Induction motors
Induction motors are single excited electrical machine which requires power supply only at its stator winding. These motors work on the principle of Faraday law of Electromagnetic induction.
Induction motors required very less maintenance that’s the reason most industrial motors are Induction motors. Thus, it is important to study every aspect of an Induction motors, like its starting, running and protection.
Induction motor has mainly two parts rotor & stator.
Rotor
Rotor is a solid round block which rotates in between the stator. Rotor includes the silicon steel stamping which holds aluminum bars in skewed shape. Rotor consists of bearings at both the ends which is been held by end covers so that rotor rotates smoothly.
There is very thin gap in between the stator & Rotor.
Stator
Stator also has the silicon steel stamping which further includes winding in it. Winding is a combination of coils connected in a particular pattern according to the power rating, Torque & RPM.
Next, In this post motor word is used for Induction motors only.
Induction motor starters
” Starters not only starts the motor but it provides protection to the motor in overload conditions. There are multiple types of motor starters such as DOL starter, Star-Delta starter & auto-transformer starters“
For small rating motors, DOL starters can be employed. But for large rating motors, DOL is not a good choice.
‘DOL stands for ‘direct online starter‘ In which Electrical power is directly connected to motors winding without any change in voltage, frequency & winding configuration“
The starting current can’t be controlled through DOL starter. In DOL starter, The starting current of the motor can reach up to 8 times the rated capacity of motor. High starting current can deteriorate the insulation of the winding.
High starting current produces high electrical stress on winding Which further can reduce the useful life of the motor.
One more big problem is linked to DOL starters; that is, For overload protection, Overload relays have been connected. But heavy Induction motors have high starting current. As a result, the overload relay can get trips at starting in DOL starters for large rating motors.
Understanding the above statements through an example below :
Let suppose, A 7.4 kw motor with rated current of 15 Amps(approx.). Suppose Overload relay for DOL starter been set at 15 Amps.
However, when mechanical load is directly connected with the motor’s shaft. And DOL starting is connected, starting current can reach up to 90 Amps.
Overload relay always trips if that much high starting current flows through it at starting. That’s why limiting the high starting current is very important.
Now the question is, Why starting current is so high ?
- Reason No.1 (Big reason) // At starting, there is no back emf in stator winding.
- Reason No.2 // Motor takes current according to the connected load. When motors are loaded permanently. At starting, It takes extra current due to that permanent 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.
Now, how to limit high initial current ?
To limit high starting current in Induction motors, Several starting techniques been employed. Such as Star-Delta starters, Variable frequency drives, Auto transformer starters. From all the techniques, Star delta starter is simple & cheapest one.
In this post, We are going to discuss star-Delta starter. For practical learning, We are using L&T made ML 1.5 F Star-Delta Starter.
STAR DELTA STARTER
In Star-delta starters, Motor’s winding connection get changed after some seconds of starting. In that process, 2 winding connections are made which are known as ‘Star’ & ‘Delta’ . That’s why this starter is known as ‘Star delta starter‘.
At the time of starting star winding connection works While after some time delta winding connection works.
An image of L&T made ML 1.5 F start-delta starter is shown below :
Winding
Winding is the softest part of Electrical motor. Winding is the unit which made from multiple coils connected in a particular pattern.
Winding pattern of Induction motor can be done in two ways. One is ‘Star‘ connection and another one is ‘Delta‘ connection.
3 phase Winding has 3 coils in which each coil has two open ends. By connecting these open ends of the coils in a particular manner, Connection can be made in Star Or Delta.
Both the winding configurations are discussed through a GIF below. Winding; in 3 phase Induction motor; is the overall symmetry of 3 coils. Connection of these 3 coils in particular manner creates the Winding configuration.
Star connections
In Star connection, one end of all the 3 coils are shorted. Which results junction get made. That junction forms the midpoint of the winding, Which is technically a neutral. In Star connection, Every coil gets a voltage of Line-Neutral(Phase voltage).
