Miller Magazine Issue: 116 August 2019

HOW TO CHOOSE THE CABLE THAT IS BETWEEN DIRECT CURRENT TRANSFORMER AND AMMETER Current transformer: (Image 5) These are electrical devices that transforms the alternative current from primary windings to secondary windings and as a result enables the measurement of high currents with electronic me- asurers. Transformation Rate: They are written on current transformers as 100/5, 75/5, 50/5. According to IEC 60044-1 standards, transformation rate shows that 1/20 of the current going through primary windings would go through secondary windings for (100/5). Accuracy Class: According to IEC61869-1 standard, it is manu- factured in classes like Class 0.5 – 1 – 3. As class value decreases, accuracy is better. Choosing a current transformer that is closer to the motor current, will make accuracy better. POWER LOSS BETWEEN CURRENT TRANSFORMER AND AMMETER If the voltage and current at the secondary part of current transformers cannot be transformed to the ammeter, correct measurement results would not be obtained. I will try to explain how products should be cho- sen by the help of the following formula: I: Current in the secondary part of Current Transformer (5A or 1A) L: The distance between the last measurement device and current transformer (meter) S: cable section (1.5- 2.5- 4- 6 mm2 etc.) Line Power Loss VA = I² / (S x 58) x L VA = 52 / (0.75x58) x 1 = 25/43.5 x 1 = 0.575 x 1 VA=0.575 VA power loss will occur. For a cable section of 0.75 mm2, if there is a distance of 1 meter between current transformer and ammeter, a current transformer of 5VA will be sufficient. VA= 52/ (0.75x58) x 50 = 25/43.5 x 50 = 0.574 x 50 VA = 28.7 VA When the cable length is 50 meters, current decreases by 28.7 VA and current transformers cannot provide this power. Ammeters cannot make the measurement, either. VA= 52/ (2.5 x 58) x 50 = 25 / 145 x 50 = 0.172 x 50 VA = 8.6 VA When the cable length is 50 meters and cable section is 2.5 mm2, current decreases by 8.6 VA. In this case, current transformer should be at le- ast 10 VA. VA= 52/ (4 x 58) x 50 = 25 / 232 x 50 = 0.107 x 50 VA = 5.38 VA When the cable distance becomes 50 meters and cable section is increased to 4 mm2, dec- rease in current is 5.38 VA. A current transfor- mer of -at least- 10 VA should be chosen here. 10 VA products can be found among 250/5 cur- rent transformers without bar. But those products would not be the best choice because roller mo- tors’ current scala is lower. As the power of current transformer and size of the cable section increase, the accuracy will also increase. Operator adjusting the roller roll may grind the raw material better. If this is the case, he can check the ammeter and can tighten up the rolls as long as the capacity of the motor allows. If he has been performing the crushing with a lower current, he can use a smaller motor. Imas-Milleral’s first priority is to ensure safety and security for humans, machines and the envi- ronment in all of the processes I mentioned abo- ve. Our second priority is to ensure longer lifeti- mes for machinery and equipment, enabling to use them with full capacity and ensuring predicti- ve maintenance and therefore increasing produc- tivity and decreasing energy consumption. Sources: TOB Akım Trafosu - Sezgin. (2019). http://www.sezgin.web.tr/urun-detayi/to-b-akim-trafosu/ Motor Current Transformer 15 KW 50/5 18.5 KW 50/5 22 KW 75/5 30 KW 100/5