Yes, the duty cycle increases as the welding current is decreased. If you looked at the duty cycle chart referenced by acrobat-ants
("... info from the miller website
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look on page 5tru 7 it talks about duty cycle."
you might notice that the graph uses a logarithmic axis rather than a linear axis. There is a formula for calculating the duty cycle at any operating current if the factory duty cycle rating is known (which is almost always listed by the manufacturer). The formula is as follows:
A = (S/I)^2 x R
where: A = adjusted duty cycle at desired welding current (%) S = specified current at rated duty cycle (amps) I = desired (or actual) welding current (amps) R = factory rated duty cycle (i.e. at current S) (%)
note that the ^2 stands for squared
(I am using non-conventional symbols here because I think it is easier than using subscripts for this text)
For the example machine which is rated at 20% duty cycle at 225 amps, the question is what is calculated duty cycle at 150 amps. Using the above formula, the numbers work out as follows:
A = (225/150)^2 x 20 A = (1.5)^2 x 20 A = 2.25 x 20 A = 45 %
Note that this results matches the graph figure from the above referenced miller manual.
The advantage of knowing or using this formula is that you only need to know the factory duty cycle rating to calculate what you want (i.e. you don't need to find the manufacturer's manual and hope they have a duty cycle chart).
For a second example, suppose that you want to use your machine at a current higher than the current listed for the duty cycle? Suppose your machine is rated at 60% duty cycle at 150 amps but you want to use it at 175 amps? Using the formula, this works out as follows:
A = (150/175)^2 x 60 A = (0.857)^2 x 60 A = 0.735 x 60 A = 44%
Hope this helps