2. The harm caused by too large starting current and too small starting torque.
The starting current of ordinary asynchronous motor reaches 5-7 times of rated current, while the starting torque is only 0.4- 1.6 times of rated torque. It can be started directly when the power grid conditions (the voltage drop of the power grid is less than 10% when the motor is started) and the process conditions (meeting the starting torque) allow. However, the starting current is too large, the starting torque is too small and the starting time is too long, which has caused great harm to the motor and the power grid.
When the starting current of the motor reaches 6-7 times the rated current, the heating value of the coil is 36-49 times that of the normal operation of the motor, and the generated electromagnetic force is also 36-49 times. Excessive temperature, excessive heating speed, excessive temperature gradient and electromagnetic force have produced great destructive power, shortening the service life of stator coils and rotor copper bars (especially the rotor often uses skin effect to reduce the starting current, and the surface temperature of rotor copper bars reaches above 350℃ when starting). For example, the main fan motors in the 29m2 sintering plant of Liugang Company are two 1000KW squirrel-cage motors, which are directly started at full pressure, and the motors are burnt out six times in one year from 1997. Starting from 1998, step-down start-up was adopted, and by 2002, it had been running for 4 years, and there was no motor burning accident. For this reason, relevant statistics show that the direct starting time of the motor is less than 0. 1% of the motor running time, but the failure rate accounts for more than 30% of the total failure rate of the motor.
The power supply device of the motor is generally selected according to the rated current of the motor. Excessive starting current often leads to heating of the contact of the power supply device and aging of the insulation around the contact, which is also a main reason for the damage of the power supply device. For example, in the propane plant of Nanyang Refinery, the power supply transformer capacity is 800kvA, and the maximum motor capacity is only 130kw, which fully meets the conditions for direct motor start. However, the excessive starting current of 200 1 year caused three short circuits of low-voltage switchgear and three shutdowns of production equipment, which caused great economic losses to the factory.
Due to the above reasons, when the power grid and process conditions are satisfied, large-capacity motors often take measures to reduce the starting current to improve the reliability of power supply and reduce the failure rate of motors. However, the starting device is expensive (especially the middle and high voltage motor, soft starter+starting switch cabinet, etc.). More than 0.5 times the price of 65438+ motor) and complicated structure not only increase the cost of users but also reduce the reliability of electric drive system. At the same time, the starting torque of the motor is proportional to the square of the starting current. When the starting mode of reducing the stator voltage is adopted, if the starting current is reduced from 6Ie to 3Ie, the starting torque of the motor will be reduced by 4 times and the starting time of the motor will be increased by more than 5 times. Therefore, it is required that the motor with large starting torque and short starting time can only be replaced by wound motor.
Because of its carbon brush, slip ring and starting control system, wound motor can not be used in occasions with high requirements for reliability and maintenance-free of traction system. For example, pumping units widely used in oil fields need large starting torque motors, but they are all installed on site, and there are a large number (in Henan Oilfield with an annual output of 6.5438+0.8 million tons of crude oil, the number of pumping unit motors exceeds 2,000, and in Daqing Oilfield it exceeds 50,000). In order to improve the reliability of motor drive, squirrel-cage motor has to be adopted. At the same time, in order to increase the starting torque, the motor capacity must be increased. So that the average load rate of the motor of the pumping unit is less than 30% of the rated power of the motor. It not only increases the cost of the electric drive system, but also reduces the inherent efficiency of the motor and wastes a lot of energy.
In order to illustrate the advantages of brushless automatic control motor, a 1400KW, ne = 1488r/min, 6KV, Ie = 154. a，Ist/Ie=6.38，MMAX/ME = 2.3； The squirrel-cage motor drives a main fan and a 1400KW wound motor to drive the same load, both of which are equipped with water resistance soft starters (the price of water resistance soft starters is only 1/2 of that of silicon controlled rectifier and magnetic saturation reactor soft starters). As an example, the design method, starting performance and the cost of the traction system are explained.
3. Squirrel cage motor and its driving system
The composition of the drive system is shown in Figure 2, which generally consists of running high-voltage switch cabinet DL 1, starting high-voltage switch cabinet DL2, disconnector K cabinet, water resistor R and its control device. Among them, the water resistance R and its control device are formed by inserting two electrodes into a conductive liquid such as salt water. Its resistance is proportional to the distance between electrodes. By adjusting the distance between the electrode plates, the size of this resistor can be adjusted to achieve the purpose of adjusting the stator current. By connecting this variable resistor in series with the stator circuit of the motor, the motor can be started smoothly.
After the motor stator coil is connected in series with a three-phase symmetrical resistor:
Start the motor after the air inlet valve of the fan is fully closed. Assuming that the acceleration torque when the motor starts is 10% of the rated torque of the motor, the table (nominal value) is 1:
Slip rate1.000.900.800.700.600 0.500.400.300.200 0.100.050.02 average.
Speed 0.000.10.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 0.950 0.98
Full voltage torque1.171.21.251.301.401.42/.
The current at full voltage is 6.38 6.30 6.216.10 5.96 5.80 5.615.38 5.09 4.53 2.571.705.14.
