6 Misunderstandings and Solutions in VFD Application

Variable frequency drives have obvious advantages in speed regulation, energy saving, soft start, PID adjustment, etc., so they are widely used in various fields of electromechanical control. In view of the many misunderstandings existing in the practical application of VFDs, in this article we analyze several common problems and propose countermeasures.

Misunderstanding 1: Connect electromagnetic switches and electromagnetic contactors in the output circuit of the VFD

In practical applications, it is necessary to use a contactor for inverter switching in some situations. For example, when the VFD fails, it switches to the power frequency operation, or when the one-drive-two mode is used, one motor fails, and the VFD turns to drag the other motor. Therefore, many users think that the installation of electromagnetic switches and electromagnetic contactors in the VFD output circuit is a standard configuration, and it is a way to safely disconnect the power supply. In fact, this approach has great hidden dangers.

Disadvantages:

When the VFD is running, if the contactor is disconnected first and the load is suddenly interrupted, the inrush current will cause the overcurrent protection to act, which will have a certain impact on the main circuit of the rectifier and inverter, and in severe cases will damage the output module IGBT of the inverter. If the load is a motor, the energy of the inductive magnetic field cannot be released quickly, and a high voltage will be generated, which will damage the insulation of the motor and the connecting cable.

Solution:

The VFD output side is directly connected to the motor cable, and the normal start and stop of the motor can be realized by triggering the VFD control terminal to achieve the effect of soft start and soft stop. If a contactor must be used on the output side of the VFD, a necessary control interlock must be added between the output of the inverter and the action of the contactor to ensure that the contactor can operate only when the VFD has no output.

Misunderstanding 2: Disconnect the VFD AC input power when the equipment is normally shut down

When the equipment is out of operation, many users are accustomed to disconnecting the AC input power of the VFD, thinking that it is safer and can save energy.

Disadvantages:

This method seems to protect the inverter from power failure. In fact, if the VFD is not charged for a long time, coupled with the influence of the on-site environmental humidity, the internal circuit board will be slowly oxidized and gradually short-circuited due to moisture. This is the reason why soft faults are frequently reported when the VFD is powered on again after a period of power failure.

Solution:

The VFD should remain powered on, except when the equipment is being serviced. In addition, the upper and lower fans of the inverter control cabinet should be turned on, desiccant should be placed in the cabinet, or an automatic temperature and humidity control heater should be installed to keep the ventilation and the environment dry.

Misunderstanding 3：In the open air or dust environment, the VFD control cabinet is sealed

The VFD cabinet installed and used in some factories, mines, basements, and open air will withstand severe tests such as high temperature, dust, and humidity. For this reason, many users will choose a sealed VFD control cabinet. Although this can prevent rain and dust to a certain extent, it also brings the problem of poor heat dissipation of the frequency inverter.

Disadvantages:

The sealed control cabinet will cause the internal components of the VFD to overheat due to insufficient ventilation and heat dissipation capacity, and the thermal element protection will act, resulting in fault tripping and the equipment being forced to stop.

Solution：

Install a ventilated rain cover with a dust filter on the upper part of the VFD control cabinet, which also serves as an exhaust port. The lower part is slotted to install a fan with a filter, which acts as an air intake. In this way, the air can be circulated and the dust in the environment can be filtered. Cooling air flow direction: from bottom to top. The lateral installation distance between VFDs should not be less than 5mm, and the cooling air temperature entering the inverter should not exceed +40℃. If the ambient temperature is above +40°C for a long time, consider installing the VFD in an air-conditioned space.

In the VFD control cabinet, the frequency inverter should generally be installed on the upper part of the cabinet, and it is absolutely not allowed to install heating elements or components that are easy to heat up close to the bottom of the inverter.

Misunderstanding 4: Connect power factor compensation capacitors in parallel at the VFD output to improve voltage quality

Some enterprises are limited by the power consumption, and the voltage quality is unstable. Especially when the large-scale electrical equipment is put into use, the busbar voltage in the plant drops seriously, and the load power factor decreases accordingly. For this reason, users usually connect a power factor compensation capacitor in parallel at the VFD output to improve the motor power factor.

