Like the transformer, there is also magnetic flux leakage in the motor, because there is an air gap of 0.5~3mm between the stator and the rotor, so the magnetic flux leakage of the stator is the magnetic flux of the stator core and the air gap into a closed loop, and the magnetic flux leakage of the rotor is the magnetic flux between the rotor core and the air gap into a closed loop. This, combined with the magnetic flux leakage characteristics of the transformer, is very well understood.
The leakage flux of the stator causes a leakage inductance L1σ in the stator winding, and the leakage flux of the rotor causes a leakage inductance L2σ in the rotor winding. Xm is denoted as the maximum inductive reactance of the rotor winding, that is, the inductive reactance of the motor at the moment of starting, then Xm = ωL2σ = 2πfL2σ. The instantaneous inductive reactance of the rotor is equal to the product of the slip rate s and Xm, i.e., X2=sXm.
Therefore, in the rotor winding, there is both resistance and inductive impedance, which together is called impedance. At room temperature, the resistance can be regarded as a constant quantity, but the inductive reactance is an instantaneous changing quantity, and the impedance triangle relationship between the two is satisfied. The impedance is equal to the sum of the square of the resistance and the square of the inductive reactance.
With the rotor impedance and the rotor induced potential, the rotor current can be found, because the rotor impedance and the rotor potential are related to the slip rate s, therefore, the rotor current I2 is a dynamic quantity, and the current I2 of the rotor circuit increases with the increase of s.
When the load is running, when the resistance torque increases and the motor speed decreases, the slip rate increases, and the rotor current becomes larger, which is reflected on the stator side, and the stator current also increases, which is the reason for the increase of motor current.
When the motor starts, the slip rate is the largest, s=1, at this time, the rotor induced potential is the largest, the rotor current is also the largest, the stator current is usually 4~7 times of the rated current, the starting impulse current and no matter what, it can reach 10~14 times of the rated current, and the switchgear is selected by 12 times, which also originates from this.
Let's look at some rotor power factor cosθ, the ratio of resistance r to impedance Z. Since impedance Z is a dynamic quantity, cosθ must be a dynamic quantity. r is constant, Z increases with the increase of the slip rate s, so cosθ must decrease with the increase of s, and the slower the speed, the lower the power factor cosθ. When the motor starts, s=1, at this moment, the power factor is the lowest.
The stator current I1 in the motor is determined by the rotor current I2. In an asynchronous motor, energy is transferred from the stator to the rotor through the rotating magnetic flux. The energy obtained by the rotor rotating magnetic field is converted into mechanical energy output by the rotor, except for a small part of which is converted into heat loss (resistance heating). In this way, the stator of the motor is linked to the rotor.
Finally, let's take a look at the output torque of the rotor, which is equal to the difference between the electromagnetic torque and the frictional torque. The torque is output by looking at the rotating shaft, and the rotor is press-mounted on the rotating shaft, and the two ends are matched with bearings, so the bearing will bring a part of the friction loss, but it is very small and basically negligible. When the motor is no-load, the output torque can be 0 by default, and the stator current is the excitation current of the motor, and its load current is 0.
The electromagnetic torque is generated by the interaction between the magnetic flux Φ of the rotating magnetic field and the rotor current I2, therefore, the electromagnetic torque is proportional to the active component of the rotor current and the magnetic flux of the stator rotating magnetic field, the stronger the magnetic field, the greater the rotor current, and the greater the electromagnetic torque, the expression is: T=kΦI2cosθ.
From the formula 4.44, it can be seen that if the supply voltage remains constant, the magnetic flux does not change, so that the electromagnetic torque is proportional to the active component of the rotor current, i.e., T∝I2cosθ. Therefore, when the resistance torque increases, the current will inevitably increase, because the output torque must be balanced with the resistance torque.
If the power supply voltage changes, it affects the magnetic flux of the rotating magnetic field, so the power supply voltage has a great influence on the output torque of the motor, and the output torque is proportional to the square of the voltage of each phase of the stator.
Disclaimer: This article is transferred from the Internet, the copyright belongs to the original author, if it involves the copyright of the work, please contact us to delete it in time, thank you