Les fournisseurs de convertisseurs de fréquence spécifiques à l'industrie pétrolière rappellent que les moteurs électriques sont actuellement les outils rotatifs les plus utilisés. Avec le développement et la popularisation des convertisseurs de fréquence, leur utilisation conjointe avec celle de moteurs électriques se généralise. Cependant, cette association engendre inévitablement de nombreux problèmes :
1. Les démarreurs progressifs de moteurs permettent-ils d'économiser de l'énergie ?
L'effet d'économie d'énergie du démarrage progressif est limité, mais il peut réduire l'impact du démarrage sur le réseau électrique, assurer un démarrage en douceur et protéger l'enroulement du moteur.
Selon la théorie de la conservation de l'énergie, le démarrage progressif, du fait de l'ajout de circuits de commande relativement complexes, non seulement n'économise pas d'énergie, mais augmente également la consommation énergétique. Il permet toutefois de réduire le courant de démarrage du circuit et joue un rôle protecteur.
Quels sont le courant de démarrage et le couple de démarrage du moteur lorsqu'un convertisseur de fréquence est utilisé pour son fonctionnement ?
En utilisant un variateur de fréquence, la fréquence et la tension augmentent proportionnellement à l'accélération du moteur, et le courant de démarrage est limité à moins de 150 % du courant nominal (125 % à 200 % selon le modèle). Un démarrage direct sur secteur engendre un courant 6 à 7 fois supérieur, provoquant des chocs mécaniques et électriques. L'utilisation d'un variateur de fréquence permet un démarrage en douceur (avec un temps de démarrage plus long). Le courant de démarrage est alors de 1,2 à 1,5 fois le courant nominal, et le couple de démarrage de 70 % à 120 % du couple nominal. Pour les variateurs de fréquence dotés d'une fonction d'augmentation automatique du couple, le couple de démarrage dépasse 100 %, permettant un démarrage à pleine charge.
Existe-t-il un lien entre la surcharge du moteur et un court-circuit ?
Il existe deux types de surcharge moteur : 1. La surcharge mécanique : elle est causée par une charge motrice dépassant la valeur nominale ou par un blocage du système de transmission, et n’a rien à voir avec un court-circuit. 2. La surcharge en charge normale : si le courant moteur est excessif, cela peut être dû à une mise à la terre locale ou à des courts-circuits entre les spires de l’enroulement moteur.
À quoi sert la régulation de vitesse à fréquence variable ? Quels en sont les avantages ?
À quoi sert la régulation de vitesse à fréquence variable ?
Il peut être appliqué aux machines tournantes nécessitant une régulation de vitesse.
Quels sont les avantages de la régulation de vitesse à fréquence variable ?
Before the implementation of variable frequency speed regulation (theoretically, it had already been realized, but the actual implementation was after the invention of power electronic devices), traditional speed regulation used direct current. The disadvantages of direct current speed regulation are:
① DC motors have complex structures and high maintenance costs
② Due to the existence of the commutator, there is not much room for increase in the power of the DC motor.
Therefore, the benefits of variable frequency speed regulation are:
① It can achieve the same excellent speed regulation performance as DC speed regulation for AC motors.
② The maintenance of squirrel cage asynchronous motors is simple and convenient.
③ There is no limitation on the power of AC motors due to the commutator.
How to measure the insulation resistance of a motor?
If it is a three-phase AC motor, measure the insulation resistance between phases and to ground of the motor's three-phase windings.
If it is a DC motor, measure the motor armature winding to ground, series excitation winding to ground, secondary excitation winding to ground, and series excitation winding to secondary excitation winding. Select the corresponding shaker according to the voltage level of the tested motor.
Measurement steps:
---Disconnect the power supply
---Ground discharge
---If it is a three-phase AC motor, open the center point (if possible)
---If it is a DC motor, lift the brush.
---Use a shaking table to measure the insulation resistance between phases and to ground separately
---Ground discharge
---Restore the line
---Record the insulation resistance and ambient temperature.
6. What is a brushless and acyclic starter?
Brushless and Ringless Starter is a starting device that overcomes the disadvantages of wound asynchronous motors equipped with slip rings, carbon brushes, and complex starting devices, while retaining the advantages of low starting current and high starting torque of wound motors. JR, JZR, YR, and YZR three-phase wound rotor AC asynchronous motors (except for variable speed and those equipped with input cameras) that originally used resistance starters, reactors, frequency sensitive variable resistors, liquid variable resistor starters, and soft starters can be replaced with "brushless and open-loop starters".
How many capacitor starting methods are there for motors?
There are two types of starting:
⑴ Capacitor starting (refers to the disconnection of the capacitor after the motor is started);
⑵ The capacitor starts and operates (the capacitor participates in operation after starting).
Can a transformer be used as a load for a frequency converter?
In principle, it should be possible, but it is not practical in practice. Frequency converters do not require transformers to boost voltage, and there should be varieties that can be used for circuits above 380V. If higher voltage is required, there are also circuits that can be directly converted to 220V or 380V and then double the voltage to obtain high voltage. Frequency converters are mainly used for load driving (such as electric motors) and are rarely used for power frequency conversion. The functions of frequency converters are far from limited to frequency conversion itself, and there are many additional functions such as various protections. If frequency converters are used to obtain frequency conversion power, it is not advisable from an economic perspective. It is recommended to use other frequency conversion circuits.
Can the frequency converter be adjusted to 1Hz, and how many Hz can it be adjusted up to for use?
