why we use star delta starter?
Induction motors typically start on-line. However, when massive motors started, using that method, they can cause disruption of the voltage on the supply lines because of massive current surges at the beginning.
To reduce the start-up current surge, the large induction motors start with a lower voltage, and then its connect to full voltage as they reach close to a speed that is rotated. Two strategies used to cut down on the starting voltage include auto transformer to state.
The Working Principle of Star-Delta Starter:
This is the lower voltage method of starting. They achieved the reduction in voltage during star delta starting through physically changing the windings of the motor, as shown in the figure below.
At the time of starting, the motor’s windings connected to each other in a star arrangement, which reduces the voltage that applies to each winding by 3. The torque also decreases by three factors. After a certain period, they reconfigure the windings into the delta, and the motor continues to run normally.
Star/Delta starters are the most commonly used low voltage starters. It employed them to lower the current that is that applies to the motor at the start-up process, hoping to decrease interference and disturbance with electricity supply.
Traditionally, in several supply regions, it has been an obligation to install an electric starter with a lower voltage on any motor that is greater than five horsepower (4KW). Its Star/Delta (or Wye/Delta) starter is among the most affordable electromechanical starters used with reduced voltage.
This Star/Delta starter comprises three contactors and a timer and thermal overload. The contactors are less that the one contactor used in a Direct On-Line starter because they control the currents through the winding. The winding currents are about 1/root 3 (58 percent) that of the total current flowing through the line.
Two contactors close to the motor during operation, and are often called Delta contactor and prime contactor. They are AC3 that are rated at 58 percent from the rating for the power source. Third contactor known as the star contactor. It is only used to carry star current while the motor is in the star. Star’s current equals one-third of delta’s current that flows through delta, so this contactor could be AC3 with a rating of one third (33 percent) of the motor’s rating.
Star-delta Starter It comprises the of the following components:
1.) Contactors (Main Delta, Star and Main Contactors) 3 no’s (For the Open State Starting) as well as 4 No’s (Close Transient Starter).
2) Time relay (pull-in delayed) 1 No.
3.) Three-pole thermal in the 1No release in its current version.
4.) Fuss elements as well as automated cut-outs to the main circuit, 3 Nos.
5.) Fuss element (or automatic cut-out) to one circuit 1No.
The Circuit for Star Delta’s Starter
Main circuit breakers functions as the primary power supply switch that provides power into the power circuit.
They connect the primary contactor to the reference power source R, Y and B to the primary motor’s terminal U1, V1, and W1.
When in operation, the main Contactor (KM3) along with the Star Contactor (KM1) are shut at first, but then, after a certain period, the Star contactor is open, and the Delta contactor (KM2) will be shut. Control of both contactors happens through the clock (K1T) integrated in the starter. It is important to note that the Star and the Delta are interlocked electronically and, ideally, mechanically interlocked too. There are 4 states to consider:
The star contactor functions to short the second connection of the motor V2, W2 to initiate the start sequence to begin the initial start-up off the motor when it is at standstill. This will provide one-third of DOL current to the motor, cutting down on the inrush current associated with high-capacity motors during start-up.
We accomplish the control of the changing star with the delta connector to the AC induction motor via a wye or star delta control circuit. The control circuit comprises push button switches, additional contacts and an alarm clock.
Control Circuit of Star Delta the Starter (Open Transition):
The ON button activates the circuit by first activating the Star Contactor Coil (KM1) of the star circuit and the Timer Coil (KT) circuit.
When the Star Contactor Coil (KM1) energized, Star Main and Auxiliary contactors change their positions between NO and then NC.
If Star Auxiliary Contactor (which is on the Main Contactor Coil circuit) was changed from the position of NO to NC it’s the complete circuit of Main contactor Coil (KM3) so Main Contactor Coil was energized, while Main Contactor’s Main and Auxiliary Contactors Change their Position from NO to NC. This happens over an interval of friction.
After pressing the ON button, the auxiliary contact on the contactor’s main coil (2) that runs parallel with the button ON changes from the NO contact to NC and provide an auxiliary latch to keep that main coil active that will keep the circuit in operation after the release of on the push.
Once Star Main Contactor (KM1) is closed, it connects motor is connected to STAR and is connected to STAR up to Time Delay Auxiliary contact KT changes from NC and NO.
After the time delay has at its set Time The timing auxiliary contacts (KT) in the Star Coil circuit will change their direction between NC to NO. At the same Time Auxiliary Contactor (KT) in the Delta Coil Circuit alters its position from NO to NC thus Delta coil is energized Delta Main Contactor switches from NO to NC. The motor terminals now change between a delta and star connection.
A normally closed auxiliary contact that is present on both delta and star contactors is also in the opposite of both delta and star contactor coils. These interlock contacts function as safety switches to stop the simultaneous activation of the star and delta coils in order to prevent one from being activated without having the other one deactivated first. Therefore, it is impossible for the delta coil to cannot be activated while that of the star contactor is in use in the same way, as the contactor of star can’t be active while that coil’s delta contacts are activated.
