How Can We Select a Suitable VFD?

Specifically, the selection of low-voltage general-purpose frequency inverters includes two aspects: type selection and capacity selection.

The basic principle of selection is to determine whether the functional characteristics of the frequency inverter can meet the process requirements. Therefore, we should pay close attention to the following factors during selection.

1. Actual working current of the motor

The actual working current of the motor is the most critical factor in the selection. When the work last a long time, the output current of the frequency inverter must be greater than the actual working current of the motor.

In a project, the motor is usually selected first, and then the frequency inverter is selected based on the motor. The actual operating current of the motor does not refer to the rated current marked on the motor nameplate. Therefore, you should first familiarize yourself with the working conditions and initially estimate the relationship between the motor's operating current and changes over time, and then determine the corresponding frequency inverter model.

① Under normal circumstances, the frequency inverter drives a constant-torque load motor, and it is selected based on the rated current of the motor;

② Under normal circumstances, the frequency inverter drives motors with loads such as fans and pumps, and it is selected based on the rated current of the motor;

③For motors that often run overloaded for short periods of time, the overload period and overload current need to be calculated.

For this load motors, the maximum output current of the frequency inverter is required to be greater than the peak current of the motor, and the parameter is within its own allowable range. During selection, it is also possible to amplify one or more gears to meet on-site needs.

Let’s take a 10kW, 20A rated current motor as an example: If the motor works intermittently, the peak current is 40A (2 times the rated current) during overload operation within 1 second, and then stops running for 20 seconds. At this time, the overload curve of the frequency inverter must be used in the selection: first, find out the curve of the motor current changing with time, and secondly, see whether the output current curve of the frequency inverter can cover the motor current curve (that is, whether the output current of the frequency inverter exceeds the actual operation of the motor current), only the  model whose output current curve covers the motor current curve is suitable for motors with heavy loads. For the selection of heavy-duty frequency inverters, there are often some empirical data for reference.

2. Environmental factors

①The influence of temperature

The temperature of the environment in which the frequency inverter is used should be taken into consideration, which is generally between -10 and 40°C. If the temperature of the working environment is higher than 40°C, the frequency inverter should be derated by 5% for every 1°C increase;

②The influence of humidity

When selecting the frequency inverter, if it works in an environment with a humidity lower than 90%, the relative humidity of the air should be less than or equal to 90%, and there should be no condensation. If the humidity is too high and the humidity changes greatly, condensation will easily occur inside machine, and the insulation performance will be greatly reduced, or even cause a short circuit. If necessary, a desiccant or heater must be added to the box.

③The influence of altitude

The frequency inverter can output rated power when installed at an altitude below 1000m. When the altitude exceeds 1000m, its output power will decrease.

④The impact of dust

It is not suitable to install the machine in places with metal conductive dust for the dust will invade the inside of the frequency inverter, it is easy to cause a short circuit in the internal circuit, and in serious cases, the machine will be burned. This factor must be taken into consideration.

3. The selection of incoming power supply

Commonly used voltages are single-phase AC 220V, three-phase AC 220V, three-phase AC 380V and three-phase AC 690V. The incoming power supply is determined by the existing upstream transformer voltage level, which should be clarified at the early stage of frequency inverter selection. The voltage level determines the different wiring methods of the motor junction box. Star connection has higher voltage resistance and smaller operating current than delta connection. The incoming line power frequency is generally 50Hz. The frequency inverter has a relatively high tolerance for fluctuations in the incoming line frequency. The diode rectifier bridge of the inverter is not sensitive to frequency.

4. Correctly select the cooling method

Common low-voltage inverters below 1000VAC are mostly internal air-cooled. During the group transmission of high-power frequencyinverters,its cooling fan makes a lot of noise. If necessary, water-cooled series frequency inverters can be selected.

5. External configuration requirements forfrequencyinverter selection

① Any frequency inverter that does not have a fast fuse to protect the silicon device in front of the internal rectifier circuit should be equipped with appropriate fuses and isolating switches between themachine and the power supply (air circuit breakers cannot be used to replace fuses and isolating switches) to avoid damage to the rectifier components of the frequency inverter due to internal short circuit.

② Select the incoming and outgoing cables of the frequency inverter according toits power. If conditions permit, it is best to use shielded cables from thefrequencyinverter to the motor, and they should be as short as possible to help reduce capacitive leakage current and electromagnetic radiation. If the actual length of the power cable used on site exceeds the output cable length allowed by the frequency inverter, it should be equipped with an output reactor to prevent stray capacitance from too long cables from affecting the normal operation.

Frequency conversion control signals and frequency inverter feedback signals should use shielded cables and be well grounded, which can reduce the interference of the frequency inverter on other instruments and control systems.

③ If an EMC filter or AC reactor is configured at the input end of the frequency inverter, it can effectively suppress the electromagnetic interference caused by the switching of the power components of the frequency inverter to meet the power grid quality requirements of other equipment under the operating conditions of the frequency inverter. Note: When the neutral point of the transformer of the power grid is not grounded, the EMC filter cannot be used at the input end of the frequency inverter.

Tips: Detailed explanation of frequency inverter selection steps

When selecting a frquency inverter, you can follow the following six steps:

1. Clarify the working mode, capacity and load type of the equipment;

2. Clarify the technology, performance indicators and control requirements of the equipment;

3. Determine the construction method, I/O interface, communication interface, etc. of the internal system. 4. Summarize various performance indicators and requirements;

5. Provide technical consultation or direct bidding on the summarized results;

6.Make a comprehensive comparison of performance, service life, price, and services between different frequency inverters.

In addition, during selection, you need to pay attention to the local voltage and check whether the motor is equipped with capacitors. The most important point is to ensure that the power of the frequency inverter is greater than or equal to the rated power of the motor. If the motor has a large operating load, then a load and amplified frequency inverter gear must be used to ensure the normal operation of the equipment motor.