Instrumentation of inverter-fed motors for hazardous atmospheres

In applications requiring hazardous area motors, the problem of increased motor temperatures, due to the nonsinusoidal supply of an inverter, requires special attention. The requirements for these motors are considerably more stringent than for standard safe area machines. In addition to the winding temperature rise, the surface temperatures of a hazardous area machine must also be considered. These temperatures, both internal and external in some cases, must not exceed those permitted by the relevant temperature classification of the equipment. The authors discuss the selection of suitable motors for hazardous areas and their protection. In particular the need for temperature detection is discussed. The use of thermistors for this purpose is also discussed. The authors conclude that thermistors provide an effective method for protecting inverter-fed motors