Two interrupt contacts stop the motor in the control circuit above. The button that is OFF shuts down the control circuit and motor when needed. The thermal overload contacts are an emergency device that automatically activates the STOP Control circuit in case the thermal overload relay identifies the current of motor overload. It avoids combustion of the motor in the event of an excessive load that exceeds the maximum power of the motor. identified via the thermostatic overload relay.
In the beginning of an operation, it’s essential to switch from the star connected winding into an ad hoc winding. They arranged circuits for power and control by using one of two methods, either an open or closed transition.
What is an Open or Closed Transition Starting?
(1) Open Transition Starters.
We know the discussion above as open transition switching because the delta state is an open state that exists between the star state and the delta state.
In open transition, they cut the power off from the motor, while they reconfigure the winding using an external switch.
When a motor is powered by the power source, either at full speed or partial speed, there’s a magnetic field that is rotating in the stator. The field is spinning at the rate of line frequency. The force of the stator field causes a current to flow through the rotor, which is reflected in an electric field in the rotor.
If we remove the motor from the power source (open transition) there is a spinning rotor inside the stator. This spinning rotor is subject to an electromagnetic field. Because of its low resistance of circuit that drives the rotor, this means that the time constant can be rather long. The effect of the rotor’s spinning field inside the stator acts as an electric generator that generates a voltage that is governed by the speed of the rotating rotor. When the motor is connected to the power supply to close, it’s re-closing onto an unsynchronized generator, which can cause an intense torque and current transient. The intensity of the transient depends on the relationship between the voltage generated and the voltage on the line at the time of closure. It is often much greater than the torque and current of DOL and could cause mechanical and electrical damages.
Starting an open transition is the simplest to do in terms of costs and circuitry. If the timing of the switchover is right, this technique is a good choice. Though, it is difficult to set the timing to operate correctly and disconnection/reconnection of the supply can cause significant voltage/current transients.
In Open transition, there are four states:
- Off State All Contactors are available.
- Star State Star State: The Main [KM3] and Star [KM1Contactors are closed, and the contactor for delta [KM2] is open. We connect the motor to the star form and produce one third of the torque DOL at a third of DOL current.
- We know the Open State kind of operation as open transition switching because it is open that exists between the star state and the delta state. They shut the main contactor while both the Delta Star and Delta Star contactors are both open. There is a voltage present on one side of the motor’s windings. However, the other one is not open, so there is no current flow. We equip the motor with a spinning rotor, and acts as a generator.
- Delta State: The Main and the Delta contactors are shut. Star Contactor: The Star contactor remains open. Motor is in the full line voltage, and all power and torque are available.
(2) Closed Transition Star/Delta Starter.
There is a way to decrease the intensity of the transients that switch. This involves the use of the fourth contactor as well as three resistors. We should place the resistors to allow a substantial amount of current allowed to flow through the motor’s windings when they are within the circuit.
We connected the auxiliary contactor and the resistors to the contactor delta. In operation, before the contactor of the star opens, the auxiliary contactor is closed and current flows through the resistors to through the contactor’s star. After the contactor has opened, it allows the current to move through the motor’s windings and to the supply through the resistors. We then cut the resistors off through the delta contacts. If the resistor’s resistance is too high, they won’t be able to absorb the voltage produced by the motor, and thus have no value.
In a closed-transition, we keep power for the motor at all times. This is accomplished by adding resistors that absorb the flow of electricity during the changing of the winding. We need a fourth contactor to install the resistor within the circuit prior to opening the star contactor, and then remove the resistors after we shut the delta contactor. These resistors have to handle the motor’s current. Along with the need for more switching devices as well, the control circuit becomes more complicated because of the necessity of carrying out the switching of resistors.
In Close transition, there are four states:
- OFF State. All Contactors are available.
- Star State. Its Main [KM3] and Star [KM1contactors are shut, and the contactor delta [KM2] is open. We attach the motor to the star form and produce one third of the torque of DOL at a third of DOL current.
- Star Transition State. It is the motor that is connected to the star, and resistances connect to the delta contactor through the aux [KM4] contactor.
- A Closed Transitional State. The Main [KM3contactor is shut, while the Delta [KM2 and Star Contactors [KM1] are open. The motor’s current flows through its windings and through the transition resistors through KM4.
- Delta State. Delta State. Main Delta and Main Delta contactors are shut. We have shorted the transition resistors out. It is the Star contactor is in the open. Full line voltage powers the motor, and all torque and power are available.
Effects of Transient Effect of Transient (Open Transient starter)
It is crucial that to ensure that the pause between the Star contactor switch off besides Delta contactor switch on is right. This is because of the Star contactor has to be successfully removed before Delta contactor can be activated. It is also essential to ensure that the timer for switching over is not overly long.
We effectively decreased the 415v Star Connection voltage to 58%, or the equivalent of 240v. It is the equivalent of 33% is, achieved by Direct Online (DOL) starting.
