This paper introduces the use of high-power thyristors for motor forward and backward control and related fault protection. AVR microcontroller is used to realize the diagnosis of thyristor fault state. It can choose to control a variety of power motors, and realize overload protection with reverse time limit, phase break and three unbalanced protection respectively.
High power reverse parallel thyristor has the advantages of no contact, no spark, fast switching frequency and so on.
It is very suitable for motor forward and reverse control. Three-phase asynchronous motor has many advantages, such as simple structure, low cost, convenient operation and maintenance, etc.
It is widely used in industry. With the improvement of automation, more and more complex process control is emerging, and the working state of motor is deteriorating.
Higher requirements are put forward for motor protection.
Intelligent motor protection is constantly emerging [1]. Considering the advantages of thyristor and the requirement of motor protection, an integrated motor protection controller is designed. The main circuit of motor protection controller adopts five reverse parallel thyristors to control the forward and reverse of conduction motor. The current sensor collects the working current of the main circuit and chooses transformers with different conversion ratios according to the pre-set motor power.
Thyristor terminal leads out terminal to detect thyristor terminal voltage. The fault of silicon controlled rectifier is divided into two kinds: silicon open circuit and silicon breakdown. The silicon breakdown is detected by the motor without starting, and the silicon open circuit is detected by the motor when it is open and when it is rotating. If a fault occurs, the corresponding markers are set for the fault state, and the thyristor is switched off and the power supply of the fault output relay is turned on in time. Output fault signal. Thyristor breakdown detection is to detect the silicon state after 100 ms delay (waiting for relays, keys and other devices to complete mechanical action) of the motor starting signal. Fig. 1 below is used to collect the voltage at both ends of SCR and detect the on-state of SCR. According to the similarities and differences between the on-state and the actual state of five-way anti-parallel SCR, it can be judged whether it is silicon breakdown or silicon open-circuit. Three-phase unbalance and Phase-break fault detection is carried out. In the program, the Phase-break detection is based on the current-break of less than 1/8 of the rated current.
Therefore, the set power of the motor must be determined before the rated current of 1/8 can be determined. Here there are nine rated power motors of 0-8 for selection. Phase-break detection starts only when the effective start signal is sent out for 100 ms. The purpose is to skip the action delay of the relay. However, Phase-break detection should be carried out in the time of large start-up current. The number of effective Phase-break fault signals detected in 2 seconds after the occurrence of a signal with a Phase-break fault sign is more than 75% of the total detection times, so it is considered that phase-break.
Signal of Phase-break fault is sent out. In the process of three-phase unbalance detection, the maximum and minimum of three-phase current in the main circuit can be found. If the maximum is more than twice the minimum, three-phase unbalance will occur. It is considered that three-phase unbalance occurs when the number of effective three-phase unbalance fault signals detected is more than 75% of the total detection times within 5 seconds after the detection of three unbalanced signals. Figure 2 below collects the current output from the current transformer in the main circuit and processes the sinusoidal voltage signal converted by resistance. After AD sampling by CPU, the current of the main circuit is obtained. Inverse time-limit protection is more similar to the characteristics of closed-circuit motor when fault occurs, and it is more advantageous to use inverse time-limit for over-current protection. Inverse time limit is widely used in overcurrent protection abroad, and a mathematical model has been established earlier. I is the multiple of overcurrent, based on rated current; R is a constant, and there are three kinds of values in the range of [3]. For motor protection, r = 2 is adopted; Q is a constant, dimension is time; T is the time of overcurrent protection.
The calculation results of the program exceed the critical value of protection action. The system software adopts multi-task design method. The tasks are: system clock setting, peripheral parameter acquisition, current acquisition, voltage acquisition, motor forward and backward input detection, fault type judgment, state indication. (1) Peripheral parameter acquisition includes: motor power setting, start-up time setting, zero detection; (2) system clock setting: 1 ms interrupt is set in timer as the clock rhythm of software operation; (3) current acquisition: main circuit current signal output by current transformer is collected through AD converter inside AVR microprocessor, 10 times for each current signal acquisition, median filtering is used, thermostatic element filtering efficiency is improved. (4) Voltage acquisition: collect the voltage at both ends of thyristor, use the high-level time of 4N35B optocoupler output to determine whether the voltage exists; (5) Motor forward and reverse input detection: use software filter to collect the reverse control signal, and interlock the keys. Only when one key is loosened, the input of the other key is effective.
The next key signal can be detected only after 60 m release of the key; (6) Fault type judgment: Fault type includes three-phase unbalance, silicon breakdown, silicon open circuit, reverse time overload and phase breakdown. According to the principle of fault discrimination, a variety of faults are analyzed through the input signal; (7) state indication: indicating the running state of the device through the indicator light, and if the fault occurs, output the fault type. The protection controller has compact structure and perfect protection function. The reverse parallel thyristor is used in the design to control the positive and negative rotation of the motor. The AVR microprocessor collects the voltage and current signals to realize the protection of the motor. After nearly one year’s actual operation in the factory, the protection controller runs steadily and reliably. Each index meets the design requirements and can be further promoted.