An application of common PLC is the speed control of AC motors with VS (variable speed) drives. The programmable controller implementation of this station will provide automatic control of speed motor through an analog interface by varying the analog output voltage, 0 to 10 VDC, to the drive.
The operator station contains of a speed potentiometer (speed regulator), a reverse/forward direction selector, a jog/run switch, and start and stop push buttons. The PLC program will consist all of these inputs except the potentiometer, which will be replaced with an analog output. The required input field devices e.g. start push button, stop push button, forward/reverse, jog/run) will be added to be application and connected to input modules, rather than using the operator’s station components. The PLC program will consist the logic to start, stop, and interlock the reverse/forward commands.
The connection uses a contact output interface to switch the reverse/forward signal, since the common must be switched. Terminal TB-1-6 must receive 115 VAC to turn ON the internal relay CR1 to activate the drive. The drive terminal block TB-1-8 supplies power to the PLC’s L1 connection to turn the drive ON. The output of the module CR1 is connected to terminal TB-1-6. The drive’s 115 VAC signal is used to control the motor speed so that the signal is in the same circuit as the drive, avoiding the possibility of having different commons (L2) in the drive, the start/stop common is not the same as the controller’s common. The contacts of motor’s overload are wired to terminals TB-1-9 and TB-1-10, which are the drive’s power connection and the output interface connection. The drive will turn OFF if an overload occurs, because the drive’s CR1 contact will not receive power from the output module. This configuration does not provide low voltage protection, since the motor and drive will start immediately after the overloads cool off and reclose. The auxiliary contacts from the drive to have low voltage protection, CR1 in terminal TB-1-7, have to be used as an input in the PLC.