If the frequency converter is used on a general AC asynchronous motor, when the frequency converter is adjusted to 1Hz, it is already close to DC, which is absolutely not allowed. The motor will operate at the maximum current within the limit of the frequency converter, and the motor will generate severe heat, which is likely to burn out the motor.
If the operation exceeds 50Hz, it will increase the iron loss of the motor, which is also detrimental to the motor. Generally, it is best not to exceed 60Hz (exceeding it in a short period of time is allowed), otherwise it will also affect the service life of the motor.
What is the working principle of the frequency regulation resistor in a frequency converter? Why can adjusting the resistance change the frequency?
The frequency adjustment resistor of the frequency converter is used to proportionally divide the 10V reference voltage of the frequency converter, and then send it back to the main control board of the frequency converter. The main control board of the frequency converter then performs analog-to-digital conversion on the voltage sent back by the resistor to read the data, and then converts it into a proportional value of the rated frequency to output the current frequency. Therefore, adjusting the resistor value can adjust the frequency of the frequency converter.
11. Can the frequency converter decouple the motor current?
Can frequency conversion be decoupled? I can't! But as long as the output frequency f and synchronous speed n1 keep the slip rate in the stable range or rated slip rate Se, it is equivalent to decoupling the motor current, because the rotor power factor is now 1, and the rotor current is the torque current that everyone needs to decouple and control! The frequency converter is a speed control device for asynchronous motors, and it cannot perform any control beyond the mechanical characteristics of asynchronous motors.
Why is the current high when starting an induction motor? Will the current decrease after startup?
When an induction motor is in a stopped state, from an electromagnetic perspective, it is like a transformer. The stator winding connected to the power supply is equivalent to the primary coil of the transformer, and the rotor winding in a closed circuit is equivalent to the secondary coil of the transformer that is short circuited; There is no electrical connection between the stator winding and the rotor winding, only magnetic connection. The magnetic flux passes through the stator, air gap, and rotor core to form a closed circuit. At the moment of closing, the rotor has not yet started rotating due to inertia, and the rotating magnetic field cuts the rotor winding at the maximum cutting speed - synchronous speed, causing the rotor winding to induce the highest possible potential. Therefore, a large current flows through the rotor conductor, which generates magnetic energy to counteract the stator magnetic field, just like the secondary magnetic flux of a transformer needs to counteract the primary magnetic flux.
In order to maintain the original magnetic flux that is compatible with the power supply voltage, the stator automatically increases the current. Because the current of the rotor is very high at this time, the stator current also increases significantly, even up to 4-7 times the rated current, which is the reason for the high starting current.
Why is the current small after starting: As the motor speed increases, the speed at which the stator magnetic field cuts the rotor conductor decreases, the induced potential in the rotor conductor decreases, and the current in the rotor conductor also decreases. Therefore, the part of the stator current used to offset the magnetic flux generated by the rotor current also decreases, so the stator current decreases from large to small until it returns to normal.
What is the impact of carrier frequency on frequency converters and motors?
The carrier frequency has an impact on the output current of the frequency converter:
(1) The higher the operating frequency, the larger the duty cycle of the voltage wave, the smaller the high-order harmonic components of the current, that is, the higher the carrier frequency, and the smoother the current waveform;
(2) The higher the carrier frequency, the smaller the allowed output current of the frequency converter;
(3) The higher the carrier frequency, the smaller the capacitance impedance of the wiring capacitor (because Xc=1/2 π fC), and the greater the leakage current caused by high-frequency pulses.
The impact of carrier frequency on motors:
The higher the carrier frequency, the smaller the vibration of the motor, the lower the operating noise, and the less heat generated by the motor. But the higher the carrier frequency, the higher the frequency of harmonic current, the more severe the skin effect of the motor stator, the greater the motor loss, and the lower the output power.
Why can't a frequency converter be used as a frequency converter power supply?
Le circuit complet d'une alimentation à fréquence variable est composé de sections de conversion AC/DC, AC et de filtrage. Les formes d'onde de tension et de courant qu'elle délivre sont donc des sinusoïdes pures, très proches de celles d'une alimentation AC idéale. Elle peut fournir la tension et la fréquence du réseau électrique de n'importe quel pays du monde.
Le convertisseur de fréquence est composé de circuits de courant alternatif continu et de courant alternatif modulé. Son appellation standard est « variateur de vitesse à convertisseur de fréquence ». La tension de sortie est une onde carrée pulsée comportant de nombreuses harmoniques. La tension et la fréquence varient proportionnellement et simultanément, et ne peuvent être réglées séparément, ce qui le rend inadapté aux besoins d'une alimentation électrique. De ce fait, il ne peut être utilisé comme source d'alimentation et sert généralement à la régulation de vitesse des moteurs asynchrones triphasés.
Pourquoi la température du moteur augmente-t-elle davantage lorsqu'on utilise un convertisseur de fréquence qu'à la fréquence du réseau électrique ?
Étant donné que la forme d'onde de sortie du convertisseur de fréquence n'est pas une onde sinusoïdale, mais une onde déformée, le courant du moteur au couple nominal est environ 10 % plus élevé qu'à la fréquence du réseau, de sorte que l'élévation de température est légèrement supérieure à celle à la fréquence du réseau.
Un autre point à noter est que lorsque la vitesse du moteur diminue, la vitesse du ventilateur de refroidissement du moteur est insuffisante, et la température du moteur augmente davantage.