Inverter motor and power frequency motor are two completely different series of products, but it is a very common practice for customers to use power frequency motor inverter.
This simple and easy method may be the most economical and fast for both motor manufacturers and ancillary equipment manufacturers, but it carries great risks: it may lead to a reduction in the life of the motor or direct burnout.
Since the use of power frequency motor frequency conversion is very common, there is no doubt that it has its reasonable side.
The speed of the asynchronous motor is positively correlated with the frequency of the power supply: that is, the frequency is high, and the speed is also high (proportional regardless of the influence of slip); The magnitude of the output torque and current is related to the load (= "dynamic load" + "static load or steady state load"), if the influence of harmonic composition is not very serious, the current increase is not large at the rated load;
When the motor is running, the frequency conversion range is not less than 30% of the rated frequency. In line with the above situation, there are often quite a lot, the actual operating speed is not so low that it affects ventilation and heat dissipation too much, and the current increase is not too large, so the motor can indeed work normally as expected by the design.
The so-called great risk is because most end users do not know that when the frequency conversion power supply, the frequency converter will output a large number of high-frequency harmonics, and the motor may resonate at a certain speed.
Therefore, when the rated load is overloaded (the current increase is too large), high-frequency harmonic breakdown wire wrap or enameled wire insulation, howling or violent vibration during speed regulation, and sometimes even the explosion of the inverter power module.
today We make a simple summary of the particularity of the inverter motor, aiming to reduce the risk of the inverter-powered motor during operation and minimize the loss in the event of an accident.
Variability in the selection of performance parameters
For ordinary asynchronous motors, the main performance parameters considered in the design are overload capacity, starting performance, efficiency and power factor, and the variable frequency motor, because the critical slip rate is inversely proportional to the power supply frequency, can be started directly when the critical slip rate is close to 1,
therefore, the overload capacity and starting performance do not need to be considered too much, and the key problem to be solved is how to improve the adaptability of the motor to the non-sine wave power supply. Therefore, the measures taken in the design process are mainly as follows:
Reduce the resistance of the stator and rotor as much as possible. This is because reducing the stator resistance reduces the fundamental copper loss to compensate for the increase in copper loss caused by higher harmonics.
In order to suppress the higher harmonics in the current, it is necessary to increase the inductance of the motor appropriately.
However, the rotor groove leakage resistance is larger, and the skin effect is also larger, and the copper loss of higher harmonics is also increased. Therefore, the size of the motor leakage impedance should take into account the rationality of impedance matching in the whole speed control range.
The main magnetic circuit is designed to be unsaturated, one is to consider that the higher harmonics will deepen the saturation of the magnetic circuit, and the other is to consider that the output voltage of the inverter is appropriately increased in order to increase the output torque at low frequency.
Structural design countermeasures
In the structural design, the influence of non-sinusoidal power supply characteristics on the insulation structure, vibration, noise, cooling mode and other aspects of the variable frequency motor can be fully considered and evaluated, and the performance of the product can be guaranteed to meet the requirements in the following ways:
Generally, it is F or higher insulation level, and the insulation strength of ground insulation and wire turns should be strengthened, especially the ability of insulation to withstand impulse voltage.
Some motor manufacturers use thickened paint film magnet wires, and standardized motor manufacturers use special magnet wires for variable frequency motors. From the analysis of the performance indicators of the magnet wire, the paint used in the frequency conversion magnet wire is different from the ordinary magnet wire, and the use of thickened wire will have a certain effect, but it cannot solve the fundamental problem. However, from the cost analysis, the price of variable frequency magnet wire is higher than that of ordinary magnet wire.
It is necessary to fully consider the rigidity of the motor mechanism and the whole to effectively avoid the vibration and noise problems of the motor, and avoid the resonance phenomenon with each force wave from the perspective of increasing the natural frequency of the motor.
Bearing insulation measures should be used for motors with a capacity of more than 132 kilowatts. Mainly because the frequency conversion motor is prone to asymmetry of the magnetic circuit, resulting in the shaft current generated when the motor is running, when the current generated by other high-frequency components is combined together, the shaft current will be greatly increased, resulting in bearing damage, so insulation measures should generally be taken.
For constant power variable frequency motors, when the speed exceeds the synchronous speed of the 2P motor, special grease with high temperature resistance should be used to ensure the lubrication effect of the bearing during operation, and special high temperature resistant bearings should be selected if necessary.
ifferent motor manufacturers have different electromagnetic schemes of the same specification, but the actual effect of motor operation will have some differences; The same product, due to the particularity of the use of working conditions, will also lead to differences in the stability of the same specification of products. Ms. suggested that motor manufacturers should try their best to fully understand the working conditions of the motor and avoid the actual use risk through the top-level design improvement of the product.
Get more information about inverter motor and normal induction motor, please contact with professional electric motor directly - Dongchun motor
