Why is the starting current of the three phase asynchronous motor large, but the starting torque is

The reason why the starting current is very large is that when it is just started, the slip rate s is the largest, and the rotor electromotive force e is also the largest, so the starting current is very large. There are two reasons why the starting torque is not large:

First, because the electromagnetic torque depends on the active component of the rotor winding current, when starting, s=1, the rotor leakage reactance is the largest, and the power factor on the rotor side is very low (left and right), therefore, the active component of the rotor winding current is very small when starting; Second, the large starting current leads to the increase of the leakage impedance voltage drop of the stator winding, and if the power supply capacity is small, it will also lead to the decrease of the output voltage of the power supply, and the result is that the magnetic flux of each pole air gap decreases, and then causes the reduction of the starting torque.

The starting current of the three-phase asynchronous motor is very large (up to 4-7 times of the rated current), but the starting torque is not large (about 2 times of the rated torque); This is due to the structure and working principle of the asynchronous motor;

When starting, the rotor does not move, the speed of the stator rotating magnetic field cutting the rotor conductor is very large, and the induced potential generated in the rotor is also very large, so the starting current is very large;

When starting, the frequency of the rotor induced current is the same as the frequency of the power supply, which is 50Hz, at this time, the rotor reactance relative to the rotor resistance is very large, and the power factor of the rotor circuit is very small, according to: m=km ICOS, COS is very small, i is larger, so the starting torque m is not too large; (Because both the current and the magnetic flux are changing, there is a phase difference between the two, and when the current is large, the magnetic flux is not large, so the torque is not large).

Because the current is 90 degrees faster than the voltage (meaning that there is an existing current and a voltage later), there is no inductance when there is no voltage at the moment of start-up, so the current is large.

The magnitude of the current depends on the differential rate, which is the largest at the beginning.

When the three-phase asynchronous motor starts under full load and no load, the starting current and starting torque are the same.

Compared with single-phase asynchronous motors, three-phase asynchronous motors have good running performance and can save various materials. According to the different rotor structures, three-phase asynchronous motors can be divided into cage type and winding type. The asynchronous motor of the cage rotor has a simple structure, reliable operation, light weight and low cost, and has been widely used, but its main disadvantage is that it is difficult to regulate the speed.

The rotor of the wound three-phase asynchronous motor, like the stator, is also provided with three-phase windings and is connected to an external rheostat through slip rings and brushes. Adjusting the rheostat resistance can improve the starting performance of the motor and adjust the rotational speed of the motor.

It's not the same. Y-shaped (power supply voltage is 380V) has a high voltage and a relatively small current; The triangle (power supply voltage is 220V) has a low voltage and a relatively large current, and the power and starting torque of the two are the same.

The rated starting current and starting torque of the motor are determined by its electromagnetic parameters at the factory. For full-load and light-load starting, the starting current and starting torque are the same, and the result is the difference in the length of the starting time. Of course, if the load torque is greater than the starting torque, the motor will not start.

DC starting torque.

The starting torque determines the starting capacity of the motor. It is related to the starting mode (such as star delta starting, frequency conversion speed regulation starting, etc.), and the direct starting squirrel cage type is generally to times the rated torque. Usually the starting torque is more than 125% of the rated torque.

The corresponding current is called the starting current, and it is usually about 6 times the rated current. For DC motors, this starting torque is particularly large, so the starting current is also very large, so it cannot be started directly.

When the motor is started, it is generally required to start at no load or light load, and the starting current is five to seven times the rated current; The starting torque, also known as the stalled torque, is twice the rated torque.

The starting current of the three-phase asynchronous motor is very large (up to 4-7 times of the rated current) when starting, which is caused by the structure and working principle of the asynchronous motor.

