View Full Version : Calculate the Voltage and VA size of the DC power supply needed to build or buy
Gerald_D
Sat 29 April 2006, 07:02
You will typically hear that a power supply is 48 Volts, 500 VA. The VA is for Volts.times.Amps, therefore that aforementioned 500VA power supply can deliver about 10 Amps at 48V.
For driving a stepper CNC router, you need to get the voltage right so that it matches the stepper motors that you selected, but this is not critical. A 45V supply will work nearly as well as a 48V supply. The VA part of the calculation is also not critical. An old surplus power supply of 800VA will drive a system for which you calculated 500VA. The physical size of the supply is determined by the VA rating - a 800VA supply would only be slightly bigger than a 500VA.
Okay, that was a broad introduction of the 2 important numbers involved, now we get to the detailed calculation:
Decide on your motor wiring configuration and get the appropriate motor current and voltage specs (http://www.mechmate.com/forums/showthread.php?t=256)...
Voltage: Believe it or not the power supply voltage should idealy be 3 to 23> 20 (http://finance.groups.yahoo.com/group/geckodrive/message/10931) times the rated voltage of the stepper motor. Most agree, the higher the better, the limit is decided by overheating motors. But, the power supply voltage must not exceed the capability of the driver - the GeckoDrive 200 family is limited to 80V max. Also, the drive wants a certain minimum voltage, and the GeckoDrive wants 24V minimum. To summarise, the power supply voltage for a Gecko-driven stepper motor must be as high as possible within 3-20 times motor voltage, above 24V, below 80V. (Typical range for steppers is 50 to 75 Volt). (Edit 10 September '07: In a discussion with Mariss yesterday he said: "The "20 times rated voltage" business is a convenient stand-in for what really counts which is V / (SQRT L) being less than 1000. This 20X stand-in works so long as there is a fixed ratio between resistance and the turns of wire on the motor. Vexta is a good quality manufacturer and the use a better fill ratio on their windings which yeilds a lower than expected winding resistance.")
VA-size ("Watts"): Again, this comes from the motors' specs. Each stepper motor will have a max. Amp rating. Add all the Amps of all the motors together, and then you will know the peak Amps that your router may draw. If you have 4 motors marked at 2 Amp, we expect the drivers to be able to supply 8 Amp. Surprisingly, a Gecko stepper driver does not need 8 Amps input to supply 8 Amps output, it mostly needs much less. There is agreement that your power supply need only be able to supply 2/3 of the driver's current requirement. Those 4 x 2 Amp motors will only need 5.3 Amps from the power supply. (Hard to believe, but we havn't proved it wrong yet.) Multiply the last mentioned Amps with the Voltage selected above and you have the VA value
Therefore, start looking for a packaged power supply that firstly meets the voltage spec and secondly exceeds the VA spec. Or consider to design & build your own (http://www.mechmate.com/forums/showthread.php?t=56).
Dirk
Fri 26 May 2006, 10:20
According to the build your own thread, when the AC current is converted to DC through the recitifier and the voltage increases by a factor of 1.414, then does the amperage remain the same, or decrease by this same factor? If so do we have to compensate?
Dirk
Gerald_D
Fri 26 May 2006, 10:48
Hi Dirk,
The amperage does decrease by the same factor. Do we have to compensate? I tried to get an answer to that question by posting the following example on the Mach Yahoo group (http://groups.yahoo.com/group/mach1mach2cnc/message/54716):
"4 stepper motors @ 2 Amp, 3V each
Conventional wisdom says power supply voltage of 75V would be good.
Power supply current of two thirds x 4 x 2 = 5.333Amp.
If using a packaged supply then 75 x 5.33 = 400VA
If building own unregulated supply:
Voltage 75 / 1.414 = 53V AC
Conventional wisdom says transformer VA rating 53V x 5.33A = 282VA,
Therefore a 50V 300VA transformer will be good alternative to a 75V
400VA packaged supply.
What am I reading wrong, or is this just one of the mysteries of the AC
versus DC world?"
From the responses it was clear that the simple truth is that we don't have to compensate as long as the transformer doesn't get hot....
Not a satisfactory clinical answer, but it is valid nonetheless. Our routers very seldom draw peak amps on all motors, and one cannot put a mathematical formula on that. It appears that the two thirds rule of thumb is actually even 41% lower, and most people's transformers are not burning out.
