Star - Delta Starter Explanation with Demo Testing Videos

Star - Delta Starter Explanation
with 
Demo Testing Videos 

An electrical motor is such an electromechanical device which converts electrical energy into a mechanical energy. In case of three phase AC operation, most widely used motor is Three phase induction motor as this type of motor does not require any starting device or we can say they are self starting induction motor.
For better understanding the principle of three phase induction motor, the basic constructional feature of this motor must be known to us. This Motor consists of two major parts:

Stator: 

Stator of three phase induction motor is made up of numbers of slots to construct a 3 phase winding circuit which is connected to 3 phase AC source. The three phase winding are arranged in such a manner in the slots that they produce a rotating magnetic field after 3Ph. AC supply is given to them.

Rotor: 
Rotor of three phase induction motor consists of cylindrical laminated core with parallel slots that can carry conductors. Conductors are heavy copper or aluminum bars which fits in each slots & they are short circuited by the end rings. The slots are not exactly made parallel to the axis of the shaft but are slotted a little skewed because this arrangement reduces magnetic humming noise & can avoid stalling of motor.

Working of Three Phase Induction Motor 

WORKING PRINCIPLE & STAR DELTA WINDING CONVERSION 

DEMO VIDEO  

Production of Rotating Magnetic Field

The stator of the motor consists of overlapping winding offset by an electrical angle of 120°. When the primary winding or the stator is connected to a 3 phase AC source, it establishes a rotating magnetic field which rotates at the synchronous speed.

Secrets Behind the Rotation:

According to Faraday’s law an emf induced in any circuit is due to the rate of change of magnetic fluxlinkage through the circuit. As the rotor winding in an induction motor are either closed through an external resistance or directly shorted by end ring, and cut the stator rotating magnetic field, an emf is induced in the rotor copper bar and due to this emf a current flows through the rotor conductor.
Here the relative speed between the rotating flux and static rotor conductor is the cause of current generation; hence as per Lenz's law the rotor will rotate in the same direction to reduce the cause i.e. the relative velocity.

Thus from the working principle of three phase induction motor it may observed that the rotor speed should not reach the synchronous speed produced by the stator. If the speeds equals, there would be no such relative speed, so no emf induced in the rotor, and no current would be flowing, and therefore no torque would be generated. Consequently the rotor can not reach the synchronous speed. The difference between the stator (synchronous speed) and rotor speeds is called the slip. The rotation of the magnetic field in an induction motor has the advantage that no electrical connections need to be made to the rotor. Thus the three phase induction motor is:

• Self-starting.
• Less armature reaction and brush sparking because of the absence of commutators and brushes that may cause sparks.
• Robust in construction.
• Economical.
• Easier to maintain.

Why we need Motor Starters?

Generally, a motor, whether industrial or consumer, must be started with either no load or full load, based on the application it is used for. If the motor must be started with no load, it requires a small torque to get past the initial inertia.

But if the motor must be started with full load (or any load for that matter), the starting torque must be sufficient enough to start the motor with load and its inertia.

Generally, three – phase motors can be started by directly connecting the supply from the mains. In this case, the starting current is high and as a result, the starting torque of the motor is high. This torque will accelerate the motor to achieve its final speed.

Since the acceleration of the motor is high (fast), the copper losses i.e. loss due to heat, which is calculated using I² x R, is considerably low.

This type of motor starting is applicable for small motor i.e. motors that are rated up to 5HP. But the same starting technique can’t be applied for higher capacity motors. The reason is explained below.

In large motors, the starting current is very high and if it is connected directly to the mains supply, there will be a huge voltage drop in the line. This voltage drop effects the behaviour of other systems and loads connected to the supply.

The starting current of large three phase induction motors can be as high as 6 times that of the maximum current (full load current).

Consider the following example for the purpose of explanation. A 415 V, 50 HP industrial motor is rated for a maximum current (full load current) of 70 A.

If this motor is started by directly connecting it to the mains supply, the starting current will be around 6 x 70 = 420 A. This is a very high current draw from the network and will definitely show an impact on other devices.

