The existing restart cabinet using PLC as the core is high in cost and complicated in wiring. The single-machine restart relay only relies on the bus voltage of the motor and the position of the auxiliary contact of the contactor to determine power loss, and requires an auxiliary power supply. The probability of restart failure is relatively high. high. In this article, senior engineer Shi Yong introduces a motor (group) power loss restart method implemented in a motor protection device. It can reliably distinguish between power failure and normal shutdown situations. It has rigorous logic, flexible setting of settings, and supports multiple control methods. The motor can be restarted without increasing wiring and cost. This method has been applied in petrochemical enterprises and can operate correctly during many power surges.
Restarting refers to the process of automatic starting of the motor in batches according to the preset sequence and time after the voltage returns to normal.
The petrochemical industry is a continuous production enterprise. Many motors in its low-voltage plant power system are not allowed to trip and stop due to the process flow. Once these key motors trip and stop, it will cause unplanned outages and bring great harm to production. Big economic losses. However, in actual operation, there are many uncertain factors (such as lightning strikes, equipment failures, etc.), which can easily affect the power grid, causing the voltage of the power supply of the enterprise's internal distribution network to decrease or restore the power supply after a short interruption (commonly known as power outage). ), causing the low-voltage motor to trip and shut down. Motors in the company's internal power grid are also easily affected by this, causing production to stop, thus causing great losses.
Currently, the commonly used restart method is to use a special self-starting cabinet or a stand-alone restart relay. The hardware cost of the self-starting cabinet is high and the wiring is complicated. The cost of a single-machine restart relay is slightly lower, but it requires additional wiring, and the probability of restart failure is also high, and false starts may occur after normal parking.
PCS-9692 and PCS-9626 microcomputer programmable motor protection devices combine the operating characteristics of motors in petrochemical enterprises to provide a reliable and economical method for restarting motors (groups).
1Restart logic after power failure
The restart logic described in this article draws on the principle of automatic reclosing in high-voltage line protection, and the restart logic is rigorous. The fully digital restart process has flexible setting values and supports multiple control methods for motor restart (such as direct starting, forward and reverse starting, and high and low speed motor starting, etc.). The input and output of the device can be configured, and the logic is all in the software. implemented inside, making field wiring very simple. The analog quantity sampling uses the 24-point full-wave Fourier algorithm, which has high accuracy. The device uses a high-performance 32-bit CPU, and each sampling interval is 0.833 ms for real-time calculation. The response speed is much faster than traditional restart devices and PLCs.
1.1 Restart logic after power loss
When the motor (group) is powered on and started while it is in a stationary state, it will have an impact on the power grid, lowering the bus voltage, causing the low-voltage protection to trip or burning out the motor due to too low voltage. After a normally operating motor loses power, it will continue to rotate for a period of time due to inertia. The starting current of the rotating motor is small and the voltage will not be pulled down too much, so the motor can be started immediately after the power is restored. The restart of a completely stopped motor needs to be restarted in batches according to the power load of the factory. If the factory power allows 10 motors to be started at the same time, 10 motors will be restarted in batches at a time.