Re: VFD's

--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:

>
>
> jmkasunich wrote:
>
> >On the other hand, if you try to run a 25 HP motor with
> >a 1 HP drive, the drive will trip out on immediately.
> >All induction motors draw a sizable no-load current that
> >is used to magentize the motor. No-load current is
> >typically 25-35 percent of the nameplate full load
> >current.
> >
> Way worse than this on the integral HP motors! On my
> mill with a 1 Hp motor, the current I see on the VFD's
> own readout goes from 3 A idling to 3.3 A at full load!
> I don't know how accurate the current sensing is on
> the VFD, but that gives you a rough idea. I haven't
> been able to put a load on my lathe motor yet (5 Hp)
> but I get very similar readings showing the idle
> current is well over 50% of rated.
>

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. Some
specific examples:

My Van Norman mill has a 1.5 HP motor. The VFD displays
current in percent. When not cutting, the display reads
between 28% and 55%, depending on temperature and spindle
speed. High speeds at low ambient temp (the mill is in
an unheated garage) draw higher "no-load" current because
of higher losses in the gearbox and more friction due to
oil viscosity when cold. True no-load current on that
motor (if I were to disconnect the belts between motor
and gearbox) would probably be around 25-27%. When making
chips, the display goes up to 50-100% (100% is only when
I'm really hogging with a 5-1/2" face mill).

In the model shop at work we have a Millport mill (B'port
clone), to which we added a VFD. I tried it today, and
the no-load current ranged from 25% to 40%, depending on
the setting of the variable speed belt sheaves in the mill
head. The belt has significant friction at high speed
settings, which loads the motor even when not cutting.
Again the real no-load current is 25% or less.

The drives on both the Van Norman and the Millport are
Reliance Electric SP500 units.

Another example, admittedly way beyond HSM sizes. We have
a 600HP dyne in the lab. I was running it Friday. Full
load amps per the motor nameplate is 686 (at 460V).
Unloaded, the current was 177A. This drive was a liquid
cooled version of a Rockwell Automation PowerFlex 700.

Getting back to HSM sized motors, a few months ago we
were testing a high-end treadmill with a 1 HP AC induction
motor instead of the traditional DC motor. We tested it
with two different drives, and several different control
schemes, trying to get the best possible low speed torque.
I don't remember the exact numbers, but I know that even
when we intentionally over-fluxed the motor by a small
amount to improve torque response, we didn't have large
magnetizing current. We had the current displayed on a
scope, and you could see it more than double each time a
foot hit the treadmill belt. The idling current was less
than half the full load current.

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.

> > So your 25 HP motor would need at least a
> >10 HP drive, to handle the no-load current. I wouldn't
> >recommend using a drive smaller than about half of the
> >motor size.
> >
> >
> I really think it is dangerous to the health of the VFD
> to use one rated smaller that the motor rating at all!
> Remember, most people here are using these as phase
> converters, too, so they should already be using an
> oversized VFD for that reason.
>
> Jon

I agree that you don't want to use an undersized VFD if
you don't have to. Remember, I was replying to a post
that asked if you run a 25HP motor with a 1HP VFD. You
can't. However you can run that motor under most
conditions with a 10HP VFD. You certainly won't be
able to load it to 25HP, or even to 10HP. You might
get 5 HP out of it.

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.

John Kasunich