MCA Detects Where Vibration Analysis Could Not

Case Study: Power Circuit

We examine how Motor Circuit Analysis (MCA) technology was employed to extract essential information necessary to drive repairs for a complicated piece of equipment.

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:

The inability to run the #2 Coal Pulverizer (“Pulverizer”) places the plant in a state of reduced reliability.
The motor was tripping off-line without material loaded into the Pulverizer.
The initial 30 second inrush set of data was inconclusive without a baseline inrush set of data with which to compare it.
Vibration analysis 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.

The Solution

The Pulverizer and its main gearbox were disassembled. It was found that the final beveled gear was not properly meshed with the main shaft gear, and a significant amount of backlash was evident. This, coupled with a binding skid plate, was causing an oscillation equal to the revolution of its final gear. The gear reduction was 5:1, putting the main shaft at 116 rpm.

MCA technology is a non-destructive testing technology that measures inherent characteristics within a motor circuit. The Demodulation Spectrum is normally used as a trending tool; however, in this case we were able to utilize corresponding data and, through direct communication with knowledgeable personnel, refer our client towards the appropriate section of a complicated piece of equipment to make the needed repairs.