Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: simpson36 on January 17, 2010, 06:29:49 AM
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My new mill has a 90V servo motor for the spindle. My current PS is linear 72V with dual rectifiers. The PS will be overloaded with the new spindle motor and the 4th axis motor, so I am contemplating buying a second PS and I am in something of a quandary of what to buy. The 90V motor runs OK on 72V, but does not achieve the speed I need.
I have axis motors running fine at higher (in one case double) than their rated voltages and I was looking for opinions on where to run a 90V spindle motor and what ramifications if any I would encounter running the motor at 130V
Also I see that dual rectifier PS can have the secondaries wired in series or in parallel for double volts of double amps. I am wondering in this case if it is possible to run one motor by taking the power in series and still be able take the regular voltage off the respective rectifiers. For example, can a dual rectifier 80V SP supply 160V for one motor and 80V for other motors by taking power from a series wiring for the HV and straight into each rectifier for the 80V?
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Steve,
My opinion is that the spindle servo motor should have a dedicated supply. Splitting a supply via the rectifiers is somewhat tricky... For the spindle variable supply, I would use a commercial line rectified and PWM voltage fed to the servo.
Bill C.
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I guess I need to justify my opinion a bit....
Two transformers in series sound OK but they must be matched very well - like made at the same factory and even among the same manufactured batch. The problem lies in the fact that when a load is placed across a pair of transformers in series, the reactance in each must be identical or one will actually pull current from the other until one will buck the other. If they're well matched - all is OK with the arrangement. The same can happen with rectifiers....and ground cannot get confused in the mix or you have the same situation as with the transformers.
The other reason for this 'opinion' is that most DC supplies are SCR type controllers where AC is converted to DC at the same time it is chopped in time for the reduced voltage. They work at line frequency for the chopping - now; filtering this voltage, (even if there are four SCRs doing the regulating in a bridge circuit) will require large 'capacity' capacitors. This quickly becomes expensive and in my opinion again - dangerous. These large capacitors MUST have bleeder resistors because of the lethal dose of voltage and current that they will dump in a microsecond. Also SCRs by their nature will latch on when triggered into conduction and will remain on until the voltage is reduced below their conduction threshold. So, if one SCR decides to go to lunch and short out, you have a bomb of SCRs going to lunch too with capacitors dumping large jolts of power into the circuit - hopefully not reaching some delicate control circuit board - but most likely it will. The smoke will all get out....
PWM is much safer and controllable. But again; this is only my opinion. I have a small drill press that I built an SCR controller circuit for that has done what I explained. I now use a universal motor on it with a triac speed controller circuit.
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My new mill has a 90V servo motor for the spindle. My current PS is linear 72V with dual rectifiers. The PS will be overloaded with the new spindle motor and the 4th axis motor, so I am contemplating buying a second PS and I am in something of a quandary of what to buy. The 90V motor runs OK on 72V, but does not achieve the speed I need.
I have axis motors running fine at higher (in one case double) than their rated voltages and I was looking for opinions on where to run a 90V spindle motor and what ramifications if any I would encounter running the motor at 130V
Also I see that dual rectifier PS can have the secondaries wired in series or in parallel for double volts of double amps. I am wondering in this case if it is possible to run one motor by taking the power in series and still be able take the regular voltage off the respective rectifiers. For example, can a dual rectifier 80V SP supply 160V for one motor and 80V for other motors by taking power from a series wiring for the HV and straight into each rectifier for the 80V?
Doesn't sound wise to me. For one thing, motors have voltage and current ratings for a reason - if exceeded, the motor can fail, sometimes spectacularly. Increased voltage also implies increased current, which, in a servo motor, can cause demagnetization. It also means increased heat, and servo motors are usually not intended for continuous heavy use, so not vented. As for "ganging" transformers, that can be risky, as someone pointed out. You're much better off buying or building a single power supply suitably sized to your application. Lotsa good transformers and power supplies available at very reasonable prices from antekinc.com.
Regards,
Ray L.
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Thanks for the comments so far. I should have provided more info.
The spindle motor is a servo motor WITH and encoder and is driven by a servo drive, so although I agree with the assessment given relative to PWM vs SCR speed controllers, it does not apply in this case.
The servo drive will limit the amps to whatever I set it at up to 35amp max. The drive is rated to 160V.
