19
Sep
Screw Compressors

What is Piston Duty Cycle

Posted on by Field Air Compressors

What is a duty cycle? Why do I need to think of that when I select a piston compressor? The duty cycle is as important as the flow/cubic feet per minute (CFM) of your compressor. In short, it is referring to the frequency of use of the piston compressor. The more your piston compressor is cycling/running, the more duty cycle you need.

When buying a piston compressor, regardless if it is for DIY/Handyman tasks or for a professional application, usually the most focus is placed on the flow/CFM or liters/minute. Commonly we think “I need this much air” and then find a piston compressor that can deliver that much air. But what is often forgotten is the regularity of use, the duty cycle. All piston compressors have a duty cycle, some as low as 25% and some as much as 100%.

What can happen if the compressor runs over its duty cycle? Well, your compressor can malfunction, the piston can seize within the piston housing as it gets too hot and as it cools down it can seize, or the engine can stop to name two common issues. However, rarely is the duty cycle identified as the underlying cause.

How can find the duty cycle of my piston compressors? You can work out the duty cycle of your machine by reading the engine plate of your air compressor. Your compressor’s duty cycle is directly related to the engines duty cycle and is expressed with an ‘S’ followed by series of numbers. Most smaller piston compressors duty cycles are measured in 10-minute cycles unless specified. For example, if your compressor states “S3-75”, S3 is referring to “Intermittent periodic use” referring to a 10-minute duty cycle, 75 is referring to the amount of running as a percentage, in this case, 75%. So if the duty cycle says S3-75 it is built to run a maximum 7.5 minutes in every 10-minute cycle.

Can a piston compressor have a 100% duty cycle? Yes, there are piston compressors that have 100% duty cycle, the engine on these compressors are rated S1. So when you size your piston compressor to your needs to make sure that you take in to account the duty cycle. If you need a constant flow of compressed air, your compressor might need to be sized larger so you can let it “rest” and you can use the stored air in the receiver tank while it is not running.

S1  Continuous duty The motor operates at a continuous load for sufficient time to enable the machine to reach thermal equilibrium. 
S2 Short Time duty Operation at a load for a time not sufficient to reach thermal equilibrium, followed by enough time for the motor to cool down.
S3  Intermittent periodic duty Series of identical duty cycles each a constant load for a period, followed by a rest period. Thermal equilibrium is not reached during the cycle.
S4 Intermittent periodic duty with starting Like S3, but there is a significant starting time within the periodic operation.
S5 Intermittent periodic duty with electric braking Sequence of identical duty cycles – starting, operation, braking and rest.  Again, thermal equilibrium is not reached.
S6 Continuous operation periodic duty Identical duty cycles with a period at load followed by a period at no load.  Difference between S1 is that the motor runs at no-load, without actually stopping.
S7 Continuous operation periodic duty with electric braking As per S6, but with a significant starting and electric breaking periods.  Again, the motor operates at no-load for a period instead of stopped. 
S8 Continuous operation periodic duty with related load/speed changes Series of identical repeating duty cycles, where within each cycle the motor operates at several different load levels and speed. There is not stopped time and thermal equilibrium is not reached.
S9 Duty with non-periodic load and speed variations     Load and speed vary periodically within the permissible operating range. Frequent overloading may occur.
S10 Duty with discrete constant loads and speeds Duty with a discrete number of load/speed combinations, with these, maintained long enough to reach thermal equilibrium.