The PLC’s AC Motor Drive Interface

Speed control of AC motors with variable speeds (VS) drives is a common PLC application. Figure 39 illustrates an operator station that can be used to control a VS motor manually. This station’s programmable controller implementation will allow for automatic motor speed control via an analog interface. The output voltage can be varied (0-10 VDC) to the drive.
Operator station for a variable speed drive
Figure 39: Operator station for a variable speed drive.
The operator station is composed of a speed regulator, forward/reverse direction selector and start/stop push buttons. All of these inputs will be included in the PLC program, except for the potentiometer which will be replaced with an analog output. The application will include the required input field devices, such as the start push button and stop push buttons, jog/run and forward/reverse. These inputs will be connected to input modules and not used by the operator station’s parts. The logic to initiate, stop, and interlock forward/reverse commands will be contained in the PLC program.
I/O address assignment
Table 16 lists the I/O address assignment tables for this example. Figure 40 shows the connection diagram between the PLC and the VS drive’s terminal blocks (TB-1).
 Connection diagram from the PLC to the VS drive’s terminal block
Figure 40: Connection diagram from the PLC to the VS drive’s terminal block.
Since the common must also be switched, the connection uses a contact input interface to switch forward/reverse signals. Terminal TB-1-6 must be connected to 115 VAC in order to activate the internal relay CR1. To turn the drive on, the terminal block TB-1-8 connects to the L1 connector of the PLC. Terminal TB-1-6 is connected to the output of module (CR1). The drive’s 115VAC signal is used for controlling the motor speed. It is connected to terminal TB-1-6.
This configuration allows the motor’s overload contacts to be wired to terminals TB-1-9 and TTB-1-10. These are the drive’s power (L1) and output interface’s connection L1. The drive will shut down if it experiences overload. This is because the drive’s contact CR1 will not receive power from its output module. The configuration does not offer low-voltage protection as the motor and drive will restart immediately after overloads have cooled off. Low-voltage protection must be provided by the PLC using the auxiliary contact of the drive (CR1 terminal TB-1-7), so it seals the stop/start circuit.
Figure 41 illustrates the PLC ladder program, which will replace the manual operator station. Because the forward and reverse inputs can only be ON at any one time, they are interlocked (i.e. they are mutually exclusive).
PLC implementation of the VS drive
Figure 41: PLC implementation of the VS drive.
The motor will run at the speed specified by the analog output when it is selected. The analog output connection allows for the output to be activated when the drive turns on. Register 4000 stores the count for the analog output to drive. Internal 1000 is used in block transfer to indicate completion of instruction.
Sometimes, VS drives require the ability to operate under manual or automatic control (AUTO/MAN). To implement dual control, several additional hardwired connections will be required. This can be done with a selector switching device (e.g., a single-throw, four-pole selector switch). It is the simplest and most cost-effective way to accomplish this. This switch allows the user to choose between an automatic or manual option. This connection is illustrated in Figure 42.

Figure 42: VS drive with AUTO/MAN capability.
The operator station is where the forward/reverse field and start/stop buttons are located. These devices are connected under the same names as the control program. (Refer to Figure 41). The PLC will control the drive if the AUTO/MAN switch has been set to automatic. If it is set to manual, then the manual station will control it.

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