Miller Magazine Issue: 116 August 2019

62 COVER STORY MILLER / AUGUST 2019 GRINDING ROLL MOTORS STARTING METHOD There are various ways to run roller grinding motors: Direct Start: Motor is connected with triangular method and single (4x*) cable used. Star- Delta: No bridging is done within electric terminal. Double (4x*) cable is used. Soft Starter: The motor is connected with trian- gular method and single (4x*) cable is used. Speed Control Device: The motor is connect- ed with triangular method and single (4x*) cable is used. Starting methods other than ‘direct’ start are all more beneficial in terms of energy saving and life utility of electrical equipment. ‘Star- Delta method is often used in practice. Advantages of ‘Star- Delta against ‘Direct Start • The motor tries to start with a sudden high torque when the ‘Direct Start method is used. This puts an excessive force on V belts and trig- ger belts. This makes harm on belts on each start. On the other hand, the motor starts with a low torque when star-delta is used. Therefore, the force applied on the belts is much lower and life utility of belts increases. • It protects bearings from sudden hits and strokes. • Excessive current is decreased to prevent overheating of motors. • It prevents overheating of cables and line links. MEASURING THE CURRENT WITHIN THE ROLLS IN THE RIGHT WAY There are often ammeters on control systems of rolls. Measuring the value of the current in the right way ma- kes it easier for the operators to adjust the roller. They can detect to which extent the capacity of the motor has been used by looking into the value of the current. There are investments to measure the current within individual machines and turnkey projects. But we spotted signi- ficant problems and shortcomings in this area which is between the electricity company and roller manufacturer. Of course, a measurement is displayed but it is hardly the exact value because of wrong measurement. Imas-Milleral has been working hard in order to get the exact value of the current on the roller for many years. You can sport the changes of real values of the current when the feeding motor is on and off. There might be approxima- tely 50 meters long cables between current transformers on MMC panels and the ammeter on the roller which cau- ses the problem. On the other hand, the distance between pneumatic, main switch current transformers and ammeter is lower and we do not encounter this problem. In addition to this, features of current transformers are also important. You can calculate cable section needed. You have to be careful to ensure that the cable reaches the same section which is between current transformer and ammeter. Image 2 Image 3 Image 4 Image 5