Optimal voltage...

[JW1977]

Hi!

I’m about to build a PSU and im thinking about optimal voltage.
According to (32 x √L = Vmax), the optimal voltage for my steppers would be 43 V if they are wired in bipolar parallel which I have intended to. The formula comes from Geckos site but ive seen that all people do not agree. One aspect is modern drives and what the can do to keep heat down and so on.
This is all theory from my side and I wonder if you experienced people :thumsup: would recommend “bumping” that up a bit? I havent given any info about my machine but im curious of how you think in general?


I’m looking at two toroidal transformers.
Second winding of 36V would give about 48V.
Second winding of 40V would give about 52V.

Below is some data from the steppers.

Cheers

24476

[Clive S]

“bumping” that up a bit? I havent given any info about my machine but im curious of how you think in general?


It depends mainly on what drives you have. If your drives can handle 80V then I would bump it up to about 68V.
So two secondary's of 24 or 25 volts. Steppers are designed to get reasonably hot. Hot is when you can't keep your hands on them 40 - 55 degrees is fine.

Some say 60 - 70

[JW1977]

I havent bought the drives yet but im leaning towards AM882 because of the stall detection feature.
Im gonna use slaved axis and from what ive read here that would be a nice feature to have, and a good driver overall.

On the downside is the shipping time from China but i guess i got the time anyway.

A 68V PSU would be much better in the long run. If or maybe when i decide to change things...

[Clive S]

I havent bought the drives yet but im leaning towards AM882 because of the stall detection feature.
Im gonna use slaved axis and from what ive read here that would be a nice feature to have, and a good driver overall.

On the downside is the shipping time from China but i guess i got the time anyway.

A 68V PSU would be much better in the long run. If or maybe when i decide to change things...

You might have trouble getting AM822's now but have you seen these https://www.ebay.co.uk/itm/2DM860H-2-Phase-Stepper-Driver-32-bit-DSP-control-AC30V-80V-DC40V-110VDC/202034368351?_trkparms=aid%3D111001%26algo%3DREC.S EED%26ao%3D1%26asc%3D20160908105057%26meid%3Db5eec 8656b7c4f8f841d5305dcf624b5%26pid%3D100675%26rk%3D 1%26rkt%3D15%26sd%3D202034368351%26itm%3D202034368 351&_trksid=p2481888.c100675.m4236&_trkparms=pageci%3Adfd3a39c-794a-11e8-ae78-74dbd180ae75%7Cparentrq%3A3c6fb9d61640aad90fd959f5 fffa0cbb%7Ciid%3A1

I t will take AC or DC for the power input .

[m_c]

A key thing to remember, is the motor is essentially a constant current device, as current = torque.
The problem is, as motor speed increases, the back EMF increases, meaning in order for the same current to flow through the motor, a higher voltage is required (the higher a motors inductance, the higher the voltage needed).

What the driver is attempting to do, is maintain the same current through the motor, regardless of speed.
So say you have a motor rated at 1A and 1V, while stationary, the driver will alter the output so 1A is flowing through the motor, which means the voltage across that winding will be 1V, regardless of the voltage being supplied to the motor.
However, as the motor starts to move, back EMF comes into play, so the drive then has to increase the voltage to maintain that 1A.

And this is where motor heating comes in to play. As motor speed increases, the more power it has to dissipate (stepper motors are pretty inefficient due to their cogging effect).
So using that example above, at idle it only has to dissipate 1W (1A x 1V) of power, however say you have a 50V supply, as speed increases to the point of reaching the maximum voltage, the motor is theoretically in constant current mode, up to the point it reaches maximum voltage, at which point it's having to dissipate 50W of power. Now once you pass that point, you enter constant power mode, as current will drop of as back EMF rises above the voltage, so the motor never has to dissipate more than 50W.

This is where selecting a supply voltage can be tricky. On something like a small mill running relatively slowly all day, you're not likely to have the motors running fast enough to get past the constant current stage to make use of a higher voltage, so even with the highest voltage possible, the motors are unlikely to see the full voltage.
However, on a router whirring around relatively fast, the power supply is going to be a limiting factor, but you're also likely to get past the constant current stage, and into the constant power stage. At that point it may sound like a good idea from a performance view, but it also means you're likely to be causing excessive motor heating. You can reduce current at this point to reduce heating, but then you lose torque throughout the speed range, so it's got to be a compromise.

It's worth noting Gecko's formula came from lots of experimentation, and is not an exact figure. It is however a very good ball park figure, and much more accurate than the multiple the motor voltage by a random figure formula.
Also, when I say dissipate power above, some will end up in kinetic energy (aka movement), while the rest will end up in heat (with a very minor amount making noise). As speed increases, the amount lost to heat increases, due to the inefficiencies caused by the cogging.

[JW1977]

You might have trouble getting AM822's now but have you seen these https://www.ebay.co.uk/itm/2DM860H-2-Phase-Stepper-Driver-32-bit-DSP-control-AC30V-80V-DC40V-110VDC/202034368351?_trkparms=aid%3D111001%26algo%3DREC.S EED%26ao%3D1%26asc%3D20160908105057%26meid%3Db5eec 8656b7c4f8f841d5305dcf624b5%26pid%3D100675%26rk%3D 1%26rkt%3D15%26sd%3D202034368351%26itm%3D202034368 351&_trksid=p2481888.c100675.m4236&_trkparms=pageci%3Adfd3a39c-794a-11e8-ae78-74dbd180ae75%7Cparentrq%3A3c6fb9d61640aad90fd959f5 fffa0cbb%7Ciid%3A1

I t will take AC or DC for the power input .
Cheap to, but no stall detection.
Static current automatically halved. That’s interesting regarding to what m_c is describing below.
What are the benefits of them being able to run on AC?
The thought to go closed loop crossed my mind. I’ve only bought 3 steppers so far.