Confusion sets in, methinks, when I apply stepper motor thinking to servo motors. If one considers the voltage rating on a stepper vs where they actually run, there is no correlation. My experience with servo motors so far shows me that a servo motor can run much higher voltage than it's rated voltage on an axis where it has plenty of rest time, but a spindle motor has a different environment and my concern is overheating. I can cool the spindle relatively easily, but I have no practical method of cooling a servo motor. It is my understanding that amperage causes demagnetization, not voltage, although I really don't understand that whole phenomena.
Ironically, the idea of running rectifiers in series for double voltage comes from Antek themselves. I should clarify that this is only applicable to a pair of rectifiers running off the same toroidal coil, as is the case with my current PS and the 1,500W models from Antek.
What I am after is to get the full RPM's from the motor that I was expecting. the belt ratios on my new mill were designed around Keling's specs, which I now suspect were exaggerated. The motor does not run at the spec speed, even after deducting 20% as indicated by several tech articles on the subject. I can easily get the speed back with higher voltage and I am going to purchase a separate PS just for the spindle, so that is the basis for the question here. My alternatives are grim; buy another motor or redesign my carefully chosen ratios on my two speed mill head. I sold my old mill and my two Minarik PWM drives went with it, so as a diagnostic I have purchased another Minaric cheap off eBay to run the spindle motor on 130V 5A to see what speed I get and if the motor gets hot.
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Take duty cycle as a guide....1/2 speed = 50% duty cycle for a continuously running servo motor. The servo motors on this mill are rated for 32 volts DC, they are being run at 88 volts - but these are for axis drives so they are not continuous as the spindle would be.
That's a tough call....The drives will protect the motor and themselves so give it a try. You can detect the heat long before it gets out of hand and does any damage. I've seen motors so hot they were smoking but were still running fine and after a shutdown and cooldown, they continued to run fine.
My opinions are from a cautiousness that I have in these situations - and my be a bit too cautious. I hoped to relate that...
AS for cooling; if the motor tends to run hot. Fabricate a set of fins that could be attached to the motor heat sink style and direct a small fan or blower to cool it. Model aircraft are doing that and these guys are blasting those little DC motors with many times their rated current and voltage....Little bit different situation but close....
Chill the current before it gets to the motor....!!!! :P
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Well, here is something interesting, I measured the voltage at the spindle motor and at full speed it is only50v. I removed the fuses to all drives except the spindle and still got only 50V at the motor at full speed. Of I extrapolate up to 90V from there, the speed is much closer to what I expected.
I have had the PS do this before and then it started putting out the full 72V again for no apparent reason. When I received the PS from Keling, the frame was very badly bent. I straightened it out and used the PS and it seemed to be fine.
My impression was that voltage was determined by the number of windings and is unchangeable. How can a 72V linear PS with a toroidal transformer put out less voltage that it is supposed to?
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Check the connections at the rectifiers. Sounds like they may be dropping out one now and then which drops the RMS voltage that the capacitors see and the maximum output - like going from a full wave rectifier to half wave....
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Interesting,
If I have this right, there are 4 diodes in the rectifier and if one goes belly up (and fails open?), then would you loose 25% of your voltage? That would fit with what I am seeing . . sort of. The PS is supposed to be 72V and I am getting 50.
Both of the rectifiers in this PS blew up and I replaced them with the identical parts . . . a bit hard to find as they are thru hole with one staggered pin.
Maybe I should check the AC voltage coming off the toroid?
On the PS question, after gathering info from a couple of trusted sources, my decision is to go with a separate 105V 800W unregulated linear (toroid from Antek) exclusively for the 90V 7.6A spindle motor.
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Yes, you could loose 25% of the full voltage but if you looked at the voltage on an oscilloscope, you would also see lots of ripple too. That's not good for a DC motor either....
105V toroid will give you 148.5V RMS. That ought to spin her!
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The motors don't seem happy . . very buzzy, so maybe this is the problem. There are two separate rectifiers, so I'll need to check the second one and see if it also is kicking out only 50V. One of these days I will have to break down and buy another scope, I suppose. I had one briefly, but that's another tale.
The 105V is on the DC side. I am buying a complete PS. I'm too busy to build stuff like that, although it seems I may be forced to repair the current one if the rectifiers are fried again.
