Stepper power supply confusion ...

Lets assume the following reasonable scenario: KFlop + KStep driving stepper motors with windings that have about 2.8mh inductance.

If I follow the recommendation on the Gecko site, using the formula (32 * SquareRoot( inductance in mh)) I end up with a "recommended" voltage of about 54 volts. Given that the KStep is rated for 48 volts and 48 volts matches nicely with what seems to be available I've decided I would like a "48 volt" power supply.

The two most popular power supply types appear to be "switching" and "linear" [I understand the difference in the two technologies] and while both might be available in "regulated" and "unregulated" [again, I understand the difference] the most available and reasonable priced flavors seem to be "regulated switching" and "unregulated linear".

Which brings me to my "knows just enough to be dangerous" conundrum.

If I go with the unregulated linear supply I must go with one that has a maximum output voltage of 48 volts or less. Regretfully this voltage will happen at zero load and will decrease as load increases ... i.e. when you actually need it. I assume the voltage could theoretically drop to as low as 34 volts under heavy loads, but even if it didn't go to the theoretical minimum it would certainly drop below the optimum voltage we defined earlier.

If I go with a regulated switching supply by definition I can stay at 48 volts under all reasonable load conditions but then must face the switching power supply detractors related to complexity ... i.e. if something goes wrong the entire device usually becomes a door stop ... and their inability to react quickly to the abrupt current needs of the motor driver. It has been suggested that the latter shortcoming could possibly be addressed by adding a large capacitor across the switching power supply output but this adds yet more complexity and adds an additional constraint on the switcher that it be able to address the ultra high inrush currents of an empty capacitor on turn on.

I would appreciate any comments to help me thru would be appreciated.

Thanks,
Arvid

Ok, I'll offer my 2 cents. I'm not an electronics engineer, so maybe someone will chine in who is, but from my experience you'll see no difference on a stepper driven cnc machine. More important will be a good system match (your driver & motors with the power supply's rating). Excessive continuous load draw on a stepper driven system isn't normal either unless you're using a power supply rated lower than your motor/driver can handle for best performance. That's not to say a linear power supply load will never drop to the levels you mention, but how often you do run all 3 (or 4) axis at or near max at the same time? In my experience linear power supply's are normally pretty heavy duty units with much bigger transformers and huge capacitors. I've used both on cnc machines and can tell no difference other than I've had a regulated power supply go bad, but never a linear supply personally. Switched supplies are normally cheaper. There are some high quality SMPS but it seems the majority used for home build cnc machines and those included with stepper "kits" are mass produced in Asia and use cheaper electronic parts. That's not to say they won't work flawlessly for years. They are normally smaller and lighter if cabinet space is an issue. Some electronic equipment is pretty sensitive to fluctuations & ripples and the regulated power supply will do a better job or may be a necessity, but stepper drivers aren't normally prone to these problems. Just don't use any power supply rated over your drivers 48v limit. If you already have one type or the other, and it's rated for your needs... use it!

If you add a large cap to a switching power supply it may not start (if its really large) it will trip the over current when you power it up, just like a short.

Example: I am using several for constant voltage battery charging and there rated for 8 amps, it will lower the voltage a little at ~10 amps and shutdown if it goes any higher, requiring a power down to reset.

Most of the time your stepper motors will not draw full power continuously, it will be mostly surges and if the linear supply is designed right it will recover with only a small drop in voltage, the switching will hold its rated voltage from 0-100% load, most are rated at 5-10% voltage fluctuation.

Switching:
- Need to be 10-20% over your max current draw to avoid tripping the protection.
- Sure there not going to be easy to fix or most likelly a brick if they go bad.
+ Regulated and have more protection than just a fuse. Under, over voltage and current.
+ Smaller and typically dont generate as much heat under a load.
+ Lighter weight, maybe not important on most machines.

Linear:
- No regulation and can have "huge" voltage swings.
- Heavy and bulky vs rated output.
- Cost, more dollar per watt than switching
+ Simple design and low part count but still can be bricked, transformers are not cheap.
+ For steppers they dont need to be higher than max current draw since theres only a fuse for protection.

I went with switching but that shouldnt influence your choice, they both have good and bad points.

I think switching supplies work well with KSTEP. A common reason that switching supplies are not recommended with some stepper drives is because of regenerative braking. In some cases with fast stops, EStop, or even a stall, the mechanical energy must go somewhere. If pushed back into the power supply the voltage can rise and trip out the over voltage circuit requiring a power cycle to recover. Or worse a damaged power supply or stepper driver. Linear supplies usually don't have over voltage protection to trip and usually very large capacitors to absorb energy. But KSTEP has an over voltage clamp circuit that absorbs the energy and avoids this problem.

The power supply should be sized to handle the worst case power situation. Even though you might not often demand a high power I wouldn't want to risk a stall if I happen to do so.

A common misconception with power supplies is to assume the motor currents are the same as the supply current. The average supply current will usually be far less. It is more of a power issue rather than current. When steppers are stopped or moving slowly (even while generating great torque) very little power is consumed. It is a conservation of energy thing. Most any stepper drive's efficiency is very high. Power from supply basically equals power to motor. KFLOP's efficiency is exceptionally high because of very efficient MOSFETs used. (This is why no heat sink is required). So virtually all the supply power goes into the motor as either a few watts of heat or work performed. To do work the motor has to be moving. It is quite hard to force high power into a Stepper Motor because as the speed increases it is usually not possible to keep the current flowing.

My 2cents ;)
HTH

Tom, cgo and coherent,

Thanks for the responses.

I have not purchased a power supply so "the best recommended solution" was what I was going to get.

To be honest, I am still vacillating between the KStep and the 203 Gecko drives. I love the simplicity, cost and excellent support associated with the KStep but like the advertised bullet proof nature ... I am still new at this ... related to the Gecko drives along with the 'if I mess one drive up all I need to replace is one drive' rather than the "all the drives on one board' nature of the KStep.

I have a KFlop and KAnalog that I will be using for another application and would use my existing KFlop in the short term development interim.

I believe one of these motors will best fit the application, the 5 amp and 48 volt requirements ... NEMA23 570oz/in 5A 1/4? Dual Shaft Stepper Motor (KL23H2100-50-8B) | Automation Technology Inc or NEMA23 570oz/in 5A 3/8? Dual Shaft Stepper Motor (KL23H2100-50-4B) | Automation Technology Inc. They are prices the same and have the same dimensions but one is 4 wire and slightly less inductance than the other 8 wire version in parallel mode so it appears to be a wash.

They will be used with precision linear class scales in a closed loop environment. The motors are dual shaft in case I need to add encoders to work effectively in closed loop.

It seems to be down to KStep with a switching power supply because that is what Tom recommends and Gecko with a linear supply as that seems to be what the documentation I have seen on the Gecko site recommends.

Arvid

Hi Arvid,

It sounds like you understand the issues perfectly.

The Geckos are absolutely, hands down, the better drives and work very well with KFLOP.

But with KFLOP you get a smaller package, less wiring, no heat sinking, voltage clamping, Isolated 24V IO, a 0-10V analog output, 16X microstepping, and at almost 1/3 the cost. You could replace the entire KSTEP for almost the same price as a single Gecko. :)

Regards

Tom, the silver tongued devil, has made a compelling argument for the KSTEP and therefore had no choice but to order one.

Thanks to all that provided guidance,
Arvid