3-phase motor current start 1

[jgeraldini]

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?

 

[fangas]

What type fuses?

RK, CC, ....

Ed

 

[jgeraldini]

the ones I am currently using are time-delay 10 amp 500V 10kA IR (Ferraz Shawmut ATQ10). I guess they are midget fuses.

 

[fangas]

it's been a long time. around 20 years. But I once had a problem similiar to yours.

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

 

[jraef]

If the FLA is 5.2A and the motor is pulling 6A, you are already in trouble, but if it has a 1.15 Service Factor, the OEM who built the system has pawned the issue off on to you in order to avoid using a 5HP motor (I am assuming it is a 3HP now, you didn't say) by designing it with a motor that needs to run into the Service Factor continuously. You probably need a bigger motor or you need to reduce the load.

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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[FreddyNurk]

In addition to jraef's excellent comments, I noted that you stated that it originally had 6A fuses, but then it just started blowing fuses.

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.

 

[LionelHutz]

Yes, you indicate 6A fuses then it began blowing fuses... Do you mean it worked fine for some period of time with the 6A fuses?

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.

 

[jgeraldini]

Hey, thanks to all of you, your answers have been very useful. Now, first question from jraef, this motor has 2.6kW an runs at 3,450 rpm. The whole stuff is a pretty good machine, very well built, the motor and the fan are Italian made and the control is German made. It has a Siemens Sirius 3RU1116-1GB0 as overload relay, has a contactor Siemens Sirius 3RT1016-1AP01, just to give you an idea. He have 5 of them, but different sizes. This one is the biggest one and, of course, more used. The OL relay is setted for 6 amps (it has adjusts from 4.5 to 6.3 amps). Then there is the logic control that I assume it is ok. As lionelhutz pointed out, if I remove the hose suction, still the start current is around 20 amps, then it goes to 4.7 amps with no load. This equipment has been working since October 2008 and it always worked very hot. I haven't checked its amperage before as it never failed before. Todays production is not bigger than we have had in the past, but it was colder (we are in Houston area). Today's rate is around 680 lb/h and we have done over 1,000 lb/h in January. Yesterday it blowed some more fuses during the day, but I could not stop, so I kept changing them. I called the manufacturer again and he told me to use 15 amps instead and rewind the motor as soon as I can. To me it is clear that this piece of equipment is undersized for this extruder, and I have asked them to substitute it for a bigger one. It is in study yet our request. The point is: why in the world it worked for about 9 months without problems and now it is giving this issues? Could it be the insulation that is going away (to much stress)? Again, thanks for you help.

 

[jraef]

"The point is: why in the world it worked for about 9 months without problems and now it is giving this issues? Could it be the insulation that is going away (to much stress)? Again, thanks for you help."
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

 

[jgeraldini]

thanks a lot jraef! I am going to find one Megger over here. I really appreciate your advices. Have a nice weekend!

 

[DickDV]

One thing jraef didn't mention. You need a 1000V megger. There are 250V and 500V models out there but, on 480V, you will want the 1000V unit. Otherwise, the test really doesn't stress the insulation adequately to be meaningful.