DC motor starter
When a motor is in steady state, no Back EMF is produced and the armature acts as a low resistance circuit.
If the motor is connected directly to the mains supply, then the armature conductors take more current than the mains supply, which may result in the following.
(i) Insulation of armature winding can be destroyed.
(ii) More sparking or sparking will occur on the commutator.
(iii) The supply voltage may decline.
(iv) A very high starting torque will be generated, which will increase the speed of the motor suddenly and can cause damage to the rotating parts of the motor and the load on the motor.
Therefore, a starter is used to limit the starting current, which reduces the voltage used at the time of starting the motor and sends it to the armature.
The simple form of a starter is a variable resistor, which is added to the series of armature while driving the motor.
As the motor speed increases and the back EMF is induced, the resistance is reduced by the armature circuit.
When the motor attains its normal speed, the resistance is completely separated from the armature circuit.
What is NVC and OLC
Before knowing about the starter of the motor we need to know about the following. The structure and function of the no-volt coil and Over Load Coil used in the starter are described below.
(i) No-volt coil
This coil (NVC) coils on Lamination. In which so many turns of thin electrified wire are given so that the full current of the motor area can flow easily through it.
Voltage less coil (NVC) is added to the series of field point Z. This coil has two main functions.
(A) Holding the starting arm on the ON stud at full speed position of the motor by magnetic force.
(B) If the supply voltage suddenly stops, then leave the starting arm electromagnetically, so that it goes to the OFF position and the motor can be safely started when the power supply comes again.
(ii) Over load coil
This coil is made up of very few turns of thick insulating wires and is connected in series of starting arm and line.
Its main function is to mix the two ends of a voltage less coil (NVC) when the current is flowing in the circuit, creating a magnetic field and raising the lever.
Whereby the voltage less coil (NVC) is non-magnetic and leaves the starting arm and the starting arm returns to OFF state with spring effect.
Thus, the overload coil (OLC) provides protection against the potential danger of over-current flow in the motor armature circuit due to a fault, with the help of a voltage less coil (NVC).
The figure shows the three point starter combined with a direct current shunt motor.
The following starter is mainly used for direct current motors.
Two types of starter are used for shunt and compound motors.
(i) three point starter
(ii) four point starter
Three point starter
The above diagram shows the electrical diagram of three point starter. This starter consists of three terminals. On which the letters L, Z and A are inscribed.
The positive end of the line is connected to L and the negative end of the line is given at a point made by mixing one end A2 of the motor armature and one end of the field Z2 together. As shown in Fig .
The second end of the motor armature A1 is connected to the point A of the starter. And the other end of the motor area is connected to the point Z of the Z1 starter.
The starter's point L is connected to the starting arm by the Over Load Coil of the starter. The portion holding the starting arm is made of insulated material.
When the motor is started, turn on the main switch and slowly rotate one starting arm from left to right.
The field is connected in direct parallel to the circuit line as long as the arm is in contact with stud number 1. And at the same time the total starting resistance is in the series of Rs armature.
The starting current taken by the armature where Rs is the starting resistance and Ra armature resistance.
Now the starting resistance decreases as soon as the starting arm is moved forward.
Now the starting arm slowly reaches the on stud cutting the resistance. That is, the motor achieves its full speed at this time. The starting resistance is then completely cut off.
The Starting Arm moves to the forward and on studs touching the various studs and is controlled by a spring.
So that when the supply is stopped suddenly or any other fault, the starting arm comes to the off condition with the help of spring.
The starting arm of the condition does not fall behind, for this a rubber tech is attached. The starting arm has a soft iron piece.
Which is attracted by the electromagnet of the no-volt coil in the ON position.
The electromagnet of the No Volt Coil (NVC) is energized by the motor's shunt field current. This coil is also called hold-on coil or low voltage coil.
It has been observed that as the starting arm moves from stud no. 1 to the end stud, the field current has to flow backwards through the starting resistance part cut from the armature circuit.
Due to which the shunt current decreases slightly, but the starting resistance (R) value is much less than the shunt field resistance. Therefore a slight decrease in the shunt current ish is negligible.
This deficiency can be overcome by placing a brass arc in the starter. As shown in Fig .
When the starting arm comes to stud no. 1. The brass arc is then attached directly to the starting arm. Whereby the field circuit is completed by brass arc.
And thus the shunt current does not flow through the starting resistance (Rs).
Four Point Starter
It is used for starter shunt and compound motors where the speed of the motor is to be controlled by the starter itself.
This starter has four points. There are 3 points L, Z and A and a fourth point N in which a negative chord of the supply line is given as shown in the figure.
In the four point starter, the field coil of the NVC is not added to the range of the motor, but the starting resistance in the side of the direct current supply is in the range of Rs.
The no-volt coil is directly on the side of the main supply to protect the contacts of the OC when there is a short supply.
For this, a protective resistance R is also placed in the range of no-volt coil as shown in the figure.
This means that when the NC is collapsed at the time of lifting the lever by the OLC coil, the destructive current will stop and the coil should be safe.
Here also, the work of a voltage less coil is the same as a three point starter.
There is surcharge horoscope O.C. There is also an over load coil which shortens the voltage less coil.
Whereas the stream rises to a specified value. Usually four point starter is fitted for speed control with an additional field weakening resistance.
Series Motor Starter
The starter for the range motor also has a suitable variable resistance that is given in the range of armature circuit when driving the motor.
Resistance is cut gradually, as the motor acquires speed.
These starters also have a no-volt coil or no-load coil.
In the diagram, internal connections of the starter are shown by a no-volt coil.
Whereas in Figure b, the combination is shown by a no-load coil. In Fig. A, the voltage less coil is added to the range of safe resistance (R).
In this situation, if the supply is stopped from the rear due to any reason, the hand of the starter goes back with the help of spring itself. In this case, the protection is done by a voltage less coil.
Because the direct current range motor rotates at very high speed at low load, the weightless coil of the starter is added to the range of armature and field circuit, as shown in Figure b. .
In this coil, less turns of the insulating wire are given and this coil also takes the same current as the motor.
If at any time the load on the motor is less then at that time there will be less current flowing in the weightless coil and at that time the handil will not be in a position to hold and the handil will automatically go to the off position and the motor will be safe.
Therefore, this type of starter is also called weightless (with no - load release). Often, the starter of a class motor is used to control the speed of the motor.