Bilge Pump Installation, Continued
Reading my colleague Lenny Rudow's piece yesterday entitled "Installing New Bilge Pumps, Read This First" made me think of an on-going issue with bilge pumps and any small motor circuit you install on your boat, macerators, bilge blower fans and such. The issue is called "locked rotor" and what happens when this occurs. A locked rotor situation is basically defined as a motor that's trying to turn, but can't, for whatever reason. In the case of a bilge pump motor any bit of debris that enters the motor housing around the pump's impeller and wedges itself between the motor housing and the impeller could jam the pump so that the impeller can't turn. The problem is, your float switch doesn't know that. When the float reaches its turn on point, its still going to send power to the bilge pump motor. what happens next is the tricky part. The motor has power so its trying to turn but can't. So what happens electrically here is the interesting part. You have a voltage source, and a device trying its best to function. It does the only thing it can do at this point, generate heat. In fact maybe even enough heat to catch on fire. The electrical current is flowing, but gets reduced based on one of the key electrical equations we have, Ohn's Law. Ohm's law states that if voltage is a constant and electrical resistance increases, amperage must go down or be reduced. That's whats happening here, the voltage supply is pretty steady from your battery, the motor is heating up and that increases electrical resistance, amperage goes down. Its a mathematical fact. The problem is, we would hope a fuse would blow in this case and shut down the power to the motor. It won't. Why? Because fuses blow based on amperage and if the fuse is not being subjected to enough amperage, its just going to sit there and allow whatever current there is to flow freely. The simple answer to this problem is to be absolutely certain the fuse for your bilge pump motor is rated exactly as the motor manufacturer recommends and that's where the installations typically go wrong. The vast majority of blower fan and bilge pump circuits are over-fused, and in the event of a locked rotor will not blow! In the photo below, you see the beginning stages of what can happen. Mind you I disconnected the power to this motor after about 20 minutes. If you turn sideways and look very carefully at the printed information on the top of the pump you'll see that Rule recommended using a maximum 11/2 amp fuse (this one is a 24 volt motor). All I did to get the melt down going was lock the motor impeller in a vise and install a 5 amp fuse. Again, within about 20 minutes the case got hot enough to start melting as you can see. Had I left the power connected, it would have eventually caught on fire. So, besides Lenny's sage advice about adding a vented loop to your near the waterline bilge pump installations, make sure the fuse you use matches what the pump manufacturer recommends, and no more.
Reading my colleague Lenny Rudow's piece yesterday entitled "Installing New Bilge Pumps, Read This First" made me think of an on-going issue with bilge pumps and any small motor circuit you install on your boat, macerators, bilge blower fans and such. The issue is called "locked rotor" and what happens when this occurs. A locked rotor situation is basically defined as a motor that's trying to turn, but can't, for whatever reason.
In the case of a bilge pump motor any bit of debris that enters the motor housing around the pump's impeller and wedges itself between the motor housing and the impeller could jam the pump so that the impeller can't turn. The problem is, your float switch doesn't know that. When the float reaches its turn on point, its still going to send power to the bilge pump motor. what happens next is the tricky part. The motor has power so its trying to turn but can't. So what happens electrically here is the interesting part. You have a voltage source, and a device trying its best to function. It does the only thing it can do at this point, generate heat. In fact maybe even enough heat to catch on fire. The electrical current is flowing, but gets reduced based on one of the key electrical equations we have, Ohn's Law. Ohm's law states that if voltage is a constant and electrical resistance increases, amperage must go down or be reduced. That's whats happening here, the voltage supply is pretty steady from your battery, the motor is heating up and that increases electrical resistance, amperage goes down. Its a mathematical fact.
The problem is, we would hope a fuse would blow in this case and shut down the power to the motor. It won't. Why? Because fuses blow based on amperage and if the fuse is not being subjected to enough amperage, its just going to sit there and allow whatever current there is to flow freely.
The simple answer to this problem is to be absolutely certain the fuse for your bilge pump motor is rated exactly as the motor manufacturer recommends and that's where the installations typically go wrong. The vast majority of blower fan and bilge pump circuits are over-fused, and in the event of a locked rotor will not blow! In the photo below, you see the beginning stages of what can happen. Mind you I disconnected the power to this motor after about 20 minutes.
If you turn sideways and look very carefully at the printed information on the top of the pump you'll see that Rule recommended using a maximum 11/2 amp fuse (this one is a 24 volt motor). All I did to get the melt down going was lock the motor impeller in a vise and install a 5 amp fuse. Again, within about 20 minutes the case got hot enough to start melting as you can see. Had I left the power connected, it would have eventually caught on fire.
So, besides Lenny's sage advice about adding a vented loop to your near the waterline bilge pump installations, make sure the fuse you use matches what the pump manufacturer recommends, and no more.
