Re: [CAD_CAM_EDM_DRO] Re: Where to buy non induction resistors

The chopper *simulates* constant current after mechanical
averaging of the pulses. I didn't explain true constant-current
drive (done simply with some transistors) because it would
dissipate (lose) just as much power as the L/R method.

Tony Jeffree wrote:

>
> '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.