Manual Fluke 190-102-III (page 75 of 98) (English)

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ScopeMeter® Test Tool

MDA-550-III Test Tool

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.

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Fluke 190-102-III Manual

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