The two basic cautions for VFD use are that the output of VFDs consists of a
fast risetime voltage waves, similar in appearance to squarewaves (as
opposed to the gradual rise/fall of sinewaves normally applied to a motor
without a VFD).

Ideally, a VFD should be used with motors rated for "inverter duty" which
means they have a higher voltage rated insulation.. but many dual-voltage
motors operate without problems when the output of the VFD is set to the
lower dual-voltage spec (at 240V rather than the 480V rating), and the
motor's power leads are arranged/wired for the lower voltage, of course.

The second issue is heat related, not so much because the output of VFDs
cause more internal motor heat, but instead, less cooling efficiency of
motors running at reduced speeds.. since most heavy duty induction motors
utilize a cooling fan (even totally enclosed cases), and when the fan spins
slower at reduced motor RPMs, the fans move less air.
Many HSMs install a separate, constant speed fan or small blower to maintain
adequate airflow over the motor at all times.

Soft-start doesn't necessarily imply that the mortor starts softly, the term
used for increasing the duration of time for the motor to reach it's full
operating speed is a feature of many VFDs, and is typically a programmable
preference for the speed "Ramp Up" or increase of time during starting.

As JK commented, the actual "soft start" feature is intended to put a
minimal strain on the machine's supply voltage, such as in installations
where the high inrush startup currents of a large HP motor would cause
problems with other equipment connected to the same supply (lights dimming
excessively, unecessary/frequent circuit breaker trips/interruptions etc).

I don't have much experience with VFDs, but this is the first time I've
heard of a recommended limited number of starts-per-hour for 3-phase motors
with VFDs.

Please let us know what solution you find which best fits your application.

--
WB
..........


"Oppie" wrote in message
...
I have a milling machine that's starting to show its age. It has a 5 Hp,
208V, 3 Ph spindle motor that's currently on a full voltage Fwd/Rev
starter.
Motor really slams when turned on and has damaged the variable speed
sheaves
and keyways.

Was thinking of adding a soft start system in series with the original
starter. Considered a VFD but have heard that using one with a motor not
designed for such makes problems. Even adding a line reactor to remove
harmonic content may be required. All told, a soft starter appears to be
the
best choice.

Does anybody have a favorite unit to recommend?
Ones I've seen so far have a SCR in series with two of the three legs and
control starting voltage and ramp-up time. If you keep them powered, they
can also control ramp-down time. A contactor bypasses the reduced voltage
system after the ramp time. All fine and well but only some of the vendors
seem to rate # of starts per hour. I would assume that this is due to
heat
generation when the SCRs are carrying the load and the need to dissipate
it.

Thanks in advance for any answers
Oppie