Re: [CAD_CAM_EDM_DRO] Re: STEPPER MTR VOLTAGE SUPPLY HELP!!!

cnc6805 wrote:

>
>
> How they regulate the current I have no idea. They use hexfets? I
>dont see a large resistor on the board. This is one board consisting
>of 4 motion controllers integrated together. What is L-R type? Can
>you expain the differneces between L-R verses chopper style stepper
>motor drives? How can one run at higher voltages and not the other?
>
>

The classic unipolar L-R drive from the 1960's used a motor with two
center-tapped windings. The center taps were each connected to a
huge power resistor and then to the positive motor supply. One
NPN power transistor was attached to each end of each phase winding.
When a transistor turned on, the current ramped up in an exponential
curve based on the voltage, resistance and inductance. Once the
current rose to a certain value, the resistance was such that it
balanced the
applied voltage. The resistor was selected, or often made adjustable, so
the proper motor current was delivered. This exponential rise in current
caused the torque to fall dramatically at low speeds, and the resistors
threw off hundreds of watts of heat. Generally, the DC power supply
was put at the absolute lowest voltage possible to reduce the heat,
meaning that the system was always on the edge of losing steps.

A chopper drive uses some type of switching technology to sense
motor current, and turn off the switch when the proper current had
been reached. There are a vast number of ways to do this, fixed on-time,
fixed off-time, fixed frequency, and on and on. Microstepping drives
make the phase current match a varying current depending on the
microstep the motor is supposed to be at that time.

Jon