Star-Delta starter and its working, Power and control circuit of star-delta starter, Advantages, disadvantages and applications of Star-Delta starter
One of them, the cheapest and popular methods is direct online (DOL) starting. Whereas the limitation of this method is, it can be used only for Low HP (up to 5HP) induction motor.
To start a high rated (greater than 5HP) 3-phase induction motor,Star-delta starter is used.
In this article, We will discuss in detail that what is star-delta starter and how it works, purpose of star-delta starter, the theory and working principle of star-delta (wye-delta) starter with the help of power and control circuit, rating and size of different parts of star-delta starter, advantages and disadvantages, and also its industrial application.
What is a Star-Delta starter?
Star-delta starter is the most commonly used reduced voltage starting method for starting medium HP 3-phase squirrel cage induction motor (SCIM). It is also known as 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 then it switches (with the
help of TPDT switch) to delta position throughout the normal operation.
[ ##eye## Core balance Current Transformer (CBCT)]
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 then 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 disturbance in 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:
1. Contactors
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.
[ ##eye## Construction and Working of LVDT]
In wye-delta starter, We use mainly three contactors and these are:
- Main contactor
- Star contactor
- Delta contactor
The main contactor and delta contactor is AC3 type and rated 58% of the current rating of the motor. These contactors are closed during the running of the induction motor
Whereas star contactor only carries star current during
starting of the motor. As 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 star delta starter are smaller than
that of single contactor used in DOL starter because these contactors
controlling 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 motor.
So overload protection is required
to protect windings from overloading and short circuit in the internal winding.
Hence all these conditions are prevented by 3-pole
thermal overload relay.
3. Timer
The main function of the timer in the wye-delta starter is to switch the
contactor from star to delta state when motor attains speed nearly about 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 fault. 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 motor. The purpose of star-delta starter is to limit 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, Current in the winding of motor is reduced to 1/3 of
rated value and starting torque also reduces to 33%.
Working of star delta starter can easily be understood with the help
of three different states:
1. Star connected state
 |
| star connected motor winding |
During starting time main contactor and star contactor will be in
close position to complete the power circuit.
[ ##eye## Transformer interview questions with answer]
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 attain 80% of full load speed then the timer circuit
disconnect star contactor first and connect 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
 |
| delta connected motor winding |
After activation of the timer, 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 figure, apart from protection fuse (F1), overload
relay (F2), the circuit consists of three contractors named as - main contractor
(K1), star contactor (K2), and delta contactor (K3).
[ ##eye## Parts of transformer and their functions]
The main contractor (K1) connects 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).
 |
| star-delta starter power circuit |
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%
then timmer circuit opens K2 and closes K3. So that secondary winding connection
shifted from star to delta configuration and motor continue 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 perform the switching operation of
contactors (K1, K2, K3) and transition 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.
 |
| star-delta starter control circuit |
In normal operation (F1, F2, and F3 are healthy), When S1 is
pressed (during starting) then 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 latch until S0 is pressed.
After the set time duration in the timer circuit, the coil K2 is de-energized and K3 is energized and so the motor continues to run in a delta configuration. When S0 is pressed or during overload trip or F1 blown out then the coil of K1 and K3 de-energizes and the motor stops.
[ ##eye## Theory and circuit design of Notch filter]
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 open and also, K2 will be energized only when K3 de-energized.
Open transition star-delta starting
If you closely observed 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 star configuration and then reconnects it with delta configuration.
[ ##eye## Causes and disadvantages of Low Power Factor]
It is easier to implement in terms of coast and circuitry so it is 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 and so disconnection and reconnection of supply may cause voltage and current transient.
The following are the four stages of operation of 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
remain open. So motor runs in star configuration.
Transition (open) state:- this is also called open transition switching because there is an open stage between 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 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 are 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 delta switching period. Also, the auxiliary
contactor and all three resistors should be connected across the delta
contactor K3.
[ ##eye## Surge impedance loading (SIL) of
Transmission line]
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 star.
Once star contactor K2 opens, then 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 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
remain open. So motor runs in star configuration.
Star transition state:- Motor is connected in star and resistors are
connected across delta contactor K3 with 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 are 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.
 |
| working of closed transition star-delta starter |
Effect of transients in star-delta starter
As discussed above, Transient effect occurred only in 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
transient current (also known as changeover transient current) which lasting
only for few milliseconds.
[ ##eye## Construction and Working of RVDT]
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 size of components of star-delta starter
Size of Overload relays: there are two overload relays are 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
motor.
Whereas the setting of 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 motor.
Hence size of main contactor = 0.58*full load current of motor
Also size of delta contactor = 0.58*full load current of motor
Size of star contactor: the third contactor is star contactor K2 that carries only 58% of the main contactor current. It means the rating of star contactor should be only 33% of motor full load current. It should also be AC3 type.
Hence size of star contactor = 0.33*full load current of 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 connections 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.
- Torque and current characteristics of star delta starter is better than other methods of starting.
- 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 starting current is 1/3 times of nominal current, 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 starter to motor.
- Star delta starter is failed to start the motor if 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.
Application of star-Delta starter
- Star delta staring method is generally preferred for low to medium voltage and low torque squirrel cage induction motor (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 drown must be minimum.
- It can be used in industries like- Textile, Food and Beverage, sugar plants, paper and printing, cold storage, etc.
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