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ScopeMeter
®
Test Tool
MDA-550-III Test Tool
69
Tips:
• High voltage peaks can damage the motor insulation and drive output circuit and cause the
drive to trip. Overshoot higher than 50 % of nominal voltage can be problematic.
• 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
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.
1.888.610.7664 sales@GlobalTestSupply.com
Fluke-Direct.com
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