Considerations regarding the impulse ignition test for level of protection “eb” and ignition test for protection levels “eb” and “ec” stator insulation systems of Ex-proof electric motors

. According to the provisions of the 5th edition of the standard IEC 60079 – 7, equipment protection by increased safety “e”, for Level of protection “eb” and “ec” electrical machines, if the rated voltage exceeds 1 kV, type tests in explosive mixtures have to be carried out, in order to certify such an electrical machine. The purpose of this paper is to provide some considerations regarding the impulse ignition test for level of protection “eb” and ignition test for levels of protection “eb” and “ec” stator insulation systems of explosion proof electric motors and also to present a case study regarding such tests conducted in the specialized laboratory of INSEMEX Petrosani, on a stator insulation winding system of a 6 kV electric motor type of protection Ex eb IIC Gb, case study which will assist manufacturers in designing appropriate stator winding insulation systems for increased safety type of protection electric motors.


Introduction
Electric motors are a vital component in numerous industrial applications, and ensuring their reliable operation is of utmost importance.Particularly in hazardous environments, where the presence of flammable gases or vapors poses a significant risk, the need for increased safety measures becomes paramount [1,2,3].In such scenarios, electric motors complying with Ex eb (respectively Ex ec), standards SR EN IEC 60079 -0, SR EN 60079 -7 [4,5] play a crucial role.One important aspect of their design is the impulse ignition test for stator insulation systems (in case of Ex eb electric motors), which help to mitigate potential ignition hazards [5].This article delves into the significance of the impulse ignition test and its role in ensuring the safety and reliabilty of electric motors in hazardous environments.

Regulatory principles
The EN 60079-7 standard establishes the specific requirements for stator winding insulation systems for both Ex eb and Ex ec electric motors.[5].
For all motor stators with a rated voltage above 1 kV, the electrical machine shall be fitted with anticondensation heaters [5].
In addition, for Level of Protection "eb" machines, if the rated voltage exceeds 1 kV, type tests in explosive mixtures, for the stator insulation system, have to be carried out.Type tests in explosive mixtures, that have to be conducted in this case, are the following [5]: -Impulse ignition test; -Steady state ignition test (over voltage test).For Level of Protection "ec" machines, if the rated voltage exceeds those shown in Table 1 for the applicable equipment groups, type tests corresponding to the steady state ignition test for the stator winding insulation system have to be carried out [5].

IIA
It is recommended that partial discharges be minimized for all high voltage windings.For windings with a rated voltage of 6.6 kV, or greater, the use of partial discharge suppressant materials is recommended [5].

Impulse ignition test for Level of Protection "eb" stator insulation systems
Insulation systems and connecting cables have to tested in an explosive test mixture as shown in Table 2.They have to be subjected to 10 voltage impulses of not less than three times peak phase to earth voltage and with a voltage rise time between 0.2 µs and 0.5 µs. and with a time to half value which is at least 20 µs.The impulse voltage has to be applied between one phase and earth with the other phases earthed and repeated for each phase [5].This is a non-standard waveform but it is believed that it is necessary to use a rise time as can practically be achieved to initiate discharge with a sufficient length to contain enough energy for ignition.This is based on the results of experiments conducted by Physicalisch Technische Bundesanstalt (PTB) in Germany [5].
This test is representative of wye (star) connected machines with the supply midpoint earthed or delta connected machines with the virtual midpoint near system earth.Other supply connections have to be subject of discussions between the manufacturer and the user to determine suitable insulation system tests.No ignition of the explosive test mixture has to occur, in order for the test sample to successfully pas these tests [5].

