voltages.
During the test the applied voltage steps incrementally by one
fifth of the final test voltage each minute for 5 minutes, taking
successive measurements.
Resistance readings for the first four ‘steps’ are displayed
under consecutive time designators ‘1m’ to ‘4m’. The 5 minute
reading is displayed by the main display. If the default 5
minute test duration is changed by the user the four readings
will not show the respective ‘1m’ to ‘4m’ indicators.
The SV test duration
can be adjusted if
desired from the 5
minute default value
using the up/down
arrows and OK to save
the setting.
The step timer will always be set to total test time divided by
five. Too short a step time may result in incorrect readings and
too long a step time may over stress a motor.
