Explosion-proof and safety-increasing motor is the "e" type motor referred to as the explosion-proof field (here refers to the low-voltage motor, with a rated voltage of 660V or less); Explosion-proof non-sparking type motor is referred to as "N" type motor. In the years of explosion-proof review and inspection, it was found that there are many manufacturers and users who do not know much about the explosion-proof principle of the two explosion-proof motors, and some users actually use the explosion-proof non-spark motor as an explosion-proof and safety-increasing motor in the dangerous places in the explosion-proof zone 1, which has caused very serious unsafe hidden dangers. The following is a detailed introduction to the explosion-proof principle, design points, inspection methods and differences between the two explosion-proof motors.
Terms and Definitions
1 "E" type explosion-proof
An explosion-proof type of electrical equipment, that is, some additional measures are taken for electrical equipment to improve its safety and prevent the possibility of dangerous temperatures, arcs and sparks under normal operation or specified abnormal conditions.
2 tE time
The time (unit: s) required for the AC rotor or stator winding to reach the rated operating temperature at the maximum ambient temperature from the time it begins to pass through the starting current IA until the temperature rises to the limit temperature.
3 STARTING CURRENT RATIO IA/IN
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4 "N" type explosion-proof
An explosion-proof type of electrical equipment, this type of electrical equipment cannot ignite the surrounding explosive gas atmosphere during normal operation and under some abnormal conditions specified in this department.
Explosion-proof principle and design points of "E" type motor
1 Explosion-proof principle of "E" type motor
For the motor that does not produce arcs, sparks and dangerous high temperatures during normal operation, if some mechanical, electrical and thermal additional protection measures are taken on its structure, and try to make the motor do not arc, spark and overheating under normal operation or approved overload conditions, the safety and reliability of the equipment can be further improved. As a result, the motors do not have an ignition source during normal operation and can be used in explosive atmospheres. This is the explosion-proof principle of the increased safety motor, and it is also the source of its name.
2 Design points of "E" type motor
2. 1 Electrical requirements
(1) Strengthen the dielectric strength of winding insulation, and increase the dielectric strength of winding between turns, phases and to the casing by 10% on the basis of the test voltage of ordinary motors
(2) The winding wire shall be covered with at least two layers of insulation, only the enamel can be a layer of insulating enameled wire, and the nominal diameter of the wire shall not be less than 0. 25mm。
(3) The winding should be impregnated by immersion method, drip method or vacuum pressure method to ensure that the gap between the wires is filled. When using impregnating agents containing solvents, at least two impregnation and drying processes must be carried out.
(4) The solid insulating material should be made of materials with low moisture absorption, good arc resistance, non-combustible or flame-retardant, and still have good mechanical properties at least 20 °C (but not less than 80 °C) higher than the continuous operating temperature of the motor.
(5) The electrical clearance and creepage distance between the conductive parts of different potentials are greater than the specified value of ordinary motors, which shall comply with GB3836. 3-2010.
2.2 Mechanical requirements
(1) The connection of the lead-in cable or wire should ensure that the wire is firmly connected, prevent loosening, and twist (such as using spring washer) The terminal can withstand the torsion test of the connector; When using aluminum wires, copper-aluminum over-joints are used.
(2) The internal wire should be soldered directly with tin-aluminum solder, and must be mechanically connected before soldering; or silver welding, brazing or extrusion connections.
(3) When the rotor guide bar and the end ring are not die-cast, hard brazing or fusion welding should be used; In order to prevent sparks between the guide bar and the rotor core during start-up, the pressure casting aluminum method can be used or additional groove lining and grooved wedge can be used for a single guide bar.
(4) Increase the air gap between the stator and rotor of the motor to reduce the possibility of rotor sweeping. According to GB3836.3 -2010, the minimum radial unilateral air gap between the stator and the rotor of the minimum radial unilateral air gap (mm) should not be less than the value calculated by the following formula when the rotating machine is stationary: minimum radial air gap, unit: mm.
where: . D a rotor diameter, unit: mm, its minimum value in the minimum radial air gap formula takes 75, and the maximum value takes 750; n a maximum rated speed r/min, the minimum value of 1000; r-1 is calculated according to the following formula, and the minimum value is 1.0.
(5) The enclosure protection level of the insulated live parts in the motor shall not be less than IP44; The degree of protection of the enclosure of exposed live parts (e.g. junction boxes) is not less than the requirements of IP54. In the case of motors installed in a clean room and frequently inspected by special personnel, the enclosure protection level allows the requirements to be reduced, with the exception of junction boxes and exposed live parts.
(6) The motor fan, fan cover and partition plate must have sufficient mechanical strength to ensure reliable fixation, and the spacing between each other meets the specified requirements, and shall not collide or rub against each other to produce sparks. Considering design tolerances, the distance between the inner and outer fans, fan covers, vent baffles, and their fasteners should be at least 1/100 of the maximum diameter of the fan, but not necessarily greater than 5mm. In any case, the spacing should not be less than 1mm. If the concentricity of the dimension and the stability of the dimension are controlled, the gap can be reduced to 1mm after the parts are machined.
(7) The cable entry port should be tightly sealed, and the cable must be prevented from pulling out under force.
2.3 Thermal (temperature) requirements
(1) The limit temperature of the surface of all parts and components in the motor that may come into contact with the explosive mixture shall not exceed the specified value.
(2) The allowable temperature rise of the insulation winding during the normal operation of the increased safety motor should be reduced by no less than 10°C compared with the allowable temperature rise of the corresponding ordinary motor, so as to improve its safety degree on the one hand and prolong the winding life at the same time.
