Re: Plugging 2 HP motor into a 1 HP VFD


Leo Lichtman wrote:
"Ignoramus27812" wrote: (clip) I am asking if it can be used to drive light
loads, but powered by a higher rated motor.
^^^^^^^^^^^^^^^^^^
A motor draws only enough electrical power to drive the load and overcome
internal losses. As long as these do not combine to overload the VFD, you
should have no trouble. IOW, it's okay.

You may run into trouble starting the motor. ROT is 3-phase induction
motors draw 6 times their full-load amperage (on the nameplate: "FLA")
for a short time while starting. If the VFD trips on overcurrent when
starting, then this is almost certainly what's going on (as long as
your wiring is correct, etc.) The modern (and not-so-modern) VFDs that
I've worked with have all had a way to "soft start" heavily
mechanically loaded motors to prevent startup trips. Even if your
motor is not have a large mechanical load attached (and I assume it
doesn't from your post) the "soft start" feature(s) may help you get
around the large current inrush at startup.

Here's a couple more ROTs for VFDs:

1) Keep all power wiring as short as possible. If you've got a choice
on where you install the VFD, you will be best served by keeping the
wires between the VFD and the motor short, and let the wires between
the VFD and the circuit breaker panel be longer.

2) Run the VFD through a dedicated (not shared with anything else)
circuit breaker in your circuit breaker panel.

3) Do NOT stop the motor by turning off the VFD, or opening the power
circuit to the VFD (e.g. by turning off the circuit breaker in your
panel). Emergency Shut Down (ESD) circuits may be an exception;
someone more familiar with the NEC than me might chime in here with
some help?

4) Do NOT stop the motor by opening the wiring between the VFD and the
motor. In general, there should not be a contactor (big relay)
anywhere in the power circuits on either side of the VFD.

You can kill yourself with these things. If you're not confident that
you know what you're doing, get some help. No sin there. :-)

Dave

.

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