Hitachi CONVENTION 14 Instruction Manual page 233

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Inverter Output Voltage Measurement Techniques
Taking voltage measurements around drives equipment requires the right equipment
and a safe approach. You are working with high voltages and high-frequency switching
waveforms that are not pure sinusoids. Digital voltmeters will not usually produce
reliable readings for these waveforms. And, it is usually risky to connect high voltage
signals to oscilloscopes. The inverter output semiconductors have some leakage, and
no-load measurements produce misleading results. So, we highly recommend using the
following circuits to measure voltage for performing the equipment inspections.
HIGH VOLTAGE: Be careful not to touch wiring or connector terminals when working
with the inverters and taking measurements. Be sure to place the measurement
circuitry components above in an insulated housing before using them.