Followup -- How do you test a DC servo motor

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

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.

Mart> OK, thanks to all. I tested all six. I was told about one of them that

In some countries the down is on and the up is off!

They ground the light to turn it on - down...

Mart> >> OK, thanks to all. I tested all six. I was told about one of them that

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!

Mart> >> >>> OK, thanks to all. I tested all six. I was told about one of them that

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

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

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

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

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

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

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

Here Ya go Ned: