What is a Constant Run Option air compressor?

What is a Constant Run Option air compressor?


If you run a compressor off a generator or bad power and it has blown a capacitor, or tripped the overload it won’t be long before the motor burns out!

Electric motors failing on work sites?

Consider the Constant Run Option.

Due to high starting inertia an air compressor draws very high current on initial startup. Under certain operating conditions the compressor could be starting frequently and these spikes in amperage (6 times the run amperage for a short period) cause the motor windings to heat up excessively, which damages the motor.

As a  remedy to avoid this frequent starting we have developed the Constant Run Option to limit the damage caused by multiple high current start-ups. This option prevents automatic shut-off.

Ideally a compressor should be sized large enough to limit frequent start-up cycles, but the limited access to three phase power means that often a smaller  than required 240V compressor is used.  If a generator is required, a squeezed budget often leads to a smaller than ideal generator being used to run it. When the compressor is undersized and the power is not good – these  issues compound to radically reduce compressor life.

The best solution is to have a big enough compressor and suitable mains power to correctly run it, or if using a generator that it is big enough to comfortably start the compressor under all conditions.

If you have bad power or a generator that is marginal, it may cope with the initial startup of the compressor, but not be able to sustain multiple starts per hour. The more the generator struggles to start the compressor, the more harm is done to the electric motor – until it fails.

To combat this, we have adapted the pump unloading system commonly used on our petrol driven compressors to offer a solution for the electric compressors. We call this the Constant Run Option. We can now offer CRO on all C-Series, AF-Series and MC-Series compressors manufactured in our Melbourne factory.

How the Constant Run Option works:

Effectively, the CRO provides dual control for the compressor. With CRO selected the compressor has to be started only once per session. After this initial start-up it will cycle on/off load without a stop in rotation. The system will go off load when the top limit of the operating pressure is reached and the pumped air is vented to atmosphere through a silencer.

When the tank pressure drops by around 2 bar, the compressor is brought back on load, directing pumped air into the tank again. The cycle repeats until the work session is over. At this point the compressor can be turned off using the pressure switch.
If the CRO, dual controlled compressor is being used in the workshop/factory with good electricity, then the stop-start setting can be selected. Under this condition the compressor will operate as a standard electric unit switching on and off through a pressure switch controlled cycle.

This MC15 is an example of the CRO variant. It doesn’t look much different but could save you a lot of money in production downtime, and motor repairs or replacement.

Our Constant Run Option will give to customers the choice of:

A) Running the compressor Stop-Start with the pressure switch as usual, if connected to an adequate power supply in a workshop,

B) Switching to Constant-Running mode to relieve starting strain on the motor if operating on a marginal generator or on a bad electrical power supply.


Due to the challenges of generator selection caused by different methods of rating power output and sometimes inflated power claims, we prefer not to make recommendations as regards to the compressor / generator match-ups.

As for generators, while it seems fair to say, ‘Bigger is better’, it is also wise to remember, that ‘You get what you pay for’.

Copyright © McMillan Air Compressors 2019
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