PDA

View Full Version : help please with toroidal transformers selection

Tom J
04-01-2017, 12:50 AM
Here is the link from UK supplier I found on this forum (no relation to merchant)

1. Common design with 50VDC Drivers - lets focus on voltage first

50V - (10%50V)=50V-5V=45V safe voltage

10% is deducted -EMF(slowing down motors, change direction)

Now we need calculate correct voltage of secondary coil of the transformer

24V/1.41=31.92V

From above link we can get 2x15VAC (in series 30V), 2x30AC in parallel or closest or 2x33VAC n parallel too.

I have similar one 420VA 2x30V AC
This gives me 41.5V DC after being rectified on idle
I loose about 3.5-4V, would like to have max 45-46VDC

Now how to choose power? (current figure, as voltage is known)
Power source can have lower current than current of those motors (usually 50-70% of nominal motor current) - why?
Driver takes current from psu condenserwhile PSW cycle is on, so avarage current consumption by motor is lower than motor current.
Lets say we have 3x3A motors, so no need to have 9A as 6.3A is enough
(3x3A)x70%=6.3A + 10% spare power
6.3Ax1.1=6.93A

so we choose 7A

32Vx7A=224VA
here is exact tranformer:

Does somebody used 33VAC or 36VAC?
I would like to know what DC voltage will be after Diode Bridge Rectifier and big capacitors like 2x10.000uF
and same for popular 70VDC drivers please. 70-10%=63VDC
48VDC what ?VDC
50VAC what ?VDC
55VAC what ?VDC- probably more than 63V - guessing here

Robin Hewitt
04-01-2017, 12:34 PM
AC is forever bobbing up and down either side of zero volts.

Feed it through a transformer and you change that voltage in the exact proportion to the number of turns in the primary and secondary coils. It is still bobbing up and down.

Bridge rectify it and you mirror the volts below zero volts so they show above zero volts. It is still bobbing up and down but only one way.

Add a capacitor, any capacitor and the volts will stabilise at peak volts because there is no load to pull it down.

Add a load and discover what the problem with transformers is... to sustain the voltage at something useable you need a snogging girt capacitor. Any load will reveal the bobbing, the capacitor merely determines the degree.

1 Farad will give 1 Amp for 1 second. Unfortunately capacitors are usually rated in micro Farads, millionths of a Farad.

You can get 1F capacitors but they are either for memory back up purposes of they are the size of a house.

If your drivers don't care about the Volts bobbing up and down when you apply a load then transformers are for you :tennis:

Tom J
04-01-2017, 07:04 PM
Thanks Rob I know all that from college & university.
My power supply for drivers includes capacitors 2x10.000uF so can deal with back EMF well, that is why I am so concern about leaving spare 5V before max operating voltage.
I know some guys go for max like 1-2 volts off.
My 1st panel 3 years ago had 720VA trafo with soft start and stabilized 12v for BOB lpt1.

20214

I am building another box 300x300 which is only High Voltage PSU and 24vDC
20215
here are industrial PSU 24Vdc all stabilized
20216
this is my pcb with rectifier and 2 capacitors.
20217

I need trafo to my 70VDC Leadshine EM705
60VAC after bridge and capacitors will be above 70VDC so have to get another one

I was asking for final dc voltage after rectifier and capacitors?
48VDC what ?VDC
50VAC what ?VDC
55VAC what ?VDC

Send the question to provider but no reply so far
Thanks

Clive S
04-01-2017, 08:27 PM
I was asking for final dc voltage after rectifier and capacitors?
48VDC what ?VDC
50VAC what ?VDC
55VAC what ?VDC

Are you asking for the dc after caps if so its about AC V x 1.4

Tom J
04-01-2017, 09:11 PM
Are you asking for the dc after caps if so its about AC V x 1.4

...yes Clive DC after cups but the real reading tbh.
my 30V trafo has 32VAC and after bridge and 2x10.000uF reads 41.5VDC unloaded
calculation gives 32V*1.4=44.8V 3.3V more than real test.

Clive S
04-01-2017, 09:14 PM
...yes Clive DC after cups but the real reading tbh.
my 30V trafo has 32VAC and after bridge and 2x10.000uF reads 41.5VDC unloaded
calculation gives 32V*1.4=44.8V 3.3V more than real test.

The rectifier will drop a bit of volts.

But remember the AC mains fluctuates all the time

Jonathan
04-01-2017, 09:30 PM
Jonathan this pcb is for 3x3A steppers which takes no more than 7A from my calculation, but real test by clamp meter revealed that was no more than 0.86A therefore even perforated prototype board worked fine (but should not)

You're probably mixing up the supply current with the motor phase current - the two are related, but the supply current will always be less.