Hence, we need to start a three phase induction motor using an appropriate motor starter. Since torque of the motor is directly proportional to the square of the voltage (T ∝ V²), a reduction in voltage will result in a lower starting torque. This types of motor starters are called Reduced Voltage Starters.

There are many types of Reduced Voltage Starters. Some of them are listed below.

·         Star Delta (Y – Δ) Starter

·         Resistance Reduced Voltage Starter

·         Auto Transformer Reduced Voltage Starter

·         Increment Resistance Starter

·         Part Winding Reduced Voltage Starter

·         Reactance Reduced Voltage Starter

   

Star Delta Starter (Y – Δ Starter)

Star Delta Starter, which is sometimes called as Y – Δ or Wye – Δ Starter, is a common type of Reduced Voltage Starter. Star Delta Starter can reduce the starting current without the need of any external devices.

Generally, we use Star Delta Starters for three phase squirrel cage induction motors that are normally designed to work on a Delta Connection. The major applications of motors with Star Delta Starters are fans, pumps, centrifugal chillers in ACs, etc.

In a Star Delta Starter, the initial connection of the stator windings is in the form of Star. If V_L is the Line Voltage and V_P is the Phase Voltage, then the voltage at each stator phase is given by

V_P = V_L / √3

As the motor accelerates and gains speed, the stator windings are disconnected from the Star configuration and connected in the form of Delta. The following image shows the Star and Delta connections and their respective currents.

In the second image (image on the right), the windings of the stator are connected in Delta. We know that the Line Voltage and Phase Voltage are equal in Delta Connection and let the voltage across the stator windings be V. If I is the Phase Current through the stator winding in Delta connection, then line current is I_L = √3 x I.

This is from the fact that the line current in a Delta Connection is root three times the phase current.

Coming to the first image (image on the left), the stator windings are connected in Star Connection. As V is the Line Voltage, the voltage across the windings in Star connection is given by V / √3.

As the voltage across the winding is reduced by 1 / √3 times, the current flowing in each winding is also reduced by the same amount. Hence, the phase current or the current through winding becomes I_P = I / √3. Since the Line current and phase current in star connection are equal, the Line current I_L = I / √3.

We can conclude from the above analysis that,

I_L (in Delta) = √3 x I

I_L (in Star) = I / √3

From the above two equations, we can conclude that the Line Current in Star Connection is 1/3 times that of the Line Current in Delta Connection.

Also, the starting torque of a motor is directly proportional to the square of the voltage at the windings i.e.

T ∝ V²

As we have established that voltage across the motor windings in star connection is 1 / √3 times the voltage across the motor windings connected in delta connection. Hence, the starting torque of the motor in star connection will be 1 / 3 times that of the torque when the motor is connected in delta connection.

Motor Windings in Star Delta Starter

Let us see the motor winding connections in a Star Delta Starter. Let the three stator windings of the three phase induction motor be called as U1 – U2, V1 – V2 and W1 – W2 where U1, V1 and W1 are the starting ends and U2,V2 and W2 are the closing ends (or finishing ends). The following image shows the motor windings connected in star and delta connections.

In a star connection, the finishing ends of the windings are joined together to form a neutral point. We can also connect all the starting ends of the winding to make the neutral point. In the above image, the finishing ends i.e. U2, V2 and W2 are connected together.

In a delta connection, the starting ends of one winding is connected to the finishing end of the other winding to form a structure as shown in the above image. The supply is given at the junctions. In the connection show above, the junctions are as follows: U1-W2, V1-U2 and W1-V2.

In a Star Delta Starter, the windings of the motor are first connected in Star Connection and with the help of switches, timers and contactors, the windings are connected in Delta Connection during normal running of the motor.

Automatic Star Delta Starter with Timer for 3-Phase Motor

The Star-Delta (Y-Δ) 3-phase Motor Starting Method by Automatic star-delta starter with Timer.

