Re: [CAD_CAM_EDM_DRO] How to design the power supply for steppers ???

Pavel Korensky wrote:

> Hello,
>
> anybody can help me with the following problem ?
>
> I am designing the 4-axis stepper motor driver (one Lattice ispLSI chip as
> a stepper controller, 4x 3776 chopper driver chip and MOSFET transistors).
> The design of the stepper controller is finished (and freely available, if
> you want it, just write me). Currently, I am designing the driver
> schematics and boards. The 3776 drivers are pretty simple, I am just
> confused with the proper design of the power supply stage. Let's say that I
> want up to 5 Amp per driver (per axis) and motor power supply voltage will
> be 48 V.
>
> When I suppose that I will use the full-bridge rectifier, I will need:
>
> 48/SQRT(2) = 34 V - total output voltage of the transformer
>
> 5*SQRT(2) = 7 A - output current per channel

No, you have a mistake here. When the motor is not moving, there is only the
resistive loss in the winding, plus a little iron loss, plus losses in the driver.
What this means is that, for a motor that drops 2 V at 5 A (per winding), then
there is approximately 10 W dissipated per winding. 10 W / 48 V = .2 A
Now, the current draw will increase substantially at higher speeds, but may
not ever reach 7 amps per motor. It depends quite a bit on the motors, mostly,
and what speeds you can achieve before steps are lost. It is also not likely
all 4 axes will be moving at max speed at the same time. So, it would be quite
reasonable to use a smaller power supply that the total of all drivers.

>
> 4 * 7 = 28 A - total output current of the transformer
>
> Totally, approx 1000 W toroid transformer as a power supply. Is this
> calculation right ?

I would think you could probably get by with a 15 A power supply, although your
particular application may influence how much safety factor you need. It is fairly
common to permit a properly designed power transformer to be called on to deliver
occasional bursts of power 25 - 50% above the continuous rating. The filter
capacitor and rectifier should be designed for these surge loads.

Jon