This a very interesting problem that has been sent to me by one of our readers, so let’s take a look at what is happening here. Bear in mind, I do not have the exact details of the installation, so I will try and picture the installation from what is happening.
To start off with, consider how the single phase motor will work. When we apply power to it, the starting winding will ‘kick-start’ the motor in the desired direction after which the running winding maintains the motor in the ‘run mode’.
Now let us consider the likely installation:
The pump has to pump water up to the next point which is higher than the position of the pump. In other words, it has what we call a vertical lift
(sorry all, tried getting a picture of the layout in here but seems I have an issue uploading at the moment)
So if you started the motor, it will start in the right direction and pump water to the outlet. So far there is no problem. When you have no need for it to run any longer, you stop it and it waits for the next start. Everything here is normal. There is no motor or pump fault as it works exactly as intended.
Now, you need this to run again after 5 or 10 minutes. You proceed to start it and, once again, it works as it should. Now the situation may change. After you have stopped it, you try to start it again within a very short time and there we have it, the motor runs in the wrong direction and spins off the impeller.
Why did this happen?
Very interesting problem. The issue here is not an electrical one but rather mechanical. What is happening is that the water in the pipe is running back to the pump when you stopped it, thus spinning the pump backwards. When this is happening and you start the motor, you may not have sufficient torque to stop this and the force of the water maintains the direction in which it was spinning. Once your running winding takes over, it does not care which direction this motor is running in. It simply maintains whichever direction you have started with.
The solution? Get the mechanical boys to put a non-return valve close to the pump. This will prevent the water running back and reversing your motor direction.
Hope this solves your problem and keep them coming. Not always easy to solve and I may not be able to get them all, but surely within our readers circle we will be able to answer all questions?