3 point starter
The term Starter is usually used for any device which helps in initiating any action of some other devices under its own power.Starters can be of different types, i.e. anything which initiates like motor starter, which is used for driving or initiating the operation of the motor under its own power. Now we are going to discuss about the very useful and popularly used ‘Motor starter‘. When the motor is at rest, we all know that the back e.m.f is zero and now if we provide full supply voltage across the armature of the motor, then it will draw an excessive large amount of current because of very low armature resistance. This excessive current will blow out the fuses and prior to that it may damage the respective winding, commutator and brushes etc.
Now we are going to discuss about the constructional details of this 3 point starter, how this each and every parts are interconnected and also the sub-elements or subparts used in the construction of this 3 point starter. With respect to the figure, we can easily conclude that the point ‘L’ is connected with an electromagnet, which is called Overload Release (OLR) at its one end and the other end is connected to the lower end of starter handle where a spring is also attached with it and the starter handle contains also a soft iron piece housed on it. This handle is free to move from its other side against the force of the spring. This spring brings back the handle to the OFF position by the influence of its force. Then we see a parallel path is made from the stud ’1′, given to the another electromagnet called No Volt Coil (NVC), which is further connected to terminal ‘F’. The OLR and NVC are the two protecting devices of the starter, with the starting resistance is entirely in series with the armature.
The term Starter is usually used for any device which helps in initiating any action of some other devices under its own power.Starters can be of different types, i.e. anything which initiates like motor starter, which is used for driving or initiating the operation of the motor under its own power. Now we are going to discuss about the very useful and popularly used ‘Motor starter‘. When the motor is at rest, we all know that the back e.m.f is zero and now if we provide full supply voltage across the armature of the motor, then it will draw an excessive large amount of current because of very low armature resistance. This excessive current will blow out the fuses and prior to that it may damage the respective winding, commutator and brushes etc.
So to avoid this type of incident and to check the excessive flow of current we provide a Starter which is actually a resistance, connected in series with the armature to limit the starting current to a certain safer range. Now at the same time we get to see that the starter allows a high amount of current to produce a good starting torque. The starting resistance is now gradually decreased and cut out as the motor picks up speed and thus develops the back emf. which is now used for the speed regulation. Now in this context, we are going to deal with a special type of motor starter i.e. 3 Point Starter, which is similar device which helps in initiating the rotation of other device i.e. Compound wound and Shunt wound D.C. motor, under its own power. This type of D.C. motor requires a starter initially for its excitation or starting without which they cannot gain the starting torque.
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Necessity Of 3 Point Starter
In this contrast we can also discuss about the question which arises, why we use 3 point starter in a d.c. shunt motor? This is because the utmost keyterm which is hidden behind the control of motor operation is Back EMF. Eb . This Eb , develop as the motor armature starts to rotate in presence of magnetic field, by generating action and opposing the supply voltage.
Thus we come to know that the back e.m.f Eb is zero initially and increases with motor speed and now if we provide full supply voltage across the armature of the motor, then it will draw an excessive large amount of current because of very low armature resistance.
Thus we come to know that the back e.m.f Eb is zero initially and increases with motor speed and now if we provide full supply voltage across the armature of the motor, then it will draw an excessive large amount of current because of very low armature resistance.
This is how, we know, the general motor emf equation is E= Ia.Ra + Eb ;
but as initially the motor is rest so, Eb=0,
So, E=Ia . Ra
∴ Ia = E / Ra
but as initially the motor is rest so, Eb=0,
So, E=Ia . Ra
∴ Ia = E / Ra
so, from the above expression the Ia is excessive in amount as the denominator Ra is very low.
Now this excessive current, Ia will blow out the fuses and prior to that it may damage the respective winding, commutator and brushes etc. So to avoid this type of incident and to check the excessive flow of current we provide a 3 point starter which is actually a resistance Rst, connected in series with the armature to limit the starting current to a certain safer range. Now at the same time we get to see that the 3 point starter allows a high amount of current to produce a good starting torque. The starting resistance The starting resistance Rst,is now gradually decreased and cut out as the motor picks up speed and thus develops the back emf. which is now used for the speed regulation. Hence in this way the Compound wound and Shunt wound D.C. motor starts and continue perform its action under safe condition.
