Followup -- How do you test a DC servo motor

OK, thanks to all. I tested all six. I was told about one of them that it was bad, so I marked it with a sad face, just so that I do not
forget which was the faulty one.
There were four Fanuc 6L DC servos, one Fanuc 5M DC servo, one Allen-Bradley AC servo, and one Electro-craft servo that is bad.
I tested them with a DC power supply.
What I learned about testing them (from this newsgroup and from practice) is very simple: two of the four pins on a DC servo, if picked correctly, make it spin constantly. Two pins on an AC servo make it make a small rotation and stop. If I switch to another certain pin, the motor turns back a bit.
Pictures of testing can be found here: http://igor.chudov.com/tmp/Servos/
One more question is, it would appear that many people look for a pair of matching servo motors, as opposed to just one. Would you agree with that?
Thanks to everyone.
i
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wrote:

I learned something cool today. If I move a light switch on a wall "up" the light goes on. If I move it "down" the light turns off. I'm getting very good at turning lights on and off now.
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If you had kids, you would know that it is a great discovery, that fascinates children at the age of two, or there abouts.
i
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wrote:

I wondered what the difference was between a brushless AC and brushless DC servo motors were. The AC servo motor are supposed to be wound for a sinusoidal waveform and the DC are supposed to be wound for more of a trapezoidal profile. I don't know if this is true or not maybe.
RogerN
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There is no difference, it's just that some mfrs choose to use different names. Brushless DC, if it's actually used, would be a particularly bad description -- there's no DC involved in driving the motor. The terms "brushless servo" or "AC servo," while not entirely descriptive, do not contain any contradictions. As I implied in the other thread, a more complete description would be "polyphase permanent magnet sysnchronous motor with angular position feedback." But that doesn't just roll off the tongue like "brushless" does. <g>
--
Ned Simmons

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wrote:

Greetings Ned, I've been looking at 3 phase brushless motors for RC aircraft recently and am curious about driving these motors,how they work. If the coils are energised in sequence will the motor rotate? For example, if three switches were arranged equally spaced around a spinning shaft with one switch actuating lobe and connected to power up the coils on one of these types of motors would the motor spin at the same rate as the shaft? Thanks, Eric
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wrote:

I bought some surplus brushless servo motors surplus years ago. They have 3 phases and a resolver. At the time a resolver to digital converter was over $400 so I didn't purchase one. I programmed a pic microcontroller to output PWM sin signals 120 deg per phase, used them to drive half H-bridges, and test ran my brushless motors as open loop stepper motors. The motors ran great this way. If I come up with an application where I need them I'll look for some Pac Sci drives or similar on eBay.
So, Iggy, if you need to test some brushless motors just program a PIC or similar as a 3 phase PWM micro stepping stepper driver, wire to drivers and you should be good to test.
RogerN
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On Sat, 06 Jun 2009 09:29:41 -0700, snipped-for-privacy@whidbey.com wrote:

It seems like it would, but I doubt it would be very smooth or efficient. The motors do have more in common with steppers than with brushed DC motors, but as I understand it, steppers use some funky tricks to increase the number of poles. The similarity between the two is more apparent when you consider a stepper can be set up to run as a low speed synchronous motor -- Superior Electric labels their steppers "Slo-syn."
You can run a 3 phase brushless motor as a synchronous motor from a 3 phase source, but it'll suffer some of the same limitations as a stepper. If overloaded it'll stall and probably won't recover, and may or may not be able to start without a supply that ramps up the drive frequency. I've worked on machinery that had synchronous motors that didn't have a ramped start. They started with a bang, occasionally shearing the teeth off a timing belt in the process.
A brushless servo amp gets around these problems by monitoring rotor position with dedicated sensors and adjusting its output to the 3 phases to maintain torque or speed (depending on whether the amp is in torque or velocity mode) proportional to the amp's command input. Beyond that, the amp is just a black box to me.
I mentioned in the earlier thread that with the drive set up properly, you can run a brushless motor, open loop, with a VFD.
--
Ned Simmons

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wrote:

Thanks Ned. The motors I'm looking at spin really fast and as near as I can tell the controllers use back emf to determine rotor position. Eric
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On Sun, 07 Jun 2009 13:56:53 -0700, snipped-for-privacy@whidbey.com wrote:

Got a link, Eric? I'd like to see them.
--
Ned Simmons

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wrote:

Here Ya go Ned: http://www.hobbycity.com/hobbycity/store/uh_listCategoriesAndProducts.asp?catname=All+Outrunner+Motors&idCategoryc&ParentCatY The above page has lots of motors and links to the motor controllers. Eric
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Ned Simmons wrote:

Yes, this is why this incorrect nomenclature is slowly being stamped out by the manufacturers. It probably started with the computer fans and similar stuff that actually DID have the electronic commutation circuits built into the motor, so you could treat them just like any other true DC motor.
Jon
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wrote:

So you're on par with a 2 year old?
Tell us more about the commutators in brushless motors.
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Yes, I still like to take things apart to see what they do, etc. It does not seem to do me any harm. Some people lose that interest as they age, and I consider that to be unfortunate. If that is the case with you, I am sorry about that.

Brushless motors have an electronic commutator that supplies current to different windings of armature, to make the rotor turn. The rotor has permanent magnets that interact with the armature windings. An induction polphase motor does not have such magnets.
By the way, those Fanuc servo motors, are unlikely to be brushless.
i
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wrote:

Is "electronic commutator" new code talk for fixed electrical connections aka wire?

But it has commutators if it's run off an electronic controller, right? But it doesn't have commutators if there's no electronics?
Are we to conclude all motors have commutators?
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You may want to find a commutator on a three phase induction motor.
i
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wrote:

I think you are confusing DC brush motors with DC brushless motors. Brushless motors don't have brushes, brushes are required to carry power to rotor windings unless you're talking induction motors. So, what is the difference between brushed DC servo motors and brushless DC servo motors?
RogerN
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wrote:

No problem.
you said it yourself, I'll just use an "electronic commutator" and bam, my induction motor now has a commutator.
easy!
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Ignoramus22378 wrote:

I've been tinkering with Fanuc "red cap" brushless servo motors. They have a traditional ABZ quadrature encoder plus an insane (but proprietary) 4-bit absolute encoder scheme. I have a prototype converter that converts these signals to traditional UVW "Hall" signals. I have used this with my own PWM brushless servo amp. It used the absolute encoder signals until the index pulse comes by, then counts quadrature pulses from there. This is a bigger motor than I'm used to running with my servo amp, and there are a couple quirks I need to examine. But, the converter seems to work, and I plan to make that a product. I needed to expand the CPLD in it to handle a variety of encoder resolutions that appear on these motors.
The Fanuc brushed motors are the "yellow cap" series.
Jon
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Jon, it sounds like you have done a lot of interesting work in the servo drive area. Have you worked with resolvers? I have a couple of new, old stock, modicon brushless servo motors with resolvers. I don't have an application for them right now but I thought I would use them in the future when I find a drive or perhaps a resolver to encoder or resolver to digital (or hall) converter for low $$$. Just to buy the Amphenol bayonet lock plugs for these motors isn't going to be cheap though! If I do an EMC conversion to my Anilam Bridgeport mill I thought it could be interesting to drive the knee when movements are out of quill range.
For an experiment I programmed a PIC for 3 phase sin PWM out and ran the motor like a stepper. Anyway if I come up with an application where I need them perhaps I can find a drive on eBay for reasonable. It looked like some of the Pacific Scientific drives are capable of encoder or resolver feedback and can convert the resolver in and give encoder out.
RogerN
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