At 05:06 AM 6/11/2007, W5LT wrote:
>Dick makes some good points.
>Cable capacitance and shunt capacitance of supressors will both increase the
>transient (and over-all average) current drawn from the power supply.
Another somewhat subtle problem is that with long wires, you might
get big inductive/"transmission line pulse former" spikes at the
suppressors. (on each and every step pulse) If they're MOVs, you can
burn through the life of the MOV pretty quickly, reducing the
threshold voltage, and turning them into a parallel resistor. Now,
the motor driver is putting out the right amount of current, but not
all of it is going through the motor, because of the shunt path
through the MOV. This can be very difficult to detect by simple
measurements from the drive end of the wire (since the MOV is a
nonlinear device)
If you're using back to back zeners as the transient suppressor,
there's other weird stuff that can occur. You've got a threshold
device at the end of a transmission line. The driver turns on the
juice and the current starts to rise, the threshold device trips,
sending a transient back down the transmission line to the driver,
where it confuses the current/voltage sensors in the
driver. Propagation speed in the cable is probably on the order of
0.66c, so 300 ft is about 1 microsecond round trip time.
Yet another thing to look for is some sort of resonance effect. If
the motors are being full or halfstepped, as opposed to microstepped,
(and even if the latter) you can get strange effects from an
interaction between the step rate and the mechanical resonances in
the load/step motor internal dynamics, which in turn is affected by
the L & R of the windings,cabling, and drivers.
I would think that the SteppIR is pretty well damped (lots of
friction), but this kind of thing can crop up unexpectedly.
Jim, W6RMK
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