The One Curious Case of Induction Motor

Today I am going to present a Case of Induction Motor, which might be rare in Industries. Post is to share experience, so that the problem we faced must not be faced by others

In my last cement plant, there was a Fan called Dilution Air Fan in which there was a Motor of 225 KW, 415 Volt, & 744 RPM. There were 2 such Fans in each of the 2 units of Kiln.

So After 5 year of running of plant, we thought to replace the motor with spare motor available since last 5 -6 years and take the current motor for Over-Hauling during a planned shutdown of 4 days.

So we checked the spare motor completely in workshop, taken Winding Resistance, IR Value and Trial of Motor in No Load condition and Checked Current and Vibration in No- Load. Everything was as smooth like Butter. Everything within permissible limits.

So we replaced it with current Motor and checked all the Alignment readings after being installed.

But when Motor was connected with Fan and checked with VFD, we run the fan for 1 hour in 50% load conditions and everything was normal

Now Plant Started and after 12 hours requirement of Dilution Air fan arises. So when the Fan goes to Full speed with 100% Damper open, there was heavy vibration in Fan and Motor both.

The Vibration was coming from Fan to Motor. So Mechanical team checked the Fan and its Bearing and Damper. But Found nothing abnormal. Meanwhile some problem arises during firing in Kiln and firing was stopped.

We again checked the Motor in Decoupled condition. Again every thing was normal. Then we thought that it might be due to VFD problem, as during the same time New VFD was installed for speed control. But no problem was found in VFD.

Alignment was done again and trial taken in Both Decoupled and Coupled Condition. In De-coupled Motor was as smooth as Butter, but in Coupled Condition as Motor speed increases beyond 50% vibration level starts increasing

Like always there was tussle between departments (Electrical and Mechanical). "This is not our fault ".

So to prove Mechanical wrong we put back the original motor in Place, And to our surprise and shock, every thing was back normal in both Decoupled and Coupled condition.

It was a strange problem for everyone. we checked the motor completely after removing rotor from Stator, checked each and every things but found nothing. Then after assembling it, we took the trial again. But as usual motor was running good in No-Load condition.

Now the plant was started normally and we have all the time think of every possibilities. So After 3-4 days, it striked suddenly, can there be a wrong name plate ? If yes how to check it. We checked Resistance, Inductance, IR Value and No Load trial, everything was same with that of Actual installed motor. So what was wrong. The only thing we didn't check was RPM. As we actually believe what is written on the Motor Name Plate is "A line drawn on the Rock or Patthar Ki Lakeer"

Now we checked the RPM for the First time in 15 Days and we Found the Problem. The motor was running at 1000 RPM instead of 750 RPM. And now all the Vibration levels, Damper position and Currents level problem were clear.

Normally 99% of the time we didn't check RPM and we believe what is written on Name-plate.

In the meantime, when Problem of Motor was on going, I thought the Problem has gone "Out of Syllabus"

So Nothing is out of Syllabus. It is Just the Approach

Hope it will help

All my posts are from experience only, All Practical with Theoretical Approach