PLC Motor Control: Start, Stop, Reverse, and Speed Control

Direct On-Line (DOL) Starting

Direct On-Line starting is the simplest method for starting electric motors. The PLC energizes a single main contactor, connecting the motor directly to full supply voltage. Starting current reaches 6 to 8 times rated current, with full torque available immediately. DOL is suitable for motors up to approximately 7.5 kW.

FUNCTION_BLOCK FB_MotorDOL
VAR_INPUT
    bStart : BOOL;  bStop : BOOL;  bReset : BOOL;
    bOverload : BOOL;  bFeedback : BOOL;
END_VAR
VAR_OUTPUT
    bContactor : BOOL;  bRunning : BOOL;  bFault : BOOL;
END_VAR
VAR
    bCommand : BOOL;  tonFB : TON;
END_VAR

IF bCommand AND NOT bOverload THEN bFault := TRUE; bCommand := FALSE; END_IF;
tonFB(IN := bCommand AND NOT bFeedback, PT := T#3s);
IF tonFB.Q THEN bFault := TRUE; bCommand := FALSE; END_IF;
IF bStart AND NOT bFault THEN bCommand := TRUE; END_IF;
IF bStop OR bFault THEN bCommand := FALSE; END_IF;
IF bReset AND NOT bCommand THEN bFault := FALSE; END_IF;
bContactor := bCommand;
bRunning := bCommand AND bFeedback;
END_FUNCTION_BLOCK

Reversing Circuit With Interlock

A reversing circuit uses two contactors to change motor direction by swapping two supply phases. Three protection levels prevent simultaneous engagement: electrical interlock (NC auxiliary contacts), PLC software interlock (mutual exclusion in code), and mechanical interlock (physical lever between contactors). Never rely on software alone for safety-critical interlocks.

FUNCTION_BLOCK FB_MotorReversing
VAR_INPUT
    bForwardCmd : BOOL;  bReverseCmd : BOOL;
    bStop : BOOL;  bOverload : BOOL;
END_VAR
VAR_OUTPUT
    bForwardOut : BOOL;  bReverseOut : BOOL;  bFault : BOOL;
END_VAR
VAR
    tonDelay : TON;  bChanging : BOOL;
END_VAR

IF NOT bOverload AND (bForwardOut OR bReverseOut) THEN bFault := TRUE; END_IF;
IF bFault OR bStop THEN bForwardOut := FALSE; bReverseOut := FALSE; END_IF;
IF bForwardCmd AND NOT bReverseOut AND NOT bFault AND NOT bChanging THEN
    bForwardOut := TRUE;
END_IF;
IF bReverseCmd AND NOT bForwardOut AND NOT bFault AND NOT bChanging THEN
    bReverseOut := TRUE;
END_IF;
// Direction change requires brief stop
IF bForwardOut AND bReverseCmd THEN bForwardOut := FALSE; bChanging := TRUE; END_IF;
IF bReverseOut AND bForwardCmd THEN bReverseOut := FALSE; bChanging := TRUE; END_IF;
tonDelay(IN := bChanging, PT := T#500ms);
IF tonDelay.Q THEN bChanging := FALSE; END_IF;
END_FUNCTION_BLOCK

Star-Delta Starting

Star-Delta starting reduces starting current to one-third of DOL by initially connecting windings in star, then switching to delta after acceleration.

