What is VFD?
A variable-frequency drive (VFD), also known as an adjustable-frequency drive (AFD), variable-voltage/variable-frequency (VVVF) drive, variable-speed drive (VSD), AC drive, Microdrive, or inverter drive, controls AC motor speed and torque by varying motor input frequency and voltage.
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Motion Control using VFD drives
Permanent magnet motors and servo drives were the only options for precision motion. With AC induction motors and, yes, PM motors, variable speed drives (VFDs) are now used to achieve high-accuracy motion. When properly specified and configured, motion axes based on VFDs can be very effective solutions for cost-sensitive applications with more forgiving performance specifications.
In industry, the speed of the mill is controlled based on the load in the mill.
Because of their durability and low maintenance, 3-phase induction motors are used here. Through a Variable Frequency Drive, the rotating direction and speed of a three-phase induction motor can be changed and monitored by SCADA and PLC (VFD).
For the production of their final product, industries or factories go through various stages of operation.
VFDs are an affordable option that can help optimize performance, save energy, and permanently lower machine and robotic lifecycle costs, whether they’re used in material handling, machining, or pump and fan applications. VFDs are available in a variety of basic voltage models that operate a 230 V, 480 V, or 600 V motor with 3-phase power. The motor type, voltage, current rating, input source, and input/output (I/O) requirements all influence machine drive selection. Sizing is determined by a variety of application-specific factors, such as the motor’s full-load rating and maximum voltage under full load conditions.
By varying the frequency and voltage supplied to an electric motor, VFDs allow operators to match motor speed to load requirements, operate motors at the most efficient speed for a specific application, and reduce energy consumption. Given that electric motors consume more than 65 percent of all industrial power, the importance of VFDs in everyday life cannot be overstated.
The more complex or unusual applications for VFDs, on the other hand, reveal a plethora of potential efficiencies for creative OEMs and end-users. Newer VFD applications can help solve specific motion control challenges or make them more cost-effective and profitable. Here are six real-world use cases for VFD solutions in advanced motion control applications:
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Conveyors with changing loads
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Simplifying inter-logistics
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Operate induction motors at higher frequencies
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Run induction motors in servo mode
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Run permanent magnet motors without feedback
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Reduce panel space, cable length
Conveyors with changing loads
Conveyors with changing loads are a constant challenge and a significant energy drain everywhere, from airports to factories. Conveyors that are empty do not require full power, but they must be responsive as they become loaded and the demands on the motor change over time.
VFDs can be used to run conveyors with changing loads, reducing power consumption significantly (see Figure 1). Inverters detect lighter loads and adjust the motor’s power factor to ensure efficient operation even at low load cycles. When a heavier load is added, this type of “eco-mode” reduces the amount of power used when it isn’t needed and allows the motor to power up and run at peak performance.
OEMs will have to choose between centralized implementation, where power is local to the central control cabinet, and decentralized implementation, where power is local to the motor, depending on the specific use case. VFDs that are mounted directly on top of the motor save space and provide more efficient power control.
Decentralized VFDs eliminate the time and cost (both material and labor) of running cables back to a control cabinet for large factories or automated systems that are dispersed. Furthermore, dropping the power from a power bus as close to the motor as possible is easier—and more cost-effective.
Simplifying inter-logistics
While VFDs are necessary for some situations, some systems can be improved with a simpler solution. The ability to vary the motor speeds in new inverters with multiple fixed speed selections rather than an automatically variable speed can greatly reduce the number of different geared motor combinations in inter-logistic applications.
Different conveyor speeds at different locations are required in a large warehouse where all the conveyors are connected in a large network. Traditionally, this has meant installing multiple gearboxes with different power ratios at various points throughout the system to ensure that each section of the conveyor runs at the proper speed. As a result, many different gearbox ratios are used to support the same amount of power requirements.
In this case, instead of using 20 different gearbox sizes, four or five inverter/motor/gearbox combinations might suffice. Instead of relying on across-the-line single-speed motor contactors, the frequency can be adjusted to control the speed, allowing operators to optimize each combination.
Major airports and warehouses can use fewer parts—on average, five instead of 20 and programmable inverters are even cheaper than traditional VFDs.
Operate induction motors at higher frequencies
Normal induction motors are designed to run at 60 Hz off the line, but this isn’t always the best design for a given application. OEMs can design a motor with a frequency range as low as 20 Hz for winding applications, or as high as 100 to 600 Hz for higher power density.
OEMs can design motors that are smaller but have the same power as a traditional induction motor because power is a factor of speed times torque. These higher frequency motors are two motor sizes smaller on average than their 50/60 Hz counterparts while delivering the same amount of power. VFD-enabled induction motors can also provide more dynamic system capabilities because they have less inertia in the motor.
Run induction motors in servo mode
Servo control necessitates a high level of speed and position precision. It necessitates precision. As a result, permanent magnet motors are the preferred equipment for performing servo functions. However, because they rely on rare metals, they are also very costly.