Disadvantages:

Connecting the power factor compensation capacitor and the surge absorber on the motor cable (between the drive unit and the motor) will not only reduce the control accuracy of the motor, but also form transient voltages on the output side of the drive unit, causing the drive unit to permanently damage.

If a power factor compensation capacitor is connected in parallel on the three-phase input line of the VFD, it must be ensured that the capacitor and the VFD are not charged at the same time to avoid damage to the inverter due to surge voltage.

The current of the VFD flows into the capacitor for improving the power factor, which cannot start due to the overcurrent (OCT) of the inverter caused by its charging current.

Solution:

Remove the capacitor. As for improving the power factor, an AC reactor can be connected to the input side of the VFD.

Misunderstanding 5: Circuit breaker is selected as VFD thermal overload and short circuit protection

Circuit breakers have relatively complete protection functions and have been widely used in power distribution equipment, and have a tendency to replace traditional fuses. At present, the complete sets of frequency conversion speed regulation equipment produced by many manufacturers are basically equipped with circuit breakers (air switches). In fact, there are some hidden safety hazards.

Disadvantages:

When a short-circuit fault occurs in the power cable, the circuit breaker generates a protection action delay due to its own inherent action time. During this period, the short-circuit current will be introduced into the frequency inverter, causing damage to the components.

Solution:

As long as the cable is sized according to the rated current, the drive unit of the frequency converter can protect itself, the input and the motor cable against thermal overload without the need for additional thermal overload protection equipment. Equipped with fuses to protect the input cable in the event of a short circuit, it can also reduce damage to the device and prevent damage to connected equipment in the event of a short circuit inside the drive.

Check that the configured fuse action time should be less than 0.5 seconds. The operating time depends on the fuse type (gG or aR), the impedance of the supply network, the cross-sectional area, material and length of the supply cable. If the action time of the gG fuse exceeds 0.5 seconds, the aR fuse can be used instead to ensure that the action time is acceptable in most cases. Fuses must be of the non-delay type.

Circuit breakers do not provide fast enough protection for transmission equipment because they react slower than fuses. Therefore, when fast protection is required, fuses should be used instead of circuit breakers.

Misunderstanding 6: Only the load power is considered when selecting the VFD

When purchasing VFDs, many users usually only match the inverter capacity according to the power of the driving motor. In fact, the load driven by the motor is different, and the requirements on the inverter are also different.

Disadvantages:

If the difference in the load characteristics of the motor is not fully considered, the frequency inverter may be damaged due to improper use, and at the same time, the lack of necessary braking units and filters may also cause safety risks.

Solution：

According to the characteristics and type of the load, the capacity and configuration of the VFD should be selected reasonably.

a. Fan and water pump:

This kind of load has simple requirements on the VFD, as long as the inverter capacity is equal to the motor capacity (air compressors, deep water pumps, sediment pumps, and fast-changing music fountains need to increase the capacity).

b. Crane load:

The characteristic of this type of load is that the impact is large when starting, so the variable frequency drive is required to have a certain margin. At the same time, when the heavy objects are put down, there will be energy feedback, so a braking unit or a common bus should be used.

c. Uneven load (rolling mill machinery, crushing machinery, mixer, etc.):

The load of such machinery is sometimes light and sometimes heavy, and the inverter capacity should be selected according to the heavy load.

d. Large inertia loads (centrifuges, punches, rotary kilns in cement plants, etc.):

Such loads have large inertia and may oscillate when starting, and there is energy feedback when the motor decelerates. A frequency converter with a slightly larger capacity should be used to speed up the start-up to avoid oscillation, and cooperate with the braking unit to eliminate feedback energy.

After the VFD cooperates with other intelligent equipment (PLC, DCS system), it can realize multiple control strategies and closed-loop adjustment, and it also has a relatively complete protection function. But in practical application and installation environment, there are many misunderstandings. Facing the contradiction, avoiding risks, and using it rationally are the keys to improving the efficiency and service life of the inverter.