3-phase motor current start
3-phase motor current start
(OP)
I have a vacuum conveyor for pellets of plastic raw material and it uses a 3-phase motor, 5.2 amps (nominal) and it runs on 480V. Originaly, there were 3 x 6-amps fuses on it. Then, it started blowing fuses with no apparent reason. I called the manufacturer and I have been told to replace the fuses for 10-amps time-delay fuses. The blower is running with around 6 amps on load, and its starting current goes up to 25 amps sometimes (normally goes to 22 amps). Its cycle lasts only 30 seconds every 2 or 3 minutes, so it starts and stops all day long. The body temperature went to 165F today and it burnt 6 10-amp fuses just today. Control pannel looks ok, it works fine. Voltage is around 475V.
Does anybody have any idea?
Does anybody have any idea?





RE: 3-phase motor current start
RK, CC, ....
Ed
RE: 3-phase motor current start
RE: 3-phase motor current start
i happened to luckout and have a Bussmann rep visiting. He looked at my system, sugested that my duty cycle was fairly short (It was) and the physical size of my fusing was my trouble. In that the fuses never had sufficient tine to disapate the heat of inrush. I was using CC fuses.
i changed fusing to RK, same values, never had the problem again.
good luck
Ed
RE: 3-phase motor current start
Recommended fuse size for a 3HP 460V motor is 10A TD, but that's based on a 4.8A FLC and assumes you have a full motor starter with an Overload Relay. For your motor you probably need a 12A fuse even if you get the FLC down to its rated value.
Some cheap OEMs try to get away with using the fuses alone as the OL protective device, which your original 6A sizing has me suspecting. While technically not "illegal", it is tricky at best, usually unworkable. If you have no OL relay that is tripping before the fuses blow, that is part of your problem. The next step by the way is for your motor windings to fry. Be careful, something is not right here.
Describe your entire system in more detail if you want more help. For us to help with an incomplete story is going to be difficult.
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RE: 3-phase motor current start
I'm guessing that production rates or similar have increased, but as you've discovered, even increasing the fuse size doesn't appear to be helping. You have a problem somewhere, whether its just the fact that the original motor was marginally sized, or you've developed an issue that is now blowing fuses, and may be causing damage somewhere else that isn't apparent.
Replacing or up-sizing the fuses is not likely to fix the problem, and I'd suggest further investigation.
RE: 3-phase motor current start
If true, then you need to figure out what changed. Something had to change for the 6A fuses to be OK and then for 10 fuses to be no good.
Considering this is a vacuum conveyer I'd guess this is really some type of blower fan? Maybe the blower was being started with the intake or discharge blocked and now it's being started with the blower open. Moving air while starting will take more time and energy.
Personally, I would have used Ferraz AJT8 fuses with an overload relay for that motor.
RE: 3-phase motor current start
RE: 3-phase motor current start
That's a good guess. An Italian motor is undoubtedly IEC design and they are NOT built with any service factor as we know it in the US, so running it at 6A continuously would raise the temperature significantly. The rule-of-thumb is that insulation life is reduced by 50% for every 10deg C rise over rated temp. Running a motor at roughly 13% over FLC is going to raise the winding temperature significantly, but it's difficult to say exactly how much. So your theory on cold vs warm ambient fits as well. When it was cold, the temperature rise was not as detrimental, but when it's warm, it stacks up with the overloading to accelerate the insulation breakdown.
Fairly easy to test by the way. Rent (or call someone with) a device called a Megger. It's used to test the insulation capability of the windings and if low, the inrush current will start popping your fuses. Raising the fuse size will not likely work for long, if at all.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: 3-phase motor current start
RE: 3-phase motor current start