Today I am going to present a Case of Induction Motor, which might be rare in Industries. Hence want to share the experience, so that the problem we faced must not be faced by others or can be avoided.
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, making it to total of 4 Nos Fan and hence 4 Nos motor with 1 Nos spare (in Store) between all the four.
After 5 years of running of plant, we thought to replace the running motor with spare motor, available in store since last 4 -5 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 as Butter. Everything was within permissible limits.
And we replaced it with current Motor and checked all the Alignment readings after being installed, Alignment readings were perfect.
But when Motor was coupled with Fan and checked with VFD for Trial, we run the fan for 1 hour in 40 – 50% load conditions and everything was normal. Due to certain work being carried out by mechanical, we could not take the speed to more than 50%.
Now firing of plant started after 2 days and after 12 hours requirement of Dilution Air fan arises. Now, 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, its Bearing, and its 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, everything 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.