Inverter-controlled induction motors can be used in servo mode with the right feedback, making them a much less expensive alternative to traditional permanent magnet servo motors. While VFDs are most commonly used in open-loop speed control and aren’t known for their precision, they can control the motor rotor position well enough for many servo applications.
The power density isn’t quite as good, and the motor will be slightly larger, so system requirements, motor size, and capabilities must all be carefully considered.
Run permanent magnet motors without feedback
Permanent magnet motors are among the most efficient motors available, but they have traditionally required feedback to keep track of the pole position and commutate the motor properly.
Permanent magnet motors can now be run without feedback using VFD technology, with positioning accuracy of fewer than 5 degrees. When the motor is stopped, the pole positions are calculated with VFDs, and the motor can then be commutated for proper control.
Without the need for a cable or a more expensive servo inverter, positioning applications without feedback can be replaced with a less expensive and more efficient VFD. Powered permanent magnet motors with VFD technology also allow applications to run in a speed mode when power efficiency is more important.
Reduce panel space, cable length
One of the most basic and underappreciated applications of VFDs is in space-saving efforts. On the facility floor, integrated motor-drive combinations can save space by reducing control panel space and motor cable length.
Instead of running all cables back to a central cabinet, system engineers can create a decentralized system that relies on individually-driven motors with only control cables running from the main control out to the various parts of the machinery.
Example:
Let us consider a sugar cane juice manufacturing company
To get the cane juice out of the storage, we’ll need a motor that meets certain requirements. The speed of the inductor motor connected to the VFD drive determines the flow of juice in this example.
The operator must, however, control the flow rate of the fluid passing from one point to another, depending on the requirement. The Variable Frequency Drive (VFD) adjusts the motor pump’s speed based on the setpoints provided by the control room operator.
Take a motor pump with a capacity of 1000 m3/hr, or 1000 tonnes per hour, for example. If we don’t have a VFD, however, the pump will discharge the full flow rate as soon as the motor pump is turned on.
However, we must gradually increase or decrease the fluid flow rate depending on the requirements. To do so, we’ll need to regulate the motor pump’s speed, which we can do with a variable frequency drive (VFD).
Use of controlling the motor using VFD
- Saves Energy
- Reduce demand for energy
- Turn OFF power when power is not required
- Can adjust the speed
- Can control starting, acceleration, and stopping
- Can control dynamic torque
- Gives smooth motion in elevators and escalators
How does VFD control speed?
A variable frequency drive converts a fixed AC voltage and frequency to DC, filters the DC with a capacitor bank and inductors, and then inverts the DC voltage back to AC and sends it to the motor at the desired frequency.
A microprocessor, also known as a digital signal processor (DSP), communicates with the PLC and the user (via an HMI or keypad), monitors motor operation, and checks for faults.
Control circuitry coordinates power device switching to activate power components in the proper order. The voltage and frequency supplied by the output devices change the motor speed.
Basic VFDs maintain motor torque by maintaining a constant volts-to-frequency ratio. To improve motor performance, advanced units employ more intelligent and adaptive algorithms.That means, a VFD converts one voltage and frequency to another so that motor speed can be changed without sacrificing torque.
A VFD drive system can be divided into three subsystems. They are:
- An alternating current (AC) motor, usually a three-phase induction motor but also a single-phase or synchronous motor.
- A main drive controller with a rectifier bridge converter, a DC link or filter, and a switching or inverter section made of solid-state power electronics.
- A monitoring and control interface that allows the operator or PLC to start and stop the motor, adjust the speed, and change the direction, among other things. This interface also provides information about the motor’s operation, the drive’s health, and so on. Communication with a PLC can be done using a variety of serial communications protocols or by using “old school” relay inputs and outputs with 4-20mA or 0-10V analog signals.
Variable frequency drives have shrunk over the years, thanks to the replacement of solid-state components by microprocessors. Drives frequently have a return on investment that exceeds the cost of the drive-in in many applications.
Why AC Drives?
For process applications, 85 percent of industries use 3 PHASE induction motors. AC drives, also known as variable frequency drives (VFDs), are used to control the speed, torque, and direction of these three-phase induction motors.
When to Use a Variable Frequency Drive?
VFDs are used in applications ranging from manufacturing to air conditioning that use AC electric motors. There are numerous advantages to using a variable frequency drive system.
- To precisely control a manufacturing process’s speed.
- To ensure a smooth start and gradual increase in operating speed.
- To save energy, particularly in applications where the torque and power of a load vary non-linearly. Fans and pumps with variable torque are good candidates for cost savings.
- To improve the acceleration, flow, monitoring, speed, temperature, tension, torque, and pressure processes in your application.
- To reduce mechanical and electrical stress by running a motor in specialized patterns.
The current surges required to start a motor “across the line” are significantly reduced or eliminated with VFDs. This type of start can damage mechanical components in the driven load as well as cause current surges of up to eight times the motor’s full-load current.