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The AC asynchronous motor has a large starting current, a long starting time, and a large impact on the power grid, and must be stepped down to start. Most of the rolling systems use high-voltage wound-type motors. At present, the main starting methods are: (1) rotor string resistance starting; (2) starting with thyristor soft starter; (3) rotor string frequency sensitive varistor starting; (4) rotor string liquid Starting method such as starting of the resistance starter. These types of starting methods have their own characteristics and are widely used in rolling mill production lines. In order to find a safer, more reliable, simpler and easier starting method, the AC asynchronous motor starting problem can be better solved to ensure smooth production. In this paper, the technical characteristics of various starting methods are analyzed and studied. It is considered that the liquid resistance soft starter starting method is more suitable for starting the high-voltage wound-type motor for rolling steel. Our company purchased a set of liquid resistance soft starter in 28 years, which is used for the starting of the main drag motor of the 450mm preliminary rolling mill for hot-rolled strip production line, replacing the frequency sensitive varistor, and the comprehensive starting effect is better after several years of use.
1) Wire-wound asynchronous motor, due to the special manufacturing, the three-phase symmetrical resistor can be externally connected in the rotor circuit, the resistance value is matched properly, the starting current Iq can be controlled at 1.5-2.5 times the rated current, and the starting voltage Uq is controlled. Within the rated voltage Ue, the torque reaches a maximum. Therefore, the wound-type asynchronous motor can be applied to heavy-duty and frequently-started production machines.
2) The mechanical characteristics of the rotor string resistance are shown in Figure 1:
It can be seen from the figure that as long as the appropriate resistance value is externally connected, the distance and the conductivity are easy to control, and the heat capacity of the liquid resistor is large, and the motor can be repeatedly started in a short time. Its pure resistance overcomes the shortcomings of the frequency sensitive varistor to feedback higher harmonics to the grid, so the start is smooth, stepless, and no impact. The starting current is continuously adjustable from 1.1 to 2.51e, and the starting time is adjustable from 2 to 60 seconds.
2) Working principle of high-pressure cage type liquid resistance starter , see Figure 2, where IQF is the motor running circuit breaker, high-pressure cage type liquid resistance starter short circuit breaker, and Rs is liquid rheostat. The high-pressure cage type liquid resistance starter is a three-phase structure, which is composed of three mutually insulated boxes, each of which contains an electrolyte and a pair of independent electrode plates, one of which is fixed, and the phase plate is driven by the transmission. The mechanism and PLC controller control the operation. When the motor starts, the distance between the moving plate and the fixed plate is the largest. At this time, the IQF is closed, the 2QF is disconnected, the motor starts to rise, and the distance between the moving and fixed plates is gradually approached. When the motor reaches the rated speed, between the moving and fixed plates The resistance is close to zero. At this time, 2QF is closed, the liquid resistance circuit is shorted, and the motor enters the running state too.
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