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14.02.2014
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Most induction motors are started directly on line, but when very large motors are started that way, they cause a disturbance of voltage on the supply lines due to large starting current surges.
Two methods used for reduction of starting voltage are: Star delta starting and Auto transformer starting. Traditionally in many supply regions, there has been a requirement to fit a reduced voltage starter on all motors greater than 5HP (4KW). There are two contactors that are close during run, often referred to as the main contractor and the delta contactor. The current in star is one third of the current in delta, so this contactor can be AC3 rated at one third (33%) of the motor rating. Contactors (Main, star and delta contactors) 3 No’s (For Open State Starter) or 4 No’s (Close Transient Starter).
The main circuit breaker serves as the main power supply switch that supplies electricity to the power circuit. The main contactor connects the reference source voltage R, Y, B to the primary terminal of the motor U1, V1, W1. In operation, the Main Contactor (KM3) and the Star Contactor (KM1) are closed initially, and then after a period of time, the star contactor is opened, and then the delta contactor (KM2) is closed. The star contactor serves to initially short the secondary terminal of the motor U2, V2, W2 for the start sequence during the initial run of the motor from standstill. Controlling the interchanging star connection and delta connection of an AC induction motor is achieved by means of a star delta or wye delta control circuit.
The ON push button starts the circuit by initially energizing Star Contactor Coil (KM1) of star circuit and Timer Coil (KT) circuit. When Star Auxiliary Contactor (1) (which is placed on Main Contactor coil circuit ) become NO to NC it’s complete The Circuit of Main contactor Coil (KM3) so Main Contactor Coil energized and Main Contactor’s  Main and Auxiliary Contactor Change its Position from NO to NC.
After pushing the ON push button switch, the auxiliary contact of the main contactor coil (2) which is connected in parallel across the ON push button will become NO to NC, thereby providing a latch to hold the main contactor coil activated which eventually maintains the control circuit active even after releasing the ON push button switch. When Star Main Contactor (KM1) close its connect Motor connects on STAR and it’s connected in STAR until Time Delay Auxiliary contact KT (3) become NC to NO. Once the time delay is reached its specified Time, the timer’s auxiliary contacts (KT)(3) in Star Coil circuit will change its position from NC to NO and at the Same Time  Auxiliary contactor (KT) in Delta Coil Circuit(4) change its Position from NO To NC so Delta coil energized and  Delta Main Contactor becomes NO To NC.
A normally close auxiliary contact from both star and delta contactors (5&6)are also placed opposite of both star and delta contactor coils, these interlock contacts serves as safety switches to prevent simultaneous activation of both star and delta contactor coils, so that one cannot be activated without the other deactivated first. At some point during starting it is necessary to change from a star connected winding to a delta connected winding.
Discuss mention above is called open transition switching because there is an open state between the star state and the delta state. In open transition the power is disconnected from the motor while the winding are reconfigured via external switching. When a motor is driven by the supply, either at full speed or at part speed, there is a rotating magnetic field in the stator.
When the motor is disconnected from the supply (open transition) there is a spinning rotor within the stator and the rotor has a magnetic field. When the motor is reconnected to the supply, it is reclosing onto an unsynchronized generator and this result in a very high current and torque transient.
Open transition starting is the easiest to implement in terms or cost and circuitry and if the timing of the changeover is good, this method can work well. Open State: This type of operation is called open transition switching because there is an open state between the star state and the delta state.
If the resistance of the resistors is too high, they will not swamp the voltage generated by the motor and will serve no purpose. This is achieved by introducing resistors to take up the current flow during the winding changeover. It is Important the pause between star contactor switch off and Delta contactor switch is on correct. If Star connection has sufficient torque to run up to 75% or %80 of full load speed, then the motor can be connected in Delta mode.


