best stepper driver???????

[John S]

Lee the accepted figure is 2/3 for a parallel wired motor and 1/3 for a series wired motor.
These are figures that Marriss Fremanis at gecko drive came up with and are repeated as law.

So a 3a motor in parallel take 2a and 1a in series.

However I have had clamp ammeters fitted to power supply leads and running a system under rapids and loads i have never seen these figures, they have always been far less.

In fact in many cases the figures have been very small, far smaller than I would have imagined so the 50% is a far more acceptable figure.

One machine I used to convert I used 650 VA transformers at 50 volt AC, in a cost cutting exercise, due to rising copper costs, I dropped to 500 VA and saw no difference, a drop to 400VA still didn't make a difference but going to 330 VA did,

So the drop from 650 VA to 400 VA halved the transformer cost.

[dudz]

Is there any benifit from having a USB instead of Parallel port ? ...I found this 3 Axis CNC USB Card Mach3 200KHz Breakout Board Interface Speed: 48M Win 7 | eBay
I know it is more expense, but is it better quality ? and are there benifits ?

[John S]

Is there any benifit from parallel port or USB...I found this 3 Axis CNC USB Card Mach3 200KHz Breakout Board Interface Speed: 48M Win 7 | eBay
I know it is more expense, but is it better quality ? and are there benifits ?

Not a lot to choose. More expensive and no better than a decent PP card but has the option of being able to use laptops and any USB interfaced computer.

There is an argument that PP port equiped computers are dying out, it is true but there are still 1,000's about for not much money that the argument isn't valid.
I buy Dell DX260 in bulk.



No mine but this is how they are offered, cost about £20 each with an XP license on them.

However we are trialling a couple of the USB boards, first one will be later this month and I expect that the price will drop.
The one shown is from a different company that the ones that developed them,

[dudz]

Ok.... I bought 3 of these , CNC Micro-Stepping Stepper Motor Driver 2M542 Bi-polar 2phase 4.2A Switch | eBay and I bought this ; 3 axis CNC USB Card Mach3 Breakout Board Interface 200KHz windows2000/xp/vista | eBay
I could of bought the cheap one but I really like the idea of using the USB instead of the PP port. I will let you know how I get on. Thanks both of you for all of your help.

[birchy]

Ok.... I bought 3 of these , CNC Micro-Stepping Stepper Motor Driver 2M542 Bi-polar 2phase 4.2A Switch | eBay

What's the delivery time like? I bought some from the same seller last December and they took 7 weeks to arrive. Might of been due to the Christmas post, etc. I am just wondering if they "normally" arrive sooner?

[paulus.v]

Lee the accepted figure is 2/3 for a parallel wired motor and 1/3 for a series wired motor.
These are figures that Marriss Fremanis at gecko drive came up with and are repeated as law.

So a 3a motor in parallel take 2a and 1a in series.

However I have had clamp ammeters fitted to power supply leads and running a system under rapids and loads i have never seen these figures, they have always been far less.

In fact in many cases the figures have been very small, far smaller than I would have imagined so the 50% is a far more acceptable figure.

One machine I used to convert I used 650 VA transformers at 50 volt AC, in a cost cutting exercise, due to rising copper costs, I dropped to 500 VA and saw no difference, a drop to 400VA still didn't make a difference but going to 330 VA did,

So the drop from 650 VA to 400 VA halved the transformer cost.

Shouldn't be taken into account the motor amps/torque vs. the machine weight/working material?

If I put 3A motors on a belt driven PCB milling machine it is impossible to use that power assuming to have idle-current reduction.

I think the 60% rule for the power supply works only with toroidal transformers which have an instantaneous power at least 25% higher than the nominal, and with one PSU for all motors because it is unlikely to have all axis under high loads at the same time.

John, what rated current had the motors and drives for your conversion where you used reliably a 400VA transformer?

[Jonathan]

However I have had clamp ammeters fitted to power supply leads and running a system under rapids and loads i have never seen these figures, they have always been far less.

In fact in many cases the figures have been very small, far smaller than I would have imagined so the 50% is a far more acceptable figure.

Part of the reason for your low readings is the sample frequency of the clamp meters is low relative to the frequency of the current the motors are drawing, so it tends to miss the peaks and generally not be reliable, especially if the meter is not true-rms. An oscilloscope reveals a lot more.

However, it is still true to say the rms current, and therefore power drawn, is substantially lower than the 2/3rds 'rule' implies. It's easy to show why - just look at the motor ratings to work out the power. For example the common 3Nm motors require 4.2A and 2.73V. Power is current multiplied by voltage, so P=4.2*2.73=11.466W. That's per phase though, and it's a two phase motor, so 2 times that which gives 23W. You're driving it from about 70V, so the current is pulsed such that the mean power does not exceed this rated power. Same formula again, so the mean current is I=P/V=23/70=0.33A.

That's why you see a low current (about 0.33A) per motor, not 4.2A or some significant fraction of that. The reason it helps to have the power supply rated for quite a lot more (i.e. 2/3*4.2) than this anyway is that the peak currents can be substantially greater, so you need a transformer capable of delivering that. That's not the only way to solve it though.