news:njG_q.19384$Ep3....@newsfe08.iad...
| On 2/14/2012 8:48 PM, G=EMC^2 wrote:
| > On Feb 12, 3:44 am, "Androcles"<H...@Hgwrts.phscs.Feb.2012> wrote:
| >> "
BJAC...@teranews.com"<
b...@iwaynet.net> wrote in message
|
| Andro wrote:
| >> Good -- positional accuracy
| >> Bad -- acceleration
|
| Andro has the idea! They move in steps!
| Good is positional accuracy PROVIDED they don't slip a position which
| they tend to do at high speeds and/or torques. But providing they don't
| slip they have the advantage of not needing any encoder to give you
| their position. You simply count steps!
|
Yep, and that locates sectors on floppy discs.
| Bad is actually "acceleration". But rather the fact they have too much!
An iron squirrel cage rotor in an ordinary induction motor
draws a high current at start up, it is effectively a transformer with
the secondary (rotor) winding short-circuited. When it has reached
its operating speed the current reduces. To prevent excessively high
starting currents which would trip a breaker or blow a fuse, larger
motors have a wound rotor connected to a resistive load. This also
reduces the acceleration. Such motors operate at a fixed frequency
and the rotor has a variable "slip" frequency.
With a synchronous motor, however, there is no slip and the speed
depends solely on the frequency. This means that to accelerate
the rotor a gradual increase in frequency is needed or the motor
"breaks torque" or loses a step. If it loses a step then you lose
count. Now it is true that a floppy drive does accelerate to full
speed within one sector and so a constant frequency can be used,
but that's only because the load is light.
When the load is heavy, variable frequency is needed. Acceleration
is a negative attribution for stepper motors.
| They move rapidly for each step and if there is too much inertia in what
| they are driving they can skip a step.
Which loses count, and losing count would be distastrous for
a floppy.
| Also bad is they have no speed.
| You can't drive pulses in them fast enough (inductance) to get really
| high speeds. And of course you have to limit what they are driving so
| torque is low enough they don't skip.
|
| > Good to do light work . Could work well for electric watch Possibly to
| > spin a gyro. Made very big it does not need a capacitor?? Could not
| > have the torque of a heavy iron rotor.So not good for electric cars
| > Electric cars need no transmittion. Best to keep in mind an electric
| > motor only draws the electricity needed.
|
| Electric watch. Yes. Seconds are steps! Spin gyro? No. Steppers do not
| move well continuously nor at high speeds. They do not use capacitors.
| You need to understand how stepper motor driver circuit works! Steppers
| DO have a heavy metal rotor! It's full of magnets! They are not POWER
| motors! They are positioning motors! OK?
|
Consider a robotic arm that has to move in a straight line at
a constant speed to run a bead or a weld. Put your own finger
at one side of your desk or table and move it along the edge
to the other side, watching your elbow as you do so. You'll
see it "close" or fold up and then open, so the motor of the
robotic arm has to reverse itself in mid-move. The mathematics
is three dimensional matrices controlling the motor frequencies
with limits on how fast any single motor can move.