Re: [CAD_CAM_EDM_DRO] Re: Declining motor torque with lower voltage.

cnc_4_me wrote:

>[snip]
>Well I am still trying to digest post number 78682 from Stephen Wille
>Padnos. He talks about back emf, sort of like getting free
>voltage….But until I do I will stay conventional.
>

Heh heh - I never claimed to be a good cook :)
Back EMF is really a deduction of voltage. Look at it this way - if you
attach a voltmeter to the motor leads, you will measure a voltage when
you turn the shaft by hand (it's a generator). Since this happens when
you turn the shaft, it also happens when the shaft is turned by applying
power to the windings. (ie, any time the shaft is rotating, the motor
will generate an output voltage). If you're trying to drive the motor
with a DC supply, then the supply voltage must be higher than the
voltage generated by the motor, otherwise the supply can't supply any
current (and hence no torque).

The motor is like a battery in series with a resistor. The "battery"
voltage is proportional to the shaft rotation speed. The resistor is the
winding resistance. If you have a "positive" current in the reisitor,
this applies torque, and the shaft speeds up (but the battery voltage
goes up also). That means that to continue accelerating, you have to
keep increasing the supply voltage, to overcome the back EMF.
If you want to slow the motor down, just set the supply to a lower
voltage. This makes a "negative" current in the resistor, so there's a
torque in the opposite direction, and the shaft slows down.

> Using your example
>of Z table weight I think your numbers are off. I think an empty Z
>weighs a couple of hundred pounds, and fully loaded with vises part
>ect, specs out at 750 lbs. So unless you added 300 to 400 lbs weight
>to your table to test it, you do not have real numbers yet. And no,
>you can not deliver more current with tiny spikes.
>
>What are the specs of your Z axis. Do you have an acme or ballscrew
>and what TPI.
>
>
>Wally
>
>

This page tells the weights of various parts of a Bridgeport:
http://www.frugalmachinist.com/bpwts.html
If you add the knee, saddle, and table (48"), you get 764 pounds. I
think the 42" table weighs around 250 pounds (or I wouldn't have been
able to budge mine :) ), so maybe only 650 pounds for the knee servo to
move. Then add an 80 lb vise, some clamps, and a big hunk of steel to
mill, and you're easily moving 800+ pounds. Roger's knee may weigh more
if it's one of the 12" Y movement models (vs. 9" on some models - I
don't know what mr. Frugalmachinist has)

(clearer than cowboy coffee? :) )
- Steve