U.S. MOTORS® products were supplied as equipment for the Trans-Alaska pipeline. This fostered development of a new line of motors called ARCTIC DUTY. Motors were designed to comply with FLUOR ENGINEERING specifications: ambient temperatures to -70°F and seismic forces which might result from earthquakes.
The following items point out some of the design aspects considered during the development of this line:
Insulation Systems - The major consideration in selecting materials was embrittlement at low temperatures and thermal shock. The winding insulation system had to be capable of operating over a wide range of ambient conditions and not crack or lose its dielectric strength. While the basic materials selected inherently had good moisture resistance, internal space heaters were also used. These were a flexible silicone rubber "on winding" type with low watt density and excellent moisture resistance and thermal stability over wide temperature ranges.
Bearings and Lubrication - The coefficient of expansion of ball bearing ring materials versus shaft and housing was first analyzed. Inner ring shaft fit and outer ring bracket fit were determined to ensure that the internal ball clearance was sufficient throughout the temperature range involved. Generally, there was negligible variance and clearances were satisfactory for standard motors subjected to low temperatures. Selection of a lubricant which would not solidify was an essential consideration. Grease with a synthetic lubricant base and a non-soap thickener was specified. It had a slightly higher penetration (265-320) than conventional ball bearing greases, giving it consistency between NGLI-1 and NGLI-2.
Bearing seal materials that do not become embrittled at low temperatures were used on sealed bearings. Life was ensured further, as at normal ambients, by keeping the grease moisture free. Space heaters were recommended to minimize condensation (for those units which would be inoperative for long periods).
Gaskets and Shaft Slingers - Embrittlement was the primary concern at low temperatures. Neoprene, a common gasketing material, becomes brittle below -30°F. It was confirmed by tests that silicone rubber elastomers remain resilient and functional at -70°F. Some compounds, in fact, have brittle points below -150°F. Other materials qualified by and meeting Military Specs to the standard -65°F level, were found acceptable. Other properties reviewed in our study were tear resistance, tensile strength, abrasion resistance and resistance to oil. Flexing tests, simulating the function of individual parts, were conducted at low temperature to ensure integrity.
Leads - To confirm that materials were suitable at low temperatures, Cold Bend tests were conducted to ensure that lead cable insulation did not crack. Although connection of leads by personnel didn’t ordinarily take place in these extremes (Alyeska specs. Require -31°F), embrittlement could have been a problem when motors were handled roughly in transit or when they were removed from storage.
Fans - Some of the phenolic fans proved suitable at these temperatures. Those selected did not show plastic deformation and retained their initial toughness. Actual tensile strengths increased as temperature was lowered. Metallic fans were also found to retain their strength. Most motors designed for Arctic Duty experienced -70°F temperature only while in transit to the job site. In those rare cases where equipment would operate outside and especially where it might be inoperative at times it was recommended that a protective shroud be supplied to prevent accumulation of snow and ice.
Starting Current - Starting current, or initial inrush (Is), is generated per the following relationship:
E = Voltage
X1 = Reactance
Starting Current - Current increases on induction motors as the temperature is significantly reduced. This is due to the lower resistance in the stator (R1) and rotor (R21) conductors. When the stator and rotor conductors are initially at -70°F, current increases. The increase is higher on small sizes since resistance is inherently higher in relation to reactance (Xo) and thus a more influential effect in the equation.
Although amperes are initially higher at -70°F, they diminish quickly due to the combined factors, during acceleration, of (1) speed-current relationship (current diminishes as speed increases) and, (2) I2R heating of stator and rotor conductors, causing increased resistance and return toward normal accelerating current values.
Controls and protective circuitry for motors had to be designed with the increased starting currents in mind even though the inrush KVA represented by smaller motors was minimal.
Running Efficiency - Effects upon running performance at these temperature levels was slight. Running efficiency increased slightly due to lower I2R losses in the stator and rotor.
Transmission of Torque - Shaft steels become tougher as temperature decreases. Steels do lose ductility, however, and are less resistant to impact or shock loading. Notch toughness decreases with temperature.
Since loading on shafts was torsional, and there were no high impact loads, however, this was not significant. Motor applications here were not subject to high impact conditions.
