recipe for cripy geckos?

[sixpence]

I need some help with the power supply side of things and wiring to geckos.
I have built a router " she lives again" detail in project log.
I used a board from hobby cnc because the electronics side of things still baffles me.
Since my first project was so succesful I want to move on to bigger and better things.
For this South African girl size does matter! I prefer not to turn myself into a crispy critter.
I have many more questions to follow these but these I think are enough for now?
Anyone willing to propose a shopping list?


I am interested in these motors from www.homeshopcnc.com/page3.html
640 oz/in Model # RHT34-640: $99.99

Holding torque-Nm (oz/in) +/- 10% 4.6 (640)
Rated voltage: 2.3
Rated current per phase (amps DC): 5.5
Resistance (Ohms) +/- 10%: 0.42

Induction (mH) +/- 20% typical: 3.5


I would like to use the gecko drives G201 7 amp 80volt

I have been browsing the zone and others i.e.
www.eleinmec.com/article.asp?16
http://www.campbelldesigns.com/how-t...wer-supply.php

I have followed the intructions and formulae from campbell I "think" I grasp the concept

1. Desired power supply voltage:

2.3 vdc * 20 = 46vdc(why 20x rated voltage?)

2. Transformer secondary voltage:

46vdc/1.4 = 32.86vac(why 1.4?)

3. Desired transformer VA

3x motors @5.5amps = 16.5
should not exceed 67% of total
16.5 x 67% = 11.19amps

32.86 x 11.19 =367.71VA

4. size of capacitor C=((80,000*I)/V)

C=((80,000*11.19)/32.86)
C=27,242.85uf

5. Here is where things start getting fuzzy(well maybe it started at no. 4)

From the plug 220 volts ac 50Hz- tranformer input
transformer output 32 volt
11 amps (is this correct?)

I cannot find a tranformer rated like this will it have to be
custom made or are there other standard options that I
could get away with using?

bridge rectifier: What should it be rated at?
capacitor: clueless as to what to order?

pls forgive any dunce like errors

thanks

[lucas]

Sixpence,

you did your homework and it looks fine but at point 3:

3. Desired transformer VA 3x motors @5.5amps = 16.5 should not exceed 67% of total 16.5 x 67% = 11.19amps 32.86 x 11.19 =367.71VA

A stepper has 2 phases, 5.5 amps per phase *2 = 11 amps per motor.
Final result should be 22.38 amps

So everything doubles: the caps and the transformer power ratings.

You can't find this "calculated transformer" but the Gecko's are rated at 80VDC and using a higher voltage is better, look for a transformer with output between 33VAC (46VDC) and 48VAC (67VDC).
Easiest to find could be one with 2 outputs of 24VAC and put the 2 outputs in series to obtain 48VAC.

This transformer would need to be 800 to 1000 VA or more, not exactly small....

Another option is to build a supply for each drive using a 300VA transformer, probably easier to find.

Hope this helps.

[widgitmaster]

http://transformertechnology.com/

This is a great source for transformers, give them a call! (860) 349-1061
Eric

[Evodyne]

Penny,

Hi! The geckos are great! A little more detail on the supply voltage: Geckodrive quotes (I'm doing this from memory) 24 VDC to 80 VDC for the drive itself. I've read a few times that 70 VDC may be a better top end value. On the high end your are looking at the lesser of either 80(70) VDC or 20 x your motor's rated voltage. On the low end you need the greater of 24 VDC or 4 x your motor's rated voltage. Anything in between will work with the higher values giving better torque and high end speed.

Your power supply will consist of, at minimum, a transformer, rectifier, and filter capacitor. Working backwards the capacitors you be looking for are called electrolytic capacitors. There are a few things to bear in mind:

1. The higher the capacitance (usually expressed in microfarads) the more filtering/less ripple. You would be suprised at how little is actually needed-3,000-5,000 ufarads will work if that's all you can find.

2. These caps have a second rating-their working voltage-which MUST be greater than the supplies DC voltage output. Else POOF! Doesn't matter how much greater-just greater. A few volts is actually sufficient.

3. Electrolytic capacitors are polarized, or polarity senitive. They have a plus lead and a minus lead. If you hook them up backwards (minus lead to positive voltage, plus lead to ground) they can and will explode.

4. You can use multiple capacitors in parallel. The total capacitance is the sum of the ratings of the individual units. They should all have the same voltage rating and, again, this must be greater than the DC output of the supply. By parallel I mean all the positive leads connected together electrically and all of the negative ones connected together.

Your bridge rectifier is the diode pack that turns AC from the transformer to pulsating DC that feed the capacitors. The rectifier will be subject to all the current flowing out of the supply and to the DC level present at the output. As with the capacitors, you want a bridge whose ratings exceed thee values. The rectifiers are rated for maximum forward current and peak inverse (reverse) voltage (PIV). If you had an 80 volt DC supply capable of 20 amps, a bridge with a PIV rating of 100 V and a current rating of 25 amps would suffice. But a unit with a PIV rating of 600 V and 60 amps would work too.

Your supply output can be roughly estimated by 1.414 x the transformer's AC output voltage. Thus to get close to 70 VDC out, we would look for a transformer with an output of 70 VDC / 1.414 or 49.5 volts. Great, but you will never find one. Find the closest rated a bit higher and lower and see where you end up: a 45 VAC out unit would give you about 63 VDC (so vour capacitors would need a voltage rating of 70 VD or greater). A 55 VAC secondary on the transformer would yield 78 VDC. Higher than we planned on, but under the Gecko's 80 VC max-so a likely candidate as well. Just to drive home a point the capacitor(s) for this choice would need to be rated over 80 volts. In the real world one to two volts will be lost at the the bridge rectifier, so don't be suprised when your actual output is a bit off from your calculations. I'd probably go with the lower of the two voltages to avoid stressing the drive.

Read the attached .pdf off of the Geckodrive site-there is a section on power supply considerations. Please don't omit the fuses-it's cheaper to burn one of them out than a drive!

Have fun!

Evodyne

[ger21]

Originally Posted by lucas Sixpence, you did your homework and it looks fine but at point 3: A stepper has 2 phases, 5.5 amps per phase *2 = 11 amps per motor. Final result should be 22.38 amps So everything doubles: the caps and the transformer power ratings.

This is incorrect. She was right, the Gecko will only use at most 2/3 of the 5.5A. It will usually use less. This info is on Gecko's website.

You should go with the G202. A few dollars more, but has quite a few "protections" built in. A few dollars up front may save you some trouble in the long run. The G202 doesn't need heatsinks, either. But they wouldn't hurt.

As for the 20x voltage. Gecko's website says the power supply should be between 3x and 25x the motors rated voltage. The higher the voltage, the more useable speed you'll get from your motors. When you get close to 25x, your motors will run a lot hotter. Most people use between 10x-20x. You may only need 10x to get the performance you're looking for. Unfortunately, the only way to know for sure is to actually test it on your machine. Since you don't want to build two power supplies, most people go for the higher voltage. Somewhere in the 15x-20x range should be good. If you can llok at a torque curve for the motors you'll be using, it should say at what voltage it's using. If you know how fast you want the machine to go, and what screws you'll be using, this should get you in the ballpark for what voltage to shoot for.

[sixpence]

Wow! Thanks for all the info. My dust covered calculator and I willl be working overtime
to work out all the possible variables so that on Monday I can go on the great hunt.
I will however first check in with you guys to confirm that the parts list is correct.
Thanks