Re: Power Supply for 4-axis CNC stepper driver

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Steve Stallings"
<stevesng@n...> wrote:

> Weedy,
>
> Your transformer selection is close to what I would recommend, but
> not for the reasons you expect.

Interesting. If the motors are 2.4 volts and 6 amps or 14.4 watts
times 4 motors for 57.60 watts total, why is the secondary selected
for 1,032 watts ? (43 volts times 6 amps time 4 motors ?) why not
the 60 VDC final voltage times 6 amps times 4 motors ? (1,440 watts?)

IIRC, this question was asked some many months ago, but was never
answered. Maybe one of our guru's can give the definitive answer ?

I'll put it in the files section for posterity.

The 1.8 times rating for the RMS in the DC filter is fine, but the
above ratings are worlds apart.

On another note, I had thought Mariss offered ratings (5x to 25x) for
his drives, not all drives. I have seen a few others, like the
AutomatonDirect line that have 2.4 volts motors but one of the drivers
are 150 volts DC.

I know that Mariss's numbers work pretty safely, universally, but I
had thought other manufacturers have other ratings ??

Dave

> 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