[JW1977]

A key thing to remember, is the motor is essentially a constant current device, as current = torque.
The problem is, as motor speed increases, the back EMF increases, meaning in order for the same current to flow through the motor, a higher voltage is required (the higher a motors inductance, the higher the voltage needed).

What the driver is attempting to do, is maintain the same current through the motor, regardless of speed.
So say you have a motor rated at 1A and 1V, while stationary, the driver will alter the output so 1A is flowing through the motor, which means the voltage across that winding will be 1V, regardless of the voltage being supplied to the motor.
However, as the motor starts to move, back EMF comes into play, so the drive then has to increase the voltage to maintain that 1A.

And this is where motor heating comes in to play. As motor speed increases, the more power it has to dissipate (stepper motors are pretty inefficient due to their cogging effect).
So using that example above, at idle it only has to dissipate 1W (1A x 1V) of power, however say you have a 50V supply, as speed increases to the point of reaching the maximum voltage, the motor is theoretically in constant current mode, up to the point it reaches maximum voltage, at which point it's having to dissipate 50W of power. Now once you pass that point, you enter constant power mode, as current will drop of as back EMF rises above the voltage, so the motor never has to dissipate more than 50W.

This is where selecting a supply voltage can be tricky. On something like a small mill running relatively slowly all day, you're not likely to have the motors running fast enough to get past the constant current stage to make use of a higher voltage, so even with the highest voltage possible, the motors are unlikely to see the full voltage.
However, on a router whirring around relatively fast, the power supply is going to be a limiting factor, but you're also likely to get past the constant current stage, and into the constant power stage. At that point it may sound like a good idea from a performance view, but it also means you're likely to be causing excessive motor heating. You can reduce current at this point to reduce heating, but then you lose torque throughout the speed range, so it's got to be a compromise.

It's worth noting Gecko's formula came from lots of experimentation, and is not an exact figure. It is however a very good ball park figure, and much more accurate than the multiple the motor voltage by a random figure formula.
Also, when I say dissipate power above, some will end up in kinetic energy (aka movement), while the rest will end up in heat (with a very minor amount making noise). As speed increases, the amount lost to heat increases, due to the inefficiencies caused by the cogging.

Thanks! I’ve read what you wrote a couple of times and I understand more now.
Naturally its always about finding a good compromise for what you want the machine to do.
The thought of closed loop appeals even more.

[Clive S]

Cheap to, but no stall detection.
Static current automatically halved. That’s interesting regarding to what m_c is describing below.
What are the benefits of them being able to run on AC?
The thought to go closed loop crossed my mind. I’ve only bought 3 steppers so far.

Using AC means you don't have to buy any caps or rectifier etc.
Manual here:- http://www.steppermotorcanada.ca/dm860h.pdf
I believe that stall detection only works above 300 rpm on the AM822.

I have three of these but not tested them yet but they look fine.

[john swift]

looking at the supply voltages for the AM822

the stepper driver has a simple linear power supply built in side
(bridge rectifier and capacitor)
so you can either connect it to an AC transformer or
a DC power supply


if you multiply the AC input voltages by root 2 (1.4142)
and subtract 2V DC to allow for the volt drop across the bridge rectifier

the input of 24V-80V AC will give you a DC voltage
very close to the DC input specified - 30V-110V DC

this has an advantage you cannot connect the DC supply the wrong way round !!!

John

[m_c]

Something that I've often wondered about the AC capable drives, is just how well can they handle fast deceleration?

A DC drive will dump the regenerated energy back into the supply, but an AC drive doesn't have that option. There are ways around the problem, but these drives aren't usually large enough to have much heat dissipation.

[JAZZCNC]

looking at the supply voltages for the AM822 the stepper driver has a simple linear power supply built in side
(bridge rectifier and capacitor) so you can either connect it to an AC transformer or
a DC power supply

Only the "H" series have this option. the common AM882 is only DC.


Something that I've often wondered about the AC capable drives, is just how well can they handle fast deceleration?

The cheap ones with AC/DC options don't handle it very well at all in my experience but better ones which often only have AC option I've never had a problem with. Much prefer to use DC supply with Caps on a fast router with decent weight gantry.

[waldeniojogos]

Only the "H" series have this option. the common AM882 is only DC.



The cheap ones with AC/DC options don't handle it very well at all in my experience but better ones which often only have AC option I've never had a problem with. Much prefer to use DC supply with Caps on a fast router with decent weight gantry.

Jazz, could you help me with my powerbox project, I got the same Lichuan Drivers and Motors as you did, but I am having some trouble finding some help on the installation. Could you share some of you project? You would help me a lot. Have you ever shared your project with anyone? I could not find anything here. my e-mail is [email protected] I really hope that you could help me with this! Thank you so much! And I will be looking forward to your reply!