Followup -- How do you test a DC servo motor

As a young man, (there were 5 of us kids - 4 years apart) -
The young ones we had a stick and on the end was a loop. The loop had a square top with an indention in the center -
coming down to a pair of wires into the stick. The wire was covered in WHITE wire covering. The unit would pull down and push up. Cool to teach young ones to switch the bathroom light on or the bedroom light on all by themselves - and off as well!
Martin
Ignoramus22378 wrote:

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Brushless DC motor has a commutator, and polyphase AC motors do not need one.
i
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Also, in a brushless DC motor, there are permanent magnets on the stator.
i
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wrote:

How do they get power to the windings on the armature without brushes?
RogerN
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Ignoramus22378 wrote:

Ig, A brushless DC motor does not have a commutator, hence the term "brushless". The commutation is done electronically by the controller that drives the motor. That is one of the advantages of them, no brushes to wear out, no carbon dust, etc.
-Al
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wrote:

How does "brushless" and commutator work together? I thought you had to have brushes contacting the commutator.
RogerN
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I chose very unfortunate way to say what I was thinking. I was wrong to call the controller a commutator. I was thinking that because the controller sends current to different windings that I could call it a commutator.
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On Jun 6, 8:54pm, Ignoramus22378 <ignoramus22...@NOSPAM. 22378.invalid> wrote:> I chose very unfortunate way to say what I was thinking. I was wrong

The single phase circuit is called an H bridge, but I haven't heard a standardized English name for the three phase version. At Segway the engineers just call it the motor drive.
jsw
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wrote:

Brushless motors sometimes have hall effect sensors that detect the position of the armature and control the commutation. Some use resolvers, since resolvers are absolute to 1 turn, they can use them for commutation. At work we have some motors that we have to align the resolver to the motor, others we have to run a routine that determines the position of the resolver to motor and stores the position in the drive.
Now if you have a 3 phase brushless motor and put sin of 0, 120, and 240 degrees X the voltage across the three phases, and stepped this at 1 degree at a time (1,121,241 2,122,242 etc.), you would hopefully get 1 degree of rotation per 1 degree of magnetic field rotation. If I'm thinking correctly about it, a DC servo motor might provide linear steps if you provide a linear transition between phases, instead of voltages proportional to Sin of the angle. Maybe like 0% 100%, 1% 99%, 2% 98%... would produce linear sized steps around the commutator.
RogerN
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http://en.wikipedia.org/wiki/Resolver_ (electrical)
http://en.wikipedia.org/wiki/Synchro
jsw
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Ignoramus22378 wrote:

Huh? Iggy, did you miss your morning coffee?
If a motor has a commutator, what makes the electrical connection to it? Brushes is the only way I know. So, if it has a commutator, it needs brushes, and can't be called "brushless".
The problem is this "brushless DC" thing has been thrown around for a long time, and it confuses many people. Real brushless DC motors, like computer fans, have electronic circuits inside to do the commutation, and you supple them with real DC and they run. But, except for some VERY special retrofit servo motors which had an electronic commutation circuit built in, all brushless servo motors are really AC machines of some sort, because the servo drive has to switch the polarity of the windings to make them turn a full circle. Most are permananet magnet (synchronous) motors, but there are some induction servo motors used on large machines. The latter would be used with a flux-vector drive.
The designation of "AC" or "DC" will not be much of a guide to which ones prefer a sinusoidal or trapezoidal drive profile. Sometimes the manufacturer gives this info on their pect sheets, sometimes not. I have worked with a modest number of permanent magnet motors, and I can never tell which ones will work well with my "trapezoidal" or 6-step drives until I actually try them out. A number of motors that are supposed to be sinusoidal-only seem to work VERY well with my 6-step drive. I have seen two, however, that really doesn't like the 6-step drive at all, they vibrate badly at a couple hundred RPM.
Jon
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In some countries the down is on and the up is off!
They ground the light to turn it on - down...
Martin
Cydrome Leader wrote:

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If you're building a new machine, no question. If you're looking for a spare part, not true. Most people building their own machine use smaller servos, not all.
Karl
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I have three servo's on my CNC plasma table.
Two happen to be the same, the third is small.
The other two are smaller than a coffee can - but about that size. I have some test docs on them and since one is an X and another Y the strength could be different. One moves a gantry and the other moves the torch across the gantry. I suspect volume and ease of production... made those the same.
Depends on the job and if you have a volume business - fewer parts is best.
Martin
Ignoramus23517 wrote:

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