A very important point though is that a linear power supply (tranformer/rectifier/capacitor) copes very well with short duration overloads and maintains a good voltage while pumping out a few extra amps. A switching supply on the other hand would cut its output voltage drastically to save itself from pumping out even a milliamp extra. Thus, for driving steppers in a typical router, you could use a smaller VA rated linear supply than a switching supply, and this ratio is probably near the 1.414 factor, but that is coincidence.
Mike Richards
Thu 22 June 2006, 17:38
Part of the confusion in electronics is caused by the way things are measured. For instance, the 120VAC assigned to the normal line voltage in North America is really the root-mean-square reading (or average reading). The peak-to-peak reading is 1.41 times higher, or about 170VAC. What that means is that the power company is actually supplying 340VAC because the sine wave is going both positive and negative relative to neutral. With that bit of trivia out of the way, it's easier to understand what the various components in the power supply actually do. The bridge rectifier converts the positive/negative 60 cycle per second sine wave (in North America) to a positive 120 cycle per second sine wave (assuming that we want positive voltage instead of negative voltage). Since the output is a sine wave, the capacitor acts as a battery (of sorts) to store energy between sine waves. If we use a big enough capacitor, we effectively change the 120 cycle AC sine wave into Direct Current (DC). That's why we have to use capacitors with lots of capacitance in power supplies. When the load on the power supply is greater than the capacitor can handle, we get some ripple, which is an AC component to the DC voltage. Depending on the amount of ripple, the power supply is said to be efficient or non-efficient. The important thing to know is that nothing in the circuit magnifies the voltage by a factor of 1.41. Most volt meters read the RMS value of the voltage, but an oscilloscope would show the peak to peak value of the voltage.
From a practical standpoint, a 'square' stepper motor uses more current than a 'round' stepper motor. It's always better to assume that you'll need more current than specified by stepper manufacturer so that performance doesn't suffer by draining the capacitor(s) too much between AC cycles. My test is to feel how hot the power supply is running. If it is at room temperature or slightly warmer, I don't worry. If it is too hot to touch, then it's time to add more capacitors.
reza forushani
Sat 24 February 2007, 11:58
I just ot a MONSTER power supply. I ordered a 500 VA at 70V and this thing is big. It will fill the whole top part of the board. I'll have to move everything else down. Or move the PMDX card somewhere elese? Maybe I should have gone a little smaller.
Gerald_D
Sat 24 February 2007, 13:13
Reza, can you re-package (re-arrange) that supply to make it taller instead of wider? Take photos of what goes where, strip it down and re-assemble it more efficiently for space? If you go this route, please post a photo for comment before switching it on.
reza forushani
Sat 24 February 2007, 16:52
http://www.mechmate.com/Forum/messages/17/3529.jpg
reza forushani
Sat 24 February 2007, 16:53
http://www.mechmate.com/Forum/messages/17/3532.jpg
Gerald_D
Sat 24 February 2007, 21:29
Can you tell us the supplier and part number of this beast? What voltages does it put out?
reza forushani
Sat 24 February 2007, 21:38
Hi Gerald
The Beast or MONSTER is 500 VA with three outputs
70 VDC, 24 VDC, 9VDC
Custom made for me by a guy in NJ. He wanted $150.00 but I talked him into $100.00 including postage, promissing him more busines.
His name is John and his email is john@ango.com
Check out his websites. He has many transformers, power supplies, etc
http://www.toroid-transformer.com/
remind you this thing is probably a ton?! Realy many many kilograms or pounds, very heavy.
reza forushani
Sat 24 February 2007, 21:42
I do have a question, the power supply has two black and two red leads. I guess for 110v like my situation I need to tie both blak together and both reds together running parallel. For 220v I guess would be serial? Do I need both blacks and reds? It seems I am getting the right output by just using one black and one red. see this thread (http://www.mechmate.com/forums/showthread.php?t=290) for the relay
Gerald_D
Sun 25 February 2007, 09:58
Using one black and one red will give you the right voltage output, but you will only get 250VA out of the 500VA transformer.
Why did you ask for 24V and 9V DC?
reza forushani
Sun 25 February 2007, 14:14
9V for the PMDX board and Relays, etc
24V in case I want to do some other lights, etc
Just to CMA
Jay Waters
Sat 03 March 2007, 10:47
Being that I am electronically challenged, how else would you go about supplying power to other items that need different voltages, such as the lights mentioned above without buying a power supply like Reza's? Say, for example, that I built a power supply like Gerald did. How would I get the correct voltage for the other items?
