Motor controller bipolar of unipolar?

[DimiOrla]

Hello everybody,
I planning to build a small cnc for PCB machining, and use parts that I already I have.
But I need some help with my motor controller.

My motors have 6 wire and I was planning to use them as bipolar and drive them with the 3977 or the L297-8 combination. My next choice would be tachus42's 3 Channel Discrete unipolar board
From what I have read on the wed bipolar motors have more torque than unipolar. But the graphic of the manufacturer confused me (see attachments.)

My next question is, would a 20v/4.5A/90W power supply be strong enough to drive three of them?

Thanks in advance for your help.
Dimitris.

[DennisCNC]

Those motors a 48 steps per rev, that will not give you good resolution, unless you have very low lead screws. And they are very small and might not have enough turque to even turn a screw. Get your self three of these:
http://cgi.ebay.com/3-CNC-Stepper-Mo...QQcmdZViewItem
and they will make your machine move very smooth.

[DimiOrla]

Thanks for you answer Dennis.

I am planning to use then with a 1:5 gear ratio and a 8mm screw with 1.25mm pits.
So it is more like (48*5)/1,25=0,005mm. The machine its self will be rather small, something like 60x60x20 cm.

If I am reading the graphic correctly the motors will give in unipolar 2 Nm and bipolar 1,5 Nm that is 212 and 242 oz. Is that correct?

P.S. This machine will de just for learning purposes.

[fyffe555]

Originally Posted by DimiOrla Thanks for you answer Dennis. If I am reading the graphic correctly the motors will give in unipolar 2 Nm and bipolar 1,5 Nm that is 212 and 242 oz. Is that correct?

The graphic says the max torque is 2000 and the scale is N.M. x10 -4 so thats 0.2NM or ~28ozin.

These are very small motors.

[DimiOrla]

Thanks fyffe555,

So the unipolars are better at 0,2 instead of the bipolar at 0,15. That is weird it goes against all I have read.
If I gear them down (0.2*5=1Nm), I think with minimal friction they will be good enough for one kilo of load.

P.S. Under 24v I don’t need any limiting resistors write, how about the amper’s (4.5) are they ok for three motors?

[LZ1TWB]

Hello DimiOrla,
I could tell you some tips about this motor.
First of all it is unipolar, which means it has twice as much wire in itself with twice lower cross section. Unipolar motors are easy to control as you need only 4 transistors to turn each of the 4 coils to ground / two by two coils are connected with a centre to + voltage/ Driving a unipolar motor in bipolar mode is not good as only one of the windings is used - you cannot put much current in it and the torque is then less. It is said that a bipolar motor compared with the same size unipolar has more torque because you have only two coils with thicker wire that can conduct more current before getting too hot.
DennisCNC said that it is low resolution motor 7.5 degrees step - try using some like 1.8 as they are more common, which driving it in half step gives a 0.9 step. If you intend to use a gear you have to spin it quite fast in order to have some good speed of travel. 24V is high for a cnc motor and if you drive it with a 24V supply you will certainly get nothing from it as current cannot get high enough to maintain the desired torque /coil induction issues/. It will be limited to certain rpm. Stepper motors are mostly used with a voltage many times bigger than the specified, but of course some sort of current limiter should be placed. Most steppers have a low voltage specified 2-3Volts but are normally used at 24 for example. Higher voltage means more torque at high speeds.
There are so many things that could be said about steppers, just ask if you need something, I see you are going to learn in cnc stuff, keep going, it will become very interesting to you.
I remember my first cut on my mill, I've drawn half a ball and machined it in foam and was pretty happy then.

Greetings,
Todor

P.P. - someone correct me if I've made a mistake somewhere.

[DimiOrla]

Thanks LZ1TWB this makes more sense now.

[brijkalani]

Hi,
The explaination you gave is really good and helpful. I have a querry to which you might be able to help. I am not building a cnc but using steppers for other automation purpose. i have got NEMA 23stepper motor (1.8 degree, 2.0 A) which has 5V written on the datasheet. Now the driver i have got is to be run at min 20V. I did connect to a 20v, 5.0A power supply and interfaced with my pc (c progam) but the rpm was too slow.

Can you guide me what should i do to increase the speed of the motor. i thought it was the clock speed of the computer. But i guess it should be enough to turn the motor at greater speeds.

or is it some thing else i need to change?

As the motor ran with 20V 5A supply on the driver I guess power supply should not create a problem. if you think i am doing in wrong manner please let me know.

Thanks in advance.

Originally Posted by LZ1TWB Hello DimiOrla, I could tell you some tips about this motor. First of all it is unipolar, which means it has twice as much wire in itself with twice lower cross section. Unipolar motors are easy to control as you need only 4 transistors to turn each of the 4 coils to ground / two by two coils are connected with a centre to + voltage/ Driving a unipolar motor in bipolar mode is not good as only one of the windings is used - you cannot put much current in it and the torque is then less. It is said that a bipolar motor compared with the same size unipolar has more torque because you have only two coils with thicker wire that can conduct more current before getting too hot. DennisCNC said that it is low resolution motor 7.5 degrees step - try using some like 1.8 as they are more common, which driving it in half step gives a 0.9 step. If you intend to use a gear you have to spin it quite fast in order to have some good speed of travel. 24V is high for a cnc motor and if you drive it with a 24V supply you will certainly get nothing from it as current cannot get high enough to maintain the desired torque /coil induction issues/. It will be limited to certain rpm. Stepper motors are mostly used with a voltage many times bigger than the specified, but of course some sort of current limiter should be placed. Most steppers have a low voltage specified 2-3Volts but are normally used at 24 for example. Higher voltage means more torque at high speeds. There are so many things that could be said about steppers, just ask if you need something, I see you are going to learn in cnc stuff, keep going, it will become very interesting to you. I remember my first cut on my mill, I've drawn half a ball and machined it in foam and was pretty happy then. Greetings, Todor P.P. - someone correct me if I've made a mistake somewhere.

[kreutz]

Originally Posted by brijkalani Hi, The explaination you gave is really good and helpful. I have a querry to which you might be able to help. I am not building a cnc but using steppers for other automation purpose. i have got NEMA 23stepper motor (1.8 degree, 2.0 A) which has 5V written on the datasheet. Now the driver i have got is to be run at min 20V. I did connect to a 20v, 5.0A power supply and interfaced with my pc (c progam) but the rpm was too slow. Can you guide me what should i do to increase the speed of the motor. i thought it was the clock speed of the computer. But i guess it should be enough to turn the motor at greater speeds. or is it some thing else i need to change? As the motor ran with 20V 5A supply on the driver I guess power supply should not create a problem. if you think i am doing in wrong manner please let me know. Thanks in advance.

What kind of driver are you using, chopper or Resistor current limit? If you are driving your motor without any current limit you are going ruin them, and probably your power supply too.

[brijkalani]

Originally Posted by kreutz What kind of driver are you using, chopper or Resistor current limit? If you are driving your motor without any current limit you are going ruin them, and probably your power supply too.

Thanks for your response.

I am using a microstepping chopper driver MSD542
http://www.motioncontrolproducts.co....0datasheet.pdf

The stepper motor is
http://motioncontrolproducts.co.uk/p...products_id/90


The driver is connected to the pc via a velleman k8055 USB interface board. It runs quite slow when connecting to one of the digital outputs and running with a simple looping c program.

I also tried to connect with the PWM output from the card. Again its running at very high speed and not able to control the speed with the help of program.

Thanks.

[kreutz]

Originally Posted by brijkalani Thanks for your response. I am using a microstepping chopper driver MSD542 http://www.motioncontrolproducts.co....0datasheet.pdf The stepper motor is http://motioncontrolproducts.co.uk/p...products_id/90 The driver is connected to the pc via a velleman k8055 USB interface board. It runs quite slow when connecting to one of the digital outputs and running with a simple looping c program. I also tried to connect with the PWM output from the card. Again its running at very high speed and not able to control the speed with the help of program. Thanks.

I have the same motor, it is the motor I use for testing our "Mardus-Kreutz" controller. The inductance on this motor is so high that it will lose torque at less than 200 r.p.m. (at 1/10 microstep), due to the L/R effect, with a 24 volt power supply (that is the reason why I chose that motor for testing) You will need to get a higher voltage power supply (more than 35 volts) or a controller with midband resonance compensation for higher speeds.

What is the maximum step rate you get from the USB interface board?. What is the step pulse width? What acceleration rate?

[OCNC]

Originally Posted by DimiOrla Thanks fyffe555, So the unipolars are better at 0,2 instead of the bipolar at 0,15. That is weird it goes against all I have read. If I gear them down (0.2*5=1Nm), I think with minimal friction they will be good enough for one kilo of load. P.S. Under 24v I don’t need any limiting resistors write, how about the amper’s (4.5) are they ok for three motors?

At 24V, 13 ohms each stepper is drawing .8A so for three steppers you'll need at least 2.4A. If you connect an ammeter to measure current in the steppers vs. speed you'll quickly see that your optimum range will be in the 100 to 200 pps range which for a 48 step motor is between 3 and 4 revs per sec. The trick to getting the most performance out of small motors is keeping the amps as high as possible while using the mechanicals to get the best speed. There is a definite balanced or 'tuned point' to this that can best be arrived at by the process of constructing a simple prototype. If you over drive the motors by increasing the voltage above the rated value you need to add current limiting to protect the motor coils and this in turn necessitates improving proportionately the heat dissipation for the drive circuitry (the extra voltage is converted to heat in the power transistors). This process is ultimately limited but does provide enough gain to be appropriate to a point. As a design process I would first find out what rev per sec you can achieve at .8A current and then get a feel for how much below .8A you can go before the torque falls off. Then knowing the intended rps you can chose a screw thread or gearing that will give you some speed or torque multiplication as the case may be. Then it's just a matter of keeping all of your friction and inertial loads as small as possible. I've also found that it's important to have a clean pulse source. I've been using Linistepper drivers with 1A rated motors (driving them at 1A) with the voltage set at 3 times the rated voltage (3 x 6V for 18V). I'm using Torque Spline lead screws that give me 1 inch of travel per rev. My software has been 'Master 5' demo version. In 'half step mode' at a travel rate of about 90 ipm this seems to give about 30 lbs. of linear force on the drive nut. Note that I'm working at 1.5 rev per sec. I can do no load rapids at about 2.25 rev per sec.

Chris