Anyone interested in a guessing game?
I got a call from a maintenance man at a grain elevator. They are having trouble with two small motors blowing fuses, and I am going Tuesday morning to look at it. The information he gave me sounds like something I've seen before, but I'm interested in hearing if any of you have different ideas. Here are the facts, that I have:
The complex is about 10 years old and is generally in good condition and well maintained. The service is 600 volt, three phase. Most motors are fed from an MCC and controlled by a PLC. There are about 50 motors, ranging in size from a few 100 HP to many fractional HP. The larger motors have PF capacitors at the starters, and there is a bank of individually controlled capacitors for the entire service.
The problem motors are part of a cleaning system that includes a couple larger (40 HP?) motors on drives. They run all day long without any problem, but when they turn their system on at the beginning of the day, the PLC shows "overload error" and the fuses are blown. The overloads do not trip. Some times, the fuses blow when they start the motors.
The two motors are for blowers. They have checked for material build-up that may be dragging on the blowers. The motors start well (quickly). They are both FLC = 3.8 A. The running current of one - using a crappy clamp-on meter - is:
A = 1.1 Amp
B = .8 Amp
C = .4 Amp
I don't remember the numbers for the second motor, but they were really unbalanced, too.
The MCC modules feeding the motors are breakers with 6 amp fuses below them. The wires for the motors leave the fuses and "go into the drive module". There are two other wires on two of the fuses that "go somewhere else".
I didn't ask for voltage readings, because I don't like to ask mechanical maintenance people to work on live equipment. Perhaps, I should have, but it's about 2 hours away and it's a pleasant break from my normal work.
Two motors with similar readings sound like a common problem to me. However, they are Weg motors, so anything goes, I guess. To make this interesting, if anyone guesses correctly, I will buy them a beer, but they have to come here to get it.
What you you think?
