Larger stepper motor

Hope one (or all) of you folks can give me some guidance.
Situation: I have a 'toy' cnc desk milling machine and the Z axis has
always given me problems by missing steps due to friction or ?????. I hope a stronger motor will cure this.
The present motor is a Nema 23 5-wire uni-polar rated at 24 V. & 0.18 A.
I would like to replace it with a larger Nema 23 6-wire uni-polar motor rated at 24 V. & 2 A.
The motors control uses TIP-120 resistors (can handle 5 A ??) and have Ohmite Power resistors (30 W, 5.1 Ohms) which is feeding approx. 0.29 A to the motors.
Looking at some literature on the TIP-120, my understanding is resistors are used to control the voltage to it, which in turn controls the output current.
So it seems if I want to successfully use the larger motor, I need to modify the circuit to feed the motor it's rated 2 A.
My current setup uses a 28 V. transformer with 42 VDC at the rectifier outputs.
Is this sufficient info to advise what size resistors I should use to get 2 A to the motor? I trust this is possible to do?
Thanks in advance for your help, Thermo
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thermo102 wrote:

The resistors serve to limit the current to the motor. They do this because the inductance of the motor limits the current rise time, which as the step speed increases, limits the current which limits the torque. To increase the rise time, a higher voltage is used, but then the resistors are needed to limit the current so it doesn't burn out the motor. Ideally you want really low voltage steppers, like 5V or less, with a supply voltage of 20+V. You waste a lot in the ballast resistors but you get good torque up to a much higher speed. Better still is to use a PWM circuit to limit the current without burning up the excess as heat in resistors. An even better solution is to use servomotors, once you play with some of those you'll never go back to steppers. SO much more speed, torque, and in most cases much higher resolution.
What you need to do is decrease the value of the resistors to increase the current. You can measure the resistance of one of the windings of the new motors you have and use Ohms law to calculate the additional resistance needed to limit you to 2A at your supply voltage. The rest of your driver should be fine assuming the transistors have adequate heat sinks.
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James,
Got your e-mail. I had really intended to reply to 'the group' but hit the wrong button. Sorry!
However, I just now finished cutting up a humongous heat sink I scavenged from something and now have a heat sink on each of the (4) transisters. I ordered the motor, but have not yet recieved it. Untill I get it and can meaure resistance of the windings I'll leave the electric box open. I suspect I'll need another current limiting resistor, but who knows? With the heat sinks added, maybe getting the correct resistor will be all there is to it.
Thanks again, Thermo

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