3 phase motor problem

[chaz427]

hi there,i have a problem with a 45kw squirrel cage motor running on a 415v supply,it runs a quarry screen which is quite a high initial inertia,it has always been started dol with a by pass contactor arrangement for the first 10 seconds of starting to short out the overload module to enable cold starts
i was called in today to replace a burnt out motor,which i did,but now the new motor fires up ok and after a few seconds reaches a set speed which is no where near the correct rpm,i have checked all the control gear and cannot find any noticable problems so am now drawing a bit of a blank,the motor was initially swapped as i thought it may have been a bad one but the other was the same,i have seen many faults on motors over the years but none as strange as this,any thoughts on this please?

 

[sreid]

My first guess would be binding or friction in the mechanisn that is overloading the motor.

 

[chaz427]

that was a thought,its hard to eliminate the load though as its such a heavy machine to turn over and judge if its tight but thanks

 

[davidbeach]

If the motor is intended to be connected in delta at that voltage and you have it connected in wye, you have it running at reduced voltage and it never makes the transition to full voltage.

 

[jraef]

That was my first thought as well.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

What is the current at the new reduced speed? If the current is moderate, the original motor may have been wound for a different speed.
Does it come up to speed with the belts off?
Another possibility that I have seen; we had a gravel screen with adjustable weights to set the magnitude of vibration. The bolts came loose and the weights went to the maximum shake position. This badly overloaded the motor. The overloads saved the motor and we were able to find and remedy the problem before we lost a motor.
yours

 

[ozmosis]

What type of motor have you replaced it with?
A few years ago at an aggregate company I was doing business with, they had a group policy to fit high efficiency motors throughout their sites if they failed. A lot of specialised motors (crusher motors for instance) and motors requiring a high starting torque were replaced and didn't work. We pointed out that they wouldn't as the HE motors lack the starting torque the motors replaced were capable of. It was fairly obvious on crusher motors that this should be the case but other motors were also prone, due to the starting conditions in the quarries.
What is the actual difference in speed you are seeing? If the actual speed is way below the rated speed on the nameplate, you should be seeing the current exceeding the rated current on the nameplate.

 

[stardelta]

Are you sure the replacement motor was a direct replacement. Is it possible you have fitted a motor that runs at a different speed to the original?

 

[chaz427]

thanks for the advice,after some more thorough checks it would seem that the load has become the main point of issue,the motor seems to be doing its job,it starts and runs up to a set speed but trips due to excessive currents, according to the sceen manufacturer it should be at least 55kw instead of a 45kw,and should it not have enough initial torque to get he screen up to speed, it will cause similar symptoms to what has been seen,why it has happened overnight is the odd thing,although it does hate the cold weather and have some big oil filled gearboxes in place,i think the next step is to fit a high quality motor specified by the manufacturer, and a site visit by them on monday to witness the problem,i will keep you posted

 

[jraef]

Heh, that's probably why the old motor eventually burned up!

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[ozmosis]

chaz427
I'd also double check the protection to the (new) motor. Sounds like in the past the fusegear may not have been doing its job properly.

 

[jraef]

Actually, what I would suspect (having worked in a lot of crushing operations), is that the control gear was built for 55kW by the screen OEM, but somewhere along the line the original motor was damaged and someone needed to get the screen back on-line but only had a 45kW motor. So they threw that in "temporarilly" and never changed the overloads. That happens a lot in crushing operations. You need to check for sure, but don't be surprised to find that.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[aolalde]

chaz427

The motor power in kW or HP is intended for the full speed operation. Induction motors have different speed torque curves and mainly those are fitted to properly accelerate the load connected. In USA designs A, B, C, D etc have different speed torque curves. Even some machines require a tailored design to fit the application.

That is why many people have problem replacing electric motors; they do not pay attention to the motor design some times blinded by other parameters like “high efficiency motor”.

My message is. Replace the motor with the very same original “speed-torque design”

 

[bigbillnky]

I totally agree with aolade. Most people will look at HP(blast the KW rating), RPM, and frame size when replacing a failed motor. They never take into consideration the design letter designation on the nameplate, which represents a significant difference in both starting torque and breakdown torque.

Bigbillnky,C.E.F.....(Chief Electrical Flunky)

 

[chaz427]

it seems there is a mechanical problem with the load,it has a worn drive shaft which wont allow it to reach its equired speed due to the counterweight assemblies used,so fingers crossed should be rectified as soon as the parts arrive,would have been easier to diagnose if the manufacturer had not said that this couldnt happen and was an electrical problem!