# 100VDc generation for CNC PLC.   20+ amp capacity?



## countryguy (Feb 13, 2015)

Hello All.    Back from Business and a busy month.   The son and I are now into the DC ServerMotor Power supply phase of our AjaxCNC conversion. 

what I have:  Is a .5KVA transformer that was mounted on the Mill.  the best I can do on the AC outputs is 115Vac, a 63Vac leg or a 97Vac leg. (Taps).  No Center. 

What I need - The Motor DC Main Power In on the Ajax should = the Motor Vdc settings.   So I need 100Vdc as listed on the motor itself.  Amps is Max of 10A.  

1) I see other sites that claim a .5Kva transformer will run 3 to 4 40in.Lb. motors.   That just seems like a stretch - But I do not know!  Transformers are not cheap!  So that sets up two options or even others as you arrive at some feedback and do / do-nots for the Kid and I. 

Part A)  If I can use this Transformer: I can use the 63 tap and arrive at a DC value of 87-88Volts. (diode drops)  or the 97Vac tap to arrive at 127-128VDc. (using a full wave rectifer and Cap kit).    Neither seems to ideal?  But I wondered what ya'll would think on either of these settings? 

Part B)  I good ol' best shore - Here is a 2Kva Variac, Max 20Amp setup from automation tech on sale for $104.... And give me my exact 71-72 Vac for the 100VDc out.  I just do not know enough about Variac's and the output generated for DC rectification!   Iassume these are some tupe of Triac I've long since forgotten the theory and application and not too if sure it can be used for my application.   http://www.automationtechnologiesin...riable-ac-transformer/tdgc-2km-metered-variac

Part C)  Some surplus Transformer place w/ 220 or 110 in, 70VAc out - and the KVA you think I'll need.... Uggg this keeps getting more and more $$$spensive'  

Off I go - At least I can solder on the DB9 connectors!  Thanks for any help!  Good to be home.  A few pics just to show off the kids work!  He's also now learned to Solder and heat shrink really well.  I'm impressed and yes bragging a bit!  ;-)


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## JimDawson (Feb 13, 2015)

I'm running my 140 VDC, 7.8 amp motors on a 70 VDC supply, the original supply was a 100 VDC supply.  The maximum voltage measured at the motor leads was 30 Volts @ 200 IPM rapid.

I switched over to the 70 VDC supply when I added the Z-axis stepper, have not noticed a degradation in performance.  I suggest that the 63 VAC tap would be adequate, that should provide a solid 80 VDC out.

EDIT:  I also should note that my 70 VDC supply is rated at 5 amps.  It's running two 7.8 amp DC servos and one 6.3 amp stepper, and is fused with a 6 amp slo-blow fuse.


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## RJSakowski (Feb 13, 2015)

Please excuse if I am stating something you already know. I am not an expert on servo motors but here is some input.


Do you want the transformer for isolation or just for voltage conversion?  A Variac is an autotransformer meaning you are still connected to the mains.  Further a 2 kw Variac is based upon the full usage of the windings.  If it is a 110 v., then it would be good for about 18amps.  If you are tapping at 71 volts that would be 1.3kw.  If it is a 220 Variac the maximum current and output power would be half that.

How critical is the voltage requirement?  From my experience, most motors will do well running slightly higher or  lower voltage than the rating plate.  Usually higher voltage causes the  least issue.  You may do well with running 110 volts. 

One thing to remember is that ac voltage is usually listed as RMS.  This means it behaves the same as a D.C. voltage of the same value.  The peak is 1.414 times that but as you start to load the circuit it drops down towards that RMS value.  How quickly it drops depends upon how much filtering you use.  PD.C. power supply design used to be something of an art decades ago.  Nowdays with switching supplies it is much easier.

If you want more regulation on the voltage, there are a number of options for that.  10 amps is a sizable current but not unmanageable.  I have built regulated power supplies with higher currents than that although not at that high a voltage.  If you do want to runna t a lower voltage than 110, it can be done by using a bucking transformer.  The bucking transformer is wired so its output is opposite in phase to the main input so the resultant output is the difference.  e.g., a 36 volt transformer wired for bucking, would give you an output of 110-36 or 74 volts.  The transformer still has to have the current rating but it is now 360 w instead of a 740 w.  If you search, you may be able to find such a transformer in the guise a a cheap welder.  Finally, you can wire a series of lower voltage transformers to give you the desired bucking voltage.  

Good luck with your project!

Bob


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## countryguy (Feb 13, 2015)

Thanks RJ.  never heard of a buck or boost transformer until tonight.   Thanks!   A cool site for square D xformer selection : http://www.buckboostcalculator.com/index.cfmWow never knew you could string and wire together but that makes sense.     I was simply trying to get as close to spec Vdc of 100 as I could.    The transformer is a multi VAc input of 230 408 and 575 ? I think it was 575.    Anyway, knowing that we're using a ratio of turns from input to output windings I put the 230 onto the 408 lugs of 1&4.  (or was that 475 oh Nuts! ) this input puts me at the 63Vac.     If I put on the 230 lugs I can get a the CT OUTPUT for the 97vac out.  I do not need to get precise it seems as you and Jim suggest!  GiddyUp!  I can give this a go on either setting and check case temp and amps in  DC.   





