"Oppie" writes:

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.

Given a VFD gives you lots of pluses, you may reevaluate this.
They are inexpensive. You'll get soft-start, variable speed,
and protection, all in one package.


Yes, you want a load reactor. BFD, it's off the shelf. You can
skip it, but especially on older motors, that's carrying a KICK
ME sign. Here is why....

The PoCo gives you nice smooth sine waves, gently undulating
up and down. {It says so right here in their ad...}. The VFD
gives you square waves.

Now a clever guy name Fourier proved that you can mathematically
create a waveform from a series of sinewaves....but for square
waves, some of those are HIGH voltage sine waves.

What this means is: square waves have a HV component that is
hard on motor insulation. A load reactor smoothes them down, and
gives you less-square, more sine-ish, waveforms; and thus is less
stressful on the motor insulation.