Increased Safety Ex e Motors

Increased Safety Ex e Motors #

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

Increased safety motors are generally intended for continuous duty only. Suitable protection features to disconnect the motor before the limiting temperature is reached are required for applications which involve frequent stopping and starting and/or long run-up times.

Under stall (locked rotor) conditions, the rotor surface temperature will normally increase faster than that of the stator windings, and hence the T-rating applies to both internal and external surface temperatures.

Under faults conditions, the motor must trip within the t_E time specified on the motor data plate (see below).

The principal design features are:

1. Special attention to air-gap concentricity, and to the clearance of all rotating parts.
2. Impact testing of motor frame.
3. The constructional standards give a limiting temperature for the motor windings, 
      depending on the type of insulation used (insulation class). This will be less than the   
      insulation temperature limit for standard machines, and will be less than the T-Class
        rating for a particular motor. The limiting temperature is given in IEC60079-7 for the  
        different insulation classes.
4.  Compliance with the t_E characteristic. Under fault conditions, the motor must trip  
       within the t_E time specified on the motor data plate (see below).

• Terminal blocks have specific creepage/clearance distances and locking devices.

• Minimum ingress protection to IP54 or IP44, unless covered by “special conditions of 
     use” for installation in a clean environment.

• Depending on the motor type and duty, special protection features may be required  
      in the control gear to clear faults such as overloading and locked rotor that could  
      give rise to excessive heating effects.

    Examples:

    Inverse time delta overload protection

    Phase imbalance protection for delta-wound motor

    Protection against arduous starting

              Ex e motors are generally supplied for continuous duty only

Ex e motor must trip within t_E time specified on the nameplate before the designed limiting temperature is exceeded.

t_E time #

The t_E time is defined as: ‘ The time taken to reach the limiting temperature from the temperature reached in normal services, when carrying the standing current I_A at maximum ambient temperature.’

t_E time (secs) and starting current ratio I_A/I_N are shown on the nameplate of an Ex e motor

Determination of t_E Time #

If the rotor locks as a result of a fault, motor will take full starting current and the temperature will rise rapidly and reach the limiting temperature at ‘C’ (second part of the graph).

The time taken to reach ‘C’ from ‘B’ is known as the t_E time. During fault conditions the thermal overload device in the motor stater must trip the motor within this time.

Tripping Characteristic of Thermal Overload device #

The thermal overload device will be selected according to its tripping characteristic and the t_E time and I_A/I_N current ratio marked on the motor nameplate.

Example 1: I_A/I_N = 5 and tE time = 10 secs

The above characteristic would trip the motor after 8 secs, which is within the t_E time and therefore acceptable.

Example 2: I_A/I_N = 4.5 and tE time = 8 secs

For these values the tripping time is 10 secs, which is longer than the t_E time assigned to the motor. An overload device with this characteristic would not be suitable for this motor.