When connected to Delta configuration the phase voltage increases by a ratio of V3 or 173%. During transition period of switchover the motor must be free running with little deceleration.
For a star-delta starter there is a possibility to place the overload protection in two positions, in the line or in the windings. In the line is the same as just putting the overload before the motor as with a DOL starter. In the windings means that the overload is placed after the point where the wiring to the contactors are split into main and delta. The third contactor is the star contactor and that only carries star current while the motor is connected in star.
It provides only 33% starting torque and if the load connected to the subject motor requires higher starting torque at the time of starting than very heavy transients and stresses are produced while changing from star to delta connections, and because of these transients and stresses many electrical and mechanical break-down occurs. In this method of starting initially motor is connected in star and then after change over the motor is connected in delta. High transmission and current peaks: When starting up pumps and fans for example, the load torque is low at the beginning of the start and increases with the square of the speed.
Applications with a load torque higher than 50 % of the motor rated torque will not be able to start using the start-delta starter. Low Starting Torque: The star-delta (wye-delta) starting method controls whether the lead connections from the motor are configured in a star or delta electrical connection. The transition from star to delta transition usually occurs once nominal speed is reached, but is sometimes performed as low as 50% of nominal speed which make transient Sparks. The star-delta method is usually only applied to low to medium voltage and light starting Torque motors. If the motor is too heavily loaded, there will not be enough torque to accelerate the motor up to speed before switching over to the delta position. Because I,ve known how to make control but then it confuses me on how to connect the 3 wires (3O) coming from the overload relay into the motor terminal leads. The ID fan is a high inertia load which it takes considerable time to accelerate to full speed. It seems that the connection of star delta contactor diagram above has a mistake, v2 and w2 were wrongly marked. When the supply will on only timer coil energized But not pick up Main contact whenever it not get supply. 7) When DELTA contactor get supply its contact will be change & open the STAR contactor line. We need to install variable speed drive for 1x75kW Teco motor and 1x 160kW Teco motor at our Station. Reverse outputs and accepts single-component values, two-component values, and all three-component values.
To limit the starting current surge, large induction motors are started at reduced voltage and then have full supply voltage reconnected when they run up to near rotated speed. Voltage reduction during star-delta starting is achieved by physically reconfiguring the motor windings as illustrated in the figure below.
The contactors are smaller than the single contactor used in a Direct on Line starter as they are controlling winding currents only.
This provides one third of DOL current to the motor, thus reducing the high inrush current inherent with large capacity motors at startup.
When Star Contactor Coil (KM1) energized, Star Main and Auxiliary contactor change its position from NO to NC. Thus, the delta contactor coil cannot be active when the star contactor coil is active, and similarly, the star contactor coil cannot also be active while the delta contactor coil is active. Power and control circuits can be arranged to this in one of two ways – open transition or closed transition. Due to the low impedance of the rotor circuit, the time constant is quite long and the action of the spinning rotor field within the stator is that of a generator which generates voltage at a frequency determined by the speed of the rotor. The magnitude of the transient is dependent on the phase relationship between the generated voltage and the line voltage at the point of closure can be much higher than DOL current and torque and can result in electrical and mechanical damage.


The motor is connected in star and will produce one third of DOL torque at one third of DOL current. In operation, just before the star contactor opens, the auxiliary contactor closes resulting in current flow via the resistors into the star connection. A fourth contractor is required to place the resistor in circuit before opening the star contactor and then removing the resistors once the delta contactor is closed.
The motor is connected in star and the resistors are connected across the delta contactor via the aux [KM4] contactor. This is because Star contactor must be reliably disconnected before Delta contactor is activated.
While this is happening “Coasting” it may generate a voltage of its own, and on connection to the supply this voltage can randomly add to or subtract from the applied line voltage.
I have a submersible induction Motor 30kW, this motor at no-load test started normally on star connection and during delta transition supply protection circuit breaker is tripping. The reasons for choosing home water filters range from health concerns to better tasting beverages and food.
During starting the motor windings are connected in star configuration and this reduces the voltage across each winding 3.
They are used in an attempt to reduce the start current applied to the motor during start as a means of reducing the disturbances and interference on the electrical supply. The Star and Delta are electrically interlocked and preferably mechanically interlocked as well.
The thermal overload contact is a protective device which automatically opens the STOP Control circuit in case when motor overload current is detected by the thermal overload relay, this is to prevent burning of the motor in case of excessive load beyond the rated capacity of the motor is detected by the thermal overload relay. The flux from the stator field induces a current in the rotor and this in turn results in a rotor magnetic field. There is voltage on one end of the motor windings, but the other end is open so no current can flow. The resistors must be sized such that considerable current is able to flow in the motor windings while they are in circuit. Once the star contactor opens, current is able to flow round through the motor windings to the supply via the resistors. 80-85 % of the motor rated speed the load torque is equal to the motor torque and the acceleration ceases. The remedy lies in increasing star to delta change over time, but how much is the question.
To reach the rated speed, a switch over to delta position is necessary, and this will very often result in high transmission and current peaks.
He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. The practical approach is to work out the time by running the drive in star condition with timer control disconnected (so that star to delta change over does not occur for this experimental run).
When delta contacter is energised U2 terminal must be connected to V1 terminal and V2 terminal must be connected to W1 of motor windings.
He has published numbers of Technical Articles in "Electrical Mirror", "Electrical India", "Lighting India", "Industrial Electrix"(Australian Power Publications) Magazines.
The time by which motor accelerates to full speed (which can be observed by pointer of Analog Ammeter connected) is the time of star to delta change over. Even so, many of the bacteria and dangerous heavy metals are not eliminated.Another factor is price. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. You often get what you pay for, but it’s important to first determine what you are looking to accomplish. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.



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