RJSakowski said:


> Please excuse if I am stating something you already know. I am not an expert on servo motors but here is some input. The Variac option was just an idea due to some pretty steep Xformer costs if I did need to hit the 2kva ranges!   Ouch!    For $30 less I found the Variac    Anywho!   Off to electron nite for this guy.   Thanks    Jeff and Mitch. Do you want the transformer for isolation or just for voltage conversion?  A Variac is an autotransformer meaning you are still connected to the mains.  Further a 2 kw Variac is based upon the full usage of the windings.  If it is a 110 v., then it would be good for about 18amps.  If you are tapping at 71 volts that would be 1.3kw.  If it is a 220 Variac the maximum current and output power would be half that.How critical is the voltage requirement?  From my experience, most motors will do well running slightly higher or  lower voltage than the rating plate.  Usually higher voltage causes the  least issue.  You may do well with running 110 volts. One thing to remember is that ac voltage is usually listed as RMS.  This means it behaves the same as a D.C. voltage of the same value.  The peak is 1.414 times that but as you start to load the circuit it drops down towards that RMS value.  How quickly it drops depends upon how much filtering you use.  PD.C. power supply design used to be something of an art decades ago.  Nowdays with switching supplies it is much easier.If you want more regulation on the voltage, there are a number of options for that.  10 amps is a sizable current but not unmanageable.  I have built regulated power supplies with higher currents than that although not at that high a voltage.  If you do want to runna t a lower voltage than 110, it can be done by using a bucking transformer.  The bucking transformer is wired so its output is opposite in phase to the main input so the resultant output is the difference.  e.g., a 36 volt transformer wired for bucking, would give you an output of 110-36 or 74 volts.  The transformer still has to have the current rating but it is now 360 w instead of a 740 w.  If you search, you may be able to find such a transformer in the guise a a cheap welder.  Finally, you can wire a series of lower voltage transformers to give you the desired bucking voltage.  Good luck with your project!Bob


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## countryguy (Feb 13, 2015)

Cool!  Thanks for that.  Big help!   I will finally get to reuse something. Hurray!  Lol.     I will setup on the 63vac and see how it goes.   If not, I can go to the 97ct and check into use of some of RJs notes.   

Whew.   Getting there.    The mountain via a pebble at a time!  :rubbinghands:



JimDawson said:


> I'm running my 140 VDC, 7.8 amp motors on a 70 VDC supply, the original supply was a 100 VDC supply.  The maximum voltage measured at the motor leads was 30 Volts @ 200 IPM rapid.
> 
> I switched over to the 70 VDC supply when I added the Z-axis stepper, have not noticed a degradation in performance.  I suggest that the 63 VAC tap would be adequate, that should provide a solid 80 VDC out.
> 
> EDIT:  I also should note that my 70 VDC supply is rated at 5 amps.  It's running two 7.8 amp DC servos and one 6.3 amp stepper, and is fused with a 6 amp slo-blow fuse.


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## RJSakowski (Feb 13, 2015)

countryguy said:


> Thanks RJ.  never heard of a buck or boost transformer until tonight.   Thanks!   A cool site for square D xformer selection : http://www.buckboostcalculator.com/index.cfmWow never knew you could string and wire together but that makes sense.     I was simply trying to get as close to spec Vdc of 100 as I could.    The transformer is a multi VAc input of 230 408 and 575 ? I think it was 575.    Anyway, knowing that we're using a ratio of turns from input to output windings I put the 230 onto the 408 lugs of 1&4.  (or was that 475 oh Nuts! ) this input puts me at the 63Vac.     If I put on the 230 lugs I can get a the CT OUTPUT for the 97vac out.  I do not need to get precise it seems as you and Jim suggest!  GiddyUp!  I can give this a go on either setting and check case temp and amps in  DC.



If you are using a transformer, you can use the boost/buck strategy on either the primary or the secondary.  Since you want to step down in voltage, running on the primary side would require less current capability.  As they say, many ways to skin the cat>

Bob


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## stupoty (Feb 13, 2015)

If you dont use a transformer for isolation you can rectify your ac line voltage in the same way a vfd does.

Stuart


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## countryguy (Feb 14, 2015)

Thanks Stu - Exactly what will happen.   The Full Wave DC w/ Cap output to DC Motor PLC will go on the 63Vac secondary or the 97Vac secondary.  

To RJ's point-  I'm still not sure exactly how to boost or buck on a secondary output unless the Transformere has the winding tap already?   I see they do sell boost/buck modules-  Anyway-  Will read up on that a bit later!  Thanks again for that tip.  
   So the Variac at $104 for 2000Va (or 2Kva) is a really good deal!  Checking out a 2000Va transformer and a customer 72VAc output is $300+  (torordial (sp? type).


