Variable Frequency Drives (VFDs) are electronic devices used to control AC (alternating current) electric motors. They work by adjusting the frequency and voltage of the power supplied to the motor, which changes the motor's speed, direction, and torque.

Most VFDs have screw terminals or pin headers labelled for both analog and digital input/output functions. Despite the prevalence of advanced network capabilities, many VFDs, particularly those used in smaller or remote settings, rely on digital input devices to drive their operation.

Typically, these digital inputs are generic and not dedicated to specific functions such as starting or stopping. However, you can assign functions to these signals by adjusting programmable parameters.

INDUS Engineering are the South’s leading providers of electrical engineering solutions. We specialise in a wide range of engineering services, including control panel manufacturing, special purpose machinery, and machine maintenance. Below, we explore some of the most common wiring control schemes for digital inputs.

What Are the Main Types of VFD Control Wiring Schemes?

2-Wire Start-Stop Wiring

The two-wire circuit is the simplest wiring scheme for VFD control. It involves one signal to start a function and removing that signal to stop it. This is different from the three-wire start/stop latching functions, which we will discuss later.

There are two forms of two-wire functionality in a VFD: unidirectional and bi-directional. A single input can be used to drive the motor in either the forward or reverse direction, or two inputs can be used to choose between the two. These inputs are momentary, like a jog function, and are considered two-wire.

The basic wiring concept for 2-wire forward control involves connecting a single digital input device to one of the digital inputs on the VFD, typically one of the first I/O terminals.

• For 2-wire reverse control, the single digital device is wired to the same or another input, and the parameter values are set to select the direction of motion when the run command is given.
• For bi-directional 2-wire control, two different digital input devices are used and wired to two digital inputs on the VFD, with the parameters controlling the method of operation and wire placement.

This wiring scheme is convenient for simple applications that only require basic forward and reverse functionality. The inputs can be selector switches on a control panel, sensors, or an external controller that doesn't have modern communication signals. Although specific terminals may vary by model, the basic wiring concept remains the same.

3-Wire Start-Stop Wiring

The 3-wire control scheme is similar to the 2-wire scheme, but slightly more complex. This scheme is derived from a common circuit known as "latching" or "seal-in," in addition to 3-wire. Both start and stop buttons are provided for convenient latching control, with the signals not needing to be sustained like in 2-wire.

For 3-wire uni-directional control, a normally closed "stop" signal is connected to the enable/stop digital input. A second device is connected to the next digital input to function as the "start" signal. These signals operate as expected, with pressing "start" to start and "stop" to stop. The latching function allows a short pulse to activate and stop the drive - for example, a switch or sensor may need to be tripped to start a conveyor until a separate input triggers a stop. This is ideal for 3-wire control when network signalling isn’t a valid option.

Forward/Reverse Control

A forward/reverse control wiring scheme is commonly used for motors that require operation in both forward and reverse directions, such as conveyor systems, winches, and cranes. This control scheme uses two momentary switches, typically labelled "forward" and "reverse," to control the direction of the motor.

The switches are connected to the VFD digital inputs, which are designated for controlling the direction of the motor. Once the switches are pressed, the VFD recognizes the direction control input and operates the motor accordingly.

To program the VFD for forward/reverse Control, specific parameters must be set to define how the VFD will respond to the direction control inputs. This includes defining the input terminals for the direction control, setting the speed reference values for each direction, and adjusting acceleration and deceleration rates to ensure smooth transitions between forward and reverse directions.

PID Control

A proportional-integral-derivative (PID) control scheme is used to regulate and maintain a specific process variable. For VFDs, PID control is used to regulate the speed or position of a motor by using a feedback device.

A feedback device, such as an encoder or tachometer, is typically used to provide the VFD with information about the motor's current speed or position. This information is then used by the VFD's control algorithm to adjust the motor's speed or position to maintain a desired setpoint.

The PID control algorithm consists of three main components: proportional, integral, and derivative. The proportional component adjusts the output based on the difference between the setpoint and the measured variable. The integral component adjusts the output based on the accumulated error over time, while the derivative component adjusts the output based on the rate of change of the error.

The VFD is programmed with specific PID parameters, such as proportional gain, integral time, and derivative time, which determine how the algorithm will respond to changes in the feedback signal. The goal of PID control is to quickly and accurately respond to changes in the process variable while minimising overshoot and oscillations.

PLC Control

Programmable Logic Controllers (PLCs) are often used to control multiple VFDs in a complex system. To do this, the VFDs are connected to the PLC's digital and analog inputs and outputs. The PLC is programmed to send signals to the VFDs to control their speed, direction, and other parameters. The PLC can also monitor the VFDs' feedback signals to ensure that they are operating within the desired parameters.

This wiring scheme provides a high level of automation and flexibility for complex systems. With a PLC controlling the VFDs, the system can be programmed to respond to a wide range of inputs and conditions. For example, the PLC can be programmed to adjust the speed of the motors based on the temperature or pressure of the system, or to change the direction of the motors based on the position of a sensor or switch.

Talk to the Experts in Electrical Machinery

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