Sunday, March 15, 2009

Autopilot keeps tripping the breaker

Because the weekend post this week is so short, I don't want anyone to feel cheated. So here is a bonus: solving the problem of an autopilot that kept tripping the breaker, from the summer of 2007.

Well, it happened again. The autopilot began tripping the breaker. This last happened in July of 2005. At that time, I fixed the problem by filing the relay points and putting it back in service. I also ordered two new relays and held them against future need. We now have that need.

Briefly, this problem is due to a bad design decision on the part of the EE at the autopilot manufacturer. He needed to switch a load which could be up to 15 amps with the motor stalled (I checked by stalling the motor...) but he used relays with contacts rated at 10 amps. However, each relay had two sets of contacts, and the EE reasoned that if he just connected both sets of contacts in parallel, he would have 20 amp switching capacity. Unfortunately, that's not true.

Arcing occurs at the contacts when they open, caused by the magnetic field collapse in the motor when current is interrupted. This is the toughest part of the service life of a relay contact. With two contacts in parallel, when the relay opens (here in the REAL world...), one of the contacts inevitably opens a tiny bit earlier than the other. This first contact is essentially switching zero current, since its parallel mate is still closed. But then when the other contact opens a fraction of a millisecond later, it is switching the entire current. The contact points overheat, and metal is transferred from one contact point to its mate. Sometimes the contacts are actually welded together.

Because of the way that the circuitry is laid out in the autopilot, if one of the parallel members of the pair of contacts fails to actuate when the other does, a direct short of battery power occurs, tripping the breaker. Here, I am holding the relay in the actuated condition, and the right-hand movable contact is doing what it should. But the left-hand movable contact has caught on the upper stationary contact, due to the transferred metal. This would trip the breaker.

I installed the replacement relays (each has a single set of contacts rated at 30 amps). This required some finicky rerouting of the existing wiring in the cramped confines of the autopilot housing, but it is done.

Now we will see how long these contacts last...

(NB: They're still working...)

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