I looked at a grain bin fan motor for a coop that purchased another facility just a few miles away from their operation. The "new" facility is a cobbled mess and they won't let the farmers bring grain in for safety reasons, only their company trucks transferring last years grain here to make room for new stuff coming into the original elevator. They called because the fan motor would trip the overloads right away and the motor starter is missing it's cover, also the start/stop buttons that are supposed to be in the cover are now screwed to the bottom of the can and still in use.
The starter was a nema 3 seimens with solid state overloads mounted in an old cutler hammer starter box, with a 100a disconnect and 100a fuses right above it. The Baldor motor is a 30hp 3 phase, wired for 230v, 70a, 3525 rpm, 1.15 sf, class B, code F. The first thing I did was open the motor box to check for any obvious shorts. Did not find anything so I took the splices apart so the feed wires are out and hit the starter, it held. Checked the voltage at the motor, 242 volts between all three legs and 120, 120, 205v to ground. With the motor open I decided to check for continuity to ground on the leads and found none. I even checked continuity between the windings and found no problems. I had to get a new starter and replace the sealtight to the motor (that was missing a 10" chunk in the middle of the run) anyway, so I would see if the motor will run after the new parts.
The new starter I got is a nema 3 square D with the heater style overloads, the only style they had in stock and the only heaters they had were 75a. Of course they wanted this thing going as soon as possible so I took what they had. I got the starter installed, sealtight in, and motor back together, now working by headlights, and started it up. The fan took off running, was good and loud, and no vibrations or other bad noises. I let it run while I picked up all my stuff and the guy there said to leave it on and he would be back in the morning to check on it. It ran all night until he shut it off at about 11am. About an hour later he went to start it up and said it tripped the overloads, he reset them and hit start. He said it tried to go but quit and now would not do anything.
I came out and found all 3 fuses blown. I checked the motor leads for any signs of a short and in another j-box for the same thing but found nothing. I changed the fuses and started it up and it took off running again. This time I checked the amps after the starter and got 96 amps on each leg. I was surprised it was that high and let it run. It kept running. I had to get some parts for another project there so I let it run. While I was gone it quit again. Found 2 fuses blown. Replaced them then I borrowed a co-worker's meter to test against mine. Put both meters on the same wire and started it up. The fan ran again and both fluke meters read 96 amps. I decided to check the voltage when running and got 238v between the 3 wires. Stumped, I happened to look at the amp meter and it read 0. I thought it tripped the overloads but I still had 238v on load side of the fuses and the starter was still pulled in. I shut it off and noticed the phase the meter was on was the fuse that did not blow. I thought maybe if that phase is at 0 amps then the other 2 phases would have an increase of amperage and take out their fuses.
So after my long winded background story my questions are;
Why is this motor drawing 96 amps when the FLA is 70? (direct drive squirrel cage setup, not the air over the motor)
How did it stay running all night with 75a overloads?
When the one phase was drawing 0 amps, what is going on here? I think it is dropping causing the other fuses to blow but when restarted it's back at 96a.
If I replace the motor can I expect to have such a high amp reading on it?
I thought about blocking part of the intake to lower the amps but then had the 0 amp draw problem. I did the math and came up with 87.5a for the overloads and 122.5a on the fuses. If I went with a 125a or 110a fuse then the disco needs to be changed out too.
If anyone has any thoughts, ideas, or answers it would be greatly appreciated. Thanks!
The starter was a nema 3 seimens with solid state overloads mounted in an old cutler hammer starter box, with a 100a disconnect and 100a fuses right above it. The Baldor motor is a 30hp 3 phase, wired for 230v, 70a, 3525 rpm, 1.15 sf, class B, code F. The first thing I did was open the motor box to check for any obvious shorts. Did not find anything so I took the splices apart so the feed wires are out and hit the starter, it held. Checked the voltage at the motor, 242 volts between all three legs and 120, 120, 205v to ground. With the motor open I decided to check for continuity to ground on the leads and found none. I even checked continuity between the windings and found no problems. I had to get a new starter and replace the sealtight to the motor (that was missing a 10" chunk in the middle of the run) anyway, so I would see if the motor will run after the new parts.
The new starter I got is a nema 3 square D with the heater style overloads, the only style they had in stock and the only heaters they had were 75a. Of course they wanted this thing going as soon as possible so I took what they had. I got the starter installed, sealtight in, and motor back together, now working by headlights, and started it up. The fan took off running, was good and loud, and no vibrations or other bad noises. I let it run while I picked up all my stuff and the guy there said to leave it on and he would be back in the morning to check on it. It ran all night until he shut it off at about 11am. About an hour later he went to start it up and said it tripped the overloads, he reset them and hit start. He said it tried to go but quit and now would not do anything.
I came out and found all 3 fuses blown. I checked the motor leads for any signs of a short and in another j-box for the same thing but found nothing. I changed the fuses and started it up and it took off running again. This time I checked the amps after the starter and got 96 amps on each leg. I was surprised it was that high and let it run. It kept running. I had to get some parts for another project there so I let it run. While I was gone it quit again. Found 2 fuses blown. Replaced them then I borrowed a co-worker's meter to test against mine. Put both meters on the same wire and started it up. The fan ran again and both fluke meters read 96 amps. I decided to check the voltage when running and got 238v between the 3 wires. Stumped, I happened to look at the amp meter and it read 0. I thought it tripped the overloads but I still had 238v on load side of the fuses and the starter was still pulled in. I shut it off and noticed the phase the meter was on was the fuse that did not blow. I thought maybe if that phase is at 0 amps then the other 2 phases would have an increase of amperage and take out their fuses.
So after my long winded background story my questions are;
Why is this motor drawing 96 amps when the FLA is 70? (direct drive squirrel cage setup, not the air over the motor)
How did it stay running all night with 75a overloads?
When the one phase was drawing 0 amps, what is going on here? I think it is dropping causing the other fuses to blow but when restarted it's back at 96a.
If I replace the motor can I expect to have such a high amp reading on it?
I thought about blocking part of the intake to lower the amps but then had the 0 amp draw problem. I did the math and came up with 87.5a for the overloads and 122.5a on the fuses. If I went with a 125a or 110a fuse then the disco needs to be changed out too.
If anyone has any thoughts, ideas, or answers it would be greatly appreciated. Thanks!