Have my hands full finishing up my new mill, building more 4th axises (axii?), and finalizing the pneumatic spindle lock and tail stock for it . . on top of my regular work.
FWIW, I am advised by a very reliable source that 20% to 30% it the max overvoltage one should attempt. 100% is verboten and can damage the motor coils even without overheating. Antek publishes the voltage drop on some of their units and it seems clear to me that the unregulated PS will not put out the rated voltage while there is a load on it, so I'm confident in the choice. I want 2,500 in the low range and 8,500 in the high, and it looks like I will get that with the new PS, so fingers crossed as I push forward making the pulleys as designed.
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Interesting new info. I have the feeling I am asking these question in the wrong place as this forum seems to be 99% steppers, but I'll give it another go.
I checked the PS at the output and both sides are cranking out 74V. Yet the spindle servo motor wires only show 50V at max speed.
The Minarik drive arrived and it's 130VDC output kicked the spindle speed up from 1,300 to 2,700 . . nice. Yet a voltmeter reads only 105V on the motor wires.
Both the Minarik and the servo drive are PWM, and I recall reading somewhere that a voltmeter reads PWM inaccurately. Yet it seems that at full speed the PWM should be at 100% and therefor show as the full voltage on a voltmeter.
Adding to the puzzle, the actual motor speed calculates correctly using the actual voltmeter reading and not the theoretical voltage, so it would seem that the motor also sees the voltage at the same level as the voltmeter . . i.e. significantly below the rated voltage of either the servo drive or the Minarik speed controller.
What am I missing here?
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Interesting new info. I have the feeling I am asking these question in the wrong place as this forum seems to be 99% steppers, but I'll give it another go.
I checked the PS at the output and both sides are cranking out 74V. Yet the spindle servo motor wires only show 50V at max speed.
The Minarik drive arrived and it's 130VDC output kicked the spindle speed up from 1,300 to 2,700 . . nice. Yet a voltmeter reads only 105V on the motor wires.
Both the Minarik and the servo drive are PWM, and I recall reading somewhere that a voltmeter reads PWM inaccurately. Yet it seems that at full speed the PWM should be at 100% and therefor show as the full voltage on a voltmeter.
Adding to the puzzle, the actual motor speed calculates correctly using the actual voltmeter reading and not the theoretical voltage, so it would seem that the motor also sees the voltage at the same level as the voltmeter . . i.e. significantly below the rated voltage of either the servo drive or the Minarik speed controller.
What am I missing here?
Simpson,
That would suggest your drives are current limiting. What happens if you connect the motors directly to the power supply? They should then go full speed.
Regards,
RayL.
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The servo drive will limit the amps to whatever I set it at up to 35amp max. The drive is rated to 160V.
Steve,
Servo motors will run at max RPM when the rated voltage is applied. If your servo driver is PWM (dugon 160V 35A), your PS output voltage does not have to match your motors max voltage rating, the PS out can be more, then you set the max PWM parameter on the driver so average RMS voltage to the servo motor is not more than the motors rating.
JH
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That would suggest your drives are current limiting. What happens if you connect the motors directly to the power supply? They should then go full speed.
Yes, they both are current limiting, but it was my impression that this would not effect the voltage. In any case, I don't know why it did not occur to me to connect the 74V PS directly to the motor for testing. I'll do that next.
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Servo motors will run at max RPM when the rated voltage is applied. If your servo driver is PWM (dugon 160V 35A), your PS output voltage does not have to match your motors max voltage rating, the PS out can be more, then you set the max PWM parameter on the driver so average RMS voltage to the servo motor is not more than the motors rating.
The motor does run at the predicted RPM based on the voltage I am reading on the voltmeter. i.e. the motor spec is 13.36V per K RPM and that pretty close to where it runs if you go by the measured voltage.
The quandry here is that the voltage coming off both the servo drive (PWM) and the motor controller (also PWM) does not *measure* what it should. I'm interested in knowing the reason, which I can only imagine could be;
1) ?? Full PWM is not really *full* as in full time *on*,
2) ?? There is some sort of loss in between the input and output of the PWM scheme,
3) ?? The full voltage is actually there, but a normal voltmeter cannot read it correctly becuase is is pulsing,
4) ?? A DC brush motor 'sees' the voltage in the same way as a standard voltmeter, in which case it would seem logical to size a PS higher than the motor's rating if you are using a PWM scheme, but I have not been able to find anything difinitive on that, although I have not looked very hard yet.
