MDA-550/MDA-510
Users Manual
20
•
Measure on the motor input to check the pulses on the
input of the motor and the influence of the cable.
•
When installing a filter, do the dV/dt measurement
before and after the installation to verify the
performance of the filter.
Phase-Ground
When you connect the reference lead to ground, the test
tool shows switching pulses for each phase. Typically, a
sinewave shows on top of the modulated signal as the
ground level is not the star point of the 3-phase system. Due
to the fluctuations in signal level to ground, a stable signal is
not always automatically shown in all zoom modes.
Compared to the Phase-Phase measurement, the carrier
frequency shows as the reading when you select Zoom 2,
as the waveform shows the switching of one phase, as
compared to the mix of two phases switching in Phase-
Phase.
Zoom 3 shows the same parameters as Phase-Phase and
can be high voltage peaks to ground that could damage the
motor insulation. The Phase-Ground signal can damage
different parts of the insulation. When filters are applied,
higher peaks can be visible when measuring Phase-Ground
compared to Phase-Phase.
Tips:
•
Make sure that the slope that is automatically selected
is the pulse of the PWM signal and not of an
interference. The start of the slope must be around the
0-level.
•
When a drive has a dc bus with intermediate 0-level
(middle point of DC+ and DC-) that can be accessed
with a reference lead, the same measurement can be
applied.
Phase DC- or DC+
The measurements with a dc positive or negative bus signal
as reference is the same as for Phase-Phase but with an
offset relative to the dc level. The Phase dc measurement is
also used to measure switch frequency, identify IGBT
problems, or to check if the signal is floating up and down to
indicate a system grounding problem.
Spectrum (MDA-550 only)
The MDA-550 includes spectrum analysis in voltage
modulation mode. In this mode, no hardware filter is
enabled. This function shows the spectral content of the
motor drive output voltage waveform. It performs an FFT
(Fast Fourier Transform) to transform the amplitude
waveform from the time domain into the frequency domain.
The switching frequency shows as high peaks. For phase-
phase measurement, 2x the switching frequency shows as
it is the combination of two phases switching. For Phase-
Ground measurements, only the switching frequency of the
drive shows as a peak in the spectrum.
Voltage Unbalance
The Voltage Unbalance checks for a difference between the
Phase-Phase voltages for 3-phase systems. The unbalance
value is calculated by dividing the maximum rms voltage
deviation of one of the phases and the average rms voltage
of all phases.
Voltage unbalance at motor terminals can adversely affect
motor operation and can also cause tripping of the current
overload fault protection on the motor drive.
changes the readings that show at the top of the
screen to the peak-peak values of each phase as well as
the highest Crest Factor (ratio between peak and rms value)
of one of the phases.
