Can/should I power my DM860 (non-H) stepper drive with 57VAC? - Page 1
Forum section: electronics / general

author: Langdon
author_rank: Frequent Contributor
timestamp: March 15, 2025, 08:53:48 pm
content: It has an integrated rectifier and can be powered with AC.It says max 50VAC but the rectifier cap is 100V and it says max 80VDC.57VAC rectifies to about 80VDC at no load.Is 28.5VAC a better choice?

author: Kean
author_rank: Unknown
timestamp: March 15, 2025, 09:40:26 pm
content: As discussed in your several prior threads, there is no need for maximum voltage. It may help to get faster rapid (G00) movements, but it isn't likely to be necessary for milling ops.Just try to get the thing running with the original 28.5VAC supply and stop blowing up stepper drivers.

author: Ian.M
author_rank: Super Contributor
timestamp: March 15, 2025, 09:59:46 pm
content: Also you must consider the transformer regulation factor - its secondary voltage is almost invariably specified to be at nominal voltage at its rated load current, and rises when lightly loaded or unloaded. A 5% rise wouldn't be exceptional, so if its nom. 57V AC it could easily give you closer to 85V DC before the steppers are enabled. You also need to consider the impact of high mains supply voltage (so-called 'high line' conditions) as it may rise as much as 5.8% under extreme operating conditions causing the secondary voltage to also rise.IMHO it would be extremely unwise to exceed 90% of the drive's rated max. AC input voltage - you need the margin for the worst case combo of light load and high line conditions.

author: Langdon
author_rank: Frequent Contributor
timestamp: March 16, 2025, 01:04:50 pm
content: 57VAC is no-load voltage.I'm not actually sure what the transformer's rated output voltage is; there is no sticker or identification on the transformer at all.I don't know who made the transformer, where it was made (I assume Canada), how much power it can supply, or anything about it at all.All I know is it has 3 independent windings that reach 28.5VAC with almost no load.The transformer must be pretty beefy tho; the original stepper drivers (which broke) had 2 L6203 drivers each and were so inefficient they were essentially space-heaters!

author: Ian.M
author_rank: Super Contributor
timestamp: March 16, 2025, 03:15:29 pm
content: So you don't have to worry about the transformer's regulation factor. However 'high mains' is still a concern. Even a few percent variation will put your DC bus out of spec.The 100V rated caps are a false clue as there is no standard capacitor voltage rating between 63V and 100V, and its also a bad idea to run capacitors at their full rated voltage as that typically shortens their life severely. The true limit will be the output transistors of the motor drive's H-bridge, and even if you had their specs, you don't have the data to assess why the manufacturer specced the DC bus limit at 80V. If you are lucky, the transformer may have a tapped primary to allow it to be adjusted for line voltage above or below nominal, and if that is the case, connect its highest input tap to drop the output voltage a bit,

author: thm_w
author_rank: Super Contributor
timestamp: March 18, 2025, 09:17:16 pm
content: Not sure if we determined each winding was originally driving one axis per winding?But if it was, and you had 5A per winding say, and a 4A stepper motor.If you tried to series three windings, and run three motors at a time, now you've got 12A pulled from a 5A rated winding.I'm sure its overbuilt to hell, but something to watch out for. You can probably load down the winding with the stepper motor and get a rough idea of what is capable of.https://www.eevblog.com/forum/mechanical-engineering/should-i-just-suck-it-up-and-buy-a-good-stepper-driver/https://www.eevblog.com/forum/mechanical-engineering/why-does-this-knockoff-dm860h-have-only-half-the-flyback-diodes/https://www.eevblog.com/forum/mechanical-engineering/should-i-make-an-external-pcb-with-flayback-diodes-a-good-idea/