For my 70VDC 7A motors I use the same layout but wider path on pcb.

You can avoid cutting the ground plane with one of the tracks by routing it another way ... not that it really matters for what you're doing.

Just checked current-carring capacity and 1mm2 will handle 11A.
For 16.5A i need exactly 1.5mm2 conductor, so 1mm pcb has 0.035mm of copper.
Path width=1.5/0.035=42.86mm - my entire PCB is 50mm wide, according to this calculation we both wrong

It's a thermal limit and a cable is somewhat different to a PCB - try this:

http://www.4pcb.com/trace-width-calculator.html

I like that drain resistor - how many ohms?

3.3kOhms. But that's not the right question. You size it based on the energy stored in the capacitors, time in which you want the capacitors to discharge, and acceptable power through the resistor.

Crudely, the capacitors are discharged in t=5*R*C and the resistor power dissipation is P=V^2/R. I used a 3W resistor, but to get good lifetime from the resistor and not set fire to things it's a good idea to use less than the rating, say 2W. R=V^2/P=80^2/2=3.2kOhm. t=5*3200*0.02 = about 5 minutes.

Where do you buy components if you don't mind me to ask - your price are better than mine:(

Generally Onecall (farnell,CPC) or Rapid Electronics. Also Mouser and Digikey have started doing free postage on smallish orders, so they're good bets.

Be extremely wary of components from eBay in China - rife with fakes, especially capacitors. I treated myself to an LCR meter so I can at least check the capacitors.

Doddy
04-01-2017, 09:59 PM
...yes Clive DC after cups but the real reading tbh.
my 30V trafo has 32VAC and after bridge and 2x10.000uF reads 41.5VDC unloaded
calculation gives 32V*1.4=44.8V 3.3V more than real test.

Your off-load DC voltage is largely irrelevant, other than to consider any maximum voltage for component selection. The RMS voltage (i.e. 32V) represents the effective average voltage under the specified rating of the transformer. The problem you will have with an unregulated supply is exactly that - it's unregulated and with large value smoothing capacitors you can expect that off full-load that the capacitors can hold up the DC level as the AC supply transits through the zero-crossing point, and the effective average DC voltage will increase above the full-load value (but below the peak value).

Expect to lose 1.2-1.6V (typically) across a bridge rectifier. This affects the peak DC voltage directly.

When looking for variance between your measured vs theoretical, as already mentioned in the replies - consider the actual AC line voltage at the time of test, and the ratio of the primary and secondary windings (many transformers are rated with primaries at 220VAC - if fed with 240VAC then expect the secondaries to read 240/220 x the rated voltage).k

m_c
05-01-2017, 12:44 AM
For a read of the how/why behind choosing a toroid based on steppers, have a look at my post in my Triac retrofit thread - http://www.mycncuk.com/threads/10344-Yet-another-Denford-for-the-collection-(Triac-Retrofit-thread)?p=85703#post85703

Although having just skimmed over the post, I've just realised I never mentioned about sizing the capacitor. It's a 100V 22'000uF from a reputable source.

Tom J
05-01-2017, 01:28 AM
For a read of the how/why behind choosing a toroid based on steppers, have a look at my post in my Triac retrofit thread - http://www.mycncuk.com/threads/10344-Yet-another-Denford-for-the-collection-(Triac-Retrofit-thread)?p=85703#post85703

Although having just skimmed over the post, I've just realised I never mentioned about sizing the capacitor. It's a 100V 22'000uF from a reputable source.

That kind of answer I was looking for.
33VAC - 52VDC, big boost over calculation, any idea why was like that?

My both 420VA & 720VA has 30VAC and gave 41.5VDC (calibrated Fluke 87V and 1587)
Secondary coil was ~32VAC

20226
picture shows Amp meter after calibration, voltage was set too

Clive S
05-01-2017, 10:30 AM
My both 420VA & 720VA has 30VAC and gave 41.5VDC (calibrated Fluke 87V and 1587)
Primary coil was ~32VAC

Tom you are reading too much into all this. It has been done to death many many times on the forum.

Have a look at the quote and notice you are saying primary coil is 32 AC. The secondary would be the the 32 volt coil.
It is facts like these that can mess people up.

Also you are stating a 1.57 A load is that on the primary or secondary as they would be different.

All this can be very confusing for people seeing the thread title thinking this is the right way to pick a Tx.

m_c
05-01-2017, 10:46 AM
You need to allow for mains voltage tolerance (230V -6/+10% or 216 to 253VAC), transformer load regulation (the toroid spec I just looked at was 5%), and a lower volt drop over the bridge rectifier when zero load.

