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General-purpose VFD Capacity and Type Selection Guide

Views: 16     Author: Site Editor     Publish Time: 2022-08-19      Origin: Site

How variable frequency drives work

A variable frequency drive is a power supply device that converts a power frequency AC power supply into a frequency-adjustable power supply. According to the formula of AC motor synchronous speed N=60f/p (in the formula, N is the synchronous speed of the motor, f is the frequency of the power supply. P is the number of pole pairs of the motor), as long as the frequency is changed, the speed of the AC motor can be changed. VFD is a power conversion device developed based on this principle.

The general-purpose vfd usually adopts the AC-DC-AC working mode. Relatively speaking, the low-voltage variable frequency drive is the most widely used, with mature technology, low cost and easy maintenance.

The working principles of general-purpose vfds and special-purpose vfds are the same, but the special-purpose vfds are developed according to the characteristics of professional equipment, and are more targeted.

Such as metallurgy and hoisting applications, vfds and motors are often overloaded, so the inverter capacity is required to be large enough and the overload capacity is strong.

Textile and metallurgical machinery requires high precision and high performance of frequency inverters.

Selection of low-voltage general-purpose AC Drives

The main indicators to measure the performance of general-purpose ac drives are control mode, starting torque, torque control accuracy, speed control accuracy, control signal type, speed control mode, multi-stage speed setting, carrier frequency, frequency jump function, communication interface, etc.

Figure-XiChi XFC500 low-voltage general AC Drives application case

China AC drives-XiChiElectric

The operation, maintenance and application of general-purpose vfds at home and abroad are basically similar. The difference lies in the functionality of the different brands, the communication interface, the quality, their specific definitions and their unique features. However, users do not necessarily use all these performance indicators, but select functions and indicators that can meet their needs according to their needs, and use them as the standard to measure and select general-purpose ac drives.

The correct selection of general-purpose frequency inverters is critical to the normal operation of the drive control system.

First of all, it is necessary to clarify the purpose of using universal ac drives, according to the type of production machinery, speed regulation range, speed response and control accuracy, starting torque and other requirements. Fully understand the characteristics of the load driven by the frequency inverter, decide what kind of general-purpose vfd to use to form the control system, and then decide which control method is the most suitable.

For the long-term and reliable operation of the general-purpose vfd, the communication standards, input and output interfaces of the ac drive, and the accessories are also very important.

In addition, attention should also be paid to the manufacturing technology level, life, harmonics, efficiency, power factor and sales services of low-voltage frequency inverters.

Specifically, the selection of low-voltage general-purpose ac drives includes two aspects: type selection and capacity selection. The principle of selection is: first of all, its functional characteristics can ensure reliable realization of process requirements, and secondly, to obtain a better performance-price ratio.

The selection of general-purpose vfd type should be carried out according to the load characteristics.

a. For the square torque of fans, pumps, etc., the load torque is small at low speed, and a special or common function low-voltage vfd is usually selected.

b. For constant torque loads or machinery with higher static speed accuracy requirements, a high-function low-voltage vfd with torque control function should be selected.

c. This kind of frequency inverter has low-speed torque, high static mechanical properties and high hardness, is not afraid of load impact, and has the characteristics of excavators. In order to realize the constant torque speed regulation with large speed ratio, the method of increasing the vfd capacity is often adopted.

d. For production machinery that requires high precision, good dynamic performance and fast speed response, vector control or direct torque control general-purpose vfds should be used.

In terms of capacity, most general-purpose vfds are marked with three parameters: rated current, available motor power and rated capacity. The last two parameters are usually given by the manufacturer according to the standard motor produced by the country or the company, and cannot accurately express the actual load capacity of the inverter. 

Only rated current is a key parameter that can reflect the load capacity of general-purpose vfds.

Therefore, it is the basic principle to select the variable frequency drive capacity based on the rated current of the motor not exceeding the rated current of the general vfd, and the rated power of the motor can only be used as a reference.

In addition, before determining the capacity of the general-purpose vfd, the process conditions of the equipment and the parameters of the motor should be understood, and the type and working characteristics of the motor should be considered, leaving a certain margin.

For example, the rated current of a submersible pump motor is greater than that of an ordinary asynchronous motor;

The roller table motor commonly used in the metallurgical industry not only has a large rated current, but also allows it to be in a locked-rotor working state for a short time. Because most of the roller drives are multi-motor drives, it should be ensured that the total load current is not allowed to exceed the rated current of the general-purpose ac drive in a fault-free state.

Another example is the occasion where the original reduction box is removed and a general-purpose ac drive is used for speed regulation. It should be considered to appropriately increase the capacity of the motor to meet the torque requirements of the load.

It should also be fully considered that the output of the general-purpose ac drive is rich in harmonics, which will deteriorate the power factor and efficiency of the motor. Therefore, the current of the motor will increase, generally about 10%, compared with using the power frequency grid to supply power to the motor with a low-voltage general-purpose ac drive. The temperature rise will increase by about 20%. Therefore, when selecting a frequency inverter, an appropriate margin should be reserved to prevent the temperature rise from being too high and affecting the service life of the motor.

When selecting a variable frequency drive, you can follow the following seven steps:

(1) Specify the working mode, capacity and load type of the equipment.

(2) Clarify the process, performance indicators and control requirements of the equipment.

(3) Determine the system structure, I/O interface, communication interface, etc.

(4) Summarize various performance indicators and requirements.

(5) Carry out technical consultation or direct bidding according to the summarized results.

(6) Comprehensive comparison of performance, service life, price and service.

(7) Determine the drive brand, model, specification and supplier.

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