Re: VFD's

I might agree with not using lower rated motors if it was not used in
common practice so often.

When I used to sell VFD's the 10 and 20 hp units were shipped in a
week or two, whereas the 100 hp units took many weeks, sometimes 8
weeks or more.

We would install a unit that was capable of getting the motor up
between 20% and 50% of speed, ideally 80%. these were for
centrifigual loads, fan motors to be exact. The VFD would run for
weeks or months 24/7 until the unit could be switched out with the
full rated unit. 10hp on a 100hp motor was not uncommon.

The typical approach was to spin the motor up to 50% or better yet,
80%, for full automatic control. Then when the load demanded above
the VFD max, the bypass would run the motor across the line at full
speed.

And Like I stated in the first post, all external safties are sized
for the motor.

Agreed that this is not appropriate for a machine tool and that a
closer match to motor size is desired. I think we always sized one
size over the motor hp, ie: a 10hp motor was powered by a 15 hp or
20 hp drive and a 100hp motor from a 125 hp drive.

Dave

--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
>
>
> jmkasunich wrote:
>
> >A 25 HP drive will run a 1 HP motor, if you are using a
> >simple V/Hz drive. Many of the newer drives have more
> >sophisticated control, often referred to as sensorless
> >vector or similar. These drives use current feedback to
> >improve motor performance, and drives will not work well
> >with undersized motors, since the motor current is too
> >low to sense accurately. In any case, the protective
> >functions like current limit, overload, etc, will provide
> >little or no protection, again because the currents are
> >too low for the sensors in the drive. If you can disable
> >any special control, and don't care about the loss of motor
> >protection, then go right ahead. (I design large VFDs,
> >and in our lab we use a 1 HP motor for the initial power
> >up testing of 600 HP drives, before moving the drive to
> >a high power test cell for complete testing.)
> >
> >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.
>
> > 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