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## RJSakowski (Feb 14, 2015)

To uses a buck/boost transformer, the primary is wired to your source voltage.  The secondary is wired in series with your output.  Depending upon which way the windings are connected, it will either add to or subtract from your output.  If you are going to use the Variac, keep in mind that you are still tied to the mains and that your output current will be limited as I previously described.  Also, I would make some kind of provision to lock down the rotor so you don't accidentally apply full voltage.  Otherwise, it may be a good choice because you have the freedom to dial in your specific needs.  BTW, isolation transformers in the 1 -2 kw range are easier to find  if you have a need for isolation.  They can have a 110/220 input with a 110 output.  There are also isolation transformers designed to convert one leg of 3 phase to 110/220.  The three phase side would be a 208v.  This could be run backwards to drop 220 1 phase to 208 or, if the current rating was high enough run on 110 to drop it to 104.


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## countryguy (Feb 15, 2015)

OK- I get it now!   
A great doc that put the bulb on in my mind is listed below.       I get it.  A very flexible transformer with a multitude of taps and connection inputs to outputs.    The doc link below has some visuals of both buck and boost.     Also-  The transformer I have is indeed a buck/boost w/ the same type of electrical representation image shown on the 5 inputs and 3 outputs.


https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&cad=rja&uact=8&ved=0CDoQFjAG&url=http://www.emersonindustrial.com/en-US/documentcenter/EGSElectricalGroup/products_documents/control_power_solutions/xformers/buck_boost_xformer/buckboost-xformers/buck_boost_transformer_catalog_pages.pdf&ei=ppzgVNCUD8mXNrPugogI&usg=AFQjCNH1DKKmbRn66H7TdxkeLOI_L8WgwQ&sig2=l4F1iKP6qQoLcB5zz-NjcA&bvm=bv.85970519,d.eXY


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## RJSakowski (Feb 15, 2015)

It looks like you are getting the hang of it.  A few things to remember about transformers.

The output voltage as a direct ratio to the input voltage and equal to the ratio of the number of turns on their respective windings.  You can use a transformer to generate  a different voltage than the plate voltage.  i.e. a 120 volt in/80 volt out has a ratio of .666:1 and will also work as a 100 volt in//67 volt out.  The maximum current rating remains pretty much the same as it is determined by the ability to dissipate heat, core losses,  and resistance of the windings.  Multiple transformer outputs can be wired to add or subtract, when connected together, depending upon whether they are wired in phase or out of phase.

If you have not found  the solution yet, look at isolation transformers on e-bay in business and industrial.  I had seen some isolation transformers there  with 120/208/240 in, and 120/208/240 out.  Running 120 on the 240 in winding will give you 104 out on the 208 out winding.  They are fairly common in industrial environments where they have 3 phase and want to develop a true 120-0-120 circuit.  Just remember that you will have to derate the power by 50% if running on half voltage.  

Good luck!

Bob


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## ironmonger (Feb 15, 2015)

Just to clarify this in my mind... is this DC supply to be used to operate a DC servo motor?
The power supplies for DC servo motor DRIVERS (the electronics that takes step & direction input and operates the servo motors) do not need to be regulated other than limiting the maximum input voltage. The DRIVER itself IS a voltage regulator.

paul


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## countryguy (Feb 15, 2015)

Yeah- you have it right as far as I know.  But-  On the Ajax inputs, they advise the following:  
a) Do not exceed 155VDc on the H2 input as the current/voltage source input for their internal servo driver. 
b) They do not regulate in anyway-  They advised me to try and match the H2 input Vdc source as close to the Motor spec as possible.  

Just passing along what I was told.    Q&A alway's welcome on this stuff!  so thanks! 
CG.


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## ironmonger (Feb 15, 2015)

A fullwave bridge rectifier will have a peak voltage output of about 1.4 times the RMS value fed to the rectifier. Especially with a filter capacitor on the output.

How much the voltage 'sags' will depend on the current capacity of the transformer and also on the current load  and the size of the filter caps.

The peak voltage from your 67 VAC supply, which is measured with a typical RMS reading voltmeter, will have a peak value of about 97 VAC and when rectified the DC voltage will be higher than the obvious and expected 67 volts DC volts. 

While 'they don’t regulate in anyway' is one way of looking at it, the servo drivers are essentially continuously adjustable voltage regulators that output a voltage based on what the encoders and the controller is telling it to do. What your guys are telling you is not to exceed 155 volts open circuit. You may not need anywhere that high a voltage for your application. If your voltage is to low your rapids may well be slower than you would like, but things will still move.

I don't own any part of these guys, http://www.antekinc.com/, but I have used their transformers and power supplies. This one is in my unfinished CNC project.
Tooo may hobbies...

paul


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