Variable Speed for Explosion Proof Mixers
Explosive or hazardous duty locations have been defined in classifications by hazard type. Class I locations are those where flammable gases or vapors may be present. Class II is locations that may be hazardous due to combustible dust and Class III are those where ignitable fibers may be in the air but not suspended at a rate to produce ignitable mixtures with air. A more detailed description of these classifications and managing them is available through OSHA and in the National Electrical Code Handbook.
Electric motor manufacturers produce motors designed to prevent explosions in a specific class. These motors limit ingress of environmental materials to prevent ignition and are constructed to contain an internal explosion if it were to occur. Variable frequency drive (VFD) speed controls for electric motors are generally not rated for these environments by test laboratories. To certify a control and motor “system” as explosion-proof would require testing of the specific combination of the controller with each unique motor it could be paired with. Furthermore, field wiring between the motor and control is generally not certified. A common solution for controlling electric motor speed in the field is to remotely locate the VFD outside of the hazardous area.
Another alternative for speed control in these environments is to use pneumatic motors. Since there is no source for an electrical arc they are considered to be intrinsically safe, that is they are safe because there is no source for ignition rather than because they are designed to prevent or contain ignition.