Re: VFD's

--- In CAD_CAM_EDM_DRO@y..., "doug98105" <dougrasmussen@c...> wrote:

> Dave,
>
> You say a 1hp VFD will run a 25hp motor up the max amps
> of the drive. That's kind of extreme, but I get your
> point.
>
> This is a question I've never been able to get a straight
> answer to.
> I was concerned about running a 5hp motor off a 3hp drive.
> None of the engineeers at the various motor or drive
> manufacturers could give a definitive answer or were
> willing to venture a guess as to whether there was any
> downside to doing this, other than reduced hp from the
> 5hp motor.
>
> Doug
>
>
> --- In CAD_CAM_EDM_DRO@y..., "turbulatordude" <davemucha@j...>

wrote:

> (snip)
> > Your 25 hp VSD will run your 1 hp motor and your 1 hp
> > VSD will run your 25 hp motor. No really, but only up
> > to the max amps of the drive.
> >
> > Dave

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. 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.


John Kasunich