Jay Waters
Sat 03 March 2007, 11:23
Found what I was looking for here (http://www.mechmate.com/forums/showthread.php?t=282)
Mike Richards
Sat 03 March 2007, 11:33
Jay,
You will need a voltage source for each item that requires a different voltage. A 24V lamp will need a 24V. A 5VDC device will require a 5VDC power supply.
(That's why a standard PC-type power supply generates so many different voltages. Components inside the computers require different voltages.)
One way to get multiple voltages is to buy a transformer that has multiple 'taps'. However, for a DC power supply, each 'tap' must have its own rectifier and its own capacitor. If the DC supplies are regulated, to give a precise voltage, the supply will also require a voltage regulator.
Depending on the amount of current required, a higher voltage can be regulated down to a lower voltage. I sometimes use a 24VDC power supply with several regulators to give me 24VDC, 15VDC, 12VDC and 5VDC. HOWEVER, and this is important, when you use simple regulators to reduce the voltage, heat will be produced - sometimes lots of heat. Unless you only need a few milliamps of current, it's best to either buy a switching power supply that can efficiently produce several different voltages, or buy a multi-tap transformer.
Gerald_D
Sat 03 March 2007, 11:36
Alternatively, avoid buying stuff that needs "odd" voltages. eg. If you have 115V in the system, use a 115V light.
Jay Waters
Sat 03 March 2007, 15:02
Thanks, Mike and Gerald, just trying to get things straighened out on paper and in my brain before I commit to buying anything. Everyday is just one step closer to building and the opportunity of learning something new from you guys.
bugmenot dillbert
Thu 15 March 2007, 16:18
Just out of curiosity, if the motor only ever draws 2/3 of the rating, is there any penalty for using a 2/3 of the amp rating for the motor for the driver, ie. a 2A driver running a 3A rated motor?
Gerald_D
Thu 15 March 2007, 22:25
Bugmenot, my first post in this thread was misleading - I have edited it now. The driver must match the motor current. But all 4 motors never work at 100% together, continuously.
However, I don't know what happens when the driver's "holding current reduction" is turned off - theoretically the motors would all draw max current when the machine is standing still. Let's say the conventional wisdom (and the first post) is only true if the drivers have a current reduction (min 33%) when the motors are idle.
Loren Gameros
Tue 20 March 2007, 13:33
Hi Greald,
I am having some technical difficulties in figuring out which power supply to use. I sent an email to John at Antek asking which power supply to use and this is what he said:
Hi,
"The PS-8N70 will give you 800Watt 12A at 70Vdc. It is little more than you need". the price is $120 each plus $10 shipping.
"Or I can give you the 600W 8A at 70Vdc. It is same rating as you need". The price is $110 each plus $10 shipping.
I may not have those the our site now. I can build it in a few days.
Thanks.
John
I am using the Gecko 202 and the Oriental Motor Vexta-Step type PK299-01AA motors with the PMDX-133.
So which power supply do I use?
Thank You.
Gerald_D
Tue 20 March 2007, 13:50
Loren, we run 4 of the Gecko G202 and OM PK299-01AA motors on only a 300VA power supply with no problems. As I said right at the beginning of this thread: "The VA part of the calculation is also not critical. An old surplus power supply of 800VA will drive a system for which you calculated 500VA. The physical size of the supply is determined by the VA rating - a 800VA supply would only be slightly bigger than a 500VA."
As long as you can afford the price, and fit the supply inside your case, you can use a big supply to drive a small load without a heat penalty. I start asking heat questions when guys are forced to use big supplies because their motors want lots of amps.
Loren Gameros
Tue 20 March 2007, 14:38
Ok, Thank You.
Loren Gameros
Wed 21 March 2007, 13:45
Just to follow up.
John at AnTek says that I will need a 600W 8A at 70Vdc to drive the 4 (202) geckos and 4 (2 amp) oriental motors. Being that I am a novice at this I have ordered based on his recomendation. Wish me luck.
Loren
Gerald_D
Wed 21 March 2007, 23:45
Relax, you will be okay http://www.mechmate.com/Forum/clipart/happy.gif
Bob Cole
Thu 22 March 2007, 07:58
Gerald, Anyone:
I have (4) Applied Motion Stepper motors. They are rated @ 2.47Volts, 5.9 Amps. Could someone give me an idea based on the math that you use to compute the power requirements, what size power supply should I be looking for to give me plenty of power with a "comfortable" reserve. Meaning I only want to do this once, Correctly.
thank you in advance.
Bob C.
