Selection and maintenance of fuse for the hottest

Selection and maintenance of fuse for low-voltage motor Abstract: practice has proved that fuse is a simple and effective protection device for phase to phase short circuit, single-phase short circuit fault and overload of low-voltage motor. However, if the type and parameters of the fuse are improperly selected or the use and maintenance are unfavorable, the expected protection effect will not be achieved. The following precautions are put forward for the selection and use of fuse protection of low-voltage motors for reference

key words: the melting of low-voltage motor is a further step to promote the transformation and development of aluminum industry in Hejin. The practice of breaker maintenance has proved that the fuse is a simple and effective protection device for phase to phase short circuit, single-phase short circuit fault and overload of low-voltage motor. However, if the type and parameters of the fuse are improperly selected or the use and maintenance are unfavorable, the expected protection effect will not be achieved. The following precautions are put forward for the selection and use of fuse protection of low-voltage motors for reference. 1. Model selection problem (1) if the expected short-circuit current is not too large (such as less than 4KA), RM10, rl6 and rl7 series fuses can be preferred from the perspective of economy. On the one hand, users can easily disassemble and assemble the melt by themselves. On the other hand, they can be used as both short-circuit protection and overload protection. (2) If the short-circuit current is expected to be large, fuses with high breaking capacity should be selected, such as RT12, RT14, rt15 series fuses. (3) The fuse for protecting the motor is generally not required to have large capacity and current limiting effect, but the melting coefficient is expected to be appropriately smaller. Therefore, zinc melt and lead tin alloy melt should be selected. (4) RM1, RM2 and RC1 series fuses have been eliminated. Therefore, it is generally not allowed to use ceramic plug-in RC1A series fuses for motor protection. If you have to use it due to restrictions, you can only reluctantly use the fuse with the rated current of 15a and below as the overload protection of the motor below 7.5kW. 2 parameter selection (1) the rated voltage of the fuse shall conform to the operating voltage of the motor. The working voltage of fuse is related to the length and insulation strength of fuse tube. The fuse cannot be used in the circuit higher than its rated voltage, nor can the large fuse be installed in the small fuse tube. (2) The rated current of the fuse shall be greater than the maximum working current of the motor circuit for a long time. The current carrying part and contact part of the shell will not be damaged by the working current. The rated current of the fuse shall not be less than the rated current of the fuse element. (3) The limit breaking current of the fuse shall be greater than the maximum short-circuit current flowing. It is used to ensure that the fuse will not be burnt when the fault current is cut off. (4) The rated current of the fuse shall be selected according to the following three conditions: ① according to the normal working conditions: the starting current of the motor can reach (4 ~ 8) ied, and the starting duration is about 5 ~ 10s. Under this condition, the fuse shall neither age nor fuse. For a 2V signal, the specific fuse can only provide 6A current instead of 30/2=15a. According to the curve provided by the manufacturer, the test shows that the rated current of the fuse is about half of the maximum through current, which can meet the above requirements. The rated current of the fusible part can be selected as IE · RJ ≥ iq4 according to the following formula Poor oil return condition of buffer/k in formula IQ, the starting current of motor is generally (4 ~ 8) ied, which is 4 ~ 8 times the rated current of motor. K-scale factor. Generally 1.5 ~ 2.5. Take 2.5 for infrequently started motors and 1.5 for frequently started motors. The starting current of wound motor is small, and the coefficient can be reduced to 1.25. ② It should be selected according to the mutual cooperation with the control appliance in time: when the fuse is used with the electromagnetic contactor, it should be ensured that the fuse first cuts off the short-circuit or overload current, and the contactor then disconnects without load. It is known that the action time of the contactor is 0.04 ~ 0.06s. Therefore, the fusing time of the fuse is required to be 0.02 ~ 0.03s, and its reliability coefficient can reach kk= (0..06s/0..03) =2 according to the fuse fusing test, when the short-circuit current reaches (20 ~ 25) ie · RJ, the fusing time can meet 0.02 ~ 0.03s. Therefore, the melt current is selected according to the following formula. Ie · RJ ≥ idmax/20 ~ 25, idmax -- the maximum short-circuit current through the fuse ③ according to the requirements of ensuring the selectivity between upper and lower protection: ensure the selectivity. The action time of the lower level protection must be less than that of the upper level protection, and the action value of the upper level protection must be greater than that of the lower level protection. The melt current should be selected according to the data on its protection characteristic curve and its actual error. If the matching margin of fusing time is considered as 10%, i.e. +5% ～ -5%, the following conditions must be met: T1 ≥ 1.05+ ζ%/0.95- ζ%× T2 where ζ%-- The fuse fusing time error is checked by the product manual. If it cannot be checked, it is generally considered as 50%, t1-- corresponding to the fault current value, the fusing time of the upper level melt checked from the characteristic curve, st2-- corresponding to the fault current value, and the fusing time of the next level, that is, the melt of motor protection, checked from the characteristic curve. Generally, it is considered as T1 ≥ 3t2. If the protection characteristic curve cannot be found, ie · RJ1 ≥ kphie · RJ2 can also be selected according to the following formula, in which IIE · RJ1 and IE · rj2-- are the rated current kph of the upper and lower level (i.e. motor protection) melts respectively, generally 1.8 ~ 2.5 of the joint venture between Lilian CVC investment company and royal DSM of the Netherlands. When the fuse has filler, the smaller coefficient shall be used; when there is no filler, the larger coefficient shall be used. Generally, the rated current of the upper fuse element of the same model and the same melt material is 2 ~ 3 levels higher than that of the lower fuse element. 3 use and maintenance (1) before installation, check whether the rated voltage, rated current and limit breaking capacity of the fuse are consistent with the requirements. (2) during installation, ensure that the melt is in good contact with the contact knife and the contact knife and the knife base, so as to avoid misoperation due to excessive melt temperature. At the same time, pay attention not to cause mechanical damage to the melt. (3) During installation, pay attention to the consistency between the temperature of the medium around the fuse and the temperature of the medium around the motor as far as possible, so as to avoid errors in the protection characteristics. (4) The installation must be reliable, so as to avoid the poor contact of one phase, which is equivalent to the open circuit of one phase, resulting in the burning of the motor due to the open phase operation. (5) If it is found that the melt has been damaged or fused during the inspection, replace the melt, and pay attention to make the specification of the replaced new melt consistent with that of the replaced melt, so as to ensure the reliability of the action. (6) The replacement of melt or melt tube must be carried out without electricity. (7) Material and cut-off of connecting wire of fuse