STAR – DELTA STARTER FOR THREE PHASE INDUCTION MOTOR

Introduction:

Motor Starters are types switches that are designed to start and stop the three phase induction motors by providing the necessary power to the motor and prevents the motor to draw the excess current. Star-delta starters are the most common reduced voltage starters in the 50Hz world. (Also known as Wye-Delta starters in the 60Hz world). They are used to reduce the start current applied to the motor during start for reducing the disturbances and interference on the electrical supply.

Need for motor starters

In general, a motor must be started with either full load or no load, based on its application. If a motor started with no load, it requires a small torque to get past the initial inertia. But if the motor is started with load, the starting torque will be sufficient enough to start the motor with load and its inertia.

The three – phase induction motors can start by directly connecting the mains supply. In this case, the starting torque of the motor will be high due to the high starting current. This torque accelerates motor to achieve the final speed. Since the acceleration of the induction motor is high, the copper losses i.e. loss due to the heat. and is calculated using I square x R, is considerably low.

This type of motor starting is applicable only for the motors that are rated up to 5HP. Because, in large motors, the starting current is very high and if it is connected directly to the supply from the mains, so there will be huge voltage drop in the line. This voltage drop may affect to the behavior of other systems or loads that are connected to the supply.

The starting current of large three phase motors can be 6 times higher than the full load current.

For example, the rated current of a 415V, 15 HP motor 70 A,If this motor is started by directly connecting it to the mains supply, the starting current will be around 6×70=420A. This is a very high current draw from the network so it will show an impact on other devices.

So that, we need to start the motor using an appropriate starter. If torque of the induction motor is directly proportional to the square of the voltage (T ∝ V2), a reduction in voltage will result in lower starting torque. This types of starters are known as Reduced Voltage Starters.

There are many types of Reduced Voltage Starters

• Star-Delta (Y – Δ) Starter
• Auto Transformer Reduced Voltage Starter
• Part Winding Reduced Voltage Starter
• Resistance Reduced Voltage Starter
• Increment Resistance Starter
• Reactance Reduced Voltage Starter

 Star-Delta (Y – Δ) Starter

Voltage reduction during the star-delta starting is achieving by physically re configuring the winding of the motor. During starting, the winding are connected in star configuration and this reduces the voltage across each winding 3. This will reduce the torque by a factor of three. After the delay time the winding are reconfigured as delta and the motor runs normally.

• Star/Delta starters are probably the most common reduced voltage starters. 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.
• Traditionally in many supply regions, there has been a requirement to fit a reduced voltage starter on all motors greater than 5HP (4KW).  The Star-Delta starter is one of the lowest cost electro mechanical reduced voltage starters that can be applied.

 Start-Delta starter consist of following components:

• Contactors : The Star- Delta starter circuit comprises of three contactors: Main, star and delta contactors.
• Timer: The contactors are regulated by a timer incorporated with the started.
• Interlock Switches: Interlock switches are connected between star and delta contactors of the control circuit as a safety measure so one can’t activate delta contactor without deactivating star contactor.
• Thermal Overload Relay: A thermal over-load relay is likewise consolidated into star-delta control circuit to ensure the motor from intemperate heat which might expedite motor finding fire or wearing out.

Working of Star-Delta Starter

• At the starting primary contactor and the star-contactors will shut.
• After a time delay the timer signs to the star-contactor to head off to the open position.
• The primary, delta-contactors to head off to the shut position, accordingly structuring the delta circuit.
• At the starting time, every stator stage gets voltage VL/v3, where VL is the line voltage.
• The line current drawn by the induction motor at starting will decrease to one-third as contrasted with starting current with the winding associated in delta.
• Since the torque advanced by a motor is corresponding to the square of the applied voltage.
• Star delta starter will decrease the starting torque to one third of that possible by immediate delta starting.
• The timer controls the conversion from the star connection to delta connection.
• A timer in star-delta starter for an induction motor is intended to do the move from star mode.
• Utilizing which the motor runs on a decreased voltage and the current and produces less torque.
• To delta mode indispensable for running the induction motor at its full
  power.
• Utilizing the high voltage and current to transform a high torque.

Terminal Connections in Star and Delta Configurations

 Star Configuration:

• L1, L2 and L3 are the line voltages, which are given to the primary contactor
• The main motor coils are U, V and W.
• In star-mode of the motor winding, the primary contactor associated the mains to the essential winding terminals U1, V1 and W1.
• The star contactor shorts the U2, V2 and W2 terminals, that are the auxiliary winding terminals
• Notwithstanding when the primary contactor is shut, supply will arrive at terminals U1, V1 and W1.
• As a result, the motor winding are energized in the star-mode.
• The timer is initiated in the meanwhile moment when star contactor is energized.
• After the timer achieve the time period that already specified. The star contactor will de-energize and the delta contactor will energize.

 The point when delta contactor closes:

• The motor winding terminals U2, V2 and W2 will get associate with V1, W1 and U1 individually through the shut contacts of the primary     contactor.
• For delta association, fulfilling end of one winding is to be join with beginning end of other winding.
• The motor winding will reconfigure in delta by supplying line voltage L1 to winding terminals W2 and U1, line voltage L2 to winding   terminals U2 and V1, and the line voltage L3 to the winding terminals V2 and W1.