author: Langdon
author_rank: Frequent Contributor
timestamp: March 18, 2025, 10:07:53 pm
content: The transformer has 3 28.5VAC windings.They were originally configured in parallel 28.5VAC to power 3 drivers via a rectifier.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 08, 2025, 08:37:17 am
content: During braking or deceleration, the motor delivers power back into the stepper driver. This energy is rectified by the freewheel diodes in the end transistors, and then goes to the DC bus. As a result, the voltage on the DC bus increases. As long as inertia and acceleration is "moderate", this is not such a big problem. The usual precaution is to stay well below the maximum for the motor voltage. When there is a lot of inertia and/or quick deceleration (i.e. hard braking), then extra measures are needed. VFD's often have an option to add an external braking resistor. This resistor is internally connected to the DC bus, and whenever the DC voltage on that bus gets too high, the braking resistor is turned on to bleed excessive voltage. VFD's normally also have an extra precaution built in. If the DC bus voltage gets too high, (even with a braking resistor) then the motor is disconnected (i.e. no current through the motor), and as a result the motor is freewheeling (i.e. not braking).For stepper motor controllers, the protections are usually a bit less, because the motors are supposed to be well kept within their capabilities. But quality of motors and motor drivers varies. The "cheap chinese things" are probably best used upto about 70% of their "rated" capabilities (but this is guesswork, and quality of chinese stuff varies a lot.

author: Langdon
author_rank: Frequent Contributor
timestamp: April 14, 2025, 02:12:26 pm
content: They are good drivers.I'm 99.9% sure the H and non-H drivers are actually identical, save for the fan and cap voltages.The H has 200V caps, the non-H has 100V.But otherwise, they are identical.

author: eugene
author_rank: Frequent Contributor
timestamp: April 15, 2025, 03:30:08 pm
content: Quote from: Langdon on March 18, 2025, 10:07:53 pmAnd you never said the motors didn't run fine that way, so I'll assume they did.But now you want to run at twice the voltage but one third the available current. The only reason i can think of to do that is if you want to have a power supply that's current limited before the drivers blow up. Is that your goal?

author: Langdon
author_rank: Frequent Contributor
timestamp: April 15, 2025, 03:38:52 pm
content: The original drivers did not work at all.They were 100% broken.And very inefficient!

author: eugene
author_rank: Frequent Contributor
timestamp: April 16, 2025, 03:30:00 pm
content: Quote from: Langdon on April 15, 2025, 03:38:52 pmYou're not paying attention. What was wrong with the power supply that makes you want to change it? Don't bother answering; there is no good answer.

author: Langdon
author_rank: Frequent Contributor
timestamp: April 16, 2025, 04:05:48 pm
content: I didn't change it.I'm using the original transformer.The drivers i bought have their own rectifier.I got rid of the original rectifier board.

author: eugene
author_rank: Frequent Contributor
timestamp: April 16, 2025, 04:35:44 pm
content: 1. The power supply consists in part of a transformer with three identical secondary windings.2. The power supply originally had all three secondary windings in parallel.3. You want tochangethe power supply by using two of the windings in series for twice the voltage, but only one third the available current of three winding is parallel. (No idea what you intend to do with the third winding.)How is that notchangingthe power supply? One of us appears to be missing the point in an important way, and I don't think it's me.

author: thm_w
author_rank: Super Contributor
timestamp: April 16, 2025, 09:13:46 pm
content: Quote from: eugene on April 16, 2025, 04:35:44 pmYes I already explained this above.

author: Langdon
author_rank: Frequent Contributor
timestamp: April 17, 2025, 12:38:00 pm
content: That barely counts as change.The drivers work better with a higher voltage.The third winding is in parallel with another winding.And it powers a third driver with 28.5 VAC.

author: eugene
author_rank: Frequent Contributor
timestamp: April 17, 2025, 05:38:24 pm
content: Quote from: thm_w on April 16, 2025, 09:13:46 pmI read and understood your post. At the time of writing, I was still under the naive hope that the OP could be made to understand too. We've done all that we can.