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Automation & Control

Factory Automation Design & the AC-DC Equation

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Rob Spiegel
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AC or DC?
Rob Spiegel   12/5/2013 10:53:12 AM
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Good article, David. You showed just how complex this question actually is.

William K.
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The decision between motor types
William K.   12/6/2013 8:57:24 PM
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This is an interesting post that does provide some useful insight. But the slant toward brushless low voltage motor controls is sort of obvious. So clearly the choice needs to be based at least partly on just exactly what the motor need to do. But other concerns include the cost of controls and the maintenance of those controls. A standard small three-phase motor can go for many years with no attention at all, and if something does eventually fail there are lots of folks qualified to do the repairs. Brushless motors are quite a bit more complex, and servicing their systems requires a good bit more skill and education.

So the choice is not always simple and straight-forward.

Cabe Atwell
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Re: The decision between motor types
Cabe Atwell   1/20/2014 5:36:50 PM
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Indeed, some factories require both AC and DC, which can further complicate matters.

William K.
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Re: The decision between motor types
William K.   1/20/2014 8:54:22 PM
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We have an interesting statement here, "AC motors must generate a magnetic field in order to operate. As a result, AC motors typically have high inrush currents." My experience is that ALL motors must generate a magnetic field to operate. Sort of an interesting argument. PM motors don't need to generate two magnetic fields, so they are a bit more efficient in that aspect. 

What is indeed certainis that some kinds of motors are easier to control than others, and some are much simpler to ramp up the speed with than others. But just running on a lower voltage actually reduces efficiency because the resistance losses are greater. The 24 volt DC motor is probably a lot simpler to use as an intermittant duty motor with the speed ramped up and down, and it is undoubtedly more efficient than an AC motor running constantly with a slip clutch slipping when things are halted. 

My point here is that the benefits come from the application of more complex control schemes, not because of some particular technology. Variable speed AC drives can also deliver some real savings. 

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