Dc motor starters- two point, three point, four point starters

Hello guys welcome to my blog. Hope you are all good and well.

Today we will discuss about, 

  • Why starters used in DC motors?
  • Function of starters in DC motor
  • Types of starters of DC motor- 
  1. Two point starter
  2. Three point starter
  3. Four point starter
  • Difference between three point and four point starter


Let's go to understand,

Why starters used in DC motors: -

  • Let assume a DC shunt motor connected to DC supply as shown in fig. Below.
  • When the switch turn ON, the current will flow through the armature as well as the field winding which will rotate the armature in the magnetic field (according to the DC motor working principle). 
  • When the armature rotates in the magnetic field it will produce an emf due to generator action and called back emf Eb. 

NOTE: - 
Generator action means when a conductor rotates in the constant magnetic field an emf will induce in it. 

Now, if we apply KVL to loop 1 as shown in fig. Below then we will get an equation and that will called voltage equation of the DC motor.

V = Eb + IaRa 

Where, 
V = applied voltage 
Eb = back emf
Ia = armature current 
Ra = armature resistance. 


From above equation, we can calculate

Ia = V - Eb / Ra

Where Eb is directly proportional to the speed of the motor. 

And at the time of starting, the speed of the motor is 0 (zero) means the back emf will also 0 at the time of starting. 

So, 
Ia = V / Ra

Where the voltage is high and resistance is very low due to which a high starting current will flow through the motor which will damage the motor. 

To understand this high inrush starting current assume a motor operating on 220V having the resistance of 0.5 Ohms then the current will be, 

Ia = 220 / 0.5
     = 440 amperes, which may be 10 times more than the rated current of the motor and damage it easily. 

So to avoid this damage we connected an external resistance to the DC motor at the time of starting and remove it when the motor runs at its full speed. And this external resistance is known as a starter. 

So after connecting the external resistance the current will given by: -

Ia = V / Ra + Rs

Where, 
Rs = starter resistance which will decrease with increase in speed and back emf. 

Therefore the starters are used in DC motors. 

Let's go to understand more function of starters. 

Function of starters in DC motor: -

  • As discussed above, the DC motor starters are mainly used to limit the high inrush current at the time of starting. 
  • Used to stop and start the motor.
  • The starters are provided with the overload coil which protect the motor from overloading.  

Types of starters used in DC motors: -

There are mainly three types of starters used for the different types of DC motor as given below: -
  1. Two points starter
  2. Three point starter
  3. Four point starter


1. Two point starter: -

  • The two point starter is used for DC series motor. 
  • As its name suggests it has two points name as L (line) and A (armature). