What Does I/O do on a VFD?

VFD is a motor control device used in electromechanical systems, which drives an electric motor and controls its speed and torque by changing the frequency and voltage of the power supply.

Why VFD? 

The modern VFD drives are capable of integrating networks and diagnostic capabilities for better control performance and more productivity. So, we can say that the three main reasons for choosing VFDs as a controller in an application is saving energy, intelligent motor control, and reducing the peak current drawn. They are commonly used in fans, pumps, and compressors and account for 75 percent of all the drivers operating worldwide.  

VFDs can perform the following functions: 

• It is a power conversion device that converts the fixed voltage and frequency to variable voltage and frequency, which can control the output of the AC motors.  
• They are often known as speed controllers of the motor. These variable frequency drivers can also control the torque of the motor and keep it constant by maintaining the voltage-to-frequency ratio.  
• VFDs can control the ramp-down velocity of the motor when it is stopping, so there is a controlled and precise deacceleration. We can also set the voltage and frequency of the motor at a certain level if we don’t want to operate it at full speed.  
• Usually, these drivers are simple toggle commands of forward and rewind motor operation so they can also control the direction of motion. Like, if we want to operate a simple asynchronous motor, and it is running backward, we can alter it with the help of a VFD for changing its direction of motion.  

The main reason for choosing a VFD is that it varies the power supplied for matching the energy requirement of the equipment in usage and saves energy. The driver is responsible for reducing the consumption of energy compared to Direct-On-Line (DOL) operation, where the motor runs at full speed and does not consider the specifications or requirements. It should be noted that at least 40 percent of the fuel is saved by using these types of drives. Their roll-on effect is also responsible for reducing nitrogen and carbon emissions of the application in which it is used. 

Operation of a VFD:

For controlling the RPM of the motor, VFD operates by changing the frequency and voltage of the supply. The VFD has to convert the AC voltage of the supply in two conversion stages. Initially, it has a fixed frequency and AC supply, which it feeds to the rectifier bridge that converts it into DC voltage by biasing. Afterward, it is fed to the capacitator’s’ bank and inductors to output a clean and stable wave. In the second step, the transistors in the VFD start acting as switches for converting DC to AC voltage and then send it to the motor at the required frequency. These transistors are used for adjusting the frequency, and this changing frequency can control the speed of the AC motor.

Over the last few years, power electronics have made significant advancements in VFDs. That has reduced their costs and improved their performance through semiconductor switching applications, drive topologies, and other control techniques in hardware and software.

I/O’s of the VFD:

The most important part of the circuitry of the VFD is the wiring which powers it and actually turns on the motor. The input wiring is used for accepting the line voltage, and the output is for connecting it to the motor. Different models and capacities of VFDs are available in the market. Depending on the models, there are different DC bus terminals, so the wiring is changed.

The following are the types of Input/Output Wiring Types of VFD:

1. Digital Inputs – DIs

They are most common and convenient for remotely commanding the driver to turn on the motor. They are digital, so there are two input states; True or False (0 or 1). The majority of VFDs provide a voltage, often 24 volts direct current (VDC), to their digital inputs, allowing the user to supply a dry contact like a switch. These drives can also allow the voltage to be sourced externally with the help of jumpers or a DIP switch for changing the setting.

These digital inputs can be programmed to control any driver function, including the motor’s speed or direction, etc. However, there are some unique VFDs that have specific functions associated with these digital inputs. Some of these functions are connected to application-specific parameters.

Usually, VFD’s digital inputs are used for connection with digital devices, including relay contacts, push buttons, selector switches, and PLC digital output modules. A preset function, such as start/stop, forward/reverse, external fault, and preset speed settings may be allocated to each digital input, as shown in Figure 1. For instance, if a motor has to run at three distinct speeds, a relay or switch contact might be designed to shut and transmit signals to multiple digital input locations, changing the motor speed to the preset value.

2. Digital Outputs – DOs

Digital outputs have two states (0 or 1). They are intended to turn any piece of field equipment ON or OFF. The VFD’s digital outputs have a voltage potential of usually 24 volts DC coming from them. A sample DO connection is shown in Figure 2.

