A 3 Phase Induction motor consists of a stator which contains 3 phase winding connected to the 3 phase AC supply. The arrangement of the winding is so as to produce a rotating magnetic field. The rotor of the Induction motor contains cylindrical core with parallel slots that contain conductors.
Problems Encountered During Motor Starting:
The most basic feature of an Induction motor is its self starting mechanism. Due to the rotating magnetic field, an emf is induced in the rotor, because of which current starts flowing in the rotor. As per the Lenz law, the rotor will start rotating in a direction so as to oppose the flow of electric current and this gives a torque to the motor. Thus the motor gets self started.
During this self starting period, as torque increases, a large amount of current flows in the rotor. To achieve this the stator draws a large amount of current and by the time the motor reaches its full speed, a large amount of current is drawn and coils get heated up, damaging the motor. Hence there is a need to control the motor starting. One way is to reduce the applied voltage, which in turn reduces the torque.
Objectives of Star-Delta Technique Motor Starter are:
- Reduce high starting current and along these lines forestall motor from overheating
- Provide over-burden and no-voltage assurance
Star Delta Starter:
In star delta starting, the motor is connected in STAR mode throughout the starting period. When the motor reached the required speed, the motor is connected in DELTA mode.
Components of a Star-Delta Starter:
Contactors: The Star- Delta starter circuit comprises of three contactors: Main, star and delta contactors. The three contactors are solicited to unite the motor windings first in star and afterward in delta.
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. By any chance if star and delta contactors are actuated at the same time, the motor will be damaged.
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. In the event that the temperature goes past a preset quality, the contact is open and power supply is cut in this manner ensuring the motor.
Working of Star-Delta Starter:
At first the primary contactor and the star contactors are shut. After a time interval the timer signs to the star contactor to head off to the open position and the primary, delta contactors to head off to the shut position, accordingly structuring delta circuit.
At the time of starting when the stator windings are star associated, every stator stage gets voltage VL/√3, where VL is the line voltage. Hence, the line current drawn by the motor at starting is decreased to one-third as contrasted with starting current with the windings associated in delta. Likewise, since the torque advanced by an induction motor is corresponding to the square of the applied voltage; star- delta starter decreases the starting torque to one- third of that possible by immediate delta starting.
The timer controls conversion from star connection to delta connection. A timer in star delta starter for a 3-phase motor is intended to do the move from star mode, utilizing which the motor runs on a decreased voltage and current and produces less torque – to the delta mode indispensible for running the motor at its full power, utilizing high voltage and current to transform a high torque.
Terminal Connections in Star and Delta Configurations:
L1, L2 and L3 are the 3-phase line voltages, which are given to primary contactor. The main motor coils are U, V and W is shown in figure. In star mode of motor windings, the primary contactor associate the mains to essential winding terminals U1, V1 and W1.the star contactor shorts the auxiliary winding terminals U2, V2 and W2 as indicated in figure. Notwithstanding when the primary contactor is shut supply arrives at terminals A1, B1, C1 and consequently the motor windings are energized in star-mode.
The timer is initiated in the meantime moment when star contactor is energized. After the timer achieves the specified time period, the star contactor is de-energized and delta contactor is energized.
The point when delta contactor closes, the motor winding terminals U2, V2 and W2 get associated with V1, W1 and U1 individually through the shut contacts of primary contactor. That is for delta association, fulfilling end of one winding is to be joined with beginning end of the other winding. The motor windings are reconfigured in delta by supplying line voltage L1 to winding terminals W2 and U1, line voltage L2 to winding terminals U2 and V1; and line voltage L3 to winding terminals V2 and W1, as indicated in figure.
Types of Star Delta Starter:
There are two types of star-delta starters, open and close.
Star Delta Open Transition Starter:
It is the most widely recognized strategy for star-delta starting. As the name proposes, in this strategy motor windings are open throughout the transition time of altering the windings from a star mode with a delta mode. The star-delta open move starter uses 3 motor contactors and a move delay relay.
Merits:
Open transition starter is very easy to implement in terms of cost and circuitry, it does not require additional voltage educing equipment.
Demerits:
Open transition makes a surge of current and torque at change over which stuns the system both electrically and mechanically. Electrically, the outcome of the momentary peaks in current could cause force vacillations or misfortunes. Mechanically, the expanded torque coming about because of the current spike could be sufficient to harm system components i.e. snap a drive shaft.
Star Delta Closed Transition Starter:
In this starter, the transfer from the star to delta modes is made without disengaging motor from the line. A couple of components are added to dispose of or decrease the surge connected with the open transition. The extra components incorporate a contactor and few transition resistors. The transition resistors consume the present stream throughout the winding changeover. A fourth contactor is additionally used to place the resistor in circuit before opening the star contactor and afterward evacuating the resistors once the delta contactor is closed. Notwithstanding needing additionally exchanging mechanisms, the control circuit is more confounded because of the need to complete resistor exchanging.
Merit:
There is a reduction in the incremental current surge, which results from transition. Thus closed transition starter has a smooth change over.
Demerit:
In addition to requiring more switching devices, the control circuit is more complicated due to the need to carry out resistor switching. Also, the added circuitry leads to significant rise in the cost of the installation.
Example of Star-Delta Starter:
A Star-Delta starter is generally used to reduce start current of the motor. An example is given to know about the star-delta starter.
From the circuit, we used a supply of 440volts to start a motor. And here we used a set of relays to shift motor connections from star to delta with a time delay. In this, we explained the working by using lamp instead of motor for easy understanding. Throughout star operation the lamps might glow faint showing the supply voltage over the coils are 440volts. During delta operation after the timer works the lights might glow with full intensity showing full supply voltage of 440volts. The 555 timer performs the monostable operation, the output of which is sustained to a relay for updating the mains supply from 3-phase star-to-delta.
Photo Credit:
- Motor starting period Vs Steady state running period by myelectrical
- Star Delta Motor Control Power Circuitby by s1.hubimg
- Induction motor winding terminals connected in star and Delta Configuration by myelectrical
- Full load current in Open Transition and closed transition by electricneutron
