On 2008-02-14, Jerry wrote:

"DoN. Nichols" wrote in message
...

[ ... ]

The sure indicator here is the range of horsepowers (instead of
"up to X horsepower). This is because the capacitor needs to be sized to
the horsepower of the motor which it is being used with. Too large a
motor or too small a motor will not start properly. (Of course, the
word "Static" in the name is another clue.

[ ... ]

Hi Don

Why do you write that a static converter with more capacity will have
difficulty starting a smaller motor?
I refer to your statement -
This is because the capacitor needs to be sized to
the horsepower of the motor which it is being used with. Too large a
motor or too small a motor will not start properly.

Because the phase shift from the capacitor is a function of the
motor horsepower (and related inductance) the capacitance, and the
frequency. Since the frequency is fixed at either 60 Hz (here) or 50 Hz
(UK) we have the motor inductance/horsepower and the capacitance. Too
small a capacitor will not produce enough phase shift to get the motor
started quickly enough to avoid blowing the capacitor, and too large a
capacitor will generate too much phase shift, again resulting in very
slow motor starting and likely failure.

Note that the static converter was spec'd for a range of
horsepower (in the part of the article now trimmed), not "up to
such-and-so horsepower".

This is a common limitation for a static converter -- at least of
the capacitor and relay style. So -- if the machine tool has three
motors -- one 4 HP, one 2 HP and one 1/2 HP (say for perhaps spindle, feed,
and coolant pump), then while it may start the 4HP motor nicely, it will
fail to start the 1/2 HP coolant pump if that is needed first.
Obviously (in spite of what Nick thinks) once you have the spindle motor
running, it can start the others, but if you need the smallest to start
first (say it also pumps lubricant around to the bearings before you
start moving things) you may have problems. For this, really, an
external rotary converter would be a better bet. Let the static
converter start the rotary, and it can start all of the load motors with
no problems.

Obviously -- if it is a homebuilt static converter, or if the
owner is competent to open it up and replace the capacitor with the
appropriate size for the other motor, then this can be overcome.

Also -- if he is not careful to wire it so the real 240 VAC
comes in to the terminals which feed the control circuitry, he may not
be able to get the spindle motor to start with the static converter
alone.

Enjoy,
DoN.

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