Before VFDs, the motor control industry consisted of motor starters, overloads, and relays. They had quite simple functionality and were only used to provide the on/off and fault status of the application. On the other hand, the digital outputs on VFDs not only tell us about the status of the motor but also indicate the array of conditions, depending on the value of the assigned parameter. These output terminals normally consist of form A relays, and form C relays, and sometimes a photocoupler is attached to some versions. Relay outputs also referred to as “dry” contacts, close or open another potential by switching anything externally. Normal ratings for relay outputs include both AC and DC voltages.

We have to program both the functioning of the output terminals and the normal operating state. But, the important factor is to consider the voltage and current rating of these digital terminals. Their rating should exceed more than the maximum capacity. Otherwise, it may result in damaging the internal circuitry.

Pilot lights, alarms, auxiliary relays, solenoids, and PLC digital input modules are examples of devices that may receive digital/relay outputs.

3. Analog Inputs – AIs

Analog input is a continuous field input to the VFD. It varies according to the field circumstances. The most widely utilized analog signals are 4 to 20 milliamperes (4-20mA) and 0 to 10 volts DC ( 0-10VDC). The input signal can vary between 4-20mA and 0-10Vdc depending on the field circumstances.

Almost all the modern VFDs models have a Proportional Integral Derivative PID controller built inside them, and the output is measured based on feedback input and the set point. The input source (feedback) is connected to these analog input terminals. In drive terms, we can say that analog input is alternating current or voltage with lower and upper limits that the driver can read and process. Usually, analog inputs control the output frequency, but we can give them several other functions by reprogramming them.

Adjusting the parameters also includes selecting between voltage or current input. Sometimes a DIP switch is also necessary as scaling to see that the input value and rated value of the terminal have any variation. We also need a power supply typically rated 10V or 24V DC for combining the AI and external motor.

4. Analog Outputs – AOs

Analog outputs are continuous outputs from VFD to the field. These outputs are normally modulating signals generated by the VFD and transferred to a device such as a meter, which can indicate speed or current. We usually use analog outputs when we have to check the current without looking at the keypad of the driver.

Like AI, AO also changes based on assigned functions and scaling. If the requirement is 0 to 10 Volts, when the motor’s current is maximum or 100%, the output will be 10 Volts, and 0 Volts when VFD is stopped. The same holds for the current range of values. These AOs may monitor functions such as output frequency, motor current, transducer-based feedback, and many more.

Advantages of VFDs:

Here are some of the advantages of using VFDs in our appliances: 

• There is more accurate control over the speed and acceleration of induction motors. It helps in applications containing conveyor belts that can smoothly ramp up rather than jerk to obtain maximum power. 
• AC motors are responsible for consuming more than 60 percent of energy in industries, and optimizing these operations with the help of VFDs can remarkably reduce energy consumption by up to 70 percent. Also, they are used in applications that do not require full energy and speed.  
• Speed control with the help of variable frequency is quite a good method that helps us to operate motors effectively at given speeds. The manufacturing performance levels have also increased by reducing errors. For example, using VFDs in conveyors and belts helps in the elimination of jerks and provides smooth throughput. 
• VFDs provide a complete return on investment in a very short period by improving the quality of the process, eliminating mechanical drive components, and reducing manufacturing expenses.  
• A slightly higher voltage is required to start the machine, which can be lessened with the help of using VFD. The costs can be further reduced if we start them at peak hours. Induction AC motors need less power for starting up. 
• Inrush current is the current that is required by the machine to start and is 5 to 8 times more than the normal rated current. Larger inrush currents can result in winding stress, voltage dip, and overheating. VFDs help in inrush current too by limiting it and thus reducing the chances of insulation damage. 
• A machine can experience instant shock when it is started, which can be prevented by using VFD. Also, they can help in reducing hammer effects by smooth acceleration and deacceleration. This smooth starting and stopping of the machine help in reducing mechanical stress and machinery breakdown and thus reduces the net maintenance expenses. 
• These drives can start the machine at zero volts and frequency as it keeps the winding of the machine in check for flexing or overheating. Also, as the speed increases the VFDs can start and stop the motor securely and reduce the jerks. This can help in increasing the lifetime of the motor. They also help in other motor issues such as changes in phase, heating, more or less voltage, etc. Also, the control provided by the driver helps in a longer lifetime.  
• VFDs not only control the torque of the motor but also limit them and keep it in check so that the AC motor would not cross it. That not only ensures the safety of the motor but also the product in production.  
• AC motors usually have lower power factors that can result in reactive power loss, which results in energy wastage. VFD can improve the power factor and thus increase efficiency.