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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.


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