All of our explosion-proof motors(Class 1 Div.1 Group C & D) have an automatic-reset temp. switch that is wired in the control circuit. I was under the impression that the purpose is to open the control circuit in the event that the surface temp. exceeds the ignition temp. of the flammable vapors, and therefore designed into all similar E-P motors. One of our Asian plants is specifying this temp. switch only for E-P motors over 15 HP.
Interesting post, never even gave the surface temperature a thought on thermal cut out. Don't believe thats the case on gas pumps. Seen some mighty hot cases that weren't kicking out on the internal thermal of fractional EP motors. On Submerged motors, I hope the case doesn't get that hot but I change sub. motors all the time that are cycling on off because of the thermal.
Might be interesting to use on of those infrared thermometers to check case temps although I'm not sure that gasoline would flash just because the case is over the flash point. The vapors would have to be over the flash point.
It is a desirable form of overload protection, particularly if the protected motor is exposed to above-normal ambient temperature. Winding insulation heated by routine operation AND high ambients should properly use some means of stator-temperature sensing, and that is not possible with a starter-mounted overload block, and is especially useless if the starter-mounted overload relay is "ambient compensated."
Be careful if "2-wire" control is used—there is a potential violation at 430-43. Automatic Restarting.
Motor replacement in a hazardous location is usually more expensive than for open-drip or TEFC motors, from a process-interruption standpoint.
[This message has been edited by Bjarney (edited 04-10-2003).]
Re: Internal thermal switch on 3 Ph. E-P motors#24279 04/10/0306:16 AM04/10/0306:16 AM
Redsy, I've never seen a Class 1 Div 1 motor with the intenal temperature switch. I have seen a number of Class 2 Div 1 motors with the internal temperature swich, and on those we wired the switch in seires with the stop button.
Walrus, The flash point is not the ignition point. The flash point is the temperature at which the liquid will give off enough flammable vapors to support a "flash" fire. The ignition point is hundreds of degrees above the flash point for most materials.
We have installed a lot of equipment in different classes of Hazardous Environments and found that something that most people forget (when engineering) is the Flash Point or the Temperature at which the product (which created the hazard) will ignite.
Depending on the Flash Point it may be necessary to shut down the equipment if the outside temperature reaches (usually before) the potential ignition point.
This can affect not just motors no matter what size, but lighting also.
We actually did installations in areas where they cooled the area to prevent “Potential Problems” with interlocking and safety’s to alarm and or shut down the process should the cooling shut down.
The flash point is not the ignition point. For exampe the flash point of gasoline is about -40F while its ignition point is about 850F. Yes, it is more hazardous when the liquid temperature exceeds the flash point, but you still need a temperature source at or above the ignition temperature to cause the explosion or fire. Don