Can I use WattNode® meters to monitor motors driven by variable speed drives (VSDs)?

Yes, all current production WattNode meter models work well with VSDs provided a few precautions are followed:
  1. Try to keep the WattNode at least three feet (one meter) from the VSD. Generally, we recommend placing the WattNode meter at the breaker or service panel, rather than next to the VSD.
  2. Do not extend the CT wires. If the CT wires are longer than needed, trim them.
  3. NEVER install a WattNode meter downstream (on the load side) of a VSD. The varying line frequency may overheat the power supply and the extremely high noise will affect the measurement accuracy.

Overview

Variable speed drives (VSD), also known as variable frequency drives (VFD), and motor inverters commonly refer to the same type of devices. These generate outputs with varying line frequency to speed up or slow down electric motors. They are commonly used to improve energy efficiency.

Internally, they work by generating a pulse-width modulated square-wave output, with a square-wave frequency of 1 kHz to 10 kHz. After filtering, the effective line frequency varies from just a few hertz up to 90 Hz or higher. This allows motors to start slowly and run at very low speeds or at speeds higher than possible with a 50 Hz or 60 Hz input. In some cases, the filtering to smooth the PWM square-wave is provided by a filter network (reactor). More commonly, especially in the U.S., the inductance of the motor provides the filtering.

Issues

VSDs generate substantial electrical noise, because they are switching tens to hundreds of amps ON and OFF thousands of times a second. Many early electronic power meters, including our WNA series models (no longer in production) had accuracy problems when working with VSD loads.

Our newer WNB and WNC series WattNode meters work well with VSDs or inverters.

In rare cases, the noise from a VSD is so extreme (more common with older VSD models) that it causes interference. This will generally show up as communications problems, no readings, or severely inaccurate readings. Generally this can be fixed by moving the WattNode meter and CTs further from the VSD. In some cases, a line reactor (filter) is necessary to reduce the noise (these are commonly required in the EU because of the interference that can be caused by VSDs).

See Also