Tom Trierweiler
Technical Education Committee Member
Malloy Electric Industrial Service Center
Explosion proof (XP and EX) motors are an integral and required component in maintaining safety around hazardous locations containing flammable gases, vapors or combustible dust. Prevention of an internal spark, flame or explosion exiting the motor and contacting the ambient air is required to be certified by either the Underwriters Laboratory (UL), Mine Safety and Health Administration (MSHA) or Canadian Standards Association (CSA). A spark, flame or explosion in a hazardous location may lead to a larger explosion or a series of explosions.
There are systems in place to limit external surface temperatures on motors for Class I, Group C or D and Class II, Group E, F or G. One of these systems is a thermal connected to the control circuit.
What is a Thermal Protector?
Thermal protection may be used to monitor temperature in XP and EX motors. These devices will trip the motor when a certain temperature within a range is reached and can be found embedded in the winding. A thermal protection device behaves much like a thermostat. When rated temperature is exceeded, one of the bimetallic strips will bend. The electrical contact points are now separated (or closed), which leads to the circuit being open (or closed), stopping the motor and preventing excessive skin temperature. An illustration can be seen in Figure 1.
The rating of a thermal used on a UL Class I, Group C or D and Class II, Group E, F or G explosion proof motor is from 120°C to 130°C with a ± 5°C tolerance; this equates to a range of 248°F to 266°F with a ± 9°F tolerance. Once an explosion proof motor is in for repair, governing bodies require the repairer to verify the thermal opens and closes within the designed temperature range. Monitoring heat is only one of several safeguards included in XP and EX motors to support workplace safety.
Do You Test Your Thermals?
Thermals should be tested before the motor is returned to the customer. To do so, connect the tester to the motor’s thermal leads. (See Figure 2.) The motor is then placed into a bake oven, which is set to at least 10°C (18°F) above the motor’s rated temperature. Plug in the tester, turn it “on” and press the test button to start the test. Once the thermal has reached an appropriate temperature for the contact points to separate (or close), the light on the tester will go off (or on). At this point, turn off the oven and allow the winding and its thermals to cool. Once the thermals cool sufficiently, they will close (or open, if normally open). This can be tested by pressing the test button and looking for the light to come back on, confirming the thermals function properly.
The Results of Testing
When performing this test, the temperature can be monitored in real time to confirm the exact temperature at which the thermal opened. Figure 3 shows the test light being on, which indicates the thermal being closed. The thermal being tested is rated for 248°F ± 9°F (120°C ± -12.8°C) and tripped at 252.5°F (122.5°C). When the test light shuts off, this indicates the thermal has opened. (See Figure 4.) Figure 5 shows the thermal having cooled and the test light turning on.
Performing this test allows us to confirm the thermal has opened and closed, functioning as designed. This is one of many crucial steps we take to ensure the safety of our repairs to maintain UL certification. The next time you see an XP or EX motor, remember the required practices in place to maintain a safe working environment.
Precision Is Key
The precise placement of the thermals is crucial to getting an accurate measurement. In other words, your home's thermostat does not reside outside your home. For UL Class I, Group C or D and Class II, Group E, F or G, a minimum of three thermals will either be bound to the outside of the coil end turns or wound into the end turns. In either case, the thermals should be located on the drive end of the motor; which runs hotter than the fan end.
At our business, we take precise action to ensure motors running warmer than designed will not pose risk to employees or production.
AVAILABLE IN SPANISH
Related Reference and Training Materials
Print
