Re: [CAD_CAM_EDM_DRO] Re: PWM vs Constant current

Alan, I truely don't know [ hence why I asked]. Below is a paste from him
which is off a website where he purchased his equipment. I'm sure they are
trying to promote their product however I just want the final answer that
isn't bias in any way :-)

****************
The major problem is as a stepper motor's speed increases the current drawn
by the motor and it's power output slowly drop until it reaches a certain
speed (this point varies greatly with motor type) then suddenly the current
and power drop to almost nothing, making the motor almost useless at
anything other than low speeds. This is caused by insufficient
time for each coil's field to break down before the next step.

Different systems are often employed to increase the speed of stepper motors
like ballast resistors or chopper drives. Chopper drives or PWM (pulse width
modulated) drives turn on and off (at high speed ) a power supply higher
than the motor's rated voltage.
This technique is a linear response to a non-linear problem.

This type of drive maintains motor current and power by incensing the
voltage to the motor as the motor speed rises. Constant current drives force
each coil's field to collapse quicker allowing themotor tu run much faster.

Our (K142C) Constant Current Source systems sense the drop in current and
increase the voltage to the motor and thus the current and power is
maintained with higher speeds. A common motor we tested gave similar torque
at around 300 RPM as at 1 or 2 RPM (this is as FAST as we tested with a 200
step motor).
Because of the wide voltage output range of the constant current source we
had to re-design our (K142) CNC Stepper Motor Driver Kit to cope.

************

Hope that helps I appreciate all the help you can offer.

Thanks

Jarrett Johnson
Sask, Canada

> First, I assume you're talking about a stepper based system.
>
> If so, I'm quite curious as to the reasoning of the person who suggested a
> constant current system.
>
> While there are a few theoretical disadvantages to a chopper system over a
> CC
> for managing the current into a stepper, those disadvantages are, in my
> experience, largely academic.
>
> CC systems, however, have genuine real world disadvantages, in the form of
> much bigger and much more expensive power supply requirements and WAY more
> heat generated.
>
> So I'm curious.
>
> Note that outside of being more expensive and generating lots of heat,
> there's
> nothing wrong with a CC system, but since there is a cheaper, more
> efficient
> solution to the problem ...
>
>
> Alan
>
>
> On Monday 24 October 2005 06:54, Jarrett & Heidi Johnson wrote:
>> Hello, I've been told that I should be looking for a constant current
>> system [verses a PWM one] and I'm wondering if some of you electronics
>> fellows could offer comment. First of all in what the differences mean
>> [ I
>> think I know, but would like that confirmed] and if this gentleman is
>> correct or not in regards to which route to go. I'm by no means well
>> trained in electronics and theory of that nature so go easy on me
>> please..
>> however I'm trying to learn and understand what I can.