Induction Motors Ex e

These motors are similar in appearance to standard industrial motors and inspection of the certification and rating plate is usually necessary to identify them.
These motors are not designed to withstand an internal explosion. Hence, they have special design features to prevent arcs, sparks and excessive surface temperatures occurring both internally and externally.

The principal design features are:

• Special attention to air-gap between stator and rotor
• Clearance of all rotating parts
• Impact testing of motor frame
• T2 or T3 surface temperature limitation
• Temperature rise 10 °C lower than normal
• Compliance with winding temperature rise limit, called Te
• Special terminal block with good creepage / clearance distances and locking devices on terminals
• Minimum ingress protection to IP54

A rise in motor winding and casing temperatures is a major concern with Ex e motors.

Protection must be added to the motor and its winding control to prevent temperature rise in a potentially explosive atmosphere.

To remove electrical power to the motor the supply must be isolated by an overload device, which has a thermal trip input from the motor. The timing of the trip action of the overload is illustrated below:

The motor time tE for an increased safety motor is a prime safety parameter.

The motor time tE means the motor can withstand thermal stability of winding insulation for a longer time without insulation failure during fault condition and in addition stop arcing and sparking within the windings.

The time tE of motor should be more than 5 seconds as specified in IS/IEC 60079-7 The length of time tE shall be such that when the machine is stalled it can be disconnected by the current dependent safety device before tE time is exceeded.

The thermal overload will be selected for suitability according to its tripping characteristic. The tE time and IA / IN current ratio are influential in the selection of the device and are marked on the motor nameplate.