Gerald_D
Thu 22 March 2007, 09:27
Bob, here is my stab at it:
- Volts: 2.47 X 20 = 49.4. Therefore a standard nominal 50V DC supply is just right. (The transformer will be 35V AC)
- VA: 35V X 5.9 Amps X 4 X 0.67 = 553VA if using using drivers with current reduction (Gecko's). Otherwise, the .67 factor cannot apply and you need 826VA
A nominal 50VDC 1000VA supply should laugh at anything you throw at it....hope you can fit it in the case and keep it cool.
Michael Cunningham
Sun 08 April 2007, 22:34
So do the geckos' actually increase/decrease the voltage to the steppers? I think they can handle up to 80v.
Using the example above of 49.4.. would it be okay to use a 70v 800va supply and the geckos reduce the voltage or should the supply only
supply 20X max.. ie 50v? Why?
Gerald_D
Mon 09 April 2007, 00:25
The gecko's function is actually quite complex and we cannot just simplistically say it increases/decreases this or that. The input to a gecko is simple - pure DC at a steady voltage. The output is a "digital" waveform which is "chopped" up. A normal multimeter is okay for measuring what goes in, but tread carefully to measure what comes out. (My multimeter shows about 200V coming out, which is plain impossible). Having said all that, now to answer your question.....
The maximum input voltage to a gecko is stated as 80V. That is a conservative rating and is one of the reasons why it has such a good reputation for long-life. Other producers, use the same components would rate at 100V, but they don't have the reputation for long life.
The gecko itself can do nothing about increasing/decreasing voltage - it is a current controlling device. The user can select a current output level (when running and when at standstill).
I personally don't understand where the voltage limit of 20x comes from, but I have heard often enough that too high a voltage causes the motors to overheat.
driller
Wed 10 October 2007, 16:19
If you want more information on the GECKO drives, check out the white paper on www.geckodrives.com Mariss explains how the motor works and how to size the power supply to the motor.
But as Gerald said on this thread....
Add all the motor amps together, then multiply by 0.66 and that is the AMPS you need.
Look at the motor voltage, it should be between 1 volt and 12 volts, multiply by 20 to find the maximum voltage for that motor. Do not use more voltage and don't use over 80 volts with a Gecko.
The closer your 20x and power supply are to each other the better the performance.
If you have 4 motors at 2 amps each, then 4x2=8 further, 8 x 0.66= 5.28 amps
If you want a 70 volt power supply and to deliver 5.28 amps, then multiply 70 times 5.28 to get 370 VA or 370 watts.
A 400VA power supply is all you need. If you want to over-do it, then multiply the original 8 amps times 70 volts to get 560VA. You cannot use anymore than that no matter what you do with the drivers and motors, so anything more is not possible to use. Actually, you cannot use more than the 370VA, but no matter how many times you tell some people, they will not believe you, or just want to buy something bigger for some other reason.
Why convert to watts ? To find out what power you need from the 115volt wall outlet.
400 watts divided by 115 volts = 3.5 amps. So you need 3.5 amps from your wall outlet.
- - - -
Another question was why the 80 volt limit ?
When you decelerate a motor, it acts like a generator. The motor you just powered up to move the gantry will act like a generator when it is slowing that gantry down. That voltage can easily spike up over 20% of the voltage you can get from the power supply. So, Gerald is correct in his observation that a Gecko has components rated for 100 volts, but under operation, the voltage can get close to that !
Hope this is not confusing.
Dave
Gerald D
Wed 10 October 2007, 23:16
Dave, if you thought that was confusing, try this . . . . . . . . :)
. . . . . . Look at the motor voltage, it should be between 1 volt and 12 volts, multiply by 20 to find the maximum voltage for that motor . . . .
A lot of motor manufacturers do not give a motor voltage. As Mariss recently suggested, the inductance of the square body style of stepper motor can be used to calculate optimum voltage . . .
Either take:
- the square root of the inductance in Henry (H) times 1000, or
- the square root of the inductance in milliHenry (mH) times 31.6
Have a look at this PK296-A2A Oriental Motor (Vexta) that is the first choice for the MechMate today:
458
Following the old 20X motor voltage approach would have given us 20X 2v = 40V.
Using the new square root of the inductance in milliHenry (mH) times 31.6 we get sqrt(6) x 31.6 = 77V
And that is apparently why the folk with Vexta motors discovered that more volts gives them more performance without overheating - their 20X calculated voltages were just too low to begin with in some models/configurations.
Looking at the Motionking 9801 motor:
http://www.mechmate.com/Forum/messages/15/3331.gif
. . . . the nominal motor voltage would have been 4A X 0.98ohm = 4 Volt and the power supply voltage calculated at 20 x 4 = 80 Volt.