If star connections are connected with 3 phase 415 Volt supply then each coil get a voltage around 240 Volts.
Delta connections
In Delta connection, all 3 coils are connected in a series manner. This way, A closed loop is formed and 3 nodes are made. These 3 nodes are brought out so that each coil gets double phase voltage (Line voltage).
It means each coil getting a voltage of 415 Volts when connections are connected with 3 phase 415 Volt 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, The motor runs in star connection. After when motor attains a least 75% rated speed, the connections get shifted to Delta.
Components of Star-Delta Starter
In this, All the 6 terminals of the motor are been brought inside the starter. So that connection get made easily inside the starter.
Components Used :
- 3 contactors : 1st contactor work as a main contactor . 2nd contactor work as a star. While 3rd contactor work for Delta connection.
- 1 Thermal overload relay, : Overload relay provides protections in short circuit & overload conditions.
- 1 timer : for time delay.
Power wiring of Star-Delta Starter
Power wiring of star-delta starter is been discussed below. Power wiring do not includes the control wiring.
Motor’s 3 terminals are directly connected to the main contactor . Remaining 3 terminals are connected to ‘star’ and ‘delta’ contactor parallelly.
At the time of starting, Motor runs in star connection In which Main+ Star contactor works. Another side terminals of star contactor are shorted which makes each coil getting a voltage of phase voltage.
After some time, The star contactor is exchanged with the delta contactor . In this setup, the Main+ Delta contactor works.
Another side of delta contactor are connected to power lines. Which makes each coil getting a voltage of line voltage.
Control Wiring of Star-Delta Starter
“WE CAN CREATE CIRCUITS WITH A SAME FUNCTION IN MANY DIFFERENT WAYS !”
There are many circuits for Star-Delta starters but function remains the same. In this circuit, at the time of starting, Star contactor & timer Energizes first. After that, main contactor energizes.
Next, According to the set time on timer, Star contactor get eliminated & Delta contactor comes into the circuit. While main contactor continuously remains active in the circuit.
Next, We are going to discuss the circuit diagram.
- All the contactor have a coil voltage of 415 volts. That’s why one phase X1 is directly connected to all the three contactor‘s coils and timer. While Another X2 phase is used in control circuit.
- X2 phase passes through the NC of stop button Which is also a type of NC contact of thermal overload relay. After that it is connected to NO of start button.
- When we press the start button, Supply passes through the NC of Delta. Then it passes through the NC of the timer. After that, it is connected to the timer. In parallel with the timer, Star contactor is connected.
- Timer & Star contactor get energized first. Star contactor get a HOLD from its 43-44NO .
- 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 the timer, the timer relay operates. Its 4-5 NC changes. The Star contactor and timer both cut out from the supply. In the meantime, the Main contactor remains 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 stop button (95-96NC) is pressed or motor trips. Actually, the overload relay also using the same 95-96NC in tripping as discussed above.
Actually, These all the NC’s work as a protection!
- 11-12NC of the main contactor is used. This ensures that the star contactor energizes first. The main contactor not get 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, the 11-12NC of Delta contactor completely eliminates the Star contactor . It also removes the Timer from the circuit.
- Supply to Delta contactor passes through the series combination of 11-12 NC of Star contactor & 9-8NC of Timer. Delta operates only when both the star contactor and timer are OFF; not energized.
- At last, Overload’s 95-96 NC is used in stop button & As a trip button.
SUMMARY
- Induction motors are predominantly used in industrial applications. A thorough understanding of their operation is necessary. This includes starting mechanisms and protection methods.
- Direct-On-Line (DOL) starters are effective for small motors. However, they cause excessive starting currents in larger motors. This can damage windings and unnecessarily trip overload relays.
- To address high starting current issues, methods like Star-Delta starters are used. These starters initially operate the motor in a star connection at lower voltage. Then, they switch to delta for full power.
- Star-Delta Starters employs contactor and timers to facilitate this transition, ensuring effective operation and motor protection.
HOPE ! you like the post. If you have any query or doubt please do comments. You are always welcome.