On 2008-06-11, Karl Townsend wrote:


Karl, you may be able to buy a soft start inexpensively.

I've heard of the term. What exactly is it? When you say inexpensive, i
assume used. This machine could use it. So could my Supermax, 5hp.

A soft start is a electronic device that makes the motor spin up
gradually instead of producing that infamous "starting current".
It is not a VFD and can only produce 60 Hz, but it modulates the
amount of current available so that the motor would gradually spin
up to its rated frequency.

It may not be acceptable for hard starting loads like compressors,
however, it is used widely for soft starting loads like lathes, fans,
various spindles in general etc.

They are not very expensive used on ebay, even for high HP motors.
Sometimes the sellers describe them as VFDs, which is not true and can
only be explained by ignorance.

http://en.wikipedia.org/wiki/Soft_start

Soft start is an electronics term describing any circuit which is
reduces excess current flow during initial power up.

[edit] Need for soft start

....

Soft starts are sometimes used on larger equipment as well, such as
electric motors in various applications. The current drawn by an
electric motor during a start can be 2 to 10 times the normal
operating current, and this can exceed the supply's ratings if not
controlled.

Finally soft start is widely used on hand held tools to prevent the
tool being jerked out of position when switched on. Jumping of
electric drills was a widespread issue in the 1970s when soft start
was usually not fitted.

[edit] Soft starting methods

There are various ways to implement soft starting.

The most popular for appliances is triac control which ramps up duty
cycle over several cycles.

A method used with some motors is to start with windings in series,
switching them to parallel when partial speed has been reached. This
is commonly known as Y- start.

Increasing supply impedance has a semi soft start effect on motors,
with initial current still being above run current, but by a much
reduced amount. This method is not widely used.

Current limiting and voltage ramp-up in electronic supplies are common
methods of soft starting low voltage loads.



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