Newbie- Power Supply Question

[Jesse B]

Sorry if this has been covered, I just haven't found anything that specifically answers my question.

For the purposes of this example, let's assume we have a 2-Phase stepper motor, which draws 2.0A per phase. When choosing a power supply for this motor, do you need it to be able to output 2.0A, or do you need it to be capable of supplying 4.0A, to supply both phases. Sorry if this has been covered


- Jesse

[Crevice Reamer]

Hi Jesse.

In your example you would only need enough for one phase at a time, or 2 A. To size the PSU for several motors, the formula is add up the total and multiply by .67.

Some of this basic info might be helpful to you:

http://crevicereamer.com/Page_2.html

CR.

[Jesse B]

Originally Posted by Crevice Reamer Hi Jesse. In your example you would only need enough for one phase at a time, or 2 A. To size the PSU for several motors, the formula is add up the total and multiply by .67. Some of this basic info might be helpful to you: http://crevicereamer.com/Page2.html CR.

Excellent, thank you very much!

[Crevice Reamer]

Whoops, that link was bad. Fixed it now.

CR.

[Jesse B]

Thanks!

Also, it's always beneficial to use a power supply with a rated voltage higher than the rated voltage of the motors, correct? I've seen that quite a lot, just making sure that's the case.

[Crevice Reamer]

Originally Posted by Jesse B Thanks! Also, it's always beneficial to use a power supply with a rated voltage higher than the rated voltage of the motors, correct? I've seen that quite a lot, just making sure that's the case.

Uh, Yes, but you have to calculate it. Best PSU voltage for a motor wired Bipolar Parallel, (which is best for most uses) is 32 times the square root of the motor inductance. Sometimes you can be a few volts under, but you should NEVER be over that amount.

This and other formulas are in that link.

CR.

[Jesse B]

Another quick question

Is it possible to wire together a pair of the same power supplies with the intent of doubling the voltage, and keeping the amperage the same? Higher voltage supplies seem to be a lot more expensive, and I'm trying to do this on as much as a budget as possible. I've found some supplies that are inexpensive, have adequate amperage, and just under half the voltage, so I was just wondering if this was possible or not.

Thanks,


- Jesse

[stef_d]

Also reading about PSUs for CNC, and followed the link.
I found this sentence:
POWER SUPPLY (PSU): Both types of motors run on DC Voltage. The power supply simply converts ordinary alternating current into smooth DC at a Voltage for CNC motors. Choosing the proper voltage to match drivers/motors is one of the most important decisions needed. You NEVER want to install a switch on the DC side of the power supply.

Why the last point ?
you NEVER want to install a switch on the DC side of the PSU ?
I dont get that. (i am an electrical engineerm, but not specialised in power supplies)
I have completed my first machine which does work, and does have switches on the DC side of the PSU. I am looking into building a bigger one, hence the reason i am looking into PSUs. Before i make a stupid mistake, why cant i have switches on the DC side ?
(sorry to hijack..)

[Crevice Reamer]

Originally Posted by stef_d .....why cant i have switches on the DC side ? (sorry to hijack..)

Hi Stef! On topic so no hijack.

You CAN have a FUSE on the DC side, but no switch. There is no problem if the power is cut on DC side. The trouble begins when you turn the switch ON.

The PSU allows slow buildup of power when AC is switched on--But drives may suffer catastrophic power surge when DC is switched on.

You may have been lucky so far, but should eliminate those DC switches.

Thanks for the question. I will clarify this on the site.

CR.

[stef_d]

Originally Posted by Crevice Reamer Hi Stef! On topic so no hijack. You CAN have a FUSE on the DC side, but no switch. There is no problem if the power is cut on DC side. The trouble begins when you turn the switch ON. The PSU allows slow buildup of power when AC is switched on--But drives may suffer catastrophic power surge when DC is switched on. You may have been lucky so far, but should eliminate those DC switches. Thanks for the question. I will clarify this on the site. CR.

i see, so you are worried about voltage spikes when closing the switch..
i suppose it makes sense. i will try to do my switching on the AC side in my next design.

I am using an L298. in the usual fashion, with the diodes on the output.
Wont these diodes (reverse) clamp the voltage spike ? and the 100nF cap should also smooth things out ?
http://www.st.com/stonline/products/.../1773/l298.pdf