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Author Topic: Questions on driver rating and PSU rating - newbie questions  (Read 3545 times)

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Questions on driver rating and PSU rating - newbie questions
« on: March 05, 2012, 06:58:11 AM »
Hello,

I'd like to power 3 steppers motors FL86STH80-4208A (4 phases, 4.2 A per phase), datasheet attached. I will only use 2 steppers in the near future, but would like to be able to add a third later. The wiring I am using at the moment is bipolar (parallel) connection... The steppers will be used for relatively light work (for example a sand plotter as shown by Bruce Shapiro), maybe a small mill.

I think my steppers are well over specs for my actual needs, but I have two of them and would like to avoid buying more steppers.

My question is about the power supply I need and the amp rating of the drivers
(datasheet attached) ... my online seller first told me that a 36V 10A power supply and  4.2 A (max) drivers were fine for this, ... Is this right?

From the specs of the stepper in parallel connection I apparently need 6A (4.2 * 1.4) A ... so it looks like 6 * 3 = 18A. So my power supply looks too small

As for the drivers the max amperage is 4.2A - this again looks under-powered if I use the stepper in parallel connection...

Now the seller is okay swapping my drivers (at a cost) for drivers that can handle up to 7.2A (http://www.ebay.co.uk/itm/CNC-Mill-Router-DIY-Stepper-Driver-Board-Controller-MA860H-2-6A-7-2A-24V-110V-DC-/270917084995)

Should I do this, and also get a new power supply? What power supply would you recommend?

many thanks

Thomas

Offline Hood

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Re: Questions on driver rating and PSU rating - newbie questions
« Reply #1 on: March 05, 2012, 10:46:13 AM »
Here is a pdf with some good info on power supply sizing and making.
http://campbelldesigns.net/files/power-supply-part-1.pdf

Hood
Re: Questions on driver rating and PSU rating - newbie questions
« Reply #2 on: March 05, 2012, 01:09:32 PM »
Great, thank you.
So if I just look at DC voltage I would need 3.8V * 20 = 76V minimum, and the maximum voltage 3.8V * 25 =95V
So I would need a 80V DC PSU

as for the amps, even using the parallel connection I would need maximum 3 * 6A * 0.67 or 12A

I have 36V 10A, which is below both ratings...

what will be the impact of this? Is it just going to be speed and torque? In that case it may be okay - I don't think I need all the power for a simple XY table...
Also, where can I find a 80V PSU? I found a 48V 12A PSU, marginally better than the 36V 10A one I have...

The driver my seller is ready to exchange my current ones for is here:
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&isIU=1&item=180824312186
and can take AC voltage at 60V - so I would need a transformer to supply that. It should be 80V * 12 A or almost 1000VA!

what do you think??

many thanks

Thomas

Offline Hood

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Re: Questions on driver rating and PSU rating - newbie questions
« Reply #3 on: March 05, 2012, 01:30:34 PM »
I think Geckos site has info on voltage for steppers and I am sure they say somewhere in the range of 5 to 25x but I think it depends on the motors, maybe the inductance etc. It has been a long time since I have used steppers so have forgotten the little I knew about them.

If the drives you are looking at can accept AC then use that, I would imagine they must have internal capacitance to smooth things out so all that would be needed is a transformer to drop your mains to the required voltage.

Voltage is speed and Current is torque in the servo world and I believe it is the same in the stepper world.

Hood
Re: Questions on driver rating and PSU rating - newbie questions
« Reply #4 on: March 05, 2012, 01:41:29 PM »
Thanks Hood,

Just wondering if I should spend an extra 50 quids to get bigger drivers, then extra to get bigger PSU as well...

cheers

Thomas

Offline Hood

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Re: Questions on driver rating and PSU rating - newbie questions
« Reply #5 on: March 05, 2012, 02:02:21 PM »
Probably best to wait on others that know steppers better to say what they think. I have long since moved away from steppers on my metal cutting machines, the only one that still has steppers is the wee coil winder and they work well on that.
Hood
Re: Questions on driver rating and PSU rating - newbie questions
« Reply #6 on: March 05, 2012, 02:39:05 PM »
From the Gecko website, the PSU should deliver "anything between 4 to 20 times the listed voltage" so in my case I am at 10 times, not too bad...

cheers

Thomas

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Re: Questions on driver rating and PSU rating - newbie questions
« Reply #7 on: March 05, 2012, 03:04:59 PM »
I think you would really need to see the torque curve for your motors to see what the best would be.
Hood
Re: Questions on driver rating and PSU rating - newbie questions
« Reply #8 on: March 06, 2012, 12:19:26 PM »
I believe also that modern stepper drivers use switched-mode technology, so that though they might be driving the steppers at 4.2A, the average current from the power supply is less than this (theoretically by the ratio of the actual voltage across the stepper to the supply voltage).  Also you seldom drive all the steppers at full current all the time.  In any case for "light work" you don't need to drive them at full current anyway, and modern drivers reduce the current to 50% when they are not actually stepping.  I use a 36V 11A supply for the 3 motors on my Novamill, two of which are type23s which I run at just over 1.5 A for X and Y and the third is a type 34 (what you have I think) which I run at 2.5A for Z.  There's plenty of torque on all the axes and the power supply (and the drivers) don't even get warm.  My drivers are essentially the same as yours as well.  I think you'd be fine with your current drivers and the 10A supply.
Re: Questions on driver rating and PSU rating - newbie questions
« Reply #9 on: March 06, 2012, 12:30:19 PM »
Thanks, very useful to know. I'll probably stick to my current drivers and PSU then!

Thanks to everyone for your time and suggestions!

Thomas