Circuit Diagram and Method of Operation for a Three-Point Starter
What is a three-point Starter?
An electric device known as a three-point starter is utilised to start and keep the speed of a DC shunt motor once it has been started. A graded starting resistance, an overload release, and a no-volt release are the components that make up this system. In order to control the amount of current that flows through the motor during the starting, the starting resistance is connected in series with the armature. The overload release is connected in parallel with the motor, and it breaks the connection between the two components if the current through the motor reaches a certain threshold. The no-volt release is linked in series with the field winding of the motor, and it is designed to open the circuit in the event that the voltage supplied to the motor falls below a certain threshold.
The three terminals of the starter are:
L: The line terminal, which is connected to the positive supply voltage.
Z: The zero-volt release terminal, which is connected to the overload release.
A: The armature terminal, which is connected to the armature of the motor.
Why is a starter necessary for a DC motor?
A DC motor needs a starter because when the motor is at rest, it has no back EMF (electromotive force), so it draws a very high current as the armature resistance is very low. This high current can damage the motor windings and cause the power source to overload and trip. V=Eb-IaRa, as Eb=0 Ia=V/Ra The starter is a piece of equipment that is used to slowly increase the amount of power going to the motor. This keeps the motor from getting damaged. Most starters are made up of a number of resistors that connect to the motor armature in series. All of the resistors are in the circuit when the starter is first turned on. This limits the current to the motor and prevents it from drawing too much current. As the motor starts to spin, the resistors are gradually removed from the circuit, which allows the current to increase. Once the motor is up to speed, all of the resistors are removed from the circuit and the motor is running at its full current. The starter provides a safe and controlled way to start a DC motor. It protects the motor from overload and under-voltage conditions.
3 Points Starter Construction
A Three-Point starter is made up of a starting resistance that is split into various parts and linked in series with the shunt field winding. Every piece is held together by a brass stud. It also has two safety features: a “no-volt release” and a “over-load release.” There are three ends to this starter: L, F, and A. So, this type of starter is called a “Three-Point starter.”
Terminal A is linked to one of the armature winding terminals and the starting resistance’s last stud (in the “On” state). Through the main switch, Terminal L is linked to the positive power source. So, it links the source to the series resistance by letting the excess current out.
A coiled spring S is attached to one end of the arm. When the arm is not moving, the spring holds it in the OFF position. Also, if the power goes out while the machine is going, the spiral force of this spring moves the arm to the OFF position. In the middle of the arm is a soft iron guard. The soft iron guard is drawn to the no-volt release or keeping coil when the machine is running normally.
Terminal F is connected to the last metal pin, and when the arm is connected to this point, there is no extra resistance. Terminal F is linked to one end of the shunt field turning. The negative end of the supply is attached to the other ends of the DC shunt motor’s stator and shunt field winding.
Working of 3 point starter
The following is an explanation of how a three-point starter works:
- The primary switch has been activated. The line terminal receives voltage as a result of this action.
- It is necessary to shift the starting handle to the first position. This establishes a connection between the armature circuit and the starting resistance.
- The handle of the starter is turned gradually until it is in the second position. This results in a progressive decrease in the resistance that is present in the armature circuit.
- The lever that activates the starter is now in the third position. and so on till the final position.
- Because of this, all of the resistance in the armature circuit has been eliminated, and the motor is now operating at its maximum speed.
What is the Function of No-volt Release coil?
If we don’t use it, When the motor is running at its normal speed and the arm is in the ON position, the starting resistance is taken out of the circuit. Now that the machine is operating, if the power is cut or disconnected, the starting arm will stay in the ON position. When the power comes back on, there is no back EMF in the circuit, and the armature winding is connected straight across the main supply.
So, when this happens, too much current starts to run through the armature winding, which can cause damage. So, a no-volt release is used in a circuit to pull the arm back to the OFF position to avoid this problem.
The no-release coil is made up of an electromagnet attached to the shunt field winding in a series. When the power is on, the soft iron guard attracts it. But if the power goes out or is cut off, the electromagnet loses its magnetism and a soft iron guard is released. The spring’s twisting force pulls the arm back to the OFF position, which cuts off the main supply.
No-volt release also keeps shunt field winding from having an open circuit, which is another thing it does. Because the no-volt switch is linked to the shunt field winding in a series. If the shunt field wire is broken, no current goes through the no-volt release. And it stops the coil from being magnetic and moves the arm to the OFF position.
You may like to know
What is the Function of Overload Release Coil?
To avoid an overload state, a second safety device with an overload release is connected in series with the motor. Since this device is linked to the motor in series, the same current flows through the motor and the overload release.
A coil makes up the overload release. In normal situations, the amount of current going through this coil is not enough to pull the frame up. But when there is an overload, too much current will flow through the motor and the overload release coil. And this high amount of current is enough to pull the frame up.
So, when the armature is pulled up, it shorts out the no-volt release, which makes the no-volt release coil lose its magnetism. So, the soft iron keeper came off the no-volt release, and the arm was pulled back to the OFF position. With the help of the overflow release, the motor will be cut off from the power source when it is overloaded.
Three-Point Starter Drawbacks
The speed of the DC motor is controlled by controlling resistance connected in series with field winding. The speed of the motor is inversely proportional to the flux N ∝ 1/ϕ. Hence, the motor speed can be increased by decreasing flux. And the flux can be decreased by decreasing the field current. The field current is controlled by field resistance.
The field winding is linked in series with the no-volt coil in a Three-Point starter. So, the current that goes through the no-volt coil and the current that goes through the field wire are the same. For speed control, both the current going through the field and the current going through the no-volt coil are lowered.
During normal operation, the no-volt coil creates an electromagnetic pull force that pulls the soft-iron guard towards it against the force of the spring. But in this case, the current is lower, which makes the spring force stronger than the pull of the electromagnetic field. So, because of the strain of the spring, the arm is pulled back to the OFF position, cutting the power to the motor. So, a Three-Point starter can’t be used to control the speed of a DC motor in a way that changes.
A Four-point starter is the way to make up for this loss.