(3) The limit temperature of the motor insulation winding of the increased safety motor rotor shall not exceed the specified value within the stall time tE. The tE time can generally be determined by experiment, and for motors greater than 75kW, it is allowed to be calculated by the calculation method.
When the motor is stalled, the ratio of the temperature rise △θ of the stator winding to the time t can be calculated according to the following formula:
where: j is the starting current density, unit: A/mm2;
a is a constant, (for copper windings, a=0.0065)
If a current protection device is used to prevent the limit temperature from being exceeded, the length of the tE time should be that the current protection device can disconnect the motor from the power supply before the end of the tE time when the motor is blocked. In general, these meet the above requirements if the tE time of the motor is greater than the minimum tE time in the diagram as determined by the starting current ratio IA/IN function relationship. If the tE time of the motor is less than the specified value in the figure, it is allowed to be used only after the appropriate overload protection device is adopted and its function is proved to be effective through the test. This device must be marked on the motor nameplate.
The tE value should generally be greater than 10s, and the tE time should not be less than the time required for the overcurrent delay protection device to cut off the power supply of the motor when the rotor is stalled.
In determining the tE time, it should be noted that the tE1 time of the stator winding and the tE2 time of the rotor winding are different, and the smaller of them should be taken as the tE time of the motor.
Fig. 1: The relationship between the minimum tE time of the motor and the starting current ratio IA/IN
Both the tE value and the starting current ratio IA/IN should be indicated on the nameplate so that the user can select the protection device.
Motors with overcurrent delay protection devices are generally only allowed to be used for continuous working conditions with light load start and infrequent start. For motors that are difficult to start (i.e. more than 1.7 times tE in start-up time) or start frequently, special protection devices must be used.
The explosion-proof principle and design concept of the "N" type motor
Explosion-proof principle of 1 "N" type motor
This type of explosion-proof motor cannot ignite the surrounding explosive gas atmosphere during normal operation and under some specified abnormal conditions. The so-called "normal operation and some abnormal conditions specified" means that the electrical and mechanical properties of the motor meet the requirements of the design specifications and are used within the range specified by the manufacturer.
The "N" type motor is an explosion-proof motor used in hazardous locations in Zone 2 of ground factories. In the case of Zone 2 explosion hazard sites, it refers to places where explosive gas mixtures are unlikely to be generated, and even if they are generated, they can only exist for a short time. In addition, in the case of the "N" type motor, as mentioned earlier, it is a motor that does not generate sparks during normal operation and is unlikely to produce ignition failures.
Taken together, the first is that the site is unlikely to be dangerous, and the second is that the motor is unlikely to form an ignition source, and the chance of the two occurring at the same time and meeting is very small, which is easy to achieve a safe level that we can accept. In order to have a quantitative idea, let's make a rough estimate of the probability of the two encountering. According to the relevant information of Germany, Japan, Australia and other countries, in the explosion hazard place in Zone 2, the time of the explosive mixture is not more than 10h (8000h a year), that is, the probability of the explosive mixture appearing is 1/800; assuming that the motor starts once per hour, and it takes 2s each time, the probability that sparks may be generated within LH is 1 * 2/3600 = 1/1800. The probability of the two encountering is (1/800)*(1/1800)=7*10-7, which shows that the probability of meeting is extremely small and reaches the safety level that we can accept. That's why the "N" type motors are used in Zone 2 explosion hazard locations.
2 Design points of "N" type motor
Compared with the increased safety motor, the design of the non-sparking type motor is the same as that of the increased safety type motor, except that the insulation dielectric strength test voltage and tE time are not specifically specified like the increased safety type motor.
According to foreign data reports, the non-sparking motor of the large composite structure has ignited the surrounding explosive mixture due to the electric spark generated by the circulating current generated by the stray magnetic field of the motor passing through the joints of the upper and lower butt parts of the casing.
At the end of 1985, there were two explosions caused by this cause on a British offshore drilling ship. These measures include strengthening fastening to prevent loosening; Metal sheets are added between adjacent metal pieces that are not welded together to provide a continuous path; Bolts and other fasteners must be of the right size and quantity so that they can withstand circulation and prevent excessive surface temperatures; Where metal sheets are used, the boss for installation should be welded to the plate to minimize the contact resistance; It is necessary to take measures to prevent the gradual loosening of fasteners; The metal sheet should be crossed over the various joints in the current path; Metal sheets should be symmetrically arranged; When the metal sheet is distributed in and near the bearing chamber, it must be avoided to cause a short circuit in the bearing insulation.
The difference between an "E" type motor and an "N" type motor
The explosion-proof principle and design points of "E" type motor and "N" type motor have been introduced in detail before, and the main differences of several explosion-proof motors have been summarized:
(1) Insulation dielectric strength test voltage: the increased safety type motor is higher than the non-sparking type motor.
(2) Wire winding temperature rise: The increased safety motor has tE time requirements.
(3) Use environment: the increased safety motor can be used in the dangerous place of Zone 1 (here refers to the low-voltage motor, the rated voltage is below 660V); Non-sparking motors can only be used in Zone 2 hazardous locations.
With the rapid development of petroleum, chemical, coal, metallurgical and other industries, preventing accidents and explosions has become a very prominent problem. In order to solve this problem, we need to take precautions that are reasonable, reliable and economical. First of all, understand the explosive gas environment danger area, the type and temperature group of the explosive gas in the environment, and which explosion-proof type of electrical equipment is safe and reliable to use in the corresponding area and explosive gas environment.