Reduced-Voltage-Start Motor Control using the PLC

Figure 36 illustrates the control circuit and wiring diagram of a 65% tapped, autotransformer, reduced-voltage-start motor control circuit. The reduced-voltage start reduces the motor’s inrush current (locked-rotor current), to only 42% at full speed. The timer should be set at 5.3 seconds in this example. The instantaneous contacts in lines 2 and 3, must also be captured.
(a) Hardwired relay circuit and (b) wiring diagram of a reduced-voltage-start
Figure 36: (a) Hardwired relay circuit and (b) wiring diagram of a reduced-voltage-start
Figure 37 shows the hardwired circuit, with the actual inputs and outputs highlighted. Devices that aren’t circled can be implemented within the PLC by programming internal instructions. The I/O assignment and internal assignment are shown in Tables 13-14 and 15, respectively.
Real inputs and outputs to the PLC
Figure 37: Real inputs and outputs to the PLC.
Figure 38 illustrates the PLC implementation of the reduced-voltage-start circuit. The PLC program’s first line traps the internal output 1000 timer. This internal contacts replace the instantaneous control circuit timer contacts.
Register assignment.
PLC implementation of the circuit in Figure 36
Figure 38: PLC implementation of the circuit in Figure 36.
The PLC circuit implementation doesn’t provide low-voltage protection because the interlocking uses the physical inputs M1, S1 and S2. Low-voltage protection can be provided by programming the starter’s overload contacts or auxiliary contacts. If overloads or auxiliary contacts are being used as inputs they must be programmed normally open (closed when overloads are closed and motor is running) then placed in series with contact 1000 at line 3 of the PLC software. The circuit will lose continuity if the overloads are open and M1 will turn off.

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