Just in relation to the last sections where we looked at circuit diagnostics, we will now look at introducing a method to monitor the machine function.
Consider a motor that is connected to a pump by means of V Belts. Should the belts break, the motor will still run, hence no fault is evident. That is until we start flooding the plant! So, how do we detect that the belt is broken?
This will require a bit of mechanical work at the pump drive pulley. We need to have a cam that will spin with the pulley, and it needs to have notches in it that will turn a limit switch on and off as it spins. In other words, we want the limit to turn on and off a couple of times during each revolution of the pulley. We will then use this signal to implement into our circuit as below.
You will also note that I have just added a relay in parallel to our contactor. When the pump is started, this relay will be energised at the same time as the contactor. We now use a normally open of this relay to start our monitoring side. Every time the limit swith turns on, we energise timer 1, and in the next position we activate timer 2. Should either timer time out, we will know that the pump is no longer rotating and activate the alarm
So why do we use two timers and two contacts from the limit switch? The reasoning here is that should the belts snap and the pump stop, it could stop in either position. We do not want our alarm to fail because the pump stopped with our limit switch in the open position. By using both contacts and two timers, we will detect the break in either position
When you set up the system, run it and calculate the time between pulses from the limit switch. This will give you an indication of the required setting on the timers. You will also see that I have just used a contactor at the alarm output. You can connect indication lamps or an audible alarm through this output to suit your application.
Again, as per all schematics here, you would need to click on the drawing for it to open.
Hope you find a use for this application. Untill next time, same story, Safety First and Production Second. Do Not discard safety for the sake of production!