"easy" motor controller using audio amps?
 

I was thinking about 1, 2, 3-phase AC motor control and ways to generate plenty of amps to drive a motor in a totally diy fashionl.

I look over at Pauls AWESOME DC motor control thread and i see that one of the biggest hurdles is simply handling all that power! Bigass heatsink, huge bus bars, etc...

So in my searching i have found a number of "low power" AC motor control DIY papers. Microship has some application notes for their PIC controller and the RC car crowd is all over this stuff.

So i got to thinking... what if i were to use 3 car audio amps and fed them a controlled AC 3-phase output from one of these PIC controllers? I think I'd have all of my heat issues taken care of with the amp and I'd have some pretty decent power output for low dollars. It is not uncommon to see 1500W amps (that's about 125A @ 12V) for DIRT CHEAP made in china.

One major challange would be the low voltage... if the made in china cheapo method worked, i'm sure you could find some higher voltage input amps for the SPL competitions (not sure).

Could this be an option for a DIY bike controller? Could it be expanded to be more capable?

Okay, i'll respond to my own thread...

this article
Anatomy of the Power Amplifier - Special Features - Car Audio Magazine

tells me that there is no issue with feeding only 12V to the amp. This is because the amps have their own internal DC-DC voltage upconverters. This is what allows you to get 1500W over a 4ohm speaker with only a 12V output. (1500W over 4ohms requires 77V, so it is stepped up internally to just above that). In order to feed that sort of power for any meaningful amount of time, you'd definately want a big bank of parallel batteries... no freebies here. It would sure make prototyping easier with only one batter, though :)

someone tell me i'm stupid or i'm going to have to keep researching this.

testing motor coil resistance - Article and Reference from OnPedia.com

This link tells me (in rather un-specific terms) that "most motors" have 6ohm coils, typically... that's a bit loose, but i'm in the ballpark :)

Do you suppose a freewheel diode would be needed across the terminals? I presume that these amps are producing a nice clean sine wave...

Also, I would expect that audio amps of high power (typically for subwoofers) would offer excellent efficiency around 60Hz, where the motor would want it most.

I have a fairly small amp (300Wrms)... i wonder where i could score a free single-phase AC motor to see if i could make it spin with a sin wave generator and the amp... or my computer's sound card :)

Allow me to respond to my own thread again...

Would a household fan be a simple 1-phase 120v electric motor? Why do these things usually have 3 settings? I've seen an egg beater with a diode in it before... so on "low" the diode cuts out half of the sin wave. I suppose i could wire up a household fan to my audio amplifier, make my computer output a low volume 60hz sine wave and plug it into the amp... if the fan turns, i have a proof of concept... Just gotta do it all when the wife isn't looking.

The way car amps are rated is totally misleading. So when it says 1500W that does not mean it can handle a 1500W continuous load like a motor trying to actually put out some real horsepower.

Just think of how big of a heatsink/heat pump would be required if the speaker and amp actually had to dissipate 1500W of power. Even a 150W sustained load on a normal speaker voice coil will overheat it very quickly. A true RMS value is more like what the power output of it would be but there are so many 'marketing' rms values out there it is hard to find real information on them.

Beyond the PR ratings used by amp makers, Class D amps would be ideal for this application.

On a Class A/B/AB/C amplifier, the transistor is almost always in its linear range. Picture a monkey with a rheostat. You pull on the monkey's tail (gate or base) and he changes the resistance, and the amount of current through the circuit. The problem here is that the monkey's pot is always dissipating power that you wanted to drive the load. I expect your amp will end up pretty hot if you go this way.

In a Class D amplifier, you alternate between yanking on the monkey's tail so hard that the rheostat is zero ohms or that it's essentially open. The MOSFETs in a Class D amp are driven only into saturation and cutoff, so they aren't dissipating any power (oversimplification, but almost true).

It SHOULD work with any class, but I wouldn't try it at high power. I'd also see what varying amplitude and frequency did to your motor. AC control logic is way complicated.

Those fancy multi-kilowatt ratings of car amps are for PMPO - Peak Music Power Output. It has to do with dynamic range, and ensures that sudden changes in the output (occasional excursions to extremes like a blast etc) are not clipped / distorted.

My home theater speakers are rated for 12000W, and my total contracted electrical load for the house is (including all electrical appliances, lighting and the home theater itself) 6000W. So take those ratings with a pinch of salt. When read carefully, the steady state power output (RMS power in audio terms) for each speaker is just about 50W.

PS: See you make the distinction already... but for car amps with enough power to propel the car are likely to be costlier than the car itself...

They do make class D amps that are RMS continuous rated at several KW. But you have for it, several hundred to thousand. In the end, if you're going to spend $600 on a 3KW class D you might as well spend $600 on a 300A controller, more purpose driven, lower chance of failure (not that the amp would fail, but using it for what it's not made could cause premature failure).

But I like your thinking, as a small class D rated at 500W continuous would handle a bicycle sized motor easily, albeit at the cost of a huge "controller" and added weight. Potential to be super cheap though. Better option than on/off switch.

I'm going to go research >3KW D amps now, just because I'm curious.

Okay, two things to keep in mind to keep this conversation on track:
1) i have installed a car stereo before and i am well aware of the marketting crap. If it is made in china, that 1000W rating is NOT 1000W. Also, the RMS value of the 1000W peak amp is not the same as its continuous output. If i recall correctly, RMS of a perfect sine wave is peak/root(3) - this is NOT the same as continuous RMS capacity. Lets leave those details out. I am not expecting to drive a full-sized car on a couple "Thump" brand amps.

2) I'm talking about using 3 amps to control a 3-phase AC motor! Maybe i've been searching wrong, but you can NOT find a 600 dollar 3-phase AC motor controller! They cost a LOT.

As for the AC control - i'm a computer engineer... that means i'm a huge geek. I can figure that stuff out. :) Besides, there are already plenty of application notes, and i have access to the IEEE database of research papers. To start with I wouldn't even develope an embedded system to control this (which is not that hard either) i would start by outputing a sine wave from a computer program through a 1-phase motor for proof of concept.

I measured the resistance of a fan that i have at home and it was 15ohm on the high setting... i also noticed 4 wires going from the knob to the motor - so that fan is not suitable for testing "as is" (ie, not ripped appart). I have 2 more fans in the house that i will check out.

Motor rotor induces a 'back emf' in the stator winding, thereby requiring way less current than the DC resistance of the stator would imply. This is the reason why there is a huge inrush current through a motor winding, when the motor starts - the back emf is absent at rest and slowly builds up as the rotor starts spinning...

Guess, you would likely encounter and tackle all design issues involved in a motor controller design from scratch. Will be fun to follow :)

You're better off starting from some large discarded UPSes. Or just get a surplus VFD (Variable Frequency Drive), which is already pretty much what you need!

Mike,

Please ellaborate on how an old UPS will help me produce a variable frequency and amplitude required to run a vehicle (bike, scooter, motorcycle, car, whatever).

Please show me a link to a DC-input 3-phase VFD for under $1000 capable of driving a few kW.

If my idea is totally fruitless i'll drop it. But please, ellaborate when you're telling me it isn't a good idea.


Hummingbird,

The huge inrush can apparently be avoided by driving the amplitude at the same factor as the frequency from the motor rating. Ie, if the motor is expecting 208V @ 60Hz, then when you're starting off at 0.6Hz you push 2.08volts, then when you're moving really slowly at 6.0Hz you push 20.8V, etc. Apparently deviation from this ratio allows you to control your torque for smooth take-offs and reasonable current at low rpm.