How to understand the given source and control signal source of SEW frequency inverter? 
The inverter is the main equipment for modern AC speed regulation, and its control object is three-phase (or single-phase) AC asynchronous motors. We know that the formula for the speed of a three-phase asynchronous motor is: n=60f/p(1-s). Where "n" represents the speed of the motor; "f" represents the frequency of the AC; "p" represents the number of poles of the motor; "s" represents slip (the ratio of the difference between the speed of the asynchronous motor n and the synchronous speed n0 to the synchronous speed n0).
For a finished motor, the number of poles "p" is constant, so we can only change the speed of the motor by changing the frequency of the AC, which is also the speed regulation principle of the frequency inverter.
Proficient use of frequency inverters is not a simple matter. There are many brands of frequency inverters on the market, and there are also differences in the methods of use of products from different brands. However, some aspects are common, such as the need to configure motor parameters, control methods, and setpoints. I have been asked on-site why there is a setpoint source and a control signal source in the parameters of the frequency inverter, what do they mean? In this article, we will take SEW's MDX61B as an example to explain this issue.
The frequency inverter is used to change the speed of the motor. By setting different speed values, the motor can run at different speeds. This set value of speed is usually called the "speed setpoint value," or simply "setpoint." For example, if you want the motor to run at a speed of 700 revolutions per minute, "700 revolutions per minute" is called the "speed setpoint."
In addition to "speed setpoint," you can also set "frequency setpoint." Because the frequency of the current is proportional to the speed of the motor, reducing the frequency of the current can reduce the speed of the motor.
Whether it is "speed setpoint" or "frequency setpoint," both can be referred to as "setpoint." The signal source of the setpoint value is called the "setpoint source." The P100 parameter of SEW's MDX61B frequency inverter is used to configure the setpoint source of the frequency inverter, as shown in the figure below.
It can be seen that this frequency inverter supports many types of setpoint sources. Setpoints can be configured via communication methods (such as RS485, Fieldbus, SBus, etc.), through analog signals or terminal connections (Bipolar/fixed setpoint, Unipolar/fixed setpoint, Fixed setpoint+AI01, Fixed setpoint x AI01, etc.), and also through frequency input (Frequency setpoint input/fixed setpoint) to set constant values.
Let's take two examples:
- When selecting "Bipolar/fixed setpoint", the setpoint of the frequency inverter can be obtained through the input of analog signals AI1/AI2. A positive setpoint value can make the motor run clockwise, while a negative setpoint value can make the motor run counterclockwise. In addition to analog signals, fixed speed values can also be set in parameter P16x, and then controlled through external terminals (X13) (for detailed information, see "Terminal Control" later).
- Frequency setpoint input/fixed setpoint: When the setpoint source is set to "Frequency setpoint input," the frequency inverter will detect the input frequency from digital input DI04 to control the motor speed. At this time, the parameter of DI04 must be set to "No function," and switch S14 of the DIP switch must be set to the "ON" position.
During the speed regulation process of the frequency inverter, besides setting the operating speed (or operating frequency) of the inverter, other signals are also required, such as: when to start? When to stop? Should the motor rotate clockwise or counterclockwise? These signals are collectively referred to as "control signals" for the motor. The source of the control signals is called the "control signal source."
MDX61B supports four types of control signal sources: Terminals, RS485, Fieldbus, SBus1, and SBus2, as shown in the figure below.
Terminal control uses the digital input terminal X13. The following image shows the terminal definitions for X13.
From the diagram, it can be observed that the factory default settings for X13 are as follows:
- X13:2 (DI01) is set to "CW/stop," meaning "Clockwise/Stop."
- X13:3 (DI02) is set to "CCW/stop," meaning "Counterclockwise/Stop."
- X13:4 (DI03) is set to "Enable/stop," meaning "Enable/Stop."
- X13:5 (DI04) is set to "n11/n21."
- X13:6 (DI05) is set to "n12/n22."
Note: CW stands for Clockwise, representing "Clockwise," as mentioned earlier as "Forward." CCW stands for Counterclockwise, representing "Counterclockwise," which is "Reverse."
When P101 is set to "Terminals," if the input signal at X13:2 is 1, the motor rotates clockwise; if the input signal at X13:3 is 1, the motor rotates counterclockwise. Before the motor starts rotating, it needs to be "enabled," which means the input signal at X13:5 is 1. When the input signal at X13:6 is 1, the motor runs at the speed set in "n11/n12" parameters. When the input signal at X13:7 is 1, the motor runs at the speed set in "n21/n22" parameters.
This method of controlling the motor's start, stop, forward, and reverse by controlling the input signals of certain terminals of the frequency inverter is called terminal control, where terminals serve as a control signal source.
Terminal control is the most basic and default control method, but it requires PLC output points and increases wiring workload, so communication control methods are generally recommended.
MDX61B supports RS485, Fieldbus, and SBus communication methods to control the frequency inverter, using predefined process data formats (Process Data) and controlling the frequency inverter through control bits in the control word (Control Word). Communication methods cover many aspects, which we will discuss later.
In summary, the setpoint source and control signal source of the MDX61B frequency inverter are separate. Speed setpoints can be provided via the bus (Fieldbus), while start/stop control can be achieved via terminals (Terminals), or setpoints can be provided via frequency while control is done via the bus (Fieldbus). Other combinations are also possible, such as both setpoint and control signal sources using the Fieldbus.
That concludes our discussion for today.