Extend motor life with thermal analysis

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Extend motor life with temperature analysis ( THERMAL ANALYSIS)

Temperature analysis ( THERMAL ANALYSIS)

Prolonged operation at high temperature shortens motor life. This depends on the permissible temperature rise ( Temperature rise) of the winding insulation and bearing temperature. Every temperature increase of 10 degrees can halve insulation life. High heat also affects the viscosity ( Viscosity) of bearing lubricant and may cause bearings to fail sooner through inadequate lubrication. Temperature measurement to prevent prolonged overheating is therefore advisable.
The temperature allowance is the margin between average winding temperature and the hottest point ( Hottest point) where maximum permissible temperature rise = maximum temperature – maximum room temperature – allowance

Every temperature increase of 10 degrees, based on a maximum room temperature of 40 degrees, approximately halves insulation life. Insulation life relates to temperature rise as shown in Figure 4 

Temperature measurement for trend comparison ( Trending) is a convenient, quick method of assessing overheating ( Over heat) in motors. Main causes include operation beyond the rating, seized bearings and misalignment ( Misalignment) Other abnormalities that may cause overheating include restricted cooling, high ambient temperature, excessive duty cycle ( Duty cycle) and abnormal supply conditions such as undervoltage, overvoltage or voltage imbalance.

Temperature should be measured while the motor is in a steady state ( Steady state) The recommended temperature measurement locations 4 comprise 7 positions: 3 points on the motor side surface, the bearings at both ends, the terminal box and the load coupling ( Coupling)

Motor temperature measurement positions

Various factors affect temperature analysis, including load, room temperature, solar radiation and measurement position. Temperature rise results from losses such as friction ( Bearing friction) windage ( Windage) iron losses ( Core loss) winding losses ( Copper loss or I²R loss) and Stray load loss which Stray load loss and Copper loss which depend on the load. Winding losses are the largest and appear as heat, so load conditions must be considered when analysing temperature trends at motor surfaces and bearings.

Remember that a higher room temperature raises motor surface temperature. Solar radiation during outdoor operation, colour, thickness and metal properties also affect the measured surface temperature. To avoid or reduce these influences, measure a shaded surface below the horizontal diameter line. 4

Measurement location is another important factor. Generally, the hottest point has the greatest mass and the least cooling. 4 Thus, in an open motor ( Open enclosure) the hottest location is at the centre, while in an enclosed motor ( Totally enclose) it is somewhere between the centre and the area receiving the least cooling air from the fan. Temperature varies around the circumference according to cooling patterns and the unequal gap between stator and housing. The closer the stator is to the motor frame, the more stable and accurate the readings. For surface temperature trend comparisons, measure at the same or nearby location each time, preferably where the stator is closest to the frame.

When analysing motor temperature rise by trend comparison, room temperature and load conditions must be compensated for. The load-compensated temperature rise ( Normalized temperature rise) is as follows:
Tpt is the measured temperature
Tamb is the ambient temperature
%load is the percentage of load at measurement relative to the motor's maximum load.