Using the new square root of the inductance in milliHenry (mH) times 31.6 we get sqrt(4.1) x 31.6 = 64V
So the newer formula gives a much lower voltage in this case. (The old formula could have given you a hot motor)
Mariss suggests the difference lies in the packing density of the coils. Apparently the more expensive ("higher quality") motors have tighter, more closely wound, coils.
Richards
Thu 11 October 2007, 06:23
The Motionking 9801 motor is an 8-lead motor, so connecting it using Parallel wiring would probably be most efficient. The current rating for Parallel would be 4A * 1.4 = 5.6A. The Inductance would still be 4.1mH. The maximum voltage would be SQRT(4.1) * 31.6 = 63.98VDC. So, if four motors were installed, the power supply that I would use would have a minimum current rating of: 4 motors * 5.6 A * 0.66 efficiency factor = about 15A. The maximum voltage that I would use would be: SQRT(4.1) * 31.6 * 90% = about 57 V. So, just to be on the safe side, I would use a 40VAC toroidal 800VA transformer. After rectification, that would give me about 56VDC. That would allow the motors to pull maximum current and reach near maximum speed at a little less than maximum temperature.
EDITED: Gerald, you posted while I was typing. The PK296A2A-SGxx motor is the single shaft motor. The B2A is the dual shaft version. Both the A2A and the B2A are rated at 6mH Bipolar and 1.5mH Unipolar (half-coil). I use the PK296B2A-SG3.6 motors wired half-coil with power supplies ranging from 26VDC to 35VDC with excellent results. For half-coil connections with a Gecko G202 or G203, I use the Black wire and the Yellow wire for the A coil and the Red wire and the White wire for the B coil. i.e. Term #3 = Black wire, Term #4 = Yellow wire, Term #5 = Red wire, and Term #6 = White wire.
Gerald D
Thu 11 October 2007, 07:00
Ouch, that is the second time the two model numbers made me think they were different electrically! Of course they are the same motor. And Dirk also said to use the unipolar configuration. So, the currently recommended "MechMate motor", the Oriental PK296_2A-7.2, should have a supply voltage of say 35 to 40 volts (37 from inductance formula, 28 from voltage formula) and 4 of those motors will want a power supply current of 8 Amps. That means around a 300 VA supply. Right?
Gerald D
Thu 11 October 2007, 07:19
Mike, for that MotionKing motor, I would still go for the Unipolar, even though it has 8 wires. Still 56V but on a 600 Watt supply. Can't remember now why I favour unipolar over parallel . . . .
Alan_c
Thu 11 October 2007, 10:43
Richard
What is the size (VA) of your transformer to get nice toasty steppers? I have the same motors, but in testing with the kitchen table project they hardly rise above ambient - I have them wired unipolar on a 300VA transformer.
Richards
Thu 11 October 2007, 12:16
Alan,
The power supply that I used when I last tested the PK299-F4.5 motor was an 800VA unit wired to give 35VDC. The transformer never got above room temperature, but I was really exercising the stepper motors that I had attached to that power supply. Even the 250VA toroidal transformers that I often use in the 27VDC power supplies stay at room temperature when I run three 3A motors. Maybe I should also add that 'toasty' is defined in my shop as being between 130-degrees F. and 145-degrees F., well within the design range of the motors. (I have had severe transformer heating problems when I've used standard, non-toroidal, transformers. One standard Triad F-258U transformer always ran too hot to touch, even when the four steppers had been sitting in reduced current mode for more than an hour. The stepper motors were at room temperature and the Gecko G203v stepper drivers were at room temperature, but the Triad F-258U was HOT.)
J.R. Hatcher
Wed 23 January 2008, 07:30
I have a question. Does a TA7812S voltage regulator work only with a DC input or will it work with a AC input as well?
http://www.questcomp.com/PdfDir/D8204487.pdf
Gerald D
Wed 23 January 2008, 08:42
Looking at the internal schematic, there is a diode across input and ground. AC will make the diode (at least) go *pop*. DC only......until someone else verifies.
Some ideas:
http://www.explorecircuits.com/cir_power_power_supply.htm
domino11
Wed 23 January 2008, 09:29
JR, Gerald,
Yes this is a DC Voltage regulator. Only use Dc as an input. If you only have AC then hook up a rectifier bridge and a filter capacitor to give you the proper DC for the regulator. I can provide a schematic if anyone needs it. :)
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