Using ATX PSU : Someone please check my calculations

[jeroenvw]

Dear fellow CNC-ers,

I am currently building my DIY CNC machine, and I made some calculations regarding power supplies for my Keling KLHM276-30-8B steppers + Stepperworld.com FET4 driver board.

Stepper Specs (unipolar config) :
Torque : 1.4 Nm (that's 276 oz/in am I right?)
Current / Phase : 3A
Phase resistance : 0.92
Voltage : 2.76v
Phase Inductance : 2.2

Calculation (Current):
Stepper current * nr of steppers * (2/3) = PSU current
3A * 4 * (2/3) = 8A

I would like to use two PC PSU's. The voltage will be 24v then. I know steppers work best at 20 times their
'normal' voltage, but I would like to try this first and see what happens. I am having a hard time finding a 48v power supply here in The Netherlands (Holland).

Calculation (Resistors):
Can someone please show me how to calculate the resistor correctly?

Help is really appreciated!
Thanks!

Jeroen

[jeroenvw]

Kick

[ger21]

Originally Posted by jeroenvw Calculation (Current): Stepper current * nr of steppers * (2/3) = PSU current 3A * 4 * (2/3) = 8A

Where does the 2/3 come from?

Not entirely sure, but I think that with a Stepperworld board, you need 3a*2*4 = 24Amps. You might want to ask Stepperworld what they say.

[jeroenvw]

Ger21, thanks for the reply.
I found a nice 28v 36Amps power supply on ebay, and won't be using a ATX PC power supply.

[NC Cams]

Check these threads out before you totally vacate the idea of using ATX power supplies:

Daisychaining ATX power supplies:

http://www.cnczone.com/forums/showth...015#post142015

http://forums.bit-tech.net/showthread.php?t=108208

Follow the instructions and DO NOT GET CREATIVE!!!!

[dertsap]

the stepper world board is 50v max , try 24v first , running at 48v is pushing a little close to the edge , with your fet board you should have recieved a cd and on it is a resistor calculator
, the min supply you need is 3amp/ phase , don t understand how you came to 8
don t forget you can run dual resistors , this way you can use smaller and cheaper resistors

[dertsap]

best thing to do once you get up and running is get yourself a heavy fan to keep the resistors and board cool ,your going to have some serious heat radiating off those resistors , by right the board should run cool , you will have less chance of any system failure if you keep everything relatively cool

[Torchhead]

Most of your power will go up in heat. The series resistor limits the current (and thus the voltage) across the coils. So if you increase the raw voltage then you have to increase the resistor value to hold the current to below the motor spec. You can use ohms law to get close to the resistor value (R) so if you want to hold the current to 3A the formula is R = E [applied voltage]/3 [amps]. Power dissipation is E^2/R. [E sqr/ R in ohms]. To determine the voltage drop across each component use the resistance of the motor in series with the current limit. So you are only increasing the voltage across the motor (the part that counts) by the inverse ratio of the resistor values.
For example if your motor has a resistance or 1 ohms and you need a 7 ohm resistor to keep the current to 3A the raw 24v will appear across the the series of the two values in inverse porportion to their values. In the above example you will see a ratio of about 7:1 so 21 volts gets dropped by the resistor and 3 across the motor! Power is 63 watts across the resistor!

Play around with voltage and resistor values for a while and you will see two things: It takes a large change in raw voltage to get an increase in real speed and torque since everytime you raise the raw voltage you have to raise the value and wattage of the series resistor. Only a portion of the voltage gets to the motor coil.

For this reason LR type drives (those using big resistors to limit current) are no longer used in any design. Chopper drives like the HobbyCNC or Xylotex (or higher priced and current rated Gecko's) will drive those same motors at many times the RPM you can get from the LR drive and with a power supply of about 5 to 6A. A chopper drive puts all of the power across the motor so increasing the raw voltage increases the RPM.

Steppers are current devices and the voltage is used to charge the coil. The chopper drive cuts off the voltage when the coil reaches it's full charge. The charge RATE of the coil is determined by the applied voltage. The faster you can charge a coil the faster you can spin the motor.

Tom Caudle
www.CandCNC.com

[dertsap]

It takes a large change in raw voltage to get an increase in real speed and torque .

For this reason LR type drives (those using big resistors to limit current) are no longer used in any design.

Tom Caudle
www.CandCNC.com[/QUOTE]

i disagree , every time i doubled the power the speed doubled on the fet board i was runnig !!
also there are many designs that still use resistors ,HobbyCNC or Xylotex , and gecko aren t the only drivers out there

the problem with HobbyCNC or Xylotex is they are low amperage drivers ,in my opinion too low , good for small projects , gecko is the way to go with chopper drivers
while chopper drivers are the top performers ,the fet board works very well ,
i have both gecko and the fet 3 and i am pleased with both

[pastera]

Dertsap

P = V^2/R, so doubling the voltage is 4x the power!
By doubling the power you would only get SQR(2) or 1.414x the voltage (and torque)

Aaron