How to overcome high starting currents in air compressors.

Compressors and generators how to overcome high starting currents.

Compressors are one of the most challenging loads for a generator. Let’s look at a typical 1250watt direct drive compressor of the type common in Netto and other discount outlets. One might be tempted to think that this would start up easily enough from a 2 or 3kw generator.  Wrong!

Compressors have a pressure switch which turns the motor on and off at predetermined tank pressures. It might be set to switch the motor off at 100psi and then back on again at say 70psi. For the motor to start up when it is directly connected to the compressor pump which is trying to against an existing load of 70psi is like trying to drive away from the traffic lights with the hand brake on. The resulting start up power required can be as much as 6 times the rated power of the motor. Compressor manufacturers usually make a token gesture at reducing the starting currents by having a solenoid pressure release valve that evacuates the line between the pump and the non return valve feeding the tank every time the pressure switch switches off the power to the motor. This reduces the pressure in the feed line to atmosphere. This is of limited usefulness as the volume of air contained within the feed line is minimal and will not allow many revolutions of the pump before it reaches the same pressure as the tank.

So what is the solution? Well the obvious one is to buy a petrol powered compressor but this is not always an acceptable option for various reasons.

Another way round it might be to use a generator that is capable of delivering 6 times the rated load of the compressor. Again it’s not an ideal solution as this would result in a large generator spending most of its time running a small load. Generators in this situation tend to experience problems due to carbon deposits in the head and muffler.

Sometimes you can just reduce the pressure that the tank will drop to before the compressor motor cuts in using the mechanism within the pressure switch. The switch is usually within a black plastic box the siize of a cigarette pack with an on off button on top. Springs inside are tensioned to adjust the cut in and out pressures.

Another solution is to substitute the feed line for a much longer one and I have seen this done many times. A 25m roll of copper brake pipe costs around £20 and dramatically increases the number revolutions of the pump required for the feed line to reach tank pressure. It’s definitely going to help reduce starting currents.

A  more elegant solution is to reconfigure the feed line exhaust valve so that it remains open for the first few seconds when the motor is starting.. It might be necessary to use a power on delay switch and a timer relay to achieve this. It’s not a circuit I have ever used but I’m quite sure its doable. Perhaps somebody can supply a circuit diagram.
(See my previous post on ECO throttle and heavily inductive loads)

I am always interested to hear details of any particular application of these principles or indeed any other ways of overcoming the problem of compressor starting currents.

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