Maybe I haven't had enough coffee this morning but I am stuck in a quandary this morning.
I am thinking of ways to measure the RPM of a 50hp AC motor without using a shaft encoder or any "downstream" measurement device. Ideally, I would like to have it on the line side of the power to the motor where it is less likely to break/fail/give faulty readings (stupid encoders...). I am thinking, if I measure the AC line frequency of one of the legs (3phase, 480VAC, reference 60Hz) then I can calculate out the RPM easily. Problem is, the only frequency transmitters I have found so far will only handle max 240VAC. So naturally, my thought is RELAYS! Everyone loves relays! But is there a relay that will open/close at the same frequency of the AC frequency across the coil (I have not tried to do this before, and my experience in motors is limited)?