If the Star connection has enough torque to operate up to 75 percent or% 80 of the maximum load speed, we can connect the motor to it in Delta mode.
When connected to a Delta configuration, the voltage of the phase rises by a ratio of V3 or 173 percent. The phase currents also increase in the same ratio. The line current is three times its value when connected to a star connection.
During the time of transition of switchover, the motor has to be in motion with no slowing. While it is “Coasting” could generate a voltage on its own. Upon connection to the power source, the voltage may randomly be added or subtracted from the voltage of the line. We refer this to in transients. For a brief period, it can cause spikes and voltage surges. Also known as a changeover transient.
The size of each of the parts of Star-Delta starting
(1) Dimension of the Over Load Relay:
With a star-delta starter, it is possible to put an overload protector in two locations either in the line or inside the windings.
In Line Overload Relay
In the line, it’s exactly the same as adding the overload to the motor, as in the DOL starter.
The value of Overload (In Line) is the FLC of the motor.
A disadvantage: If the overload setting is set to FLC, and it’s set to FLC, then it’s in danger of destroying the motor even though you are in the Delta (setting to x1.732 excessive).
Relay for Overload in Winding:
In the windings signifies we place the overload following the point where the wire to the contactor’s splits into delta and main. The overload will always measure the current within the windings.
The setting of the Overload Relay (In Winding) =0.58 X FLC (line current).
Negative: We have to make use of separate short circuits and overload protections.
(2) The size of Main and Delta contactors:
Two contactors close during the run, commonly called Delta contactor and main contactor. They are AC3 with a rating of 58 percent of the current rating of the motor.
Main Contactor Size= IFL 0.58 0.58
(3) The size of Star contactor:
A third type of contactor, the star contactor. known as the star contactor, and it is only used to carry star current while we attach the motor to the star. The current in the star is 1/3= (58 percent) of the delta current, so this contactor could be AC3 rating at one third (33 percent) of the motor’s ratings.
The size of the Star Contactor is IFL 0.33 0.33
Motor Starter Characteristics of Star-Delta Starter:
Starting current 33% Full Load Current.
Peak beginning the current 1.3 up to 2.6 Fully Loading Current.
Maximum torque starting at 33% Full Load.
Benefits of Star-Delta Starting:
- The star-delta method is straightforward and reliable.
- It’s fairly inexpensive when compared to other low voltage techniques.
- Good Current Performance/Torque.
- It draws 2 times the initial current of the maximum load amperes of the motor that is connected.
The disadvantages of Star-Delta Starter:
- Low Start Torque (Torque = (Square of Voltage) is also reduced).
- The Break-In Supply is a possibility–Potential Transients
- Six Terminal Motors required (Delta connected).
- It will require two sets of cables connecting the starter and motor.
- It can provide only 33% starting torque. It is not sufficient if the load attached to the motor in question requires greater starting torque when it is first started than transients that are extremely strong and we create strains when switching from delta to star connections. Because of these stresses and transients, many mechanical and electrical breakdowns take place.
- In this technique of starting, we initially connect the motor to the star, and after changing over, we will connect the motor to delta. The starter forms the motor’s delta, not at motor terminals.
- Current and transmission peaks that are high at the beginning of starting up fans or pumps, for example, the torque of the load is lower at the beginning of the process and grows by the proportion of speed. At around. 80-85% of motor nominal speed, the torque of the load is the same as the motor’s torque, and the motor’s acceleration stops. To get to the maximum speed, the switch to the delta position is required and will often lead to significant current and transmission peaks. In certain instances, the current peak may get larger than an initial D.O.L start.
- Applications that have a load torque that exceeds 50% of motor’s rating cannot start with the start-delta started.
- Low starting torque: The star-delta (wye-delta) starting method determines we set the lead connections that connect to the motor up with a delta or star electrical connection. They must make the first connection in the star pattern, which reduces the line’s voltage by 1/3 (57.7 percent) in the motor’s direction. In addition, we decrease the current by 1/3 to the total voltage. However, the initial torque is reduced, or 1/5 is the starting torque.
- The shift from star to delta occurs after the speed at which we achieved nominal, however it is occasionally performed at 50% of the nominal speed. This can cause temporary Sparks.
The Star-delta star features are a part of the starting
- For high- and low-power three-phase motors.
- Lowered beginning actual
- Six connection cables
- The torque of the start is reduced.
- Current peak occurs when you switch of star into delta
- Mechanical load during changeover of star into delta
The Star-Delta Starter Application:
- The star-delta method is only used for low to medium voltage and lighter starting Torque motors.
- The starting current received is around 30 percent of the current that is used to start during a direct on line starting and we reduce the torque at which you start to approximately 25% of the torque that is available during the D.O.L start. This method of starting only works if we do not load the system at the beginning. If the motor has been heavily loaded, it will not have enough torque to get the motor to its maximum speed prior to switching to the delta position.