Measuring Volts and Amps

Voltage, current and power measurements for ac motors powered with and Adjustable Frequency Drive (AFD) are problematic. AFD’s use Pulse Width Modulation (PWM) (see image). In most cases, the current reading is the most important to determine if the motor is overloaded or has a single phase fault. The inductance in motor windings filters the current so that it can be read with a Root Mean Square (RMS) clamp-on ammeter.

Voltage is more difficult to measure. The motor inductance does not filter the voltage pulses. The motor leads can also cause wave reflections which may change pulse amplitude at the connection points where voltage may be measured. In general, RMS Digital Multi-Meters (DMM) read 20 to 30% high. Some DMM’s are specifically designed to read voltage produced by AFD’s. These are specialty DMM’s and are advertised as for AFD’s or VFD’s. Products are available from Fluke, Flir, Keysight, and others.

Of course, we all know that power drawn by an ac motor cannot be measured with a single Voltage and Current measurement. Remember Power Factor? You will need a power meter.

All vector drives have built-in current, voltage and power measurements. Provided the drive is operating correctly, these measurements can be trusted, but are not as precise as one would expect from calibrated DMM’s.


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