Power of MCA

Background and Problem

During routine operation of a coal fired steam plant within a client’s facility, a 100 HP motor that drives one of the two coal pulverizers was tripping off-line shortly after start-up.

Industry:	Corn Milling
Motor:	Siemens
HP:	100
Fault Zone:	Power Circuit
Voltage:	460
Speed:	585 rpm

We began our investigation by considering several variables brought to light with the incident:

1. The inability to run the #2 Coal Pulverizer (“Pulverizer”) places the plant in a state of reduced reliability.
2. The motor was tripping off-line without material loaded into the Pulverizer.
3. The initial 30 second inrush set of data was inconclusive without a baseline inrush set of data with which to compare it.
4. Vibration analysisThis Predictive Maintenance technique is widely used to evaluate mechanical rotating equipment to determine if any undesirable changes are present that might give an early indication of imminent failure. Uses transducers to translate a vibration amplitude and frequency into electronic signals to determine the equipment’s actual condition. This may lead to the recommendation of a logical course of maintenance actions to correct the problem before secondary damage or catastrophic failure can occur. Additionally Vibration Analysis can be used for the modeling, prediction, measurement and analysis of structural dynamic response in design and root cause failure analysis. In design, vibration modeling and prediction is used to anticipate and avoid undesirable dynamic response. In root cause failure analysis it is used both to understand undesirable response and as a factor in determining true root cause. was detecting increased vibration levels; however, due to the configuration and construction of the equipment, a source could not be isolated.

The Investigation

Using Motor Circuit Analysis (MCA) technology, the load current was captured during the first few minutes of operation (Figures 1 – 4). An initial 10 amp oscillating load current was discovered that gradually increased to 70 amps before it tripped the motor overloads at 168.9 amps. Peak-to-peak time measurements of these oscillations were very uniform at 0.51 ±0.01 seconds.

The motor was restarted and a Demodulated Spectrum captured (Figure 5) to further narrow down whether or not this issue could be load related or a rotor issue.

At 1.934 Hz, there was a 2.8 dB peak having a mechanical speed of 116 rpm. The 116 rpm correlated with the 0.51 ±0.01 second load oscillations captured during the inrush current spectrum. After conferring with the client’s M&R Department, it was learned that the Pulverizer’s main shaft rotated close to 2 times per second. The recommendation was then made for the client’s M&R Department to look into gear misalignment or rubbing associated with the equipment’s main shaft.

Armed with useful data, the client’s M&R Department was able to focus their search towards the Pulverizer’s main shaft and associated gear reducing section.