From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


>From: "Bob May" 
>
>Only one problem with this thread tuning into a discussion of DC motor
>problems is that the motor described in the original post is a stepper motor
>which doesn't get pulses to move it but rather DC levels which get switched
>about to move the shaft of the motor!
>If you remove the power from a stepper, it immediately loses its position if
>the motor is moving something.  There's a whole different process to move a
>stepper motor than from almost any other motor.
>Bob May
>http://nav.to/bobmay
>bobmay{at}nethere.com
>NEW! http://bobmay.astronomy.net
>

Hi
 This is not always true. There are two kinds of steppers.
Some have permanent magnets and will hold ( with reduced torque ) their
position with the power off. Others require the power to always be on to
hold. This also assumes that one stops at full steps and isn't micro
stepping.
 Even those that have the permanent magnets require the step
to finish and the stepper to stabilize before removing the power. Removing
the power early will often cause it to jump.
 Anyway, I wasn't following the original post. I came in
when a fellow said that he'd made a DC control with PWM and he was
wondering why it was singing. He was also wondering why it didn't have as
much torque as low speed as at full speed. I then went into describing the
right way to control a DC motor such that it will deliver full torque at
low speed.
 One fellow mentioned that one could use a LM317 but that
has the same issue that you need the feedback of the current to get the
desired voltage to speed type control. Dwight

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