The supplier of frequency converter supporting equipment reminds you that the frequency converter is an electrical energy control device that uses the on-off function of power semiconductor devices to convert the power frequency power supply to another frequency. It can achieve soft starting, frequency conversion speed regulation, improve operating accuracy, change power factor, overcurrent/overvoltage/overload protection and other functions for AC asynchronous motors. What should be paid attention to when using a frequency converter?
1. Shielded wires should be used for signal and control lines to prevent interference. When the line is long, such as a distance jump of 100m, the wire cross-section should be enlarged. Signal and control lines should not be placed in the same cable trench or bridge as power lines to avoid mutual interference. It is better to place them in conduit for better suitability.
2. The transmission signal is mainly based on current signals, as current signals are not easily attenuated or interfered with. In practical applications, the signal output by sensors is a voltage signal, which can be converted into a current signal through a converter.
3. The closed-loop control of frequency converters is generally positive, meaning that the input signal is large and the output is also large (such as during central air conditioning cooling operation and general pressure, flow, temperature control, etc.). But there is also a reverse effect, that is, when the input signal is large, the output is relatively small (such as when the central air conditioning is working on heating and the heating hot water pump in the heating station).
When using pressure signals in closed-loop control, do not use flow signals. This is because pressure signal sensors have low prices, easy installation, low workload, and convenient debugging. However, if there are flow ratio requirements in the process and precision is required, a flow controller must be selected, and appropriate flow meters (such as electromagnetic, target, vortex, orifice, etc.) must be selected based on actual pressure, flow rate, temperature, medium, velocity, etc.
The built-in PLC and PID functions of the frequency converter are suitable for systems with small and stable signal fluctuations. However, due to the built-in PLC and PID functions only adjusting the time constant during operation, it is difficult to obtain satisfactory transition process requirements, and debugging is time-consuming.
In addition, this type of regulation is not intelligent, so it is generally not used frequently. Instead, an external intelligent PID regulator is selected. For example, the Japanese Fuji PXD series and Xiamen Antong are very convenient. When in use, simply set the SV (upper limit value), and there is a PV (operating value) indicator during operation. It is also intelligent, ensuring the best transition process conditions, making it ideal for use. Regarding PLC, various brands of external PLCs such as Siemens S7-400, S7-300, S7-200 can be selected according to the nature, number, digital quantity, analog quantity, signal processing and other requirements of the control quantity.
Signal converters are also frequently used in the peripheral circuits of frequency converters, typically consisting of Hall elements and electronic circuits. According to signal transformation and processing methods, it can be divided into various converters such as voltage to current, current to voltage, DC to AC, AC to DC, voltage to frequency, current to frequency, one in multiple out, multiple in one out, signal superposition, signal splitting, etc. For example, the Saint Seil CE-T series electric isolation sensors/transmitters in Shenzhen are very convenient to apply. There are many similar products in China, and users can choose their own applications according to their needs.
7) When using a frequency converter, it is often necessary to equip it with peripheral circuits, which can be done in the following ways:
(1) A logic functional circuit composed of self-made relays and other control components;
(2) Buy ready-made unit external circuits (such as those from Mitsubishi Corporation in Japan);
(3) Choose a simple programmable controller logo (this product is available both domestically and internationally);
(4) When using different functions of the frequency converter, a function card can be selected (such as the Japanese Sanken frequency converter);
(5) Select small and medium-sized programmable controllers.
8. Proper selection of frequency converter supporting equipment can ensure the normal operation of the frequency converter drive system, provide protection for the frequency converter and motor, and reduce the impact on other equipment.
Peripheral devices usually refer to accessories, which are divided into conventional accessories and specialized accessories, such as circuit breakers and contactors, which are conventional accessories; AC reactors, filters, braking resistors, braking units, energy feedback devices, DC reactors, and output AC reactors are specialized accessories.
When multiple water pumps are connected in parallel for constant pressure water supply, a signal series connection method is used with only one sensor, which has the following advantages.
(1) Save costs. Just one set of sensors and PID, as shown in Figure 4.
(2) Comme il n'y a qu'un seul signal de commande, la fréquence de sortie est constante, c'est-à-dire la même fréquence, donc la pression est également constante et il n'y a pas de perte par turbulence.
(3) Lors de l'alimentation en eau à pression constante, le nombre de pompes en fonctionnement est contrôlé par l'automate programmable en fonction du débit. Une pompe est nécessaire au minimum, deux pour des débits moyens et trois pour des débits plus importants. Lorsque le convertisseur de fréquence est hors service, le signal du circuit (courant) est présent (un signal circule en entrée, mais aucune tension ni fréquence n'est fournie en sortie).
(4) Plus avantageux encore, comme le système ne possède qu'un seul signal de commande, même si les trois pompes sont branchées sur des entrées différentes, la fréquence de fonctionnement est la même (c'est-à-dire synchronisée) et la pression est également la même, de sorte que la perte par turbulence est nulle, ce qui signifie que la perte est faible et que l'effet d'économie d'énergie est bon.