Soft Start

[Roger Mills]

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?

I have a Lidl compressor which, according to my power meter, takes about

750 watts[1] when running.

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?

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

[dom]

You can add soft start to machinery - with an inverter (I guess the same as in your genny, just with lots of user settings on it) - but I don't think it will reduce the current draw.

I bought an old 3-phase bandsaw, and added an inverter to that (an Omron IIRC), and it has endless settings for things like ramping up the Hz - and every other imaginable characteristic of power management.

Does an excellent job, but not cheap.

[Tabby]

well... compressors are not normally soft started, they need a good current surge to get going in the face of high pressure. You can soft start them, but would need to take the pressure off them until theyre upto speed. How easy it is to arrange the additional mechanics on your compressor I dont know.

It will also work on soft start as is if you start with an empty tank, and switch it off once full so it doesnt try to top it up. Might or might not be practical though.

[Bob Minchin]

This type of inverter can ONLY be used to run 3 phase motors and will be useless for the OP.

Bob

[John Rumm]

Possible, certainly.

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

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.

[Steve Walker]

Does it start ok when there is no pressure in the receiver? If so then you could add a three port solenoid operated valve between the compressor and the receiver. The valve can vent the compressor output to atmosphere when the motor power is removed and re-connect the compressor output to the receiver a few seconds after the motor is powered up. It wouldn't be cheap though.

SteveW

[Bob Minchin]

Aldi/Lidl type compressors I have seen already have a dump valve arrangement fitted.

Bob

[Roger Mills]

etc

Dunno - I'll try it.

If so then you

An interesting solution. I'm not going to spend much money on anything though, because it was just an experiment to see whether I could operate it away from a mains power supply - but I have no compelling need to actually do that.

[Roger Mills]

Mine has a ring-pull valve to de-pressurise the receiver - is that what you mean?

It *might* work with the genny if I turn it on and off manually, completely exhausting the pressure each time - but that wouldn't be very efficient.

In normal operation the motor is controlled by a pressure switch with some hysteresis between on and off pressures - so it cuts out when it gets to max pressure, and turns it on again when the pressure falls a bit (but still way above zero).

[John Williamson]

If it's a squirrel cage motor or capacitor start, then it'll take a massive surge at start-up. The only generators I've met that will cope have been old ones with a massive flywheel on the engine.

[John Rumm]

Yup, anything from 5 to 9 times the normal operating current is quite common for an induction motor.

[Harry Bloomfield]

Roger Mills laid this down on his screen :

That is a safety valve.

[Steve Walker]

My experience is with big process gas compressors (a small drive motor was around 300kW, a large one 8MW!) The typical way of starting such compressors is to remove the load either by forcing the compressor's inlet valves to remain open or by bypassing the discharge back to the inlet. For an air compressor, you do not need to do this, instead, you just isolate the receiver and vent the compressor's discharge.

Don't think of the compressor and its receiver as one unit, think of of it as a compressor feeding a receiver via a pipe. A valve in that pipe can isolate the feed to the receiver, preventing air escaping and at the same time vent the compressor discharge so there is no back pressure. Once the motor is up to speed, the valve can operate, closing the vent and reconnecting the compressor to the receiver. This will allow the normal stop start nature of small compressors, while starting the compressor with virtually no back pressure and therefore little load every time.

SteveW

[H. Neary]

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