So taking a 33VAC toroid, applying 10% more power (mains voltage here is at the higher end of tolerance) gives 36VAC (we'll round for simplicity) at rated load. Add 5% load regulation, which means we bump our basic unloaded voltage to 37.8AC.

Multiply by root 2 (or 1.41), gives us 53.3VDC. Take away a little bit volt drop for the bridge rectifier, and we have in the region of 52VDC.

Tom J
05-01-2017, 12:26 PM
Tom you are reading too much into all this. It has been done to death many many times on the forum.

Have a look at the quote and notice you are saying primary coil 32 AC the secondary would be the the 32 volt coil.
It is facts like these that can mess people up.

Also you are stating a 1.57 A load is that on the primary or secondary as they would be different.

All this can be very confusing for people seeing the thread title thinking this is the right way to pick a Tx.

yes , sorry corrected to "secondary coil was ~32VAC"

1.57A has nothing to do with transformer, just a picture. VOLT/AMP meter red/blue LCD shows the same voltage as calibrated Fluke - that was my point.
I had only picture with Amp displaying on Fluke which also was calibrating. I know I confused enough but some guys say that my meter might be off.

JAZZCNC
05-01-2017, 03:41 PM
This F@C£ING STUPID and Dangerous.! . . . LEE STOP It PLEASE. . . . Far too many errors and misunderstandings.

Tom I'm not having a go at you personaly here and see your just trying to elp and give back but this is potentialy dangerous to others and shouldn't be put over has " How To Guide".
I'm helping and dealing with people young and old all the time who never post or ask questions, which they should but don't for what ever reason. Some do have the good sense to ask privately and this is my concern.

I'm for ever helping people, esp the young who haven't got the patence to read past first few posts and when the First post comes over in manner where looks like OP knows what doing they take it has Gospel.
They then proceed to go out and buy everything mentioned without any regards to if correct or not and follow to the letter whats wrote.
This isn't so much problem if it's frame related or Even some components like ballscrews rails etc because worst case they end up with under performing or over expensive machine.
But when comes to Mains electrics like this then it's Bloody dangerous in the wrong hands so unless the OP is Expert which clearly isn't the case here then shouldn't be allowed to proceed any further.

Anyone who's been on here long enough should know how much I hate Moderation and Censorship, Bloody hell I've come close to being banned in protest to it so it should show you how strongly I feel about this because this NEEDS STOPPING or at least Moderating closely.!

Lee Roberts
05-01-2017, 04:41 PM
Hi guys,

After receiving a couple of reports on the original thread (it needing attention), I decided to split the discussion, allowing it to continue regarding the selection process, with a new discussion for the PCB side of things, as both topics deserve the proper observations.

You can find the links to each discussion below.

Tom J
Thank you for your efforts and suggestion, however due to errors and inconsistencies in your original approach to this, I feel it is in the best interest if we strip this thread of any kind of guide/tutorial type wording or suggestion it may have, as it's just not ready for that yet.

Please don't be discouraged in your efforts, the original idea was a good one and would be an asset to the forum, however due to the severity, that kind of article/document needs to be correct in its absolute entirety before it can boast as being the definitive. Please try again once you have everything worked out.

Thank you to everyone involved, feel free to continue both discussions:

.

Neale
05-01-2017, 11:43 PM
I don't want to get hung up on this transformer voltage business (except that I was surprised by the toroidal transformer I bought recently that measured about 5% above nominal voltage when off-load - the "regulation" factor that's mentioned above which I hadn't expected) but I am interested in the practical choice of smoothing capacitors. I am assuming here that the linear PSU is feeding only the stepper drivers and nothing else, which I think is generally the case.

Stepper drivers can cope with fairly wide voltage ranges. From memory, the EM806 which I am using is rated at 24-80V absolute min/max, for example, although the Leadshine recommendation is to stay at least 10% away from either extreme. We are not building audio amplifiers here, so we're not interested in hum levels on the output. The capacitors are being charged at 100Hz (assuming the usual bridge rectifier) but the steppers are pulsed at a few KHz, so any given pulse will see only the variation in voltage during that pulse - small compared with the variation across a half-mains cycle? On top of all that, one of the stepper driver's main jobs is to control and limit current through the stepper coils, so one thing it can do very well is cope with a range of input voltages while trying to maintain the target current through the stepper coils. So do I really care if the DC output voltage sags by a few volts, across a half mains cycle? In practice, then, just what size capacitor will do an adequate job without going OTT? What is the effect of a too-small capacitor? Don't know the answer myself, but once my machine is back running I'll stick an oscilloscope on the PSU output and see what's happening, just in the interests of science.