It was determined that shaft failure would not occur at -70°F even with high external impact loads which might occur in shipping or handling. Tests, both torsional and impact, were run at -100°F. Also, drawing from previous successful experience with other steels at even lower temperatures, notch toughness levels compared favorable. Cryogenic pump shafting of 17-4 stainless has proven well at temperature as low as -270°F with notch toughness levels equivalent to the shaft steel used for -70°F Arctic Duty applications.
Brackets, Frame, Outlet Box - Grey iron has increased tensile strength at 70°F and impact strength does not change significantly since ductility decreases gradually. Impact tests were run at -100°F to ensure that castings would withstand abnormal forces which might occur during shipping or handling. It was determined that the toughness of grey iron - when subject to forces resulting from torque generation, overhung loads, seismic forces or abnormal handling was adequate, especially when considering our safety factors already designed into castings at normal temperatures (one higher grade of iron than that needed for normal stress levels is generally specified as a standard practice).
Fastening Devices - SAE Grade 5 bolts met all ordinary operational requirements to -70°F. The American Society for Metals Committee on Bolts recommended this grade at these levels. It was determined that bolts would be marginal only for abnormally large impact loads, not prevalent in the application considered.
Subsequent seismic proof tests on a shake table also confirmed the ability of the motor structure and fastening devices to withstand both operational and impact forces far above those normally anticipated. The change in strength of materials at lower temperature was considered in this comparison and analysis.
Hazardous Duty Motors:
Flame Paths - The first consideration was whether flame path dimensions between close fitting metal to metal joints (shaft to bracket, bracket to frame) would be affected adversely due to different contraction rates for dissimilar metals. Analysis showed that the effect was minimal since the rate of contraction of commonly used materials was close. Grey iron coefficient = 6.55 x 10-6 in/in/°F; steel = 6.33 x 10-6 in/in/°F.
Analysis was confirmed by subsequent tests at Underwriters’ Laboratories.
Sealants - Where motor lead wire passes through frame openings, sealants are used. This material had to have adequate strength to resist internal explosive forces and also retain its physical properties at reduced temperatures. Also critical were its resistance to shrinkage and cracking. Several compounds were tested, and some of the two-part polyurethane resin systems exhibited acceptable characteristics. Their advantage lay in their ability to maintain a certain degree of resilience at low temperature. Thus, they "gave" slightly as other materials (embedded lead wire and surrounding housing) contracted and expanded. Both simulated and final configuration tests were run - the latter at Underwriters’ Labs (explosion test at -70°F).
Motor Structure - Hazardous Duty motor structures are designed to contain those forces resulting from an internal explosion. Tests at Underwriters’ Labs at low temperature revealed that these forces increase substantially when compared to normal temperature conditions. Forces were approximately 1.8 times as high at -70°F. Underwriters’ Labs explained as follows:
"There appear to be a number of reasons for the higher explosion pressures observed in the low ambient tests. Included are increased density of the mixture, increase available energy…"
To retain those safety factors of strength required by U.L., it was necessary to increase cast iron tensile strengths for most frame sizes.
Other parts also required investigation and changes, in some cases, due to the higher explosive forces: Conduit boxes, bolts, drain fittings, separate outlet box fitting, etc.
Ability to withstand seismic disturbances - 55 basic designs, ranging from frame 56 through 5800 were analyzed extensively in order to meet Fluor spec SP-4459-40-3. This spec delineated the design spectra (i.e., characteristics of earthquake input motion) for various locations along the pipeline. Two motor sizes were tested on a shake table at full speed and load. Calculations were then performed on all remaining designs in order to determine fragility limits of various components.
As a result of these efforts, motors have operated successfully on the Pipeline for several years. Arctic Duty design ratings are shown in the Price Book as type TCA (Totally Enclosed Fan Cooled) and LCA (Explosion-proof). Generally, features are:
A. CORRO-DUTY® Modifications
B. Special Gasket and Slinger Materials (TCA)
C. Special Lubricants - Low temperature grease
D. Special Lead Potting Compound (LCA)
E. High Tensile Strength Cast Iron (210 Frame and UP - LCA)
An IEEE paper was developed as a result of the Alaskan pipeline project and can be referenced for further information: Paper number PC1-76-32, copyrighted 1977, by R. Cole and V. Binns.