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Re: (ET) E15 Alltrax died
At 04:09 PM 12/3/2007 -0500, Bob Murcek wrote:
Others will know a lot more but the third wire is for braking, using it
wouldn't reduce spiking. Braking probably even adds some spikes.
Braking shouldn't add more spiking.
A short diversion, apologies to those who know this already
The basic source of the spike is the motor inductance. Inductors 'attempt'
to keep the current flow in them constant (It has to do with the magnetic
field). To increase or decrease the current in an inductor requires a
voltage, the rate of change depends on the voltage. The higher the voltage
the greater the rate of change in the current.
Now consider what happens when you open a contactor connected in series
with a running motor. All of a sudden the current must fall to zero very
quickly(1). In order for this to happen the voltage must rise very
high. This is the same mechanism used with an ignition coil to generate a
spark.
Motors and contactor coils are quite good inductors so you can get quite
hefty voltages spikes.
There is a second mechanism in play as well. The sudden change in current
produces a magnetic field that induces current into neighbouring
wiring. So you don't actually have to be connected to the problem circuit
to see an effect.
Braking operates quite differently. It acts to short a PM generating
producing a rather large braking force.
Robert
1- Theoretically it has to drop to zero current in zero time. This would
produce an infinite voltage. In practice the voltage rises enough to
produce an arc across the contactor tips sustaining the current until the
tips separate sufficiently. A suppressor diode provides another path for
the current to take with a much lower voltage increase. So a suppressor
diode also acts to protect the contactor tips but at the expense of slowing
the rate of current decay.
http://www.aeolusdevelopment.com/
From the Divided by a Common Language File (Edited to protect the guilty)
ME - "I'd like to get Price and delivery for connector Part # XXXXX"
Dist./Rep - "$X.XX Lead time 37 days"
ME - "Anything we can do about lead time? 37 days seems a bit high."
Dist./Rep - "that is the lead time given because our stock is live.... we
currently have stock."