Maximum industrial motors are Induction motors, That's why it is important to study every aspect of induction motor like; it's starting, running and protection.

Starters employed for motors not only to start the motor but it provides protection to the motor in overload & phase loss conditions.

For small rating motors up to 7.4 kW, DOL starters can be employed. But above 7.4 kW, DOL is not good choice specially when load directly connected to the motor because starting current can't be controlled through DOL starter; As starting current of motor may reach up to 8 times the rated capacity of motor in DOL.

High starting current can deteriorate the motor winding by producing electrical stress which further reduces the useful life of the motor. & one more big problem is associated that is, For overload protection overload relays have been connected but due to high initial starting current overload relay always trips at starting in DOL starters.

For example

We have 7.4kw motor, rated current is 15 Amps(approx.). Suppose Overload relay for DOL starter been set at 15 Amps But Starting current may go up to 90 Amps when load is directly connected to the motor's shaft. Which results, overload relay trips if that much high starting current flows through overload relay at starting.

That's why limiting the high initial current is very important.

Now the question is, Why starting current is so high at initial stage ?
Actually there are two reasons:
Reason No.1 (BIG REASON)
At starting, there is no back emf generated in the winding that's why extra initial current exists.

Reason No.2

Motor takes current according to the connected load. When motors are loaded at initial stage of starting, it takes extra current also due to that connected load Because at the time of starting there is no moment of inertia. Once load starts moving it's inertia or in some cases flywheel aid the motor in the movement.

To limit the high initial current in induction motors, Several starting methods been employed for example- Star-Delta starters, VFDs, Auto transformer starters. From all the methods, Star delta starter is easier & cheapest one. In this post, we are going to discuss star-Delta starter.

For practical learning, we are using L&T made ML1.5 Star-Delta Starter.

Star Delta Starters

Connections of induction motor can be done in two ways; one is Star connection and another one is Delta connection. Every Induction motor has 3 coils in which each coil has two open ends, By connecting these open ends of the coils in a particular manner, we can create the connection in Star Or Delta.

Star connections

One end of all the 3 coils are shorted so that one junction get formed with all the 3 coils, that junction forms mid point of the winding which is technically a neutral. In Star connection, every coil get voltage of Line-Neutral.

If star connections are connected with 3 phase 415Volt supply then each coil get a voltage around 240Volts.

STAR & DELTA Connections of 3 phase Induction Motor

Delta connections

All the 3 coils are connected in a series manner & 3 terminals are brought out in such a wat that each coil get double phase voltage(Line-Line).

It means each coil getting a voltage of 415 Volts when connections are connected with 3 phase 415Volt supply.

In Star connection, Motor getting a supply voltage 1/1.732 times the Line voltage. While in delta connections, Motor getting a supply voltage equal to line voltage.

Low operating Voltage in Star connection leads to low power & low current consumption. That's why star connection takes current 1/3 times the current in Delta connection. Idea of reduced current in star connection is used in this starter. At the time of starting, motor run in star connection after some time when motor attain its 75% speed; connections get shifted to delta.

Star-Delta Starter/Type-ML1.5F byL&T

Star-Delta Starter| ML 1.5 By L&T

Components of Star-Delta Starter

In this, all the 6 terminals of the motor are brought inside the starter.

Components Used :

// 3 Contactors

1st contactor work as a main contactor.
2nd contactor work as a star while 3rd work for Delta connection.

// 1 Overload relay

// 1 timer

Motor's 3 terminals are directly connected to the main contactor. Remaining 3 terminals are connected to star and delta contactors parallelly. At time of starting motor runs in star connection In which Main+ Star contactor works. After some time star connection get changed in delta connections & star contactor get exchanged with delta contactor in which Main+ Delta contactor works.

Components of star-Delta Starter

Wiring of Star-Delta Starter

"WE CAN CREATE CIRCUITS WITH SAME FUNCTION IN MANY DIFFERENT WAYS !"

There are many circuits for Star-Delta starters but function remains the same. In which at the time of start when we press ON button; star contactor & timer Energizes first after that main contactor energizes. Next, According to the set time on timer, star contactor eliminated & Delta contactor comes into the circuit while main contactor continuously remains in the circuit.

Below, we are going to discuss the circuit diagram which is printed on the starter.

Key Diagram Star-Delta Starter| MK1.5 By L&T

Since coil voltage of all the contactors is 415 volts that's why one phase X1 is directly connected to all the three contactor coils and timer. While Another X2 phase is used in control circuit.
X2 phase passes through the NC of stop button after that it is connected to NO of start button.
When we press start button, supply passes through the NC of Delta then NC of timer & then connected to the timer. In parallel with timer, Star contactor in series with NC of main contactor is connected.
Timer & Star contactor get energized. Star contactor get a HOLD from its 43-44NO which is connected in series with NC of Main contactor.
After star contactor, 23-24NO of Star get closed which results main contactor(H) get energized. Main contactor get a Hold from its 43-44NO.
According to the set time on timer; When timer relay operates its 4-5 NC get changed & Star contactor & timer both cut out from the supply. In the mean time, Main contactor still energized.
After the cut out of star contactor & timer, Delta contactor get a supply through NC of Star & NC of timer.
This circuit; Main contactor & Delta contactor hold until 95-96NC stop button is pressed or motor trips or power supply get cut out. Actually, the overload relay also using the same 95-96NC in tripping.

Actually, These all the NC's work as a protection!

11-12NC of main contactor is used so that star contactor Energized only when Main contactor is not Energized; at the time of starting.
11-12NC of Delta contactor is used so that star contactor & timer operates only when Delta contactor is not Energized. In Reverse we can also say that when Delta contactor operates this 11-12NC of Delta contactor completely eliminate the Star contactor & Timer from the circuit.
Supply to Delta contactor passes through the series combination of 11-12 NC of Star contactor & 9-8NC of Timer so that Delta only operates when star contactor & timer both are OFF; not energized.
At last, Main 95-96 NC is used in stop button & As a NC of overload relay.

HOPE ! you like the post. If you have any query or doubt please do comments. You are always welcome.