Switch/motor starter question
***Questions are in bold font*** I’m redoing the electrical on an old 12″ craftsman thickness planer that I got for free. The previous owner had a lot of frightening wiring set up so much of it had to be replaced but I will be using the 3hp max rated motor starter that came with it(this should be fine slightly oversized right?). It has a 2hp single phase motor that will run on 240v. I will need to get a momentary on/off switch to accompany the motor starter. I have watched and learned a lot from this video showing the general schematic of my older style motor starter. https://www.youtube.com/watch?v=kyX95sW2FmQ&t=991s Between minute mark 14:05 and 18:45 he shows 3 drawings depicting the off, momentary start button pressed, and momentary start button released positions. At minute mark 21:37 he shows an older style starter that is similar to mine. Although his is 3 phase and mine is single phase the idea is the same without the 3rd leg. Something that has me hung up a bit is that besides the overload protection portion of this contraption one purpose of these starters is to act as a larger magnetic switch that can be activated and deactivated by a much smaller switch. My motor will be drawing up to 12 amps when up and running. What is a bit confusing to me is that the starter circuit that runs from L2 through the coil and the momentary switch and finally returns to L1 will be connected to the same circuit that the motor draws 12 amps from. How can you get away with using a “smaller” switch rated at less than 12 amps if this is the case? The only thing that makes sense to me at the moment is that the circuit running from L1 through the contactor to the motor and back to L2 has much less resistance in comparison to the coil circuit going to the switch… am I correct in assuming this and that the amperage seen by the switch will ultimately be much lower than the amperage seen by the hot wires going to and from the motor because of the difference in resistance between the two? Thanks
Replies
I don’t think the purpose of the switch in this case is to have a low power switch to activate a high power circuit. It’s only purpose is to keep the motor from starting if it’s plugged with the switch on. I don’t think you need to release the start button as he does at 21 minutes by pressing it twice, this is a non latching switch , if it stayed on this would defeat the purpose.
Thanks for the response. After posting this question I came across this video which gives a good explanation. In the video they connect milliamp meters to the coil/switch circuit as well as the load circuit. It shows the difference in current between the 2 circuits and how the coil to switch current is much lower than the load current allowing you to run large amp loads with a small amp rated circuit. Although there is continuity between the 2 circuits when the contactor is close I am guessing the difference in current is due to the high resistance of the coil? Hope this helps someone with the same problem I had down the road. https://www.youtube.com/watch?v=Ed3_zbZ0xaA
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