Inverter control using a PLC programmable controller
Currently, in industrial automation, the use of asynchronous motors as a drive in various types of machines and devices has become a standard. Inverters are used to control the engine speed, speed change and soft starter. Inverters are typically controlled by PLC controllers. Various communication methods are used to ensure communication between the inverter and the PLC.
Theoretical information about PLC controller.
The main task of a PLC is to respond to changes in inputs by calculating outputs according to programmed control rules. In order for the controller to effectively perform this task, it must work in the so-called real time. This means that the controller’s reaction in the form of calculated control in response to an input change must occur within a specific time, acceptable from the point of view of the requirements for this control. This is achieved by cyclically reading the state of input signals, executing a user program written in the CPDev environment and updating output signals.
Photo of the SU controller
Theoretical information about the inverter.
A frequency converter, i.e. an inverter, converts single-phase or three-phase alternating current energy with a specific frequency into alternating current energy with a different, adjustable frequency in order to change the rotational speed of the asynchronous motor. The smoothing process creates a signal with a shape similar to a sinusoid with an adjustable frequency. Nowadays, inverters are advanced microprocessor devices. The output frequency range is typically from 0.5 Hz to 500 Hz. The inverter has a built-in or external anti-interference filter, a braking system (a system responsible for receiving and converting into heat the energy generated by the electric motor during braking), digital and analog inputs and outputs with programmable purposes, and a communication interface enabling the operation of the inverter to be controlled and its status monitored. PLC controller.
Photo of the inverter
Communication between the PLC controller and the inverter can be ensured in 3 ways, using:
- digital inputs and outputs,
- analog inputs and outputs,
- communication interface.
Using the binary inputs of the inverter, the PLC controller controls the motor speed or changing the direction of rotation, the value of which has been previously programmed in the inverter. The engine speed is controlled by changing the logical levels of the binary inputs of the inverter. However, the inverter sends messages to the PLC via the inverter’s binary outputs. This way of communication is limited because the way of communication is very limited.
However, the speed control method using the analog inputs and outputs of the inverter allows for smooth regulation of the engine speed. We can change the engine speed via a standardized voltage (0..10V) or current (4..20mA) signal via the inverter’s analog input. The inverter’s analog output (also current or voltage), we can inform the PLC about the current engine speed. This method of communication is very limited because we can only change the engine speed.
Another way for the PLC and the inverter to communicate is to use the above two methods simultaneously. Analog and binary control will simultaneously ensure smooth regulation of engine speed and sending and receiving commands via the inverter to the PLC controller. The disadvantage of this solution is the need to provide a large number of connections between the inverter and the PLC controller.
The greatest possibilities of controlling the engine speed can be provided via a communication interface, e.g. RS-485 (MODBUS protocol), CAN BUS, ProfiBUS. The use of a communication interface allows for smooth regulation of the engine speed by the PLC programmable controller, changing the direction of engine rotation and appropriate interpretation of messages sent by the inverter to the PLC controller. This method of connecting the inverter provides the greatest opportunities for communication between the PLC and the inverter, because a very large amount of data can be transmitted in this way. Also, in this way we ensure communication with a minimum number of connections.
SU 1.5 driver appearance (animation)