PSU Output needed per motor???

[GrahamIT]

Having read just about every post since the start of time, my eyeballs are almost square but I've been unable to find the initial info I really need to get my CNC router project underway. Perhaps someone would like to help me.

I've just bought 3 Pac-Sci NEMA 23 motors from eBay that are brand new. I've been in contact with the manufacturers outlet here in the UK and they said that they were a high power for their size type with 140 oz in. holding torque. Pleanty of power for my little Trend router, all it will be doing is cutting and shaping MDF for dolls houses and the like.

Written on them is the following,

Powermax II M21NSHS-LNN-NS-02 3.5A BIPOLAR PARALLEL

Driving just one motor with a Bipolar chopper driver at 35V would the current required from the power supply be 3.5A max or is the rating per phase so I would need a 35V 7A power supply.

[ger21]

With a Gecko or Xylotex, you would need 2/3 of 3.5a per motor, which would be about 7a total for 3 motors. Other drivers would probably be different.

[GrahamIT]

Accepting the fact that different driver board chipsets absorb different amounts of power, why would a stepper motor only take two thirds it's rated current under full load. I'm having a little trouble understanding that.

It was my understanding that if there was not enough current available to power the motor to it's maximum current rating, then the motor would not be able to develop it's maximum torque and possibly start to loose steps as a result.

The driver chipset of my design is an L297 L298 pairing, with the L298 section of the driver board doubled up to provide a constant current capability of 4A @ 100% duty cycle. Which would leave me with pleanty of leeway with regard to power output.

[jeffs555]

The simple answer is that a chopper driver is kind of like switching power supply. It takes a higher voltage lower current input from the PSU and converts it to a lower voltage higher current to the motor.

[ger21]

Like I said, that's ONLY with a Gecko or Xylotex. Read the white papers at http://www.geckodrive.com for more info.

[lt paul]

the l297 is a non-microstepping bipolar chopper controller. though the l297 does not do the power handling it uses a current sensor through a pair of resistors to generate a chopper signal.

[GrahamIT]

It paul

The L297 will do half steps and for my initial steps (pun intended) into the world of CNC it'll do for now, when I eventually make the machine I really want, then that'll be the time to take micro steps. Hopefully by then and with a great deal of help from this and other like minded news groups, I'll be able to afford to use high speed servo motors like the grown-ups do. But until then my toys will just have to use NEMA 23's with torque multipliers and low feed rates.

[GrahamIT]

ger 21

Thanks for the info, I've read the white paper and everthing is starting to make a little more sence now. I'll start designing my power supply tonight...

[GrahamIT]

jeffs555

I thought the idea of using a PSU in the order of 25 times the voltage rating of the motor was because current lags voltage in a stepper, so in order to get the current into the motor as fast as possible you forced it in with a higher voltage until your current limiter clamped the pipe?

[jeffs555]

Yes, that is the main reason you use a high voltage supply. But the side effect with a chopper drive is that the duty cycle of the current is far less than 100%, so the average DC current from the power supply is less than the phase currents in the motor.

[fyffe555]

Graham,

If it helps, I 'm using those motors and like Pacsci generally. They are usually most effective wired bipolar parallel but as you know they require a bigger power supply in that config and more amps per phase than the other two wiring schemes but develop more torque too. I've run mine up to 48v but found no significant advantage on my small machine over 35-38vdc ( 24vac recitified), which makes sourcing transformers and drivers much cheaper.

Ger21 has it right, if you're going to use one powersupply for all three motors then size at 2/3x max load per motor or 7A and as high a voltage as your driver will stand if you're going to use half

A note on chopper drivers, in the simplest form the driver uses a resistor to measure the current drawn by each motor phase when its powered. It compares the drawn current to the max current set by the driver. High voltages, several times the supposed motor rating are used so the driver can get the phase current up to the specified maximum as soon as possible. Once the max current is reached the chopper starts to 'chop' the supply on and off so the effective max current is maintained. At this point the effective voltage should be closer to the motors rated. A higher supply voltage means the max current is reached quicker. so developing power sooner and for longer in each phase switch on. Chopper drives also often do some clever stuff with the motors back EMF too, passing it back to the supply or dumping it to a heatsink.

You need to set the maximum current delivered to the driver at the rated max current. Motors don't much care what voltage they get (within reason or 10-25x rated dependant on brand) but max current isn't something you want to ignore. If you're doubling L298's then you're a half an amp high and because the voltage is higher than rated the motor could draw the drivers 4a maximum. I'd look to see how you might limit the current to 3.5a or watch your motor temps closely.

As Ger21 says, the Gecko doc's are a huge and much appreciated guide to this stuff.

rgrds

Andrew

[GrahamIT]

Hi Andrew

Thanks for that. I've read all the Gecko files and most of the posts on this board too. I must say it's been a blast getting the little grey cells working again.

I've not done any electronic design work for 25 years, but it's coming back to me, slowly...

Thinking about the over current motor heating thing, I could design an automatic current limiter circuit with a motor mounted thermistor , so each motor only ever just reached it's optimum operating temperature, but I think that's going a bit too far. I'll just run each motor on it's own with an amp clamp at full load and make sure the current limiting trim pot is in the correct position.

Lets see if I've got this bit right. While decelerating, a stepper motor will act as a generator and charge the smoothing caps, therby lowering the current drain on the PSU transformer, if that's the case and if all three motors decelerated at the same time, the voltage developed at these caps could make them blow. So I should go for caps at least 3 times the normal operating voltage and use high temperature ones just to be on the safe side I think.

Any takers?