[The New Steve H]

I'm building my first CNC tool: a small lathe with 2 steppers. I decided to use a Kflop/Kstep combination. Now I'm looking for a power supply.

I would like to be able to run up to 4 570 oz-in steppers off this setup, because I may want to add another machine later. For that reason, I want the power supply to be bigger than the lathe requires. Obviously, I would not be running both machines simultaneously.

The 570 oz-in steppers are rated at 5 amps. I have seen various references saying I will never need more than 2/3 of the maximum rated current, so if that's true, I should be fine with anything over 13.33 amps. But someone on a forum is telling me the steppers will draw maximum current all the time, so I need something that will provide at least 20 amps. He says the torque will be derated if I go below 5 amps.

Is he right? I know nothing about CNC, but it seems like everyone disagrees with him. I have read that you don't need max current for microstepping. I found a power supply that will put out over 16 amps, and I was thinking I would go ahead and buy it.

[Al_The_Man]

. He says the torque will be derated if I go below 5 amps.

. I have read that you don't need max current for microstepping. .

Current drops using micro-stepping, but Torque is directly related to current.
Al.

[The New Steve H]

I keep reading about this. I visited the famous Hoss G0704 page, and he says he would be happy using a 48V/7.3A power supply to run three 570 oz-in motors, and he says he would go with 12.5 A for up to five axes. I guess I don't need 20 amps.

[Alexthehun]

The reason you need less than the combined max current for your power supply:
The stepper is rated at its operating current at a much lower voltage that the voltage of the power supply you will be using.
The stepper motor driver acts as a step down converter, transforming the much higher power supply voltage to the lower voltage needed for the stepper. A side effect of this is that the current requirement at the power supply side is reduced by a similar factor.
Another way of looking at it: the low stepper voltage X stepper operating current = stepper power. That power is extracted from a supply working at a higher voltage, the power needed, ignoring driver efficiency, remains the same so the power supply current is lower to arrive at same power.
The Hoss numbers you mention should give you plenty of margin still.

[Al_The_Man]

The only reason for the higher voltage supply is to maintain the rated current, current will drop with increase in rpm due to the motor inductive reactance increasing with increase in stepper rate or rpm.
Stepper drives now are PWM in nature, and allows the higher supply voltage than motor rated voltage as the rated CURRENT is maintained throughout the operating range.
Al.

[The New Steve H]

Thanks for the help. Now I just need to choose between switching and non-switching. The difference is ten bucks.

[The New Steve H]

I went with a 16.7A toroidal job. Is there any hope that someone could point me to a source listing the connectors and crap I will need to connect this to a Kstep? I don't expect anyone to write a page and a half. A simple link would be a big help. I will also need to know what kind of cables are needed to hook steppers to the Kstep.

[The New Steve H]

It looks like people are connecting power supplies with plain old wire instead of special connectors, and the steppers will work with any 4-conductor stranded wire over 20 gauge. I guess I can get it going.

[TomKerekes]

You might read this Thread:
http://www.cnczone.com/forums/dynomo...confusion.html

I think it is more of a power thing than a current thing. When stopped a Stepper will consume less than 20Watts and even at speed under load it is hard to put even 100W into a stepper.

Please make sure the unloaded supply voltage doesn't exceed the max allowed 48V of KSTEP or the KSTEP voltage clamping will draw a lot of power.

Regards

[The New Steve H]

Thanks for the help. As it happens, the supply I ordered is 48V.

The Kflop and Kstep arrived today. Now if the power supply will just get here, I can start making a mess. I can't even choose an enclosure until I see the PS.

[TomKerekes]

Sometimes linear supplies can be much higher than their rated voltage when not under load. So you might consider checking the voltage with a voltmeter without any load.

Good luck!
Regards

[The New Steve H]

How much does the voltage have to exceed 48 before I have to worry?

[TomKerekes]

KSTEP's clamp can be set for 49V. So the supply should be 48.5V or less.

Regards

[The New Steve H]

Would a 36V supply have been a better choice? The seller hasn't shipped my PS yet, so I can still cancel.

[TomKerekes]

That would be safer. This is one of the reasons I usually prefer regulated supplies.
Regards

[The New Steve H]

Thanks again. I am trying to get up to speed on this stuff, and I thought unregulated supplies were preferred. So hard to tell what's right and what's BS!

[The New Steve H]

I'll let you rest if you answer one more question! I found a 48V regulated supply I like. Is there any risk with that, or should I look for something lower? The AC around here is right at 125V, so I'm wondering if the regulated supply will go over 48V.

I'm sure I can find a 36V job with similar current if I keep looking.

These are 425 oz-in motors. Parallel voltage: 4.1. Current: 2.8A. Resistance 1.5 ohms.

[The New Steve H]

I am pretty ignorant about steppers, but I found some stuff you wrote on a Yahoo group, and I am trying to use it to advantage.

I assume the current level should be set to the first level above the rated current, and that would be 3.13A (first choice higher than 2.8A). The resistance is 1.5 ohms. Multiplying these figures gives a voltage of roughly 4.7V. Twenty times that is about 94V, so I am GUESSING that a 48V supply will not be inappropriate for these motors or the bigger motors I may want to use on a second board. I am thinking I may want a mill later, with 4 somewhat bigger motors (this is the reason I want plenty of current available). It looks like your literature approves of 48V supplies, so hopefully, a 48V regulated supply will work without making anything explode.

I found an interesting supply on Ebay. It's 48V with a second adjustable rail which can go as low as 1.8V. I am thinking that might be a good way to get around using a wall wart for the logic supply, assuming the supply isn't likely to go nuts and shoot 48V into it.

48V 12 5A Multiple Rail Switching Power Supply Second Rail Adjusts 1 8 25V | eBay

[Al_The_Man]

This is from the Gecko stepper info site:

An unregulated power supply will be sufficient and is recommended for most applications because of its simplicity. If a motor with a large inertial load decelerates quickly it will act as an alternator and send voltage back to the drive which then sends it back to the power supply. Because many regulated power supplies feature protection circuitry this may cause the power supply to fault or reset; however, if the supply is unregulated it will simply get absorbed by the filter capacitor.

To make your own power supply you must have three key components: a transformer, bridge rectifier, and filter capacitor. The transformer’s current rating must be sufficient to adequately run all motors that will be run from it using the above current formula. The DC output voltage will be 1.4 times the transformer’s AC voltage rating of the secondary. For example, a 24VAC transformer secondary will provide about 34VDC at the output of the supply. The bridge rectifier’s voltage and current ratings must exceed what the supply will deliver. Finally the minimum filter capacitor size must be calculated. Use the following equation to do this:

(80,000 * I) / V = C

Although SMPS Swithing supplies may be cheaper, they are less rugged than their linear counterpart and harder to fix, especially when components are used that have unknown ratings or value.
Al.

[The New Steve H]

I appreciate that, but if the guy who makes the Kflop likes regulated supplies, who am I to argue?