Nowadays induction motors are being used in all industries across the globe. So there are various different methods used to start the 3-phase induction motor like star-delta starter. One of them, the cheapest and most popular method is direct online (DOL) starting. The limitation of this method is, that it can be used only for Low HP (up to 5HP) induction motors. To start a high-rated (greater than 5HP) 3-phase induction motor, a Star-delta starter is used.
What is a Star-Delta Starter?
Star-delta starter is the most commonly used reduced voltage starting method for starting a medium HP 3-phase squirrel cage induction motor (SCIM). It is also known as a wye-delta starter. In this starting method, a 3-phase induction motor is connected in star throughout the starting and when speed reaches nearly about 80% of full load speed it switches (with the help of a TPDT switch) to delta position throughout the normal operation.
In simple words, we can say that a wye-delta starter starts a motor with star-connected stator winding. When the speed reaches 80% of the rated speed it begins to run with delta-connected stator winding.
Star-delta starter is used for more than 5 HP and available up to 180 HP induction motor.
Why Star-Delta Starter is used instead of DOL Starter?
Most of low HP (up to 5 HP) induction motors are started directly online with a DOL starter. But when large motors (more than 5 HP) are started in that way then it causes a disturbance in the voltage of the supply line due to a very large starting current surge (6 to 10 times of full load current).
So, To limit the starting current surge (inrush current), a reduced voltage starting method is required for starting of large HP rating induction motor. Since both auto transformer starting and star-delta starting is reduced voltage starting methods of 3-phase induction motor. Hence for large motors, We use a star-delta starter instead of a DOL starter to reduce the starting current surge of the motor without using any external devices.
Types of Star-Delta Starter
- Manual star delta starter
- Semi-automatic star delta starter
- Automatic star-delta starter
Components of Star-Delta Starter
These are the main components of a wye-delta starter:
A contactor is a heavy-duty relay with a high current rating used to power up the induction motor. The current rating of contactors varies in the range from 10A to several hundred amperes. In the wye-delta starter, We use mainly three contractors and these are:
- Main contactor
- Star contactor
- Delta contactor
The main contactor and delta contactor are AC3 type and rated 58% of the current rating of the motor. These contactors are closed during the running of the induction motor Whereas the star contactor only carries star current during the starting of the motor. As the starting current becomes 1/3 times of rated current so star contactor can be of AC3 type and 33% of the full load current rating. All three contractors used in the star delta starter are smaller than that of the single contactor used in the DOL starter because these contactors control the winding current only which is 33% of the full load current in the star delta starter.
2. Overload Relay
As in the case of induction motor, Most of the windings’ failure is because of overloading, operation on unbalanced voltage supply, and also single phasing due to loss of phase, which may lead to excessive heating and degradation of winding insulation of the motor. So overload protection is required to protect windings from overloading and short circuits in the internal winding. Hence all these conditions are prevented by a 3-pole thermal overload relay.
The main function of the timer in the wye-delta starter is to switch the contactor from star to delta state when the motor attains speed nearly more than 80% of full load speed.
4. Fuse Unit
3 nos of fuse are used in series with the motor circuit to protect the motor from external over current and short circuit faults. Also, 1 fuse is used to protect the control circuit of the wye-delta starter.
5. Push Buttons – to start (NO type) and stop (NC type) the motor.
Working Principle of Star-Delta (Wye-Delta) Starter
Star-delta starter is the most commonly used reduced voltage starter for induction motors. The purpose of a star-delta starter is to limit the starting current surge ( 6-10 times of rated current ) by reduced voltage with star-connected windings. In this method, Stator windings are connected in star and when speed reaches up to 80% then it switches from star to delta with the help of triple pole double throw (TPDT) switch. Thus during starting, the Current in the winding of the motor is reduced to 1/3 of the rated value, and the starting torque also reduces to 33%. The working of a star delta starter can easily be understood with the help of three different states:
1. Star Connected State
During the starting time, the main contactor and star contactor will be in a close position to complete the power circuit. So in this state, induction motor stator winding will be connected in star and hence the voltage across the motor winding will reduce to1/√3 times of line voltage.
When the motor attains 80% of full load speed the timer circuit Disconnects the star contactor first and connects the delta contactor into the circuit.
2. Open Transition State
During the switching from star to delta, the contactor circuits become open and the motor remains neither in star nor in delta state. So this state is known as an open transition state.
