Re: [CAD_CAM_EDM_DRO] Re: It's more VOLTAGE needed to overcome back emf in the coil

I understand it to be when AC is rectified to DC the voltage increases by a factor of 1.414 and the current/amperage decreases by a factor of 1.8.

Ron

turbulatordude <dave_mucha@...> wrote:
--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Steve Stallings"
wrote:

> As I stated in my original post, it is the VOLTAGE to the motor coils
> that will increase as the motor spins faster and develops more back
> EMF. The chopper circuit will continue to control the maximum
> current, but the net result is that more power (Voltage times
> Current) will be delivered to the motor as it spins faster under load.
>
> The 1.8 times factor relates to the heat buildup in the transformer
> when connected to a full wave bridge rectifier and capacitor filter.
> Most transformer vendors will recommend that the RMS rating of the
> transformer be 1.8 times the CONTINIOUS current that you intend to
> draw from the DC output of the supply. CNC motor drivers applications
> seldom draw maximum current, so it is possible to design for a lower
> current and base your ratings on the expected duty cycle of the load.

I have seen the 1.8, but it was associated with capacitors. when
using a transformer for a DC supply and using capacitors, derate the
trasnformer by using the 1.8. but, if using inductors, do not derate.
That was one of those simple charts on a transformer page. I'll read
the link and get a better idea of what is going on.

I had thought the 1.8 was to select the current from the transformer
so the rectified current was avaiable. a 10 amp AC will drop by 1.414
to 7.07 amps DC. 10A times 1.8 or 18 amps AC will drop to 12.7 amps
when reftified and filtered with a cap......

Dave

> For more information about the 1.8 factor, duty cycles, and
> transformer efficiency/regulation see:
>
> http://www.plitron.com/pages/technote.htm
>
> Plitron is a major supplier of toroidal transformers suitable for CNC
> motor driver power supplies.
>
> Regards,
> Steve Stallings
> www.PMDX.com
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "turbulatordude"
> wrote:
> > --- In CAD_CAM_EDM_DRO@yahoogroups.com, Mark wrote:
> > > Voltage is the force (water pressure), current is the amount of
> > > volume (water - think water current) resulting from the voltage
> > > overcoming resistance or back-flow. Regardless of the current
> > > available from the power supply, only the amount that the voltage
> > > can stuff thru the circuit (motors) will be drawn. / mark
> >
> >
> > Correct, but this is one of those mysteries to those of us who do
> not
> > have the education in the electronics field and who don't have a
> nice
> > array of scopes and things to test with.
> >
> > I built a machine with a nice 30 amp power supply, re-wired and
> > swapped out the supply for a much smaller one. the unit runs fine,
> > but it is a non-contact on X and Y, just positioning and Z is a
> drill.
> > So, even though X and Y can run at the same time, they don't have
> > much need for power. So, in my experiance...... I was able to get by
> > with a much smaller power supply.
> >
> > My next machine was smaller and I used the motor watts for the power
> > supply and that worked also.
> >
> > Seems I have not been needed high power.
> >
> > So, anyway, it seems that power supply DC voltage times motor amps
> is
> > the starting point.
> >
> > Steve mentioned 1.8 times voltage to allow for the transformer
> > inefficiencies, and I keep seeing Mariss's 0.6 multiplier.
> >
> > So, the last piece of the puzzle is about transfromer efficiencies
> and
> > filtering of power.
> >
> > With the formula of (80,000 times current) / voltage we get the
> > capacitor sizing needed for the power supply. So, that brings up
> the
> > question of how closly couplied the transfromer is to the load when
> > the capacitor is used.
> >
> > If the pulse to the motor is in the 5uS range, wouldn't the cap
> filter
> > that to the point the transformer never sees that ?
> >
> > Dave
> >
> >
> >
> >
> >
> > >
> > >
> > > > Date: Mon, 12 Sep 2005 01:51:04 -0000
> > > > From: "turbulatordude"
> > > > Subject: Re: Power Supply for 4-axis CNC stepper driver
> > >
> > > >
> > > > Hi Guys,
> > >
> > > > Noboby argued with what Steve had said that more current is
> needed to
> > > > overcome the back EMF in the coil, so I'll work with that.
> > > >

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