# what is duty cycle?

Discussion in 'General Electronics Chat' started by PG1995, Aug 14, 2011.

1. ### PG1995 Thread Starter Active Member

Apr 15, 2011
753
5
Hi

I found these two definitions of duty cycle:

1: The duty cycle of a machine refers to how long it can keep operating before it needs a rest, or what percentage of the time it's designed to be in use. For instance, a machine gun might only be able to fire continuously for 100 rounds before it needs to be allowed to cool.

For some machines, you could wear out the mechanism well before it came to the end of the warranty period if you left it printing continuously.

For example, a printer that is capable of printing 30 copies per minute would probably wear out in weeks if it was always printing. Therefore the manufacturer quotes a duty cycle such as "10,000 copies per month".

2: In engineering the duty cycle of a machine or system is the time that it spends in an active state as a fraction of the total time under consideration.

A motor runs for one out of 100 seconds, or 1/100 of the time, and therefore its duty cycle is 1/100, or 1 percent.

What is the definition of duty cycle in your opinion? Please keep it simple so I can understand it.

I didn't understand at all that motor example. Isn't motor running all the time? Is the example speaking of DC motor or AC?

Thank you for all the help.

Regards
PG

2. ### atferrari AAC Fanatic!

Jan 6, 2004
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A technical definition is not an opinion.

3. ### JingleJoe Member

Jul 23, 2011
185
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When concerned with a square wave it's the amount of time it's on (high) for, in relation to off (low), which is usually expressed as a percent.

4. ### MrChips Moderator

Oct 2, 2009
12,252
3,284
duty cycle = 100 x (High duration) / ( High duration + Low duration) %

5. ### wmodavis Well-Known Member

Oct 23, 2010
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+1 to atferrari & MrChips

Dec 26, 2010
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Like many expressions, "duty cycle" has more than one meaning - or at least there are nuances to it. In the context of a Pulse Width Modulation system, the duty cycle is indeed the proportion of "on" to "off" time of a waveform. NB while it may be pedantic to say this, such a wave form is only "square" at a duty of 50%, more generally it is a rectangular or pulse wave. A motor driven by by a PWM voltage supply, at say 75% duty cycle may rotate continuously, but at less than full power.

A quite separate sense of duty cycle applies to machines to indicate how they are to be used - continuously at 100% duty cycle or intermittently at some lower duty, typically because they would otherwise get too hot. This may be acceptable for devices which do not need to run all the time - as they can often be made smaller, lighter and cheaper that way.

You may find a small domestic coffee grinder will be rated for a few minutes per hour, but somebody running a restaurant might expect a more robust machine with a much bigger duty cycle. Similarly, the starter motor for an internal combustion engine has only an intermittent rating, but the traction motor for an electrically-propelled vehicle needs to be continuously rated, at least for cruising speed.

7. ### GetDeviceInfo Senior Member

Jun 7, 2009
1,571
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consider the task to be performed by a motor, is it continous or intermittent, and could one save by applying different motors to different tasks. Code also considers the duty of a load when spec'ing the electrical supply.

It typically all comes down to heat. Excessive heat buildup is destructive.