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Salvage It! - A Vacuum Pump From Junk

Every young techie needs a vacuum pump. They rate alongside the air compressor, multimeter and soldering iron as one of the most useful tools to take their place in the arsenal. This guide will show you how to make a serious vacuum pump for next-to-nothing. Add a vacuum tank for much the same price and you are loaded for bear.

By Neno Stojadinovic

It used to be that every young scientist needed a vacuum pump. There was metal to sputter, glass envelopes to evacuate when producing X-ray tubes and investment to ‘de-bubble’ when making castings.

Nowadays, there are commercial products available to replace these venerable old staples but hobby scientists have compensated by expanding the scope of their activities.

Click for larger image
Fig.1: Wiring diagram for a typical fridge compressor. Note the over-temperature cutout.

I don’t sputter my own telescope mirrors but I do pot my own coils and ultrasonic transducers and use vacuum to remove all air bubbles from the resin.

I don’t make X-ray tubes but I do make robot parts with ‘prepreg’ carbon fibre that uses a process called vacuum bagging to mould the material.

And call me funny but I find a strange fascination with the science of refrigeration. In refrigeration systems the flow rate and pressure can be considered analogous to current and voltage in electronic circuits.

With the advent of LP gas as a refrigerant and cheap manifold gauges, I find a lot of tech heads like me sitting around watching ice form on their home made evaporator coils.

Where do you get it?

Vacuum pumps live in the bottom of every refrigerator, except they are cleverly disguised as refrigeration compressors. Countless refrigerators are thrown out due to gas leaks, faulty thermostats or some other minor fault, leaving a perfect compressor that is just ripe for a new purpose in life. Refrigeration compressors of this type are commonly referred to as “sealed units”, and are a simple piston pump running in an oil bath for longevity.

Liberating the compressor is simple enough but it needs to be approached with a bit of caution. Most discarded fridges I’ve seen are devoid of gas but the gas lines can possibly be still under pressure and may also be partly full of refrigeration oil. Thus it is important to put on goggles before you begin, and don’t point the pipes at yourself as you’re cutting them.

So to begin: peer under old fridges until you find a compressor that doesn’t have capacitors near it. (It’s not difficult to drive a capacitor motor but there are plenty available that don’t use them at all, so I avoid the capacitor jobs as a needless complication).

Cut all of the gas lines to the compressor, leaving a handy length to work with. Use a pair of side cutters or a tube cutter to sever the lines, as a saw will introduce fragments of metal into the compressor.

Cut the electrical connections, unbolt the compressor and the prize is yours!

Test drive

Firstly, find out if it runs. Most simple compressors use a split phase start winding which is energised by a solenoid that is connected in series with the main, or “run” winding.

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