Re: Power Supply for 4-axis CNC stepper driver

Weedy,

Your transformer selection is close to what I would recommend, but
not for the reasons you expect. Your formula included a factor of 2
for the 2 phases of the motor. This is not required because on a
microstepping driver the phases run sine and cos factors times the
maximum phase current. When one phase is max the other is min, so the
total current is never a factor of 2. The 66.6% times one phase
current factor number is an imperical one recommended by Mariss
Freimanis of Geckodrive. It is a conservative rating that allows for
the maximum mechanical output from the motor under load. It is rare
to extract maximum mechanical power from a stepper motor, and even
more rare to do it to all four axes of a CNC machine at the same time.

The reason I say you are close is that your formula does not include
another factor that calls for a larger transformer. The RMS current
ratings of power transformers are intended for resistive loads. The
input stage of most CNC motor power supplies is a full wave bridge
rectifier followed by a large capacitor. This arrangement causes the
current to flow in a very non-sine wave fashion with high peaks when
the capacitor is charging and very low current when it is not
charging. Since the heating of the transformer is primarily due to
the winding resistance the main concern is the power lost due to
current flow through the resistance. This power is: I(current in
amperes) squared times resistance. The squared (I times I) factor
causes the effect of the short term current peaks to be much worse
than a steady current would be. Transformer manufacturers generally
recommend that you compensate for this by applying a factor of 1.8
times the desired DC output current to specify the RMS current rating
needed for the transformer.

With all things considered, I would recommend an 800 VA or 1000 VA
transformer for your application. Also, don't forget that your bridge
rectifier will need heat sinking. The typical voltage drop through
the bridge is 1.5 volts. With a load current of 20 amperes RMS, this
would result in 30 watts of heat generated in the bridge.

Commercial transformers are available in a wide variety of voltage
and current so no one recommendation can be made. Do keep in mind
that you should allow for higher than nominal line voltage being a
possibility. You should make sure that if the line voltage is 10%
high you still do not exceed the voltage rating of you motor drivers,
bridge rectifier, or capacitors.

Bridge rectifiers are readily available in 100 volt and 200 volt
ratings and there is little cost premium. Higher current rated
bridges generally have sturdier connections and cases that better
conduct heat to your heatsink.

The 20% safety factor that you mentioned for the capacitor voltage
rating is reasonable. Different styles of capacitor families have
different "standard" voltages so there is no magic recommendation.
Capacitors also have a ripple current rating that should be
considered. You should allow for a ripple current rating of 50% or
more of the total load current planned.

Regards,
Steve Stallings
www.PMDX.com

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Weedy" <weedy@e...> wrote:
> Hi, I need some help in constructing my Power Supply (my input
voltage is
> 220 VAC, 60 Hz) for my stepper drivers. Here are the details:
>
> Stepper Motors specifications (3 or 4 pcs., if optional 4th axis
will be
> installed):
> 2.4 VDC rated voltage,
> 6 A/phase rated current,
> 0.4 ohm resistance/phase,
> 3.4 mH inductance/phase,
> 4.2 N.m holding torque,
> 4 wires (bipolar)
>
> Stepper Drivers specifications (3 or 4 pcs., if optional 4th axis
will be
> installed):
> Output current - 7.8A max, 2.8A min,
> Supply voltage - 90 VDC max, 18 VDC min
> Microstepping - 2,4,8,16,32,64,128
>
> Since I want to construct my own power supply (PS), can someone
check the
> following calculations:
> 1) I want to go 25 times of motor rated voltage (2.4VDC), so the PS
output
> voltage (Vo) = 60 VDC
> 2) Transformer secondary voltage (Vt) = Vo/1.414 = 60/1.414 = 43 VAC
> 3) Going for 4-axis, current = 4 motors x 6A/phase x 2phase/motor =
48 Amp
> 4) Normal current draw being 66.67% of total current, so Current
(It) =
> 66.67% x 48 = 32 Amp
> 5) Transformer VA = VAC x It = 43 x 32 = 1376 VA
> 6) So, Transformer should be: Primary - 220 VAC, 1376 VA (Watts),
> Secondary - 43 VAC
> 7) Capacitor = (80,000*It)/Vo = 80,000 * 32 / 60 = 32,000 uF and
voltage
> 120% of Vo
> 8) So, Capacitor should be at least 32,000 uF, 72V or higher
> 9) Bridge rectifier at least 32 A, 72V
>
> If the above are correct, what are the commercially available
specifications
> for:
> 1) Transformer
> 2) Capacitor
> 3) Rectifier
>
> TIA!
>
> Weedy