3. Delta Connected State
After activation of the timer, the motor switched from star to delta state. In delta connected state stator phase voltage will be equal to line voltage. Hence line voltage will appear across stator winding and the motor will run normally at its rated full-load speed.
Power Circuit of Star-Delta Starter
Referring to the figure, apart from the protection fuse (F1), and overload relay (F2), the circuit consists of three contractors named – main contractor (K1), star contactor (K2), and delta contactor (K3). The main contractor (K1) connects the supply line voltage (R, Y, B) to the primary terminals (U1, V1, W1) through the primary fuse (F1) and overload relay (F2). Also, secondary terminals (U2, V2, W2) are connected with star contactor (K2) and delta contactor (K3).
In operation during the starting, the main contactor (K1) and star contactor (K2) will be closed initially, so that the motor will start with the star-connected secondary. After a time delay when speed reaches more than 80% the timmer circuit opens K2 and closes K3. So the secondary winding connection shifted from star to delta configuration and the motor continued to run in delta throughout the operation.
All the controlling of contactors, overload relays, and protection fuses is performed by inbuilt control and timer circuits of the wye-delta starter.
Control Circuit of Star-Delta Starter
The control circuit mainly performs the switching operation of contactors (K1, K2, K3) and transitions from star to delta configuration throughout the starting and running of SCIM. The control circuit consists of fuse, NO-NC start (S1), and stop (S0) pushbuttons, star-delta timer (K4), and auxiliary contacts of contractors and overload relays.
In normal operation (F1, F2, and F3 are healthy), When S1 is pressed (during starting) timer coil K4 energizes the coil of star contactor K2 and it also energizes the coil of line contactor K1. So this will drive the motor in star configuration. The NO contact of K1 is wired in parallel to S1 such that the circuit remains in the latch until S0 is pressed.
After the set time duration in the timer circuit, the coil K2 is de-energized and K3 is energized so the motor continues to run in a delta configuration. When S0 is pressed or during an overload trip or F1 is blown out then the coil of K1 and K3 de-energizes and the motor stops.
Star contactor K2 and delta contactor K3 are mechanically and electrically interlocked using NC contacts such that both contractors should not close at the same time. This ensures that coil K3 will be energized only when K2 opens and also, K2 will be energized only when K3 de-energized.
Open Transition Star-Delta Starting
If you closely observe the above-discussed control circuit operation then you will find that the motor gets disconnected during the transition from star to delta configuration. This means the starter momentarily disconnects the motor from the star configuration and then reconnects it with the delta configuration. It is easier to implement in terms of cost and circuitry so it is more widely used than a closed transition star delta starter. If the timing of changeover is good then this method works very well. But in practice, though it is difficult to set the timing correctly so disconnection and reconnection of supply may cause voltage and current transient. The following are the four stages of operation of the open transition state wye-delta starter:
OFF State:- all contactors are open
Star State:- Main contactor K1 and star contactor K2 are connected and K3 remains open. So the motor runs in a star configuration.
Transition (Open) State:- This is also called open transition switching because there is an open stage between the star and delta. In this state, main contactor K1 is closed and star contactor K2 and delta contactor K3 are open. There is a voltage on the primary end of the motor winding and the secondary winding open. So no current can flow in the secondary winding. Since the motor has a spinning rotor and so it behaves like a generator.
Delta State:- Main contactor K1 and delta contactor K3 are closed and star contactor K2 is open. The motor is connected to full line voltage and hence full power and torque are available to output.
Closed Transition Star-Delta Starting
There is a technique to reduce the magnitude of switching transients in the transition state. It requires an extra 3-pole auxiliary contactor and three resistors of appropriate rating so that a considerable amount of current can flow through the resistors during the delta-switching period. Also, the auxiliary contactor and all three resistors should be connected across the delta contactor K3.
In operation, just before the opening of star contactor K2, the auxiliary contactor closes resulting in the continuous flow of current through the resistors into the star. Once star contactor K2 opens, the current continues to flow around through the motor winding to the supply via resistors. These resistors are then shorted by delta contactor K3.
Hence in closed transition star-delta starting, a continuous power is maintained to the motor at all times.
The following are the five stages of operation of the closed transition state wye-delta starter:
OFF State:- all contactors are open
Star State:- Main contactor K1 and star contactor K2 are connected and K3 remains open. So the motor runs in star configuration.
