Okay here's the problem. I have several machines using 3/4 HP electric motors. all of them should run clockwise to make the machine work but once I turned off them for launch and when I came back and turned them on, one of them worked counter clockwise. I turned it on/off few times and it worked CW. again when I turned it off/on worked CCW. a bit strange. also I noticed even when it runs correct it's slower than the others.Can you guys explain me what has happened? Thanks in advance.
If they have capacitors, I bet the renegade has a bad capacitor. Run capacitors not only determine the direction of rotation at startup, they are crucial to developing proper torque.
An AC Synchronous motor gets its starting direction from the starting capacitor and the shading coil in the start circuit. If the starter relay has one bad contact and one good contact, you can get a reverse start in the motor. Since it is aliasing its field (skipping accross the wave-tops going the wrong way) on the applied ac sinewaves, it will run at about 87% of rated RPM's to to the back-slip effect of going to the next peak in the field in an aliasing synchronous motor. This can also occur if a 3-phase motor is single-phasing (a misnomer, since it is running on two phases out of 3, not one out of three). It can start rotating in either direction and it will run slow, and HOT. - this would mean a bad winding, a bad main motor contact for one phase, or just a loose wire connection for one of the 3 phases. Starting Cap, Shading Coil, Start Relay - it's one of those.
The slow run is a problem with the start winding not disengaging so the run circuit can operate. This is probably the cause of the reverse start also. Best solution is to replace the motor with a new one. Unless it is a very special electric motor rated more than 7.5 Hp it is not worth a rebuild charge at a professional shop. Cheaper, faster and easier to RR the unit.
I had one situation with conveyor belt at a freight company, small three phase motor connection to oversized VFD. Drive always showed correct output rotation, but motor started alternating directions each time. Found stake-on terminal on terminal strip in junction box that the wire was was not tight in crimp connection and pulled out when tugged, replaced with new crimp, started correct direction every time afterward. Can't explain why. Just what.
