Re: [CAD_CAM_EDM_DRO] Re: VFD's

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