Star Transition State: The motor is connected to the star and resistors are connected across delta contactor K3 with the auxiliary contactor.
Closed Transition State:- The main contactor K1 is closed and delta contactor K3 and star contactor K2 are open. Current flows through motor windings and transition resistors via an auxiliary contactor.
Delta State:- Main contactor K1 and delta contactor K3 are closed and star contactor K2 is open. Also, the transition resistors are shorted. The induction motor (SCIM) is connected to full line voltage and hence full power and torque are available to output.
Effect of Transients in Star-Delta Starter
As discussed above, the Transient effect occurred only in an open transition state. In this starting, there is an open transition state during which star contactor K2 and delta contactor K3 both remain open whereas main contactor K1 is closed. During this switchover period, the motor must be free running with a small de-acceleration (coasting).
So it may generate its own voltage and in connection to supplying this generated voltage may randomly add or subtract with the main supply line voltage. This causes a voltage surge and spike in the transient current (also known as changeover transient current) which lasts only for a few milliseconds. Hence to minimize the effect of transients on open transition wye delta starter we use an extra auxiliary contactor and three transition resistors across delta contactor K3.
Ratings and Sizes of Components of Star-Delta Starter
Size of Overload Relays: There are two overload relays used for overload protection and these are used in line and in the winding of the motor.
So the Rating of inline overload relays = Full load current of the motor.
Whereas the setting of the overload relay in winding = 0.58 times of full load line current of the motor.
Size of Main and Delta Contactor: The main contactor K1 and delta contactor K3 are AC3 type and rated as 58% of the full load current of the motor.
Hence Size of the Main Contactor = 0.58*Full load current of motor
Also Size of Delta Contactor = 0.58*full load current of the motor
Size of Star Contactor: The third contactor is star contactor K2 which carries only 58% of the main contactor current. It means the rating of the star contactor should be only 33% of the motor’s full load current. It should also be AC3 type.
Hence the size of the star contactor = 0.33*full load current of the motor
Motor Starting Characteristics of Star-Delta Starter
- Available starting current: 33% of full load current
- Peak starting current: 1.3 to 2.6 times of full load current
- Peak starting torque: 33% of full load torque
Features of Star-Delta Starter
- Star delta starter can be used only for Low to high power rating
3-phase squirrel cage induction motor
- 6 connection cables are required for the induction motor terminal box.
- Reduced starting torque
- Reduced starting current
- Current peak on the changeover from star to Delta
- Mechanical load on the changeover from star to Delta
Advantages of Star-Delta Starter
- It is comparatively cheaper than other reduced voltage methods of starting of induction motor.
- The torque and current characteristics of the Star Delta starter are better than other methods of starting.
- The starting current is 1/3 times of direct online (DOL) starting current.
- No tap-changing device is required to use
- It produces high torque per ampere of line current
- The design and operation of the star delta starter are simple and rugged.
Disadvantages of Star-Delta Starter
- Star delta starter can only be useful for 6 terminal delta connected induction motor where all 6 terminals can be accessed.
- As the starting current is 1/3 times of nominal current, the starting torque is also reduced to 1/3 times. Hence it provides only 33% starting torque and if the load connected to the motor requires higher starting torque then very high transient and stress are produced while changing from star to delta connection. Hence because of these transients and stresses many electrical and mechanical break-down occurred.
- It requires 2 sets of cables from the starter to the motor.
- Star delta starter fails to start the motor if the load connected with the motor has load torque higher than 50% of motor-rated torque.
- In this method of starting during the transition from star to delta, if the motor does not reach at least 80% of its rated speed then the current peak will be high same as in the DOL starter. Thus it may cause harmful effects on the contacts of contractors. Hence it would not be reliable.
- If the motor is too heavily loaded then there will not be enough torque to accelerate the motor up to the desired speed before switching over to the delta position.
Applications of Star-Delta Starter
- The Star Delta starting method is generally preferred for low to medium-voltage and low-torque squirrel cage induction motors (SCIM).
- Star-delta starter is also used in centrifugal compressors.
- It is most suitable for applications where the required starting current is low and also the line current drowns must be minimal.
- It can be used in industries like- Textile, Food and Beverage, sugar plants, paper and printing, cold storage, etc.
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