A visit to the commercial city of Onitsha where most welding machines in Nigeria are made gave me so much information about the techniques they are using to repair damaged electrical motors.
In the same location, I took notice of how these technicians are rewinding damaged coils and using them to replace the coil of engines that are damaged. The common type was the coil of power machines used for welding works. Other technical works at the same location include the making of welding transformers and car battery chargers, etc. but we shall be explaining this post on the technics in the repair and rewinding of electrical motors.
Though there are two broad types of electrical motor in terms of their power input, they are;
The single phase motor and the three phase motor, while the first can rotate in two different directions, the rather can only rotate in one direction and requires a three phase alternating current.
The starting point of the job is to get the motor disassembled and bring out all the parts one after the other. The whole parts of the motor were subjected to test separately until the faulty parts are identified, if the faulty parts has minor defect like a small cut in few lines of the wire in the coil, then they can be fixed and insulated without loosening the coil, but a situation of severe damage will lead to the rewinding of the coil and that will be costly as new copper wire will be required for the coil and insulating materials may also need to be changed.
The techniques employed by these men after dismantling the electric motor by parts are;
THE USE OF DIGITAL MULTIMETER
They make use of digital multimeter by setting it at the resistance reading, this will enable them to detect any discontinuity in the wire since the meter will not show any reading when placed at the two ends of a wire that discontinuity exists. If the meter indicates a particular reading it would be proportional to the length of the wire even though there is no discontinuity in the wire.
The same meter can be adjusted to the voltage reading during the testing period of the coil to know the amount of voltage entering into the coil.
THE USE OF MICROMETER SCREW GAUGE
From my notice, it will be impossible for the technicians to rewind any coil without first taking the measurement of the damaged wire and using the same measurement to look for the wire that will be used for rewinding of the damaged coil.
They use the micrometer screw gauge to take the external diameter of the wire by bring out one end of the wire and placing it in-between the two metallic points of the micrometer with one being adjustable to accommodate any diameter of the wire placed in them for reading, the reading is usually obtained from the calibrated area of the micrometer screw gauge, the value obtained will be according to the last reading in the calibrated area while its fraction will be from the reading in the smaller calibration.
Another material seen with those technicians were a typical type of rope which resembled the same type that was removed from the coil, the function of the rope was to help in the holding of the wires used in the coil. The holding of the wires is just holding it in the lines the formed in the coil.
The rope is applied to the electric motor coils in spiral directions such that it is able to force all the lines of wire that formed the coil to stay together. The process is continued until all the parts of the coil are completed.
Thermoset is the type of plastic usually applied for electrical works due to their inability to melt when exposed to heat, rather at a severe temperature they may start to burn, that was what we were thought in the classroom but there I observed the practical aspect of the classroom theories.
The plastic insulators used by the technicians were made with thermoset, and the quantity in stock are much, such that a lot were packed in one of the stores nearby, however, I took a closer look to observe how these plastic insulators were used.
It didn’t take up to 60 minutes I saw how the damaged wire in a particular coil was completely removed and after the damaged wire had been removed, I saw black color substance which seemed to be a burnt carton paper but rather it was the plastic insulator that got burnt due to overheating that led to the burning of the coil.
The plastic insulators were used to separate the metallic sheets used in the coil making from having direct contact with the wires that run around them in circular directions. The number required is determined by the thickness of insulation needed in the coil.
NUMBER OF TURNS
Another serious practice I observed was the proper and careful calculation of the number of turns each wire passed through the coil during the unwinding or removal process of the wire.
From my observation this process seemed more difficult because of the carefulness involved, the technician will have to mark a particular position in the coil and will start counting as the damaged wires are coming out to determine the number of times the wire passes the marked point during removal.
CLEANING AND ASSEMBLING
The observable activity before the commence of coupling back the motor, is the thorough cleaning of all the parts of the motor with a volatile liquid and checking of all the gaps in between the coils, at this stage all the abnormal lines of wire in the coil were properly arranged and alignment of the motor parts are done simultaneously as the parts were assembled together.
After the whole assembling the motor is not usually coupled completely until another test of current and voltage is conducted using the multi-meter and the values obtained were used to compare to the manufacturers specified reading of voltage and current for the motor any alteration means a mistake in the coil winding which may lead to repetition of the job, but due to the care in the whole initial procedure the possibility of such occurrence is minimum.
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