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Home / Technical Articles / PLC application for reduced voltage-start motor control

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.

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

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 Type Rack Group Terminal Description
 Input 0 0 0  Stop PB (NC)
0 0 1  Stop PB (NO)
 Output 0 3 0  Motor Starter M1
0 3 1  S1
0 3 2  S2

Table 2 – Internal address assignment

Device Internal Decription
 — 1000  Trap Timer Circuit
 Timer 1001  Timer

Table 3 – Register assignment

Register Decription
 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 –

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Vince T
    Dec 20, 2014

    TR1 is a timer relay.

  2. biswajit dey
    Dec 08, 2014

    You’ve mentioned that,at the time of PLC programming, one should consider OL as NO contact. I think it should be “examine if open” (XIO) or NC contact. If not,how could you start the motor without energising the OL !

  3. 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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