Abbreviations: ( FOR Star Delta 3-phase Motor Automatic starter with Timer)

R , Y, B = Red, Yellow, Blue ( 3 Phase Lines)

C.B = General Circuit Breaker

Main = Main Supply

Y = Star

Δ = Delta

1a = Timer

C1, C2, C3 = Contatcors (For Power & Control Diagram)

O/L = Over Load Relay

NO = Normally Open

NC = Normally Closed

K1 = Contactor (Contactor coil)

K1/NO = Contactor Holding Coil (Normally Open)

Automatic Star Delta Starter with Timer Wiring & Installation Diagram

Explanation of the above Star Delta Automatic Starter with Timer Wiring Installation:

From the left you have the Main Contactor with the Pneumatic Timer because your Main Contactor is always energized, in the Middle you have the Delta Contactor with a Thermal Overload for Motor Protection in case the Motor exceeds the Amp rating set on the Thermal Overload, on the right you have the Star Contactor which is the first Contactor to be energized with the Main Contactor then when the Timer reaches its Time limit the Star Contactor de-energizes and the Delta Contactor energizes and the Motor is running at full load.

Operation and Working Principle of Star Delta 3-phase Motor Automatic starter with Timer:

From L1 The phase current flows to thermal overload contact through Fuse, then OFF Push button, On Push button Interlocking Contact 2, and then C3. This way, the circuit is completed, as a result;

1. Contactor coil C3 and Timer coil (I1) is energized at once and the motor winding then connected in Star. When C3 is energized, its auxiliary open links will be closed and vice versa (i.e. close links would be open). Thus C1 Contactor is also energized and Three Phase Supply will reach to the motor. Since winding is connected in Star, hence each phase will get √3 times less than the line voltage i.e. 230V. Hence Motor starts safely.
2. The close contact of C3 in the Delta line opens because of which there would be no chance of activation of contactor 2 (C2) .
3. After leaving the push button, Timer coil and coil 3 will receive a supply through Timer contact (Ia) , Holding contact 3 and the close contact 2 of C2.
4. When Contactor 1 (C1) is energized, then the two open contact in the line of C1 and C2 will be closed.
5. For the specific time (generally 5-10 seconds) in which the motor will be connected in star, after that the Timer contact (Ia) will be open (We may change by rotating the timer knob to adjust the time again) and as a result; 

• Contactor 3 (C3) will be off, because of which the open link of C3 will be close (which is in the line of C2) thus C2 will also energize. Similarly, When C3 off, then star connection of winding will also open. And C2 will be closed. Therefore, the motor winding will be connected in Delta. In addition, Contact 2 (which is in the line C3) will open, by which, there would not be any chance of activation of coil 3 (C3)
• Since the motor is connected in Delta now, therefore, each phase of the motor will receive full line voltage (400V) and the motor will start to run in full motion. 

CIRCUIT DIAGRAM

STAR-DELTA STARTER CIRCUIT CONNECTION AND OPERATION

 DEMO VIDEO

Advantages of Star Delta 3-phase Motor Automatic starter with Timer

• Simple Design and Operation
• Comparatively cheaper than other voltage controlling methods
• Torque and Current performance of the Star delta starter is well.
• It draws two times starting current of the FLA (Full Load Ampere) of the connected motor.
• It reduced the starting current to one-third (approximately) as compared to DOL (Direct ON Line Starter) 

Disadvantages of Star Delta 3-phase Motor Automatic starter with Timer

• Starting Torque is also reduce to one-third because starter reduce the starting current to one-third of the rated current [as Line voltage also reduced to 57% (1/√3)]
• It required Six leads or terminals Motor (Delta Connected)
• For Delta connection, the supply voltage must be same as the rated motor voltage.
• At switching time (From Star to Delta), if the motor does not reach at least 90% of its rated speed, then the current peak may be equally high as in Direct ON Line starter( D.O.L), thus it may cause harmful effects on the contactors contacts, so it would not be reliable.
• We may not use star delta starter if the required (application or load) torque is more than 50% of the three phase induction motors rated torque