Go below the surface to monitor motor temperature

Using a digital infrared thermometer to check motor bearing temperature can help monitor winding temperature.

04/28/2012


Using a digital infrared thermometer to check motor bearing temperature can help monitor winding temperature. Courtesy: EASA“We had a motor rewound, and now that it’s back in service it’s running hot. The frame is getting so hot that we can’t put our hands on it. Did the service center rewind the motor incorrectly?”

Has this ever happened to you? After installing a freshly rewound motor, your maintenance crew tells you it’s running hot. Although no design change was involved and the test results look good, you’re on the verge of calling the service center to demand another rewind.

Before picking up the phone, though, you should investigate for yourself to see if there’s really a problem. Quite possibly the motor always ran “hot,” even when it was in good working order. It’s also unlikely that anyone on-site ever checked for this by touching the frame.

You can’t tell by frame temperature

There’s no way to determine the winding temperature of a motor just by feeling the frame. Good practice and safety considerations dictate that the temperature of any part of a motor should be checked with a temperature measuring device. The maximum temperature rating, based on the class of insulation, applies to the winding temperature at the hottest spot inside the motor. As a general rule, the temperature of the frame can be 20 to 40 C cooler (less or more), depending on the design of the frame and the enclosure.

Insulation class

The insulation class of a winding is determined by the maximum temperature it can withstand before complete breakdown. Many motors manufactured today use a Class F (155 C) system. Most service centers use a Class H (180 C) system on all rewinds. Assuming that the ambient temperature is 40 C, NEMA Standards MG 1 states that at 1.0 service factor, the maximum winding temperature rise above ambient for a Class F system is 105 C.

It’s very important to remember that even though the insulation system may be able to withstand very high temperatures, for every 10 C rise in total winding temperature, the insulation life will be cut in half.

Example

Though it may seem counterintuitive, the frame can be quite hot even if the motor is running perfectly fine. As an example, assume a high-efficiency design motor with Class F insulation has a 40 C ambient temperature and operates with a Class B temperature rise—i.e., an 80 C winding temperature rise.

At full load, that means the total winding temperature would be somewhere around 120 C (80 C + 40 C), well below the design limit for the Class F insulation. If we estimate that the frame temperature is 40 C lower than the winding temperature (pretty liberal), then the surface temperature of the frame is going to be about 80 C, or 176 F. Suffice it to say that not many people could or should hold their hand on a frame that hot for any length of time.

Check the amps

The best way to determine if the motor is running properly is to check the current draw with an ammeter. If the current is less than or equal to the nameplate current, then the motor is probably not overheating unless the ventilation system is not providing enough cooling air.

If the current is higher than the nameplate reading, then it’s possible that the motor is running too hot. Overloading, high or low voltage, restricted ventilation, or high ambient temperatures are the most common causes of motor overheating.

Older motors

In general, the old NEMA U-frame motors ran cooler than today’s T-frames, even those with a high-efficiency rating, mainly because their insulation systems had lower temperature ratings–e.g., Class A (105 C). If a T-frame replaces a U-frame motor, it can be alarming to see how much hotter it runs, even though it’s actually running more efficiently than an old U-frame.

Explosion-proof motors

In some cases, it’s very important that the surface temperature of the motor frame be kept somewhat cool. For explosion-proof motors, the surface temperature must be below the ignition point of the hazardous materials in the environment. Some of these motors come equipped with temperature sensors that will trip the motor offline when the winding temperature exceeds a certain level. In the U.S., UL sets the standard temperatures based on the gas, vapor, or dust present in the environment. However, there is no standard for other general-purpose motors.

Conclusion

The best way to determine if a motor is running within a safe operating temperature is to monitor its winding temperature with temperature sensors like thermostats or resistance temperature detectors. As mentioned before, you can also monitor the current to get a basic idea of how the motor is running.

Relying solely on the surface temperature of the frame, however, is not an accurate or safe way to determine the “safe” operating temperature of the motor winding.

Bishop is a senior technical support specialist at the Electrical Apparatus Service Association (EASA), St. Louis, Mo.; 314-993-2220 (phone); 314-993-1269 (fax); www.easa.com; easainfo(at)easa.com. EASA is an international trade association of more than 1,900 firms in 58 countries that sell and service electrical, electronic, and mechanical apparatus.



No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
Strategic outsourcing delivers efficiency; Sleeve bearing clearance; Causes of water hammer; Improve air quality; Maintenance safety; GAMS preview
World-class maintenance: The three keys to success - Deploy people, process and technology; 2016 Lubrication Guide; Why hydraulic systems get hot
Your leaks start here: Take a disciplined approach with your hydraulic system; U.S. presence at Hannover Messe a rousing success
Flexible offshore fire protection; Big Data's impact on operations; Bridging the skills gap; Identifying security risks
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
Getting to the bottom of subsea repairs: Older pipelines need more attention, and operators need a repair strategy; OTC preview; Offshore production difficult - and crucial
Applying network redundancy; Overcoming loop tuning challenges; PID control and networks
Putting COPS into context; Designing medium-voltage electrical systems; Planning and designing resilient, efficient data centers; The nine steps of designing generator fuel systems
Package boilers; Natural gas infrared heating; Thermal treasure; Standby generation; Natural gas supports green efforts

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role that compressed air plays in manufacturing plants.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
This article collection contains several articles on strategic maintenance and understanding all the parts of your plant.
click me