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Author Topic: Would like opinions on servo motor voltage and PS secondary in series  (Read 8094 times)

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Offline simpson36

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Re: Would like opinions on servo motor voltage and PS secondary in series
« Reply #10 on: January 19, 2010, 10:58:44 AM »
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.


Offline simpson36

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Re: Would like opinions on servo motor voltage and PS secondary in series
« Reply #11 on: January 22, 2010, 01:54:59 PM »
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?
Re: Would like opinions on servo motor voltage and PS secondary in series
« Reply #12 on: January 22, 2010, 02:05:28 PM »
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.
Regards,
Ray L.
Re: Would like opinions on servo motor voltage and PS secondary in series
« Reply #13 on: January 22, 2010, 04:53:36 PM »

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

Offline simpson36

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Re: Would like opinions on servo motor voltage and PS secondary in series
« Reply #14 on: January 22, 2010, 05:03:26 PM »
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.

Offline simpson36

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Re: Would like opinions on servo motor voltage and PS secondary in series
« Reply #15 on: January 22, 2010, 05:18:10 PM »
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.



Re: Would like opinions on servo motor voltage and PS secondary in series
« Reply #16 on: January 22, 2010, 05:26:23 PM »
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.
Regards,
Ray L.

Offline simpson36

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Re: Would like opinions on servo motor voltage and PS secondary in series
« Reply #17 on: January 23, 2010, 06:38:43 AM »
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.
  
« Last Edit: January 23, 2010, 06:49:55 AM by simpson36 »
Re: Would like opinions on servo motor voltage and PS secondary in series
« Reply #18 on: January 23, 2010, 09:59:29 AM »
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.
Regards,
Ray L.

Offline simpson36

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Re: Would like opinions on servo motor voltage and PS secondary in series
« Reply #19 on: January 23, 2010, 11:00:45 AM »
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.