On Thu, 05 May 2011 22:33:21 +0100, John Rumm
wrote:

On 05/05/2011 19:39, Roger Mills wrote:

Anyone know whether it's possible to add soft start to a tool that
wasn't born with it - maybe by using some external plug-in wizardry?

Possible, certainly.

I have a Lidl compressor which, according to my power meter, takes about
750 watts[1] when running.

Probably not an ideal candidate for electrical soft starting. Chances
are it would just stall on startup.

I tried to run it today from my Honda 2kW (peak - 1.6kW continuous)
inverter-based generator, and the genny didn't want to know - presumably
because of the high start-up current.

Would a soft start help and, if so, can it be retro-fitted?

I would guess if you could find a way of unloading the motor on startup,
that alone may fix the problem. If not then you could probably
electrically soft start that, and then reload it once running.

[1] Probably 1000VA with a non-unity power factor because the indicated
current was about 4 amps

If you have a nice large none reactive load like a kettle or heater,
stick it in series with the compressor and see if it runs.

If it fails to run and doesn't trip, find a lower resistance load and
try again..

If it runs and builds to a reasonable pressure on its limited current,
then either use a pressure switch to short out a series resistor
equalish to your kettle, or a timer.

You could even do a "start run" switch configuration that you switched
through from stop to run. Use make before break contacts if you try
this route.


The surge current that trips your inverter occures when the motor is
stationary and generating no back EMF. Removing any or all of the
mechanical load is a none starter. The bare motor would trip your
inverter. Electrics is quick motors isn't.

On a final note, soft starters were available for single phase motors
when I was involved with them, they were very effective and I would
assume quite inexpensive for the load you describe.

I never saw a plug in version, but if you can grasp the basics of a
terminal driver and the terms supply and load, then you would not have
a problem.

HN