Re: [CAD_CAM_EDM_DRO] Stepper motor voltage VS amps.

On Mon, 19 May 2003 10:28:02 -0000, you wrote:

>I am looking and making a small 3 axis CNC mill for wood routing and
>such. I have been surfing through Ebay and it's endless supply of
>varied stepper motors. The question I have is this. I see several
>postings that say for the power source to use 20-25 times the motors
>rated voltage. So if the motor says 2V you should use 40-50V. I
>understand this is done for responce and such but what about the
>amperage? If it says 2V 3A should you supply 40-50 Volts and 3 amps?
>Or do you supply it the same wattage as the motor plate adds up to?
>2V x 3A = 6Watts. This seems to low.
>
>If any one can shed some light on this subject for me I would greatly
>appreciate it.

The most stringent limit is the current rating of the motor.

Motors can be run three ways. Either connect them to the "proper"
voltage, in which case, they draw the proper current, or use another
scheme.

Unfortunately, motors have inductance, so the current cannot change
instantaneously. However, the faster it can change, the more torque
you have at higher RPMS.

One method to do this is to have a higher voltage supply, and insert a
large resistor in the coil lead (CT for unipolar). This limits
current, gives you the effect of a higher voltage, and wastes power.

A second (and preferred) method is the "chopper" driver. This uses a
resistor to sense current, and a comparator to actually turn off the
drivers when the current exceeds the maximum. Circuitry is included
to make the driver turn on when the current falls below the desired
maximum. So the voltage to the motor is actually a series of pulses,
which average out to the proper current, and the higher voltage.

You get much better performance.

Much of this circuitry is built into the driver chips, or can be. A
popular (low end) driver setup is the L297 controller with the L298
driver chip (bipolar, H bridge). The driver is good for roughly 35
volts (being conservative) and 2 amps, I think.

I think that the camtronics drivers are made with this driver chip,
although there might be a different controller chip used... Some
controller chips support microstepping.

Hope that this helps...

Harvey

>
>Thanks
>Scott
>