Optimal control and monitoring of stirring motors in fine chemical industry

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The stirring operation is one of the most common processes in the chemical, petrochemical, pharmaceutical, and food industries. Its purpose is to ensure maximum contact between two or more substances, thereby enabling the necessary mixing, mass transfer, heat transfer, or reaction processes to take place within a specified time frame； The main component responsible for carrying out the mixing process is the mixer, while the motor is an essential device that provides power to the transmission mechanism. The quality of the motor has a crucial impact on the final outcome of the mixing operation. Common faults of mixing motors and mixing devices: broken shafts, detached blades, motor overload, motor operation at no load, and motor shutdown due to faults. Harm caused by failures in the mixer motor or mixing mechanism: If there is no mixing in the reactor, the reaction will slow down or come to a stop, and unreacted chemicals will accumulate inside the tank. If chemical substances do not come into contact with each other, they cannot react. In exothermic reactions, this represents a serious danger that can lead to accidents. If the mixer is restarted, a large amount of unreacted material will begin to react, and the reaction rate may be very fast, preventing the cooling system from removing heat quickly and making it difficult to control the temperature of the reactor. Improvement measures: 1. Use the relationship between frequency and torque to monitor load changes. Taking a squirrel-cage asynchronous motor as an example: below 50 Hz, the motor torque varies proportionally to the frequency ； When the frequency reaches 50Hz, the motor reaches its rated power and rated torque ； When the frequency is greater than 50 Hz, the torque varies inversely with the frequency. http://www.chemicalsafety.org.cn/uploads/allimg/200425/7-200425191225262.jpg △G=（Gz*Hz%-G Out)/Gz*Hz%>2% Where Gz is the given frequency value, Hz% is the percentage of the given frequency, and G Out is the output frequency. When △G>2%, it indicates that there are fluctuations in the load, and the operator should be alerted to pay close attention. If △G remains above 2%, and considering the changes in current to determine whether there is a bearing jam, the mixer and related motors should be inspected. 2. The load conditions and equipment operation status can be determined by observing the current. △I = (t2 – t1)/t2 > 10%; where t2 is the current value in the first ten seconds and t1 is the current value in the last ten seconds. The no-load current is calculated as 30% of the rated current, while the overload current is calculated as 1.3 times the rated current. When △I remains above 10% for an extended period, by observing the changes in current, a decrease in current may indicate light load or no load conditions, suggesting that there might be blades that have fallen off or that the shaft is broken. 3. The current should use no-load current and overload current as the upper and lower limit thresholds for alarm or interlock. The motor contactor feedback signal, no-load current, and overload current are used as interlock conditions. The operating time of the motor and the mixing device is counted using the motor’s feedback signal, thereby determining the appropriate timing for regular maintenance of the motor and mixing device. Through the above monitoring measures, early warning signals can be issued for potential hazards in the mixing motor and mixing device. Predict the operating status of equipment in advance to ensure safe production and reduce the occurrence of accidents.