Ed Sherman explains how a bilge pump can melt, given the wrong combination of mechanical and electrical problems.
Question: Recently I lifted the floor boards on my boat to check my bilge and see how much cleaning I needed to do down there. As I stared at my bilge pump I really couldn’t believe what I saw. The pump looked melted. The pump hadn’t been run for a while since the boat is out of the water in winter storage and the batteries are disconnected. The picture below shows my pump after I took it out of the boat and cleaned it up. What’s going on here?
Answer: This photo illustrates a classic problem that can happen with any DC (battery-powered) motor on board a boat. The problem is what is known as “locked rotor.” In this case something must have gotten lodged into the rotor assembly on your bilge pump so the impeller could not turn. In addition, I think you’ll find that the fuse in the power supply line to the pump was rated too high for the motor.
Technically, here is what happens: Once the pump rotor gets jammed and can’t turn, it gets activated when the float switch raises to supply power to the pump. The motor will immediately begin to heat up as it is still trying to turn over. As it heats up, the actual current supply to the motor goes down. This is due to one of the basic laws of electricity, OHM’s Law.
Simply put, if the voltage supply to the pump is a constant (let’s say 12 volts), and the electrical resistance inside the pump increases (which it does as it heats up), then the amperage flow must go down. In fact in this case it goes down below the trip point of the inline fuse. So it won’t blow the fuse because the amperage that is flowing in the circuit is less than it takes to melt the fuse; but power continues to flow to the moto. As a result it heats up—and in this case it heated up enough to actually melt. Fortunately, in your case it did not catch fire, as we have seen in some cases.
So, how to avoid this? Make sure the fuse for any of your onboard DC motors (macerator pumps, bilge blower fans and the like) is not rated at more than the motor manufacturer recommends. In the case of bilges, it helps to keep them free and clear of any debris that might jam the pump impeller.