Steady state ignition test for Levels of Protection 'eb" and "ec" stator insulation systems
Insulation systems and connection cables have to be tested in an explosive test mixture, as shown in Table 2 with a sinusoidal voltage of at least 1.5 times the rated r.m.s line voltage for at least 3 min.The maximum rate of voltage rise has to be 0.5 kV/s.The voltage has to be applied between one phase and earth with the other phases earthed, and repeated for each phase.In this case also, no ignition of the test mixture has to occur [5].Description: The insulation systems and the connection cables were tested in an explosive test mixture, according to the Table 2 (IIC -(21 ± 5) % hydrogen).They were subjected to 10 voltage impulses of not less than three times peak phase to earth voltage (14.7 kV) and with a voltage rise time between 0.2 µs and 0.5 µs and with a time to half value which is at least 20 µs.The impulse voltage has to be applied between one phase and earth with the other phases earthed and repeated for each phase.where Un is the rated voltage of the motor and U1-U2, V1-V2, W1-W2 are the three phase windings of the motor.
Description: Insulation systems and connection cables were tested in an explosive test mixture, as shown in Table 2 (IIC -(21 ± 5) % hydrogen), with a sinusoidal voltage of at least 1.5 times the rated r.m.s.line voltage for at least 3 min.The maximum rate of voltage rise was 0.5 kV/s.The voltage was applied between one phase and earth with the other phases earthed, and repeated for each phase.
Tests carried out: Tests in explosive mixtures for the stator winding insulation system (Group IIC).High voltage testing at 1.5 x Un = 1.5 x 6 = 9 kV in explosive mixture (21 ± 5) % hydrogen).The voltage (9 kV) was applied for a time period of 3 min between the U phase winding and earth (the windings of the phases W and V were shortcircuited and earthed together with the stator core).The voltage (9 kV) was applied for a time period of 3 min between the V phase winding and earth (the windings of the phases W and U were shortcircuited and earthed together with the stator core).The voltage (9 kV) was applied for a time period of 3 min between the W phase winding and earth (the windings of the phases U and V were shortcircuited and earthed together with the stator core).
-connection of the winding test sample terminals was made according to Figure 2.

Fig. 2. Over voltage test
The over voltage test was carried out at: 1,5 •  = 9 kV, where Un is the rated voltage of the motor, HV stands for High Voltage and "miez" is the stator core of the motor.

Results obtained
), led to the explosion of the testing mixture corresponding to Group IIC (21 ± 5) % hydrogen).The decision then, was taken to continue with the over voltage testing of the winding test sample, but for Group IIA (5.25 ± 0.5) % propane, instead of hydrogen, in this case, the test mixture (propane-air) having a higher minimum ignition energy.Three sets of tests were carried out successfully in this case, between each winding and earth, with the other windings connected to the stator and earthed.Then the over voltage was raised to 9.9 kV (corresponding to a rated volage Un = 6.6 kV) and a new set of tests corresponding to Group IIA were carried out successfully again.c) After concluding all the tests required by the standard SR EN 60079 -7, the stator winding insulation system qualified for certification for Group IIA application of gases and vapors.

Conclusions
In hazardous environments where the presence of flammable gases or vapors poses significant risks, electric motors complying with Ex eb (or Ex ec) standards provide increased safety measures.The impulse ignition test and the over voltage test for stator insulation systems play a crucial role in the assessment and enhancing the safety and reliability of these motors.By subjecting the insulation system to high voltage impulses and over voltages and ensuring that it can withstand potential transient electrical stresses without causing sparks or arcs, the impulse ignition tests, as well as the over voltage tests mitigate ignition hazards and contribute to the safe operation of electric motors in hazardous environments.Manufacturers and users alike should recognize the importance of these tests and their role in promoting the safe and reliable operation of electric motors with type of protection increased safety Ex eb and / or Ex ec.
This paper was carried out through the "Nucleu" program of the National Research, Development and Innovation Plan 2022-2027, supported by MCID, project no.PN23320102.
Equipment used: Baker Station type DS30R and PP -impulse generator.Parameters of the impulses for line voltage Un = 6 kV: -voltage value: 14.7 kV -frequency: 50 Hz -testing period: 10 sec -50 impulses -voltage rise time: 0.1-0.2s -connection of the winding test sample terminals was made according to Figure 1.

Table 1 .
Stator insulation systems tests of Level of Protection "ec" machines

Table 2 .
Explosion test mixturesTesting programme for winding test sample for 6 kV rated voltage.Tests for the certification of the stator winding insulation system, according to EN 60079 -7, for Ex eb IIC motors, with a rated voltage Un = 6 kV.The test sample object is the winding test sample.

Table 3 .
Results for the impulse test and the over voltage test

Table 4 .
Description of each test carried out