Current Spikes on Crusher Motors at Power Plant

[TM519]

A few months ago we replaced the Coal Crusher motors at one of our plants, the nameplates match up the same as the old ones, nothing has changed the motors were just really old. We are seeing occasional current spikes that we were not seeing before. It makes sense to me that we would see occasional spikes just because of it being a crusher motor and there can be larger/harder pieces that get tossed in to be crushed, but the spikes we are seeing are above and beyond what we have seen in the past, and the HP and Torque rating of the motor matches the motor that it replaced. Any ideas?

Currently we have a high alarm at 111A (the Full Load Amp rating on the nameplate) and a high-high alarm at 115A. The DCS requires amps to drop by 4A to clear the alarm. Is there a recommendation that anyone here has for the high alarm above nameplate amps? Just wondering what others are used to seeing and if they've had to make adjustments to their DCS system for their crusher motors as well. Appreciate any insight.

 

[Jraef]

A not uncommon thing to happen with crushers is that someone bought a “crusher duty” motor and the old ones were not, they were basic Design B motors (in terms of torque-speed curves). Some crusher motors are built as Design A, which results in higher torque at a similar step change in speed.
So the full load torque and speed is the same, thus the HP is the same, but the motor’s reaction to load is different .

 

[zbang]

How old were the old motors (pre NEMA?)? I'm sure that Jreaf can comment on this, but IIRC some of the old ones had amazing torque for the HP rating; has something to do with the armature diameter. (I recall back in the late 1970s when someone replaced 1930's 3HP fan motors with new ones of the same HP. A month later they installed 7.5hp motors so the fans would actually work. 'course there was nothing wrong with the old ones other than age.)