VFDs: Variable Frequency Drives

John
Henderson

Variable frequency drives (VFDs) are a type of motor controller that drives a motor by varying its input frequency and voltage. This allows VFDs to control the speed and power of various induction motors. In this article, you will learn about the types of VFDs, their functions and applications, construction details, and repair and maintenance considerations.

Types of VFDs

A VFD takes an AC input signal and converts it into a DC signal using a rectifier. A DC link filters and stores the DC signal. And finally, the signal is converted back to an AC signal at the desired temperature and frequency using an inverter. The inverter circuit of the VFDs can have several designs. The most common types are VSI, CSI, and PWM.

Voltage Source Inverter (VSI)

A Voltage Source Inverter (VSI) uses a capacitor in its DC link to store energy. It then uses an inverted switching circuit, consisting of special transistors, to convert the stored energy in the capacitor into an AC signal. These special transistors are known as insulated-gate bipolar transistors (IGBTs). The IGBTs allow VSI type VFDs to be well suited for fast changes in motor speed and torque. VSI type VFDs also are capable of having multiple motors connected to one VFD. Due to its simple design, VSI type VFDs are cost effective and have a large mean time between failure rates in many cases.

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VSI drives do have some downsides. The load motor faces jerking during starting and stopping operations due to the cogging effect in induction motors. VSI drives also tend to have many types of harmonics and noises that can interfere with the motor. The overall power factor in VSI drives decreases when the motor speed is decreased. Pulse Width Modulation (PWM) drives solves some of the problems listed including the cogging effect. However, PWM solutions are often more complex and more costly.

Current Source Inverter

A Current Source Inverter (CSI) uses an inductor as its DC link. This allows the CSI to control the current rather than voltage. The inverter circuit of a CSI consists of gate turn-off thyristors (GTOs) or symmetrical gate controlled thyristors (SGCTs). This allows CSI type VFDs to have more control on the torque of the motor. CSI type VFDs are also better suited to support motors with higher horsepower ratings. Like VSI drives, their design is simple and cost-effective.

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There are other considerations for CSI drives. CSI drives cannot have multiple motors attached to one drive. The cogging effect is still active in CSI drives causing the same jerking effect that VSI drives seen. Also the overall power factor in CSI drives is very poor especially when operating low RPM.

Functions and Applications

In industrial systems, VFDs control fan systems, compressors and pumps. In fan systems, VFDs increase or decrease the frequency of fans to increase or decrease fan speed. For compressors, VFDs changes the frequency to change the pressure of the fluid in the compressor. In pumps, VFDs change the frequency to change the flow of the fluid being pumped.

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VFDs are energy efficient and a low-cost setup, providing low power consumption for larger horsepower motors. VFDs also limit the inrush current, preventing damage to the motor.

Construction

When constructing VFDs, the environment is a major concern to the VFD’s performance and longevity. The main environmental factors to consider are temperature, dust, and corrosion.

Temperature

Like many electrical systems, VFDs are prone to overheating. Overheating reduces the life span of the components of the VFD. Eventually, this will cause severe damage and possible replacement. To mitigate temperature risk, cooling systems are installed. If the VFD overheats, a high-temperature control typically shuts down operation. The temperature is also controlled so that the relative humidity does not rise to the point of condensation. If condensation happens, it can lead to VFD failure.

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Dust

Dust concentration is another factor to consider and monitor. If the dust concentration is too high, it will clog the cooling system. This often leads to overheating in the system and mechanical damage. The worst case scenario is the intrusion of conductive dust. At that point, flashover problems are likely, leading to catastrophic VFD failure. To prevent this, the VFD and its cooling systems require regular cleaning.

Corrosion

The presence of corrosive materials and gases can cause damage to VFDs. If the concentration of corrosive materials is too high, it can lower the efficiency of the heat sink. This creates another overheating condition and can cause damage to VFD components. To prevent this, VFDs and other equipment is kept in an sealed VFD cabinet. VFD cabinets also help control the temperature and protect the VFD from other environmental factors including precipitation.

Enclosures

Based on the environment of the VFD, an enclosure for the main VFD is selected using the NEMA rating system. The NEMA system divides enclosures into 10 categories based on their strengths and applications. It determines which environment the enclosure would perform best, whether indoor or outdoor, and which contaminate the enclosure is best at keeping out.

Enclosures are fitted with cooling, filtering, and scrubbing equipment to allow for easy cleaning. Most VFD cooling systems are air-cooled. However, for larger power ratings, liquid cooling systems are more common for VFD. Some enclosures allow for HVAC systems to control temperature and humidity. However, there must be sufficient airflow over the heatsinks. If there is poor airflow, the temperature will rise significantly. Enclosures can be installed in both indoor and outdoor environments. However, outdoor enclosures have more environmental challenges to protecting the VFD. Therefore outdoor enclosures tend to be larger and more airtight. Often, the VFD enclosure is placed inside of a larger building with a lineup of other electrical equipment. Such a building is often referred to as an electrical enclosure or an electrical house.

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Repair and Maintenance

VFDs require maintenance and need to be easily accessible for repairs. The environmental factors listed above need to be monitored to prevent damage to the VFD. VFD cabinets are fitted to have proper cooling, filtering and scrubbing equipment. Therefore the VFD needs to be cleaned regularly and the temperature needs to be controlled. VFDs also need to be inspected regularly. Inspectors need to look for loose connections, damaged components, and other signs that the VFD is not functioning properly. Measuring instruments can help ensure the VFD is functioning properly.

Alternatives

There are some situations where VFDs are not the best fit for the application. VFDs require a lot of investment especially if the plant has a lot of high horsepower motors. Therefore, some companies look for alternatives for VFDs. The most common alternative to VFDs is eddy current drives. Eddy current drives use a constant motor speed as the input. Then torque is transmitted to a variable speed output proportional to a DC signal applied to a set of coils on the output’s rotor. Eddy current drives are installed between the motor and the driven equipment. They are more costly to install but require less maintenance. Other alternatives to VFDs also exist, depending on the application. For example, pump systems sometimes use control valves to control flow through pressure rather than motor speed.

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