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This changeover in winding connection leads to DELTA connected motor. When TPDT switch is put at Run position, the winding terminals (A2, B1), (B2, C1) & (C2, A1) are connected together as shown in schematic diagram. This is the reason we go for star delta starter.Ĭase-2: TPDT Switch is at “Run” Position and MCB is ON. Thus we see that, the starting line current for DELTA connected motor has reduced to (1/3) times of what it would have been for DOL starting. Ration of Line current in Start Delta Starting and DOL starting Since motor is DELTA connected, therefore line current will be equal to the √3 times the phase current. In this case, the per phase motor starting current (I 2) will be Now suppose, instead of using star delta starter, we are directly feeding the DELAT connected motor through supply which is the case in DOL (Direct On Line) starting. Since motor is STAR connected, hence phase and line currents will be same. = Phase Voltage / Standstill per Phase Leakage Impedance I 1 = Starting current when motor is STAR connected If V L be the line voltage and Z be the standstill per phase leakage impedance, then per phase starting current (I 1) flowing through the motor will be given as below. You might think what we are achieving by making motor STAR connected during starting? The straight forward answer is to limit the magnitude of starting current. Thus the motor is now STAR connected, fed through the power supply when TPDT switch is at Start position. Thus the motor winding STAR point is formed by the Switch while the other winding terminals A1, B1 and C1 are connected to R, Y & B phase of power supply through MCB. Under this assumed condition, motor winding terminals A2, B2 and C2 are shorted by the switch as can be seen in the schematic diagram. Let us now consider two cases to better understand the functioning of this starter.Ĭase-1: TPDT Switch is at “Start” Position and MCB is ON. This TPDT switch has two positions: Start and Run. Star delta starter consists of a power MCB and Three Pole Double Throw (TPDT) Switch. Schematic diagram of star delta starter is shown below. This nomenclature can easily be seen in the motor terminal box. Sometime it is also marked as (U1, U2) for R phase, (V1, V2) for Y phase and (W1, W2) for B phase. The winding terminals corresponding to R, Y & B phases are generally designated as (A1, A2), (B1, B2) and (C1, C2) respectively. This itself means that all the six winding terminal of motor must be taken out and connected to the starter. Working Principle of Star Delta StarterĪs discussed earlier, star delta starter automatically changes the motor winding configuration from STAR (during starting) to DELTA (when the motor reaches its rated speed). Star Delta Starter is one among the various starting methods. In order to limit this starting current to some reasonable value, some methods are adopted to start three phase induction motor. Due to this voltage dip, the other equipment connected to the supply may get effected. This high value of starting current may result in considerable voltage dip of the supply line feeding the motor. This means, if the rated current of a motor is 20 A then during starting it will take around 100 to 140 A. So it that doesn't work, you must either go to a closed transition star-delta scheme, which adds a 4th contactor and a resistor bank to absorb the transient, or start over using a soft starter.As we know that when a 3 phase induction motor is started at its rated voltage, it draws high inrush current whose magnitude is around 5 to 7 times of the rated motor current. Do it twice only, no point in doing it a third time because you go back to the original. What this does is to take advantage of any possible slight differences in impedance between the windings and the phases, which might keep the transient below a tolerable level. Your only cheap solution is to "roll" the conductors, in other words change which winding they are connected to, but keep the same rotation pattern. If you google "star delta motor starter transient" you should find some articles on it. The result can be a voltage transient that can then create a current transient that in theory, can be as high as 2200% of the motor FLA, far exceeding the mag trip settings of the breakers. Depending on the magnetic permeability of the motor cores, the impedance of the windings and the phase angle at which the exact transition point occurs, it can cause the motor to connect in Delta while the fields are not yet collapsed, so it's like connecting two out-of-phase generators. There is a phase shift that occurs in transition from star to delta. This is a known issue with Star-Delta open transition starters and the reason I never recommend them.
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