Clive S
06-01-2017, 12:13 AM
Very well put Neale some sense is prevailing at last.:encouragement:

Tom J
06-01-2017, 12:27 AM
In practice, then, just what size capacitor will do an adequate job without going OTT? What is the effect of a too-small capacitor? Don't know the answer myself, but once my machine is back running I'll stick an oscilloscope on the PSU output and see what's happening, just in the interests of science.

1000uF for 1A, voltage is important (20% more than operating voltage and you are safe, min 100V caps for 80V system.)

Small capacitor won't take enough power from stepper when slow down or change direction (EMF). I am scared to write anything now:frown:

m_c
06-01-2017, 12:34 AM
I can't remember the specifics, but ultimately capacitor sizing comes down to load, and how quickly it charges. I'm hoping somebody more knowledgeable will be along with the more exact details, but I'll give a basic view.

As you know the capacitor has to provide energy during the period between charging cycles. The more load for any given capacitance, the more the capacitor voltage will drop between cycles.
Now if you choose too low a capacitance, you get a high voltage ripple on the output.
Most stepper drives will probably handle this, however by having the capacitor voltage fluctuating too much, you risk overheating the capacitor. As with nearly everything, nothing is 100% efficient, so every time you charge/discharge a capacitor, energy is lost to heat.
This is why large electrolytic caps normally have a vent, so if they do heat up and pressure builds up inside them, they can vent in a safe manner rather than exploding. For completeness, if you look at the end of small electrolytic capacitors, you'll see a scored line, which is a built in weak point, for the same reason.
Another thing is during motor deceleration, a smaller capacitor will not absorb as much energy, so over-voltage becomes a risk.

In terms of a powered circuit, other than cost, there's not really such a thing as too much capacitance, but you will reach a point where extra capacitance won't provide any tangible benefit.
With a large capacitance, you have the issue of start up surges, as a fully discharged capacitor will appear as a dead short at initial power up gradually increasing in resistance until fully charged. The more capacitance, the longer the initial charge will take during power on.
Then you need have to deal with discharging the capacitor after power has been removed. As Jonathan mentioned somewhere already, you could end up with quite a lot of heat being generated by a discharge resistor in order to discharge the capacitor(s) in a reasonable time frame once power is removed.

There are calculations that will work all this out, but it's not something I've done for a while. However I'm sure a google search for capacitor ripple voltage will turn up some detailed results.

m_c
06-01-2017, 12:48 AM
1000uF for 1A, voltage is important (20% more than operating voltage and you are safe, min 100V caps for 80V system.)

Small capacitor won't take enough power from stepper when slow down or change direction (EMF). I am scared to write anything now:frown:

The thing is, that 1000uF per A is an arbitrary figure provided as a guideline.
I just had a quick scan through the Gecko Drive guide, and they list the following formula for minimum capacitance in uF-
(80'000 * I)/V

I'd imagine they've done some calculations and research to reach that figure, and it'll involve the maximum voltage ripple their drive internal capacitors can handle safely under various conditions.

Clive S
06-01-2017, 11:28 AM
Anyone wanting to make one Jonathan is selling these http://www.mycncuk.com/threads/10475-PSU-Rectifier-Capacitor-smoothing-board-for-toroidal-transformer. A bargain I think.

Jonathan
06-01-2017, 03:35 PM
I don't want to get hung up on this transformer voltage business (except that I was surprised by the toroidal transformer I bought recently that measured about 5% above nominal voltage when off-load - the "regulation" factor that's mentioned above which I hadn't expected) but I am interested in the practical choice of smoothing capacitors.

Another thing to play with is adding or subtracting turns from the transformer to tweak the voltage. But I wouldn't recommend that by design, only if you the insulation materials and enameled wire lying around... It's a neat way to make an auxiliary supply though - just add on an extra winding.

Stepper drivers can cope with fairly wide voltage ranges. From memory, the EM806 which I am using is rated at 24-80V absolute min/max, for example, although the Leadshine recommendation is to stay at least 10% away from either extreme.

Having scoped the turn-off overshoot voltage of the MOSFETs in a DQ860MA, I have no qualms about running them at 80V. They use 100V mosfets and the overshoot was only a few volts, so still some headroom. They do skimp on the high frequency decoupling, so I added an extra ceramic capacitor on the empty footprint. I can't comment on other drives though.

We are not building audio amplifiers here, so we're not interested in hum levels on the output.

Yes!

The capacitors are being charged at 100Hz (assuming the usual bridge rectifier) but the steppers are pulsed at a few KHz, so any given pulse will see only the variation in voltage during that pulse - small compared with the variation across a half-mains cycle?

