Why is it possible to increase the starting torque by inserting resistors in series in the rotor cir

The wound rotor asynchronous motor increases the rotor circuit impedance by the string resistance in the rotor circuit.

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It can be seen that the starting current varies with the series resistance r'The 2st increases and decreases, and the rotor loop string resistance also decreases the rotor loop impedance angle 2=arctan[x2 (R2+R2ST)], thereby increasing the rotor loop power factor.

cosψ2；The reduction of starting current reduces the leakage voltage of the stator; Electromotive force.

e1 increases, making the air gap flux.

Increase. The starting torque is proportional to the air-gap magnetic flux, starting current, and COs 2.

Although the starting current decreases, the air-gap flux and COs 2 increase, resulting in an increase in the starting torque.

If the string resistance is too large, the starting current is too small, and the starting torque will also be reduced.

Wound rotor asynchronous motor rotor circuit series reactor.

The rotor circuit impedance is increased, and the starting current can be reduced. At the same time, it also increases the rotor circuit impedance angle 2=arctan[(x2+xst) r2], and CO2 decreases, so that the active component of the rotor current is i'2cos 2 is reduced, which in turn reduces the starting torque even more.

Serial reactors do not improve starting performance across the board.

This one is incomplete.

The series resistance will indeed reduce the starting current, because after the resistance is connected into the rotor circuit, according to the coupling effect of the magnetic field, like a transformer, the resistance of the entire circuit will be increased, and the natural current will be small under the condition that the voltage is unchanged.

However, too much or too little resistance will cause the starting torque to decrease. It is precisely because of this that in the string resistance speed regulation, it is necessary to calculate the resistance value of the resistance box to ensure that the t-n curve of the motor and the horizontal line n=0 intersect and have the maximum t value, that is, when the speed is 0, the starting torque is as large as possible.

Because after the resistor is inserted in series, the voltage of the rotor coil becomes larger, and the torqueProportionalBy the square of the rotor coil voltage, the coil becomes larger, and the torque becomes larger. Rotten ears

The wound motor is equivalent to an asynchronous motor that can change the rotor parameters, and when the rotor resistance of the motor (not the capacitor inductance) is changed, the maximum torque of the asynchronous motor is determined to be constant.

The solid line is the characteristic of the rotor resistance when it is relatively large, and the dotted line is the characteristic of the small resistance. The torque rotor is connected in series resistance when starting, and the starting torque can be increased from the torque characteristics.

On the other hand, the series resistance also bears the rotor power loss during start-up, reduces the loss of the rotor, and reduces the excessive rotor current impact on the brush and slip ring for the wound motor.

Furthermore, by matching the size of the resistor, the starting torque can be adjusted to prevent the impact of excessive starting shock torque on the equipment.

Asynchronous motor.

How it works:

Asynchronous motors are also known as induction motors.

The rot is caused by the air gap rotating the magnetic field and the rotor windings inducing current.

An AC motor that interacts with each other to generate electromagnetic torque, thereby converting electromechanical energy into mechanical energy.

Three-phase asynchronous motor.

It is mainly used as an electric motor to drag various production machinery, such as: fans, pumps, compressors, machine tools, light industry and mining machinery, threshers and pulverizers in agricultural production, processing machinery in agricultural and sideline products, etc. The structure is simple, easy to manufacture, inexpensive, reliable, durable, high in operation efficiency and has applicable working characteristics.

Is the starting resistance in the sub-circuit as large as possible? In the process of starting, why start.

The resistance should be cut off step by step?

Answer]: In order to reduce the starting current, the three-phase wound rotor asynchronous motor is started by the method of serial resistance in the rotor circuit, which not only reduces the starting current, but also improves the quiet starting torque. Before starting, adjust the resistance to the maximum, close the switch after the rotor begins to rotate, with the increase of speed, gradually reduce the resistance, after the motor speed is stable, the starting resistance is short-circuited, that is, cut off the full start of the missing part of the starting resistance.

Missing Answer]: c

In order to reduce the start-up current, the three-phase wound rotor asynchronous motor is started by the method of serial resistance in the rotor circuit, which not only reduces the start-up current, but also increases the start-up torque. Before starting, adjust the resistance to the maximum value, close the switch and the rotor will start to rotate, with the increase of speed, gradually reduce the resistance, and after the motor speed is stable, the starting resistance will be short-circuited, that is, all the starting resistance will be cut off.

The decrease in the starting current is well understood, and when the induced electromotive force is unchanged, the resistance increases and the current is naturally smaller.

Because the stator and rotor current is very large when the motor starts, the leakage resistance of the rotor is very large, and the power factor is very low; In addition, due to the large current at the stator side and the large leakage voltage drop, the main magnetic flux becomes smaller, and the current is very large when the motor is started, but the starting torque is not large.

After the series resistance is introduced into the rotor, on the one hand, the power factor of the rotor is increased, so that the active component of the torque is increased, and on the other hand, the starting current is reduced due to the series resistance, the leakage pit voltage drop on the stator side is reduced, and the main magnetic flux is increased. The two aspects work together to make the starting torque of the motor when the resistance is in series greater than the torque when the resistance is not in series.

Reduce the starting current, but do not increase the starting rotation.

When the non-string resistance is started, because the rotor winding is net, the speed of cutting the magnetic field lines relative to the stator rotating magnetic field is high, the voltage generated by the rotor winding is high, and the rotor current is large, the torque is large, and the stator current is large.

After the rotor is connected with the resistor, the greater the rotor current at start-up, the greater the voltage drop of the resistor, so that the rotor winding voltage decreases, the current decreases, and the torque decreases. After start-up, the speed of cutting the magnetic field lines decreases due to the close speed of the rotor and stator, the voltage generated by the rotor windings decreases, the current decreases, and the voltage drop of the resistance also decreases.

1: You can understand it this way; The motor is a resolver, the stator coil is the primary winding, and the rotor coil is the secondary winding. When the rotor coil is connected to the resistor, the load on the secondary side of the transformer is reduced, and the primary current is also reduced.

For motors, the stator current decreases.

2: Due to the resistance of the rotor coil, the slip increases. According to the motor torque calculation formula: mn=9550*pn nn

mn - torque.

pn - Frontal power.

nn - Rotational speed, when the rated power remains the same, the rotational speed decreases and the torque increases.