Motor Misalignment Is Costly

By Howard W Penrose, Ph.D., CMRP, MotorDoc

In addition to the reduced reliability of a misaligned electric motor, there is an energy and emissions impact. The lost energy is absorbed by components such as bearings and the coupling, as well as airgap losses, which shorten the life of those components. The losses increase as the machine deviates from a “perfect” alignment, which can be defined as a precision alignment, or tighter tolerances than those identified in ANSI/ASA S2.75-217 (Shaft Alignment Methodology, Part 1: General Principles, Methods, Practices and Tolerances).

The severity of misalignment can be tied to an increase in machine losses. Figure 1 resulted from an EPRI study of Electrical Signature Analysis (ESA) using a 10-hp., 1,785-rpm electric motor.

A larger 600-hp, 1,789-rpm, 460-V, 652-A motor, directly coupled to a pump, was showing misalignment and initial bearing defects. Shim location resulted in a soft foot situation on the stator side of the torqued bolts on all feet, causing misalignment and static eccentricity.

Reviewing the losses associated with the soft-foot situation (described above), as detected with ESA, Fig. 2 shows that static eccentricity totals 1.05 kW on a variable-frequency drive operating at 41.8 Hz and a total of 176 kW. Bearing losses totaled an additional 1.2 kW for 2.25 kW demand (1.3% of total load) with an output of 165 kW (shaft torque). The 11 kW of losses (Fig. 3) are 20.5% more than a proper alignment.

The kilowatt losses associated with static eccentricity from alignment soft foot are calculated as follows. If the motor operates at this load and speed for 6,000 hr./yr., $10/kW demand, $0.10/kWh usage, and the national average of 0.707 metric tonnes CO2/MWh we find:

6,000 hr. x 2.25 kW = 13,500 kWh

2.25 kW demand x $10/kW x 12 mo./yr. = $270 in additional demand charges

13,500 kWh x $0.10/kWh = $1,350/yr.

13.5 MWh x 0.707 metric tonnes CO2/MWh = ~9.6 metric tonnes CO2/yr.

This results in a total cost of at least $1,620/yr. plus ~9.6 metric tonnes of CO2/yr. from poor alignment practices. Not included is the additional load of the coupling and pump bearings. Even in smaller motors, the results are similar in relation to their size and load. EP

Howard W. Penrose, PhD, CMRP, is president of MotorDoc LLC, Lombard, IL (motordoc.com). He chairs the wind-power standards and government relations participation for American Clean Power (ACP/AWEA), holds various IEEE standards positions, and is a past chair of SMRP. Reach him at info@motordoc.com.