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Device guide / Circuits

10

Drive control with a PLC

The PLC starts the drive through its digital inputs, gives the speed reference from an analogue output and reads the ready signal from a relay output.

Power circuit: supply L1–L3 to drive U1, and the motor on U, V, W. Control: PLC relay outputs Q1–Q3 bring +24 V to inputs DI1–DI3, AQ1 gives the speed reference to input AI1, and RO1 tells input I4 that U1 is ready.

How it works

1Ready

U1 is ready to run, so RO1 is closed and PLC input I4 is on.

2Run

Q1 brings +24 V to DI1, and the motor accelerates along the ramp. When 10 V is 50 Hz, 5 V from AQ1 is 25 Hz.

3Direction

Q2 brings voltage to DI2, and the motor runs in reverse.

4Fault

U1 develops a fault and RO1 opens. Input I4 drops out, and the program removes the run command Q1.

A PLC controls a variable speed drive in three ways: digital inputs give the commands, an analogue input gives the speed reference, and a relay output reports the status of the drive back.

Digital inputs

The contact of a PLC relay output brings +24 V to a digital input of the drive. The functions of the inputs are set with parameters or a ready-made macro. In the ACS580 factory settings DI1 is run and DI2 is direction. In the Johdota game tasks DI3 is used for reset.

Speed reference

The analogue output of the PLC gives the speed reference over a screened cable. The scale is set with parameters, for example 0 V is 0 Hz and 10 V is 50 Hz. Then 1 V equals 5 Hz.

Ready signal

+24 V is connected to the common terminal of the relay output, and the NO terminal to a PLC input. The contact is closed only when the drive is ready, so a broken wire also shows up as a fault to the PLC.

Reset

A fault is reset with a rising edge on a digital input, and the reset succeeds only once the cause of the fault has gone. In the Johdota game the run command is removed for as long as the fault lasts, so that the motor does not start by itself after the reset.

Sources