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Old 02-11-2009, 06:17 AM   #1 (permalink)
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Let's talk... AC!

Now I've read a lot of threads, especially Paul's thread about his DIY controller, and I realize that DC is a common motor/controller choice due mainly to budget...

But since I have no money for either, let us bloviate at length on AC setups, their workings and advantages.

I sort've get how a DC controller works, but not how an AC controller works.

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Old 02-11-2009, 07:24 AM   #2 (permalink)
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Could you just not add a DC to AC converter (such as you would have in a car adapter) to the output from an ac controller? This seems really simple so it probably wont work.
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Old 02-11-2009, 10:30 AM   #3 (permalink)
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Converting from AC to DC or DC to AC results in major losses of power which is why it's not that simple to do in a project like this. Plus the components needed to handle that much power and heat would make it not worth the effort.
AC is a much better method of powering a motor (IMO) but your're right, the controllers require more electronics to handle the PWM at that power level so it's more expensive. Plus you would have to add a DC to AC inverter before the controller since there is no way to to store true AC.
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Old 02-11-2009, 11:43 AM   #4 (permalink)
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I've been thinking about designing a controller as well (or at least the control portion) and i've been thinking about AC. The gist of it is this - frequency of the wave determines rotation speed of the motor. rpm = 60* pole pairs * Hz. Amplitude determines current which relates to torque. I believe, however that there is also a relationship between rotation speed and the voltage required to get a particular amount of current - ie, at a stop, 60VAC will use more current than 60VAC at high speed.

From what I gather, the control portion is NOT hard, just the power portion is hard. Generating 3 AC waves from 3 PWM ports on a processor is easy. Relating RPM and throttle position to frequency and amplitude is "not hard".

I have been meaning to find a small fan to try to power with an audio amplifier (excellent DC to AC converter with variable freq and amplitude, optimised for under 85Hz)
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Old 02-11-2009, 12:51 PM   #5 (permalink)
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That's the trouble with AC is that everything is 3 times as complicated (expensive!)

Tom G. is converting a Dodge Neon to AC, using more or less off-the shelf industrial equipment. Of course he is also a electrical engineer and builds robots for fun.

I have had lots of questions from people about his project, but I am just NOT a knowlegeble guy when it comes to AC.

The next video I make with Tom will have to be about the AC drive he is going to be using in his car.

I do believe that the DC to AC inversion is a built-in feature of the drive that he is using.
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Old 02-28-2009, 07:02 AM   #6 (permalink)
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The question that's on my mind is this. From Paul's controller build thread, I've gleaned that watts are meaningless to torque, and that the more amps you can cram into a motor the more torque you will get, at least as far as DC motors are concerned.

What I'd like to know is if AC adheres to this rule as well, or, can I double my torque by doubling my voltage, drawing the same amps but double the watts?

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