Re: VFD's

I am surprised that it smoked your VFD. odd to say the least.

I would never RECOMEND the use of any power device that is not
properly sized. Especially when we are taking about less than $100
for most differences. The difference between a 3 hp and 5 hp is
often less than $100, it would be foolish to not match the drive
specs to the motor specs.

And in all fairness, it was in the 80's and early 90's that I
installed drives, and at that time, 6 step were common and the PWM
drives were only coming into wide acceptance. in all cases the motor
protection was separate heaters. Yes, there were internal safeties,
but that was for drive protection more than motor protection.

If I remember, it was either a 20 or 30 hp drive, over 7 feet tall,
at least 18 inches deep, and nearly 30 inches wide. Made by Graham,
last 6 step drive I sold. too embarrassed to sell another when the
same thing would have been a suitcase in PWM configuration.

The reason for the demand for VSD's in the industry was and still is
the huge cost savings over a non-vsd motor. The power curve drops
dramatically. at 80% motor RPM on a CENTRIFUGUL load. This is like
a 100 hp fan motor that you spin a few RPM by hand, NOT a high
starting torque load like a compressor that you cannot move ONE rpm
by hand on a 2 hp unit. 50% of the power in needed at 80% speed, and
20% at 50% speed. HUGE dollar savings on your energy bill.

Back to the on topic stuff. A 2 hp Bridgeport should not be run
with a 10 hp drive. as was stated, the internal safeties for the
drive itself are out of range.

Machine tools require full torque on start. imagine trying to
reverse a tap without full power. something will burn up.

We used to call them constant torque and variable torque
applications. I know the names have changed over the years as have
the drives. They were definitely different beasties inside. But
either would run a fan with great energy savings.

My question is are they really that different today ?

Dave

--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
>
>
> turbulatordude wrote:
>
> >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.
> >
> >
> Yes, on the larger motors, this seems to work, especially in a
pinch.
> But, for motors in the
> 1-3 Hp range, the magnetizing current seems to stay pretty high,
even at
> no load. Or, maybe
> this is only on machine tool and compressor motors, which may have
a
> greater service
> factor designed into them. My Bridgeport 1J with 'pancake' motor
draws
> 3.0 amps no
> load and 3.3 A at full load, according to my VFD's internal
readout. It
> smoked my
> 1/2 Hp VFD, although that was a pretty inferior unit.
>
> Jon