Troubleshooting series: High operating temperature

It’s 2:01 pm and there’s four hours left until the last batch of product will be ready to ship. You — and your equipment — have been running at 120% of capacity to make this shipment. Then, at 2:55 pm your phone dings with a new notification: “Motors@Work detects Air Compressor #1 Motor running at 322°F, nearing max operating temperature allowed per OEM spec.”

Heat is perhaps the most damaging stress your motor experiences. Yet, in my experience, many motor users rely on built-in thermal protection devices (e.g., RTDs) to protect their motors from heat damage — without any early warning or feedback about why the motor tripped off.

Here’s how Motors@Work helps you identify, troubleshoot, and resolve motor-damaging temperature issues that otherwise would have gone undetected without our timely condition-monitoring alerts.

About an hour later your phone dings with a new notification: “Motors@Work detects Air Compressor #1 Motor running at 333°F, 4°F above max ambient temperature allowed per OEM spec. PROVIDE EXTERNAL COOLING OR DERATE MOTOR & SHED LOAD IMMEDIATELY.”

ABOUT MOTOR INSULATION RATINGS 
Four degrees over the OEM specification may not sound like much, but every 10°C (50°F) rise in operating temperature cuts your motor’s expected life by half.This well-cited rule-of-thumb originates from two standards — 1) the National Electrical Manufacturers Association (NEMA) Motors & Generators standard, commonly referred to as MG1; and 2) the International Electromechanical Commission (IEC) Standard 60085 — and the Arrhenius equation governing how epoxy insulation chemicals decay versus temperature.

The NEMA & IEC standards state that the life expectancy of motor winding insulation when operated continuously at the maximum temperature rating of the insulation should be at least 20,000 hours under “otherwise ideal” conditions. These ideal conditions include

  • Load less than service factor;
  • Ambient temperature below 40°C (105°F;
  • Within ±10% of rated voltage;
  • Phase-to-phase voltage unbalance of less than 1%; and
  • Total harmonic distortion of less than 4%.

It used to be that maintenance staff simply touched a motor surface (skin) to see if it was “running hot.” Any motor too hot to touch was overheating. But, today’s motors run hotter than human hands can tolerate. The table below lists allowable temperature rise above ambient rating by motor type and NEMA insulation class (IEC insulation classes list the allowed temperature rise in degrees Celsius right on the nameplate) per MG1 § 12.43.

MOTOR TYPE TEMPERATURE RISE [°C]
A B F H
Medium (1.5- to 500-Hp, 2- & 4-pole machines, + 1.5- to 350-Hp, 6-pole) machines with…
…a 1.0 service factor 60 80 105 125
…a 1.15 or higher service factor 70 90 115
…a totally enclosed non-ventilated enclosure 65 85 110 130
…encapsulated windings 65 85 110
Larger machines with a 1.0 service factor rated…
…all sizes with temperature measured by resistance method 60 80 105 125
…1,500 Hp or less with an embedded detector (e.g., RTD) 70 90 115 140
…1,500 Hp or more for 7000 Volts or less with an embedded detector (e.g., RTD) 65 85 110 135
…1500 Hp or more for over 7000 Volts with an embedded detector (e.g., RTD) 60 80 105 125
Larger machines with a 1.15 service factor rated…
…all sizes with temperature measured by resistance method 70 90 115 135
…1500 Hp or less with an embedded detector (e.g., RTD) 80 100 125 150
…1500 Hp or more at 7000 Volts or less with an embedded detector (e.g., RTD) 75 95 120 145
…1500 Hp or more at over 7000 Volts with an embedded detector (e.g., RTD) 70 90 115 135

Note, motor surface temperatures typically run much cooler than internal winding temperatures. For example, the surface of totally enclosed fan-cooled motors averages 20°C to 25°C less than the winding hot spot; for open drip-proof and weather-protected enclosures, the differential may be greater than 60°C. The maximum internal “hot spot” temperatures by insulation class are 105°C for Class A, 130°C for Class B, 155°C for Class F, and 180°C for Class H.

You can also make the Arrhenius relationship work to your benefit: every 10°C decrease in motor operating temperature doubles motor life. That means operating a Class H motor operated continuously below Class F’s limit of 155°C extends its life to more than 100,000 hours.

You can’t afford to have this motor go down now, so you dispatch a maintenance technician. The technician finds that the compressor room well above the 105°F maximum ambient temperature due to a closed door and closed air vents. Moreover, someone parked a crate right behind this motor, inhibiting its ability to circulate air and cool itself.

Moving the crate, opening the room’s air vents and doors, and adding some floor fans quickly bring the room — and motor — temperatures back into the normal range.

Motors@Work’s continuous, near-real-time condition monitoring algorithms watch for these motor-damaging temperatures and send out content-rich alerts that facilitate troubleshooting — so you can keep your motor-driven systems operating reliably.

How will condition monitoring benefit your organization? Email Nicole at info@motorsatwork.com to learn more.

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