Re: Where to buy non induction resistors

'Fraid your description of a constant current drive is actually a
description of a chopper. A constant current drive continuously varies the
voltage across its output in order to keep the output current constant; it
doesn't switch the output on & off like a chopper. Hence, as pointed out
elsewhere in this thread, the constant current circuitry tends to be
trickier to design than a chopper circuit, mostly because the power stage
has to dissipate significant amounts of power in order to achieve this.

Regards,
Tony



At 05:21 29/03/2002 +0000, you wrote:

>L/R drive just uses a constant voltage and the maximum current is
>set by the series resistance. Because of the motor inductance,
>applying say 12V causes the current not to be maximum instantly,
>but to ramp up to it. If the pulses are short, then the current
>doesn't get up very high, so motor torque is less at higher speeds
>when the pulses are shortest.
>
>Constant current works by using higher voltage (like 80V) on the
>windings, and no resistors. The current ramps up faster thru the
>motor windings because of the higher voltage. When the current
>reaches a maximum, a monitoring circuit removes the voltage.
>This action is pulsing, and with various systems, the frequency
>can be from 200Hz up to 50kHz or more. Maximum current and torque
>is maintained at maximum motor speeds. The method uses no resistor
>to limit the current, so you don't get the heating problems.
>
>lenskeeper2000 wrote:
> >
> > Okay, so it sounds like I'm wasting my time worrying about resistor
> > induction. Sound like I can just stick in normal power resistors and
> > get about the same result. (I tried upping my voltage with what I
> > had around the house and really liked the improvement but this was
> > just a test was cooking the resistors)
> >
> > What is a constant current drive? Isn't that the same thing as a L/R
> > Drive circuit? (I my have just demonstrated my ignormace)
> >
> > Roger
> >
> > --- In CAD_CAM_EDM_DRO@y..., Russell Shaw <rjshaw@i...> wrote:
> > > The inductance of *any* resistor will be *much* lower than the
> > > inductance of any motor. The only way is to use higher supply
> > > voltage, lower inductance motors, or constant current drive.
> > >
> > > lenskeeper2000 wrote:
> > > >
> > > > Where can I buy non-induction power resistors? I'm using a simple
> > > > L/R style drive circuit and want to step up my voltage but have
> > found
> > > > I have too much inductance to get my speed rate up. I'l thinking
> > if
> > > > I can reduce the effect of the wire wond reisitor's im using this
> > > > will help.
> > > >
> > > > I've seem lots of power resistors for sale (allied etc.) but none
> > > > seem to be non inducting. Looking for 25 watt in the 20-25 hom
> > > > range. Any suggestions?
> > > >
> > > > I'm trying to keep on a budget and am trying to learn electronics
> > > > while upgrading my desktop CNC mill. so holding off going to a
> > > > chopper curcuit as I want to design myself (will leikely
> > eventually
> > > > give in and by Gecko drivers).