Correct. The stepper driver is a current controller with a switching frequency of 10-20kHz, so the current control bandwidth will be plenty sufficient to deal with some input voltage ripple.

On top of all that, one of the stepper driver's main jobs is to control and limit current through the stepper coils, so one thing it can do very well is cope with a range of input voltages while trying to maintain the target current through the stepper coils. So do I really care if the DC output voltage sags by a few volts, across a half mains cycle?

Probably not, for a few volts...

In practice, then, just what size capacitor will do an adequate job without going OTT? What is the effect of a too-small capacitor?

That is the question everyone should be asking - the answer is not straightforward.

Don't know the answer myself, but once my machine is back running I'll stick an oscilloscope on the PSU output and see what's happening, just in the interests of science.

Maybe I'll try it this evening, if I can find that spare stepper driver...

Meanwhile, I just dug out these graphs I captured whilst repairing a stepper driver, showing the voltage across one mosfet Vds (blue) and the line current through the same (yellow).

The first graph shows a normal situation - we see the current rising when the voltage is applied, and falling otherwise with the inductance of the motor smoothing this current. Observe how little overshoot there is on the voltage.

20240

For this one I removed the dc-bus capacitor (can't remember if it was the small or large one) and supplied from a lab PSU. You can see the voltage and current now oscillating horribly.

20238

Same again, but with a lower supply voltage - notice the current is the same (so the regulation is working) and the duty cycle has increased.

20239

This doesn't tell us anything about sizing the supply capacitor, still it's mildly interesting...

The thing is, that 1000uF per A is an arbitrary figure provided as a guideline.
I just had a quick scan through the Gecko Drive guide, and they list the following formula for minimum capacitance in uF-
(80'000 * I)/V

This is what I like to call a "Forum Formula": formula's commonly distributed with little concern for how they were derived. Often empirical, commonly including a seemingly arbitrary constant designed to make the user think they are learning something.
Just like the formula's in the first post to estimate the supply current - these bare little relevance to reality.

Where does the 80,000 come from? Which is the I, motor or supply current? etc...

The 1000uF per amp is derived from \frac{i}{C}=\frac{\mathrm{d} v}{\mathrm{d} t}. The gecko formula will be something similar.

There is some better information in this (http://www.mycncuk.com/threads/5075-Confirming-PSU-spec-for-steppers) thread. Post #2 is pertinent.

I'd imagine they've done some calculations and research to reach that figure, and it'll involve the maximum voltage ripple their drive internal capacitors can handle safely under various conditions.

But do they apply to every driver - maybe close enough, maybe not?

Anyone wanting to make one Jonathan is selling these http://www.mycncuk.com/threads/10475-PSU-Rectifier-Capacitor-smoothing-board-for-toroidal-transformer. A bargain I think.

Thanks Clive - I still have two left and more capacitors on the way.

Neale
27-04-2017, 11:29 PM
A few months ago I said that I would have a look at the output of my power supply with an oscilloscope to get a feel for what was happening to the supply voltage with the router in operation. I was able to do this recently. I had some gcode running that was winding X and Y backwards and forwards at full rapid speed (gantry probably weighs in at around 50Kg, rapid speed on X and Y 4.5m/min, NEMA23/ballscrews, to give an idea of mechanical load), and I hung the oscilloscope off the supply to one of the drivers. Basically, there was the best part of 10V mains frequency ripple, with maybe a couple of volts of high-frequency noise on top of that. Mains ripple was presumably what the smoothing capacitors could not remove, and the high-frequency noise was the combined result of four unsynchronised switching drivers. That's all on top of a nominal 68V DC supply - the output voltage was swinging either side of that. The amount of ripple didn't change very much when the steppers were stationary although the stepper drivers are set to half-current mode. Clearly that doesn't make much difference to the PSU ripple.

Edit - have now seen that PSU has 2x6800uF smoothing capacitors, 100V/105C rated. Voltage limit is fine for this job and it's good to see the higher temp capacitors being used rather than the cheaper 85C - electrolytic capacitors are not the most reliable of electronic components but the higher-temp versions are better.

On the face of it, that doesn't sound like a great result. I was surprised by the amount of ripple - much more than expected. I can't read the capacitor values where they are, unfortunately, so I'm not sure how generous they are (the original PSU is the PS806 from Zapp although with a replacement transformer). On the plus side, the stepper drivers clearly don't give a damn and just keep on doing what they are supposed to be doing, and do it very well. Maybe, just maybe, it's possible to get a bit too hung up on the details sometimes and worry unnecessarily.