Suppose that I have a 2 HP three phase motor, driving a very light
load (usually below 1/3HP), that is plugged into a VFD that is rated
for 1 HP.

Would this setup work at all? I am not trying to get two real HP out
of 1 HP VFD, I am asking if it can be used to drive light loads, but
powered by a higher rated motor.

thanks

i

"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.

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

FWIW
I bought a 5 hp VFD for my lathe project. When I went to the salvage yard
I found a perfect 7 1/2 hp three phase motor for the job. (In fact two
identical brand new, so i got both) After many trials, I gave up, no way
would that VFD start the larger motor. The instant you try to start a motor
you get an inrush of current. The VFD brain says this is more than it should
be and shuts down. This is on the Hitachi VFD drives. This little learning
experience cost me $1000. School of hard knocks is a good teacher.



--
(©¿©) An apple a day keeps the doctor away.
Two apples a day gets the doctor's OK.
Five a day makes you a fruit grower like me.

Karl Townsend

On 29 May 2006 00:55:05 -0700, "LowEnergyParticle"
wrote:


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")
snip Dave's good stuff
Dave

Three phase induction motors take a certain amount of amps to
establish the magnetic field, regardless of power draw. I've seen it
run between 40-50% of FLA. It may be that you're too near the FLA for
the VFD before you get your load moving. This is assuming you're
feeding with three phase power. If you're feeding single phase, I
wouldn't even think about it. That usually calls for derating a drive
by up to 50%.

Pete Keillor

On Mon, 29 May 2006 04:18:01 GMT, "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.


Not quite. It can take several times rated power while accelerating
from a stop to running speed.

Even when running, it only draws the power you describe, but this can
still result in too much current for the VFD because lightly-loaded
induction motors still draw significant current, albeit at low power
factor.

I have tried it with several drives, it doesn't work. Even if you can
get the motor running by ramping up very slowly youi will find you
can't load it up much without tripping the over current protection.

stan