Need Help!- NEWBIE question Stepper Voltage Duration

[albertsteptoe]

Hello

I'm an Electronic Engineer and currently starting my own CNC project.
I'm designing all the control and power circuitry and have a couple of questions to which i would really appreciate some help.

1. Lets say the machine is cutting a line through a piece of wood very slowly.
This means only 1 motor is controlling the movement for that direction.
Does the motor for the other direction and the motor for the cutter up/down axis need to have voltage applied to keep them locked into position? or do you just turn off power to the unused motors completely?
Normally i would have just turned off power to an unused motor but as a lot of vibration can happen with milling, i thought perhaps a voltage would need to be applied to one phase of the unused motors to keep everything locked and steady?
Hope i've explained that right and not sounded too stupid!

2. I'm using three 16A 12v power supplies to drive the motors, mainly because i got them for nothing!
I'm not looking for speed, only running at a frequency of between 2Khz - 4khz, 2000/4000 pulses per second on a 1.8 degree motor.
The motor voltage is rated at 3.8v, 3A per phase and a holding torque of 21Kg/cm.
This motor looks ideal, and will be running on 12v not 3.8v.
My question is to ask your opinion, as i'm not looking for speed and only milling light plastics & wood, will this motor be okay?

Thanks for looking and your help.
AlbertSteptoe

[irving2008]

Hi,

You need to keep power on a stepper to keep it locked in place, else cutting forces will move it. having said that most good stepper drivers will reduce the current in the motor when no steps are being applied to about 50% of the rated current.

The z-axis motor will definitely need to have power applied as it is supporting the weight of the router even if its not in vertical motion.


a 3.8v 2.1Nm motor is probably OK, although you need to work out your forces, inertias and any gearing requirements depending on the leadscrews and working forces/capability required. What governs speed is the coil inductance v resistance... the higher the voltage the faster the rise time of the coil current and the higher the motor torque at speed. All stepper motors lose torque as the speed increases... maximum torque is at or near stationary.

A 3.8V 3A motor has a phase coil resistance of 1.27ohm approx. If you apply 12v to this you will be pushing close to 10A through the coil and either smoke ensues or the motor demagnetises! You must provide current limiting, either by a chopper action or by PWM to keep the rms current in the coil = 3A (the other solution, series resistance, isn't practical on a 3A phase = 2.7ohm, min 50W rated per phase!)

For a good tutorial on stepper motors and drive techniques go to http://www.cs.uiowa.edu/~jones/step/index.html

[CJL5585]

My Recommendation:

Wire the three power supplies in series for 36 Volts DC and 16 Amps of current.

Most stepper motor systems have power supplies rated at or near 20 times the voltage rating of the stepper motor.

This will give you a mid-range power supply for any type stepper system that you decide to build. It will power almost any steppers from the smallest up to the 400 in/oz units. You should also have some decent to very good speed capabilities using a 36 volt power supply.

Put some high quality electrolytic capacitors on the output with a voltage rating of 65 VDC or greater. Total capacitance on the output should be in the 20,000 to 30,000+ uF range.

Hope this helps.

EDIT: Your post did not indicate whether the stepper is uni-polar or bi-polar. In simple terms the steppers should have 8 wires total (bi-polar). A uni-polar stepper has 4 wires.

If the stepper motor has 8 wires, then you have the option of wiring them in series or parallel. There are no options with uni-polar motors.

Jerry

[albertsteptoe]

Many thanks for your fast replies chaps, really appreciated.

The motors are to be powered in Unipolar mode but by using alternate pulsing of two coils to increase the torque.
Not half stepping but full stepping by using two coils, i'm sure you know the method.

The Ballscrews have a 5mm pitch and will cover 10cm/second when working on a pulse frequency of 4 Khz, this will be the fastest speed unless it can cope with higher, its worth a try.

Unfortunately i'm unable to series the power supplies for 36v because the supplies are Switch Mode type, the negatives are linked to the chassis earths.

I just missed out on some 1.8v motors, thay would have been better because my 12v would have gone further, dam!
Not to worry, i've not purchased any motors yet but if i see some lower voltage ones i will grab them.

Thanks for your help.
Albet Steptoe