3 phase motor problem
3 phase motor problem
(OP)
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?
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?





RE: 3 phase motor problem
RE: 3 phase motor problem
RE: 3 phase motor problem
RE: 3 phase motor problem
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: 3 phase motor problem
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
RE: 3 phase motor problem
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.
RE: 3 phase motor problem
RE: 3 phase motor problem
RE: 3 phase motor problem
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: 3 phase motor problem
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.
RE: 3 phase motor problem
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: 3 phase motor problem
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”
RE: 3 phase motor problem
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)
RE: 3 phase motor problem