Constructional Details of 3-point starter
A 3 point starter is mainly containing a variable resistance Rst, connected in series with the armature to limit the starting current to a certain safer range. The starter is subdivided in many variable resistance and this section of resistance contains the contact points called Studs, which are calibrated seperately from OFF, 1, 2, 3, 4, 5 …. ,RUN. The main reason behind the name of 3 point starter is the 3 main points of the starter which are as follows :
- Line Terminal ‘L’, which is to be connected to the positive of supply.
- Field Terminal ‘F’, which is to be connected to the field winding.
- Armature Terminal ‘A’, which is to be connected to the armature winding.
Now we are going to discuss about the constructional details of this 3 point starter, how this each and every parts are interconnected and also the sub-elements or subparts used in the construction of this 3 point starter. With respect to the figure, we can easily conclude that the point ‘L’ is connected with an electromagnet, which is called Overload Release (OLR) at its one end and the other end is connected to the lower end of starter handle where a spring is also attached with it and the starter handle contains also a soft iron piece housed on it. This handle is free to move from its other side against the force of the spring. This spring brings back the handle to the OFF position by the influence of its force. Then we see a parallel path is made from the stud ’1′, given to the another electromagnet called No Volt Coil (NVC), which is further connected to terminal ‘F’. The OLR and NVC are the two protecting devices of the starter, with the starting resistance is entirely in series with the armature.Working principle of 3-point starter
As we provide the d.c. supply to the motor, to make it switch on, the handle starts moving against the spring force to make a contact with stud No. 1, which was initially at OFF position. Thus field winding gets supply through the parallel path provided to starting resistance with the help of NVC. This is how we see the entire starting resistance comes in series with the armature and armature current which is high initially, gets checked to certain limit. Similar way the handle starts moving towards the other studs 2, 3, 4 etc., thus reducing the starting resistance gradually from the armature circuit and reaches the RUN position making the starting resistance Rst equal to zero and the motor keeps on rotating at its normal speed.
On further discussion a confusion came out while consulting about the movement of the handle is done manually, and the obvious question is how handle will remain in the ‘RUN’ position till motor is running? On this perspective we
get the clean idea about the use of NVC.
On further discussion a confusion came out while consulting about the movement of the handle is done manually, and the obvious question is how handle will remain in the ‘RUN’ position till motor is running? On this perspective we
get the clean idea about the use of NVC.
- Operational detail of NVC of the 3-point starter :
As earlier we have noticed that field winding gets supply through the parallel path provided to starting resistance with the help of NVC. Thus the NVC gets magnetised by the flow of the field current. When the handle is in the ‘RUN’ position, soft iron piece connected to the handle gets attracted by the magnetic force produced by NVC. Thus the design of NVC is such that it holds the handle in ‘RUN’ position against the force of the spring as long as supply to the motor is proper. NVC holds the handle in the ‘RUN’ position and hence its called hold on coil. But when there is stop or gap in the field current, the current through NVC gets affected and It looses its magnetism and hence not in a state to keep the soft iron piece on the handle, attracted. Thus due to the spring force, handle comes back to OFF position, switching off the motor. This way we see that with the help of spring and NVC, the starter handle comes to OFF position when there is a supply problem. This prevents the damage of the motor caused due to accidental starting, by making the entire starting resistance in series with the armature when attempt is made to start the motor everytime.
- Operational detail of Overload release of the 3-point starter :
Under overload condition, when high current is drawn by the motor from the supply which passes through OLR, which is an electromagnet, below which there is an arm that is fixed at its fulcrum and normally resting in horizontal position. Under overloading case, high current through OLR produces enough force of attraction to attract the arm upwards. Normally magnet is so designed that up to a full load value of current, the force of attraction produced is just enough to balance the gravitational force of the arm and hence not lifting it up. At the end of this arm, there is a triangular iron piece fitted. When the arm is pulled upwards the triangular piece touches the two points which are connected to the two ends of NVC. This shorts the NVC and voltage across NVC becomes zero due to which NVC looses its magnetism. So under the spring force, handle comes back to the OFF position, disconnecting the motor from the supply. Thus motor gets saved from the overload conditions.
What is the main disadvantage of 3-point starter ?
In the 3-point starter, the NVC and the field winding are in series. So at the time of controlling the speed of the motor above rated, field current is reduced by adding an extra resistance in series with the field winding. Due to this, the current through NVC also reduced, which further reduces the magnetism, which may release the handle from its RUN position to OFF position, switching off the motor. Thus to avoid this, there is a small change in four point starter is made, in which NVC and the field winding are connected parallel
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