Before troubleshooting the 3 phase induction motor, we must have to know about the 3 phase induction motor, star delta starter and it’s connection diagram. So let’s take it one by one:

Here we are giving you only overview on 3 phases induction motor. 3 phase induction motor have 3 winding; assume U, V and W. Every winding has its own resistance. But all winding have same resistance. Total number of wire coming from motor are 6 i.e. U1, U2, V1, V2, W1 and W2. When these three winding are connected in STAR or DELTA connection (Star delta starter), motor is ready to start. Connection of star and delta is showing in figure.

star delta connection diagram of 3 phase induction motor
star delta connection diagram of 3 phase induction motor

Now may be some questions are coming in your mind like

Q1- How we check motor is in working condition or not?

Ans- Open all connection of motor. Here we use multi-meter or series tester to check. But by series tester we can check only continuity not accurate resistance.

  • If we found infinity resistance or no continuity in any winding, motor is faulty.
  • If we found continuity between earth and winding, motor is faulty.
  • Check resistance of winding. If we found resistance unbalanced or not same. Motor faulty

Q2- Why motor is drawing current more than rated current?

Ans- Motor is drawing more than rated current due to following reason:-

  • Check input AC voltage of motor. Voltage should be in specification range.
  • Check connection of motor.
  • Check resistance of winding. If resistance of winding is unbalanced, motor is faulty.
  • Check mechanical load on motor, Load should be smooth or not in jam condition.
  • Disconnect Motor from load, and check current. If current is okay check load again.
  • Check rotor by hand or manually, rotor should be free or no abnormality.
  • If Rotor is not smooth change bearing of motor.
  • Check rotor alignment.

Q3- Motor overheating


  • Improper ventilation or high ambient temperature.
  • Low voltage or single phasing.
  • Motor is taking current higher than rated current without trip.

There are following types of starting method of motor.

  • Direct online starter:- apply direct voltage to motor
  • Star delta starter:-reducing voltage at starting time
  • Auto transformer starter:-Part winding starting
  • Soft starter:-reducing voltage at starting time

Star Delta Starter

Star delta starter is most commonly used starter after direct online starter in electrical world. It’scost effective and simple operationas compare other voltage reduced starter.

Most induction motors are started using direct online starter, but when very large power motors are started using DOL, they take large starting current. Due to large current, they cause disturbance in supply voltage. To limit the starting current flow, large induction motors are started at reduced voltage and when motor achieve own rotation speed,  full supply voltage reconnected. Two methods are used for reduction of starting voltage are star delta starting and auto transformer starting

Star delta starter used to reduced starting voltage.This also reduces the torque. In this starter motor is running in star winding during start, when motor get full rpm, winding convert in to delta winding.

We know that in star connection winding voltage is equal to 1/√3 of line voltage but winding current is equal to line current. That’s why torque is reduced by factor of three.On the other hand in delta connection winding voltage is equal to line voltage and winding current is equal to 1/√3 line current.

Power connection diagram:-

6 wire of winding is coming from motor as U1, U2, V1, V2, W1, W2. We have 3 phase line voltage R, Y, B. To make power connection diagram we require 3 contactor , 1 overload relay and 1 MCB. Here overload relay is use for protection from heavy current.

Lets start to make power connection diagram

RYB 3 Phase Ac supply
Figure 1:

Step1:- line voltage(R,Y,B,) connect in MCB input and then MCB output connect to overload relay input.

                FIGURE1:- STEP 1 AND 2 or initial connection

Step2:-overload relay output connect to Main contactor input (C1) and main contactor output (C1) connect motor three wires as U1, V1 and W1. It means R, Y and B phase connect to U1, V1 and W1 through MCB, overload relay and main contactor. Show in figure 1

Step3:-Now we connect U2, V2, and W2 in star contactor output (C3). And all input of star contactor short with wire. Using this short wire we make star winding showing in figure 2.

star connection
Figure 2: star connection

 Figure2:- Step 3 or star connection

Step4:-Now we connect R1, Y1 and B1 with delta contactor (C2). There we use our mind how we connect U2, V2 and W2 in delta contactor because we want to make delta winding when this contactor on. Connect wire showing in figure 3. Like U1 connect to W2, U2 connect to V1, V2 connect to W1.

delta connection
Figure-3: delta connection

Step5:- now merge all above. Now power connection is ready.

After merging all step showing in figure 4.

power circuit diagram of star delta starter
Figure 4: power circuit diagram of star delta starter

                Figure4- power circuit diagram of star delta starter

Control circuit diagram:- Before making circuit diagram we know how its work or working principle.

In this starter, when motor started, motor runs in star connection, it means first of all, main contactor and star connector on. After some time or motor achieve full speed, motor winding connection convert in to delta connection automatically. It means star contactor off and delta contactor on. Here we also provide some interlock for motor protection in control circuit. Interlock as following:-

  1. If main contactor doesn’t on, star contactor and timer should not be on.
  2. Till when star contactor off, delta contactor not on.
  3. If overload relay trip, starter should be off.

 For making control circuit diagram, we require one NO push button and one NC push button. If our all contactor and timer is operated on 220v-50hz, then control supply require 220v single phase. Here we can also use 24v operated items, but for 24v supply, also require 24v SMPS. For cost cutting and easy maintenance we always use 220v operated items.

Now we are making control circuit diagram with following steps:-

Step1:- here we have AC supply 220v- 50hz (L, N). Line voltage connect to NC point(21) of Overload relay , and goes to NC push button, NC push button use for stop the starter, and goes to NO push button as per figure:-

Step - 1

Step 2:- Now connect to NO push button in this circuit, and also connect to coil supply (A1) of main contactor.

Step 3:- now connect NO point of main contactor in parallel of NO push button for holding main contactor as per figure:-Step - 3

Step4:-now connect timer coil with NO point of main contactor(C1) to X point.

Step5:- and connect star contactor (C2) with NO point of main contactor and NC point of timer to X point. For interlocking with delta contactor also connect NC point of delta contactor (C3). Do Connection as per in figure:-

Step - 5

Step6:- now connect delta contactor with NO point of timer and NC point of star contactor.

Step - 5

How control circuit works:-

Step1:- Here line voltage comes to Start push button(PB2) through overload relay(OL) and stop button(PB1). When we push PB1 button supply goes to C1 contactor coil supply. The auxiliary contact of the main contactor coil (C1) which is connected in parallel across the Start push button(PB2) will become NO to NC, This is providing to hold the main contactor coil activated which maintains the control circuit active even after releasing the start push button(PB2) switch.

Step2:– When main contactor activated, star contactor and timer are also activated. Timer activates through auxiliary NO contact of main contactor. Star contactor activate through auxiliary NO contact of main contactor and NC contact of delta contactor.

Step3:– here we set time of timer, after completion of time .timer contact changes from NO to NC and NC to NO. So NC point which is connected with star contactor in series, becomes NO. star contactor deactivate and when star contactor off, delta contactor on because timer NO contact becomes NC.


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