FUNCTION_BLOCK FB_MotorStarDelta
VAR_INPUT
    bStart : BOOL;  bStop : BOOL;  bOverload : BOOL;
    tStarTime : TIME := T#8s;
END_VAR
VAR_OUTPUT
    bMainK : BOOL;  bStarK : BOOL;  bDeltaK : BOOL;
    bRunning : BOOL;  bFault : BOOL;
END_VAR
VAR
    nState : INT := 0;  tonStar : TON;  tonTrans : TON;
END_VAR

IF bStop OR (NOT bOverload AND nState > 0) THEN
    nState := 0; bFault := NOT bOverload;
END_IF;

CASE nState OF
    0: bMainK := FALSE; bStarK := FALSE; bDeltaK := FALSE;
       IF bStart AND NOT bFault THEN nState := 1; END_IF;
    1: bMainK := TRUE; bStarK := TRUE; bDeltaK := FALSE;
       tonStar(IN := TRUE, PT := tStarTime);
       IF tonStar.Q THEN tonStar(IN := FALSE); nState := 2; END_IF;
    2: bMainK := TRUE; bStarK := FALSE; bDeltaK := FALSE;
       tonTrans(IN := TRUE, PT := T#80ms);
       IF tonTrans.Q THEN tonTrans(IN := FALSE); nState := 3; END_IF;
    3: bMainK := TRUE; bStarK := FALSE; bDeltaK := TRUE;
END_CASE;
bRunning := (nState = 3);
END_FUNCTION_BLOCK

Speed Control via Variable Frequency Drive (VFD)

A VFD controls motor speed by varying frequency and voltage. The PLC interface uses digital signals (run command, VFD running feedback, VFD fault) and analog signals (speed reference 0-10V from PLC, actual speed feedback to PLC). The 0-10V reference maps to 0-50 Hz for a standard motor:

nSpeedRefRaw := REAL_TO_INT((rDesiredSpeedHz / 50.0) * 27648.0);
IF nSpeedRefRaw < 0 THEN nSpeedRefRaw := 0; END_IF;
IF nSpeedRefRaw > 27648 THEN nSpeedRefRaw := 27648; END_IF;

Motor Protection: Overload and Thermal

Multiple protection layers prevent motor damage:

Thermal overload relay: trips when current exceeds set value, opens NC contact for PLC
Electronic overload in VFD: built-in thermal model based on current measurement
PLC-based monitoring: current via analog input from current transformer

rMotorCurrent := ScaleLinear(nCurrentRaw, 0.0, 50.0, 0, 27648);
IF bMotorRunning THEN rRunHours := rRunHours + 0.001 / 3600.0; END_IF;
bCurrentWarning := rMotorCurrent > (rRatedCurrent * 1.1);
bMaintenanceDue := rRunHours >= 2000.0;

Running hours and current monitoring predict maintenance needs before failures occur.

Practical Example: Running a Conveyor Belt at Two Speeds

A packaging conveyor runs at slow (15 Hz) for loading and fast (40 Hz) for transfer:

VAR
    bStartCmd : BOOL;  bStopCmd : BOOL;
    bSlowSpeed : BOOL;  bFastSpeed : BOOL;
    bVFDRun : BOOL;  nSpeedRef : INT;
    bVFDFault : BOOL;  bFault : BOOL;  rTargetHz : REAL;
END_VAR

IF bSlowSpeed THEN rTargetHz := 15.0;
ELSIF bFastSpeed THEN rTargetHz := 40.0;
ELSE rTargetHz := 0.0;
END_IF;

IF bStartCmd AND NOT bFault THEN bVFDRun := TRUE; END_IF;
IF bStopCmd OR bVFDFault THEN
    bVFDRun := FALSE;
    IF bVFDFault THEN bFault := TRUE; END_IF;
END_IF;

IF bVFDRun THEN
    nSpeedRef := REAL_TO_INT((rTargetHz / 50.0) * 27648.0);
ELSE
    nSpeedRef := 0;
END_IF;

The operator selects speed, the PLC outputs an analog voltage, and the VFD smoothly ramps the motor using its built-in acceleration and deceleration ramps.

Summary

Motor control ranges from simple DOL starting for small motors to VFD speed control for advanced applications. Reversing circuits require three levels of interlock. Star-Delta reduces inrush current through a timed transition. VFDs provide full speed control via analog references. Every motor circuit must include overload protection and feedback monitoring. The conveyor example shows how these elements combine in typical industrial use.