“As the number of rolls and KW values of motors increase, the importance of control systems for electrical circuit and automation also increases. Earthing is vital in order to protect electronic components of the machine. Feeding and grinding roll motors and roller bodies must be earthed. Inexperienced and untrained individuals should not be let to get close to the machine. Emergency stop button on the control panel is also vital for safety of people. When trying to solve the problems with rolls, people often intervene manually to rolls and belts. Therefore, the master and the machine operator should have a clear communication. After the operation is completed and roller coverings are closed, the machine should be cranked.”
In flour mills; milling, grinding and crushing processes occur in roller mills. There are two roll systems within those machines: feeding rolls and grinding rolls. Those systems are integrated into a single machine body. Those machines may have various features depending on models and customer demands. They are controlled either manually or by PLC or electronic card systems. Density of the raw material is checked through sensors, load cells etc. Speed control device checks the motor and adjust the speed of feeding rolls in order to spread incoming raw material on the grinding surface better.
Grinding rolls are rapidly moved against each other by the help of the pneumatic system when the raw material comes from the feeding system. When the amount of the raw material decreases, grinding rolls move away from each other and feeding system stops. There is also a sensor under the grinding roll to stop the system in order to prevent stuck of the rolls in case of a full load.
Feeding and grinding rolls are monitored through a rotation monitor. The automation system halts the process when an uncontrolled slowdown occurs with pulley speeds and protects belts and other components of the machine from breaking down. Roller bearing temperatures are also tracked and if a value above the threshold level is noticed, the system is stopped and an alert is given. Central automatic lubrication system performs the adequate level of lubricating in order to prevent human error.
Features described above are implemented in a similar or different way by manufacturers. Roller mills can be sold within complete and turnkey projects or as individual machines. They are controlled via MCC panels (see Image 2 and Image 3) on the control units. As the number of rolls and KW values of motors increase, the importance of control systems for electrical circuit and automation also increases.
Earthing is vital in order to protect electronic components of the machine. Feeding and grinding roll motors and roller bodies must be earthed. Inexperienced and untrained individuals should not be let to get close to the machine. Emergency stop button on the control panel is also vital for safety of people. When trying to solve the problems with rolls, people often intervene manually to rolls and belts. Therefore, the master and the machine operator should have a clear communication. After the operation is completed and roller coverings are closed, the machine should be cranked. The emergency stop button is the key for all those processes. It prevents from uncontrolled operations. (Image 1)
There are some features to be present on MCC panel for machines to be run better and for an extended period of time:
• Earthing of the panel and earthing connections for motor cables.
• Discharge fans for decreasing the temperature within the panel.
• Voltage tracking, halting the system when phase loss or extreme voltage values are spotted.
• Choosing the copper bars within the panel according to specific needs.
• Stopping feeding motors when machines without by-pass feature within the grinding part of the mill stops.
• Stopping only feeding motors for grinding part stopovers and preventing unnecessary halting and running actions for grinding motors and preventing overcurrent.
• Connection between start and emergency stop buttons on the roller control system with MCC panel.
• Condensation system. (See Image 4) Capacitors balance phase shifting of the current (COSØ) and voltage that stems from the structure of the motor and enables using the energy with the most efficient and effective way. Motors can gain more power with less current. The electricity always chooses the easiest way. Capacitors help protecting motors and other systems from possible harmonic and peaks from electricity network and increase energy efficiency.
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 triangular method and single (4x*) cable is used.
Speed Control Device: The motor is connected 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 trigger 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 makes 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 significant 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 approximately 50 meters long cables between current transformers on MMC panels and the ammeter on the roller which causes 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.
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 measurers.
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).
According to IEC61869-1 standard, it is manufactured 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 chosen 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 least 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, decrease in current is 5.38 VA. A current transformer of -at least- 10 VA should be chosen here. 10 VA products can be found among 250/5 current transformers without bar. But those products would not be the best choice because roller motors’ 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 environment in all of the processes I mentioned above. Our second priority is to ensure longer lifetimes for machinery and equipment, enabling to use them with full capacity and ensuring predictive maintenance and therefore increasing productivity and decreasing energy consumption.
TOB Akım Trafosu - Sezgin. (2019).