In article ,
Jon Grimm wrote:
I'm not particularly knowledgeably about electrics, but couldn't you just
tong one of the motor leads to see what the ACTUAL current is?

A clamp-on ammeter will probably show the same thing that the
VFD's readout shows. The problem is that the current is out of phase
with the voltage, so only a percentage of it is actually doing work.
This is called the "power-factor", and an accurate power measurement
requires measuring the voltage and the current and the phase shift
between them. The clamp-on ammeter will measure *only* the current.

As the motor gets closer to full nameplate power being
delivered, the phase shift will reduce so the current will bear a closer
relationship to the power being delivered.

The VFD *could* calculate the power, as it has both the voltage
and the current data -- but nobody seems to bother to program them to do
so -- and in terms of heating the motor windings and fuse blowing or
circuit breaker tripping, as well as power lost heating the external
wiring, the current alone is what really matters.

Enjoy,
DoN.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---