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Author Topic: Would like opinions on servo motor voltage and PS secondary in series  (Read 8090 times)

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Offline simpson36

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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?

Re: Would like opinions on servo motor voltage and PS secondary in series
« Reply #1 on: January 17, 2010, 07:37:26 AM »
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.
Re: Would like opinions on servo motor voltage and PS secondary in series
« Reply #2 on: January 17, 2010, 03:22:04 PM »
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.
Re: Would like opinions on servo motor voltage and PS secondary in series
« Reply #3 on: January 17, 2010, 04:06:12 PM »
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.
Regards,
Ray L.

Offline simpson36

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Re: Would like opinions on servo motor voltage and PS secondary in series
« Reply #4 on: January 18, 2010, 05:47:39 AM »
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.


 

 
Re: Would like opinions on servo motor voltage and PS secondary in series
« Reply #5 on: January 18, 2010, 08:08:18 AM »
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

Offline simpson36

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Re: Would like opinions on servo motor voltage and PS secondary in series
« Reply #6 on: January 19, 2010, 02:51:54 AM »
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?


Re: Would like opinions on servo motor voltage and PS secondary in series
« Reply #7 on: January 19, 2010, 05:30:52 AM »
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....

Offline simpson36

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Re: Would like opinions on servo motor voltage and PS secondary in series
« Reply #8 on: January 19, 2010, 07:44:32 AM »
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.
Re: Would like opinions on servo motor voltage and PS secondary in series
« Reply #9 on: January 19, 2010, 09:12:39 AM »
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!