Interestingly, The Minarik drive is rated at 130VDC output, yet the instructions for setting the various pots uses 90V motors as examples. Is there a message there?
I'll have another piece of the puzzle when I do as Ray suggested and run the motor straight off the 74V PS.
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That would suggest your drives are current limiting. What happens if you connect the motors directly to the power supply? They should then go full speed.
Yes, they both are current limiting, but it was my impression that this would not effect the voltage. In any case, I don't know why it did not occur to me to connect the 74V PS directly to the motor for testing. I'll do that next.
Simpson,
Motor RPM is a function of voltage. Current is a function of load. But, higher voltage, all else being equal, means higher current. So, the drives seem to be current limiting, unless there is a drive parameter for setting maximum motor voltage? In any case, your motor seems fine, and you need to figure out why the drive is limiting, be it voltage limiting or current limiting.
Regards,
Ray L.
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Both drives are specifically current limiting. The Minarik motor control has no voltage limiting, but as mentioned by JH, the servo has a 'max PWM' setting which I would speculate effects the average voltage output, if in fact the motor *sees* it that way ( I have not gotten an answer to that question yet).
I am collecting info on this issue from a couple of sources, and an interesting tidbit that I learned is that the PWM scheme does impart a voltage drop of perhaps 10%. I got a detailed explanation for this that is over my head, but the 10% loss is what I was interested in knowing. The remaining 15% delta in measured voltage may be attributable to the inaccurate measurement via voltmeter (which has been confirmed).
Learning and understanding this stuff is essential for a venture I have coming up. I suppose that over my career I have gotten lazy by being surrounded by other engineers of different disciplines to hand things to. Now I wear all of the hats and some fit better than others. The one that says 'EE' is still in the shape of a dunce cap . . .LOL!!
Perhaps worse yet, I no longer have the resource of the shop where I could walk out and talk to the mechanics and machinists to get the 'real' story of what works and what doesn't. That's where forums are so valuable.
Rambling again . . sorry.
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Simpson,
The motor will basicallly respond to RMS voltage. If you looked at the voltage waveform, and measured the area under the waveform, the RMS voltage would be the constant DC voltage having the same area. How accurately your DVM will read the PWM waveform depends a lot on the PWM frequency, and exactly how the DVM is designed. It should at least be reasonably close in most cases. How much voltage loss there is depends heavily on the design of the controller, so there's no way to answer that question without knowing the circuitry of the specific controller in use.
Regards,
Ray L.
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The motor will basically respond to RMS voltage.
This is how I though it must work, based on what *seems* logical and also what I was observing
How much voltage loss there is depends heavily on the design of the controller, so there's no way to answer that question without knowing the circuitry of the specific controller in use.
I measured with a DVM and also an analog VM and got fairly similar results. Actually it was the low tech analog VM reading that made me think that a simple DC motor winding probably 'sees' the voltage in the same way.
I'm not concerned about how much loss there is, only with how to measure it for the purpose of oversizing a PS an appropriate amount to compensate. However, if you are familiar with the design of PWM power devices, would you know if there is a way to smooth the PWM output in order to obtain a smooth running motor? I have observed a very wide range of smoothness from different PWM devices, which leads me to conclude that there is no intrinsic and unavoidable roughness inherent in the PWM process, but rather that the designers have different approaches that yield different results.
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Simpson,
An analog voltmeter will, if anything, give you a more accurate reading of voltage on a PWM controller, as the analog meter movement will "filter" the response.
I would not expect to see any difference in "smoothness", unless the PWM frequency is very low. Did you say you had a Minarik controller? If so, I would expect it's probably using 120Hz, derived from rectifying the line frequency, as the PWM frequency. A higher quality controller would likely use a higher frequency. But there are also other factors, like I-R compensation, and the voltage and current feedback loops that can also affect smoothness. I don't recall if any of these are adjustable on the Minarik or not. A good PWM controller, well-matched to the motor, should be perfectly smooth, until you get down into very low RPMs, where the motor may start "cogging" due to commutation. This is where BLDCs, excel.
Regards,
Ray L.
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Thanks for all of the replies. I have the info I need now and a fairly good understanding of the various relationships I was asking about and everything makes sense.
Just a matter now of finding time to do the testing . . . .