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

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.

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"
<dave_mucha@y...> wrote:

> --- In CAD_CAM_EDM_DRO@yahoogroups.com, Mark <mfraser@h...> 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" <dave_mucha@y...>
> > > 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.
> > >