Re: [CAD_CAM_EDM_DRO] Re: VFD's

jmkasunich wrote:

>Something isn't right with the currents you see on your
>mill motor. I've never seen a motor that draws more than
>about 50% no-load current, in 10 years of working with
>VFDs and induction motors from 1HP to 1000HP.
>

Well, your work experience is a LOT broader than my meager experience, but I
have 2 data points from recent observations. As I said, the current
readout on my
mill's VFD shows idling current of 3.0 A, and the motor's full load
rating is 3.3
A, and that shows up on the VFD at a reasonable load. Now, maybe the VFD's
display is way off!

I measured current on a capacitor-run single-phase motor on my air
compressor.
This compressor has an unloader, which is not the same as no load, but
is pretty
low, maybe 1/4 Hp. The motor has a 2 Hp rating. Current is about 9.5 A
unloaded, and rises to 11.4 A just before the compressor goes to unload.
I know that single-phase motors have worse power factor than 3-phase.
I used a clamp-on for this measurement.

<snip>

>That and other experience lead me to say that no-load
>current (magnetizing current) over 50% almost certainly
>means something is wrong. Either there is actually a
>load on the motor (belt friction, bad bearings, etc.), or
>the drive is misconfigured and is overfluxing the motor.
>For example, applying 240 volts to a motor rated for 208
>volts can drive the flux high enough to cause partial
>saturation, and the magnetizing current can rise to
>70-80% of nameplate full load current.
>
>

This is quite possible. I will mention that the motor runs remarkably
cool, even
after running for many hours. Motor and most head bearings were
replaced when
I rebuilt the head. The belts, even though it is a step-pulley version,
still draw a
bit of power. I will drop the belt off it and see what I get, and also
try a clamp-on
meter. I don't know how reliable that reading will be, though.

I will also fire up my lathe motor that has a really nice Toshiba VFD,
and see what
it reads, no load. I don't have an easy way to load it, as the
headstock is off the bed
at the moment.

>Getting back to real applications. I agree with you that
>you should always try to match the drive to the motor.
>Using a drive that is two or even ten times too big for
>the motor should work fine, although you will lose motor
>overload protection if the drive is too big. Using a
>drive that is a little undersized (say a 2HP drive on
>a 3HP motor) will also work, with the understanding that
>you will only get about 1.5HP from the motor. Using a
>drive that is seriously undersized (like 1HP drive with
>a 3HP motor) will not work at all due to no-load current.
>
>

The point I was originally trying to make, way back in the beginning, is
that using
a VERY large motor on a small VFD has one other side effect that is
quite serious.
The VFD is tuned for the expected inductance range of the motor. If the
inductance
is too low, like a big motor, then the on-times of the transistors can
saturate the
motor magnetics, because current will rise much faster than expected.
This is what
blew up my first VFD, when I tried to run a 1 Hp motor with a 1/2 Hp
VFD. It
was ok at full speed, but passing through a low speed range, it would
trip, and eventually
popped a transistor. Later, I got help from Magnetek on using my
air-bearing
drilling spindle motor on their VFD. The solution was to put inductors
in series
with all the motor output lines. Even still, there are certain
combinations of
V/Hz that create long on-times of the transistors that make the
inductors literally
jump off the table and fault the VFD. The optimal setting seemed to be
75 V@400 Hz,
but I had to set it for 80 V@400 Hz to avoid the jumping inductors. The
air-bearing
motor was designed for very low inductance, as it is really designed for
operation
in the range of 400 - 800 Hz.

Jon