When starting, the rotor does not move, the speed of the stator rotating magnetic field cutting the rotor conductor is very large, and the induced potential generated in the rotor is also very large, so the starting current is very large;

When starting, the frequency of the rotor induced current is the same as the frequency of the power supply, which is 50Hz, at this time, the rotor reactance relative to the rotor resistance is very large, and the power factor of the rotor circuit is very small, according to: m=km ICOS, COS is very small, i is larger, so the starting torque m is not too large;

How it works. When a symmetrical three-phase alternating current is introduced into the three-phase stator winding, a rotating magnetic field is generated that rotates clockwise along the circular space of the stator and rotor at a synchronous speed n1. Since the rotating magnetic field rotates at n1 rotational speed, the rotor conductor is stationary at the beginning, so the rotor conductor will cut the stator rotating magnetic field and generate an induced electromotive force. Since the two ends of the rotor conductor are shorted by short-circuit rings, under the action of induced electromotive force, the induced current in the rotor conductor will be basically consistent with the direction of the induced electromotive force.

It is lower.

The torque is proportional to the flat of the voltage, so the torque decreases. Motor speed n=(1-s)*60f p,s is the slip rate, which is the difference between the speed of the stator rotating magnetic field and the rotor speed, when the voltage decreases, s increases, and the speed decreases.

In addition, the voltage reduction current is not all increased, when the motor is in no load or light load, the torque output of the motor is very small, and it only needs to maintain its own loss to rotate normally, so the voltage decreases, and the current will also decrease.

Torque. The moment that turns a mechanical element is called the rotational torque, or torque for short. Mechanical components will produce a certain degree of torsional deformation under the action of torque, so torque is sometimes called torque.

Torque is the basic load form of the transmission shaft of various working machinery, which is closely related to the working capacity, energy consumption, efficiency, operating life and safety performance of power machinery, and the measurement of torque is of great significance to the determination and control of the transmission shaft load, the strength design of the working parts of the transmission system and the selection of the capacity of the prime mover.

There are two reasons why the starting torque is not large:

First, because the electromagnetic torque depends on the active component of the rotor winding current, when starting, s=1, the rotor leakage reactance is the largest, and the power factor on the rotor side is very low (left and right), therefore, the active component of the rotor winding current is very small when starting;

Second, the starting current is large and the leakage impedance voltage drop of the stator winding increases, if the power supply capacity is small, it will also lead to the decrease of the power supply output voltage, and the result is that the lead gear will reduce the magnetic flux of each pole air gap, and then cause the reduction of the starting torque.

Induction motor is also known as "asynchronous motor", that is, the rotor is placed in a rotating magnetic field, and under the action of the rotating magnetic field, a rotating torque is obtained, so the rotor rotates.

The exterior and internal structure of the induction motor.

Rotors are rotating conductors, usually in the shape of a squirrel cage. The stator is the non-rotating source of the electric motor, and its main task is to generate a rotating magnetic field. Rotating magnetic fields are not hail, they are always achieved by mechanical methods.

Instead, alternating current is passed through several pairs of electromagnets, causing the polarity properties to change cyclically, so it is equivalent to a rotating magnetic field. This kind of motor is not like DC motor has brushes or collector rings, according to the type of alternating current used, there are single-phase motors and three-phase motors, single-phase motors are used in washing machines, electric fans, etc.; Three-phase electric motors are used as the power equipment of the plant.

The starting current of the three-phase asynchronous motor is very large (up to 4-7 times of the rated current), which is caused by the structure and working principle of the asynchronous motor;

When starting, the rotor does not move, the speed of the stator rotating magnetic field cutting the rotor conductor is very large, and the induced potential generated in the rotor is also very large, so the starting current is very large;

When starting, the frequency of the rotor induced current is the same as the frequency of the power supply, which is 50Hz, at this time, the rotor reactance relative to the rotor resistance is very large, and the power factor of the rotor circuit is very small, according to: m=km ICOS, COS is very small, i is larger, so the starting torque m is not too large; (Because both the current and the magnetic flux are changing, there is a phase difference between the two, and when the current is large, the magnetic flux is not large, so the torque is not large).