The no-load torque of the fan is 0.11.09 0.08 0.08 0.100.120.150.180.230.260.3/kloc-0.
Soft start current is 2.70 2.49 2.35 2.28 2.19 2.17 2.16 2.041.701.19 0.96 2.03.
Motor voltage 0.42 0.40 0.38 0.37 0.37 0.38 0.39 0.40 0.38 0.46 0.56 0.41.
The voltage drop across the resistor is 0.89 0.89 0.88 0.88 0.88 0.87 0.86 0.85 0.82 0.75 0.64 0.44 0.80.
The motor output torque is 0.21.190.180.180.20 0.21.25 0.28 0.33 0.36 0.40 0.25.
Water resistance force 2.40 2.20 2.08 2.001.921.891.871.831.681.28 0.77 0.
In this state, the starting time of the motor is 69 seconds, and the average power consumption is 2300kw, and the power consumption is about 44kwh (which can raise 475kg of water from 20℃ to 100℃). After the motor is started, the operator (or time relay) should first close the running high voltage switch DL 1, then close the start switches DL2 and K, and then open the fan load valve, otherwise it will threaten the safe operation of the motor and the starting device.
The motor price of the drive system is about 6.5438+0.7 million yuan, the starting cabinet and isolating switch cabinet are about 6.5438+0.2 million yuan, and the three-phase rheostat needs about 6.5438+0.2 million yuan. Adding three cables, each 30 meters, plus six cable heads, will cost about 45,000 yuan. If the cost of infrastructure [rheostat+starting and isolating switch cabinet] is excluded, it is about: (1.7+1.2) ×1.2× 2.8m3], and the total charge is still as high as 455,000 yuan (.
4. Wound motor and its driving system
The winding motor drive system is shown in Figure 3: When the motor is started, a resistor R is connected in series in the rotor circuit, and after starting, the series rotor resistor is short-circuited. The principle of water resistance change of wound motor is the same as that of squirrel-cage motor, but the difference is that water resistance is connected in series in the rotor circuit of motor, which reduces the starting current of motor and increases the starting torque of motor. After the rotor of wound motor is connected with three-phase symmetrical resistor in series, the biggest difference between the above formula and squirrel-cage motor is that the starting resistance of squirrel-cage motor only increases the denominator of starting torque, that is, it reduces the starting current and starting torque of motor; Moreover, the motor is an inductive load in the starting process, and the power supply voltage is all added to the starting resistor, which increases the power consumption of the starting resistor. The winding motor just overcomes the above shortcomings. RST "(X1+X20+X2), the torque of the motor is inversely proportional to Rst/S, and the current is inversely proportional to Rst/S; The output voltage of the rotor is directly proportional to the slip ratio S. According to this relationship, on the basis of the starting resistance torque of the fan and the rated torque of the motor, the motor is started at an acceleration of 100% of the rated torque, and Table 2 of power loss versus motor speed, torque, current and resistance can be made.
Slip rate1.000.900.800.700.600 0.500.400.300.200 0.100.050.02 average.
Speed 0.000.10.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 0.950 0.98
Motor current1.1.091.081.081.0810/kloc-0.
Voltage drop across the resistor1.000.900.800.700 0.600 0.500.400.300.200 0.100.050.020.46
Motor output torque1.1.091.081.081./kloc-0. 438+08 1.23 1.26 1.3 1 1. 15
Water resistance power1.11.98 0.86 0.76 0.65 0.55 0.45 0.34 0.24 0.120.060.030.56438+0.
It can be clearly seen from Table 2 that when the starting current of the wound motor is reduced by nearly 65,438+0/2 times compared with the squirrel-cage motor, the starting torque is increased by more than 4 times, the starting time is reduced to 65,438+0/65,438+00 times (6.9 seconds), and the power consumption on the starting resistor is reduced by about 33 times (65,438).
Drag system: the winding motor needs about 230,000 yuan, and the three-phase rheostat+short-circuit switch cabinet needs about 6,543,800 yuan; It is necessary to add three 3×240mm2 low-voltage cables as rotor current outgoing lines, each with a length of 30m and six cable heads, which will cost about 25,000 yuan, and the total cost will reach 355,000 yuan, excluding the capital construction cost.
5. Brushless automatic control motor and its realization method
The brushless automatic control motor drive system is shown in Figure 4. It does not need to add a starting control cabinet and complicated control devices, and only needs a running switch cabinet to realize the soft start of the motor. Its main electrical performance is equivalent to the wound asynchronous motor with water resistance soft starter. The difference is that the starting resistance in series with the rotor automatically decreases with the increase of the motor speed. When the motor speed reaches about 90% of the rated speed, the starting resistance decreases to zero. It ensures that the starting device is completely synchronized with the starting process of the motor, and overcomes the harm caused by the complexity and high cost of the slip ring and carbon brush starting devices of wound-wound motors and squirrel-cage motors.
Patent application number: 200920092058.8
Henan Quanxin Liquid Starting Equipment Co., Ltd. owns independent intellectual property rights and exclusively produces 0373-6506 158.