[moki]

I intend building the Solsylva 10 x 9 CNC machine. I have 3 Nema 23 stepper 5 lead motors rated as follows:
Rated Voltage = 3.6V
Current 2A
Phase Resistance = 1.8 ohm
Step angle 1.8 deg
Holding Torque 9kg/cm

If I use a 12v power supply I presume I will have to use a dropper resistor. Will this resistor be 12V/2A = 6 ohms to limit the current to 2A and will this resistor be connected to the common lead?
I have a controller from Chromation Systems.

Apologies in advance if I have posted this in the wrong place !

[Toinvd]

Hoi Moki, is this the stepper driver which you have ?
Full 3 Axis CNC Kit with Driver, Stepper Motors, Power Resistors - $89.99 : Chromation Systems, Electronic Kits, LEDs, Microcontrollers, Infinity Mirrors, CNC Driver Kits
http://www.chromationsystems.com/ima..._Kit_setup.png
In that case, they use indeed resistors to limit the current. To do this, you need a total of 6 ohms. The coils are 1.8 ohm, so you need a resistor of 4,2 ohm which can handle at least the power which is about 17 Watt.
If you connect that to the common lead, it will limit the current to your wanted 2A. However if you switch on 2 phases at a time, they will only get just over 1A each.
If you use four resistors of 4,2 ohm connected to each phase lead, you will always send 2A through each activated phase. This will give most power and speed on the stepper motor.
Good luck, Toin

[moki]

Thanks for the response Toin! I only have the stepper motor controller from Chromation systems, not the kit. I bought the motors seperately elsewhere. They are unipolar motors with 5 leads.
Rated Voltage = 3.6V
Current 2A
Phase Resistance = 1.8 ohm
Step angle 1.8 deg
Holding Torque 9kg/cm

Regards

[Toinvd]

Moki, go for it. Try to get some 4.7 ohm 20 watt resistors and connect between common lead and the +12V
When using, check the moter temperature. I like them to be at max about 60 degree ( you can touch, but just not keep them in the hand ) This is a very safe temp for the motors, the specs usualy give far higher values.
If you find the temp to high, increase the resistor value. If the temp is low, and you want more speed / power from the motor, carefully lower the resistor value.
regards, Toin.

[moki]

Toin

My stepper motors have one common lead. So am I correct in saying that for full stepping 2 coils are on at the same time, so the resistor will have to handle 4 amps? So the resistor will have to be 12V-3.6V/4A = 2.1 ohm and the wattage will be 4A x 4A x 2.1ohm = 33.6W. Am I correct?

Regards

[kreutz]

No, the correct way to drive it, using current limiting resistors, was indicated by Toinvd in message #2.

Regards,

kreutz

[moki]

Thanks for pointing that out to me Kreutz. Much appreciated..

Regards

Moki

[Toinvd]

Hoi Moki, unfortunate, you are wrong.
Only half of the time there are two coils on.
If you number the coils A and B on one side and the coils C and D on the other, you should get
step 1 coil A on
step 2 coils A and C on
step 3 coil C on
step 4 coils B and C on
step 5 coil B on
step 6 coils B and D on
step 7 coil D on
step 8 coils D and A on
start with the 4.7ohm and only if you motor is working, and stays cool, you can lowwer the value
The only way to get the same current in the coils when one or two coils are on is when you use four resistors each connected to the coil wires, and no resistor to the common lead.

Regards, Toin.

[moki]

Thanks for that Toin.

This is now starting to make sense! Now I better make a start with the CNC build that I have been putting off for some time now!
Just as a matter of interest how effective are the constant current source circuits for stepper motors? Are they efficient and what about power dissipation? Have you used any of these??

Regards

Moki

[kreutz]

By using current limiting resistors and a high motor power supply voltage (10 to 20 times higher voltage relative to the value of Imax(phase) * R(phase )) you are indeed using a current source to drive the motor.

There is also an active current source circuit which employs power bipolar transistors to drive a stepper motor and is even capable of micro-stepping (look for the J. Newton's Linistepper drive). Normal PWM (and also pulse frequency modulation) current regulation is also a way to implement digital current source.

Where:
Imax(phase) is the maximum phase current as specified in the motor's data sheet.
R(phase) is the phase coil resistance as specified in the data sheet

kreutz

[Toinvd]

hoi Moki,
Yes I use stepper drivers with active current limiting. I use 2M542 stepper drivers for the motors on my mill. I also have some experience on the very cheap Pololu A4988 which are used in many 3D printers. I have driven a 2A stepper motor on these without problems. But these two can't be used with your motors which are unipolar.
The switching current limit prevents the need of a current limit resistor. Al the power lost in that resistor won't be lost in a switching current limiting system. It only uses a much smaller current sensing resistor. The cirquit then switches off the power to the motor when max current is reached, and switches on when the current gets to low. In your case, you would gain about 17 watt on each motor not "waisted" in the resistor.

[kreutz]

The "linisteppers" (Analog 0-2 Amp/phase) I was talking about in my message are unipolar drives, also the Mardus-Kreutz (1- 8 Amp/phase) drives are unipolar. Both are open source.

kreutz

[moki]

Many thanks for all the responses, advice, corrections and education!! I really appreciate the help. You guys are a wonderful bunch. I always learn something new on CNCZONE. All that's left for me to do for now is start building my cnc machine. When? I really don't know. Work takes up a lot of my time. I will definitely be back with more questions sometime later on!

Moki
:banana: