PLC application for reduced voltage-start motor control

PLC application for reduced voltage-start motor control (photo credit:

Reduced-voltage-start motor control circuit

Figure 1 shown below illustrates the control circuit and wiring diagram of a 65% tapped, autotransformer, reduced-voltage-start motor control circuit. This reduced voltage start minimizes the inrush current at the start of the motor (locked rotor current) to 42% of that at full speed.

In this example, the timer must be set to 5.3 seconds. Also, the instantaneous contacts from the timer in lines 2 and 3 must be trapped.

(a) Hardwired relay circuit and (b) wiring diagram of a reduced-voltage-start motor

Figure 1 – (a) Hardwired relay circuit and (b) wiring diagram of a reduced-voltage-start motor

Hardwired circuit (real I/O)

Figure 2 illustrates the hardwired circuit with the real inputs and outputs circled. The devices that are not circled are implemented inside the PLC through the programming of internal instructions.

Real inputs and outputs to the PLC

Figure 2 – Real inputs and outputs to the PLC

Tables 1, 2, and 3 show the I/O assignment, internal assignment, and register assignment, respectively.

Figure 3 illustrates the PLC implementation of the reduced-voltage start circuit. The first line of the PLC program traps the timer with internal output 1000. Contacts from this internal replace the instantaneous timer contacts specified in the hardwired control circuit. This PLC circuit implementation does not provide low-voltage protection, since the interlocking does not use the physical inputs of M1, S1, and S2.

Table 1 – I/O address assignment

I/O Address
Module TypeRackGroupTerminalDescription
 Input000 Stop PB (NC)
001 Stop PB (NO)
 Output030 Motor Starter M1
031 S1
032 S2

Table 2 – Internal address assignment

 —1000 Trap Timer Circuit
 Timer1001 Timer

Table 3 – Register assignment

 4000 Preset register value 53, time base 0.1 sec. for 5.3 sec. (timer output is 1001)
 4001 Accumulated register for timer output 1001

PLC implementation of the circuit in Figure 1

Figure 3 – PLC implementation of the circuit in Figure 1

If low-voltage protection is required, then the starter’s auxiliary contacts or the overload contacts can be programmed.

If the auxiliary contacts or the overloads are used as inputs, they must be programmed as normally open (closed when the overloads are closed and the motor is running) and placed in series with contact 1000 in line 3 of the PLC program.

If the overloads open, the circuit will lose continuity and M1 will turn OFF.

Resource: Introduction-to-PLC-Programming –


Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears. Professional in AutoCAD programming and web-design. Present on Google+.

One Comment

  1. paul deelen
    Jun 04, 2014

    The picture is a horror, it is real bad practice and the comments are given in for long time.
    the timer should be set to the needed time to have the motor running at normal speed.
    this can be from 2 to 30 seconds.
    hardware circuit is correct
    i/o address assignment line 2 should be start button
    in program the O/L should be detected, when resetted the motor will have full power as M1 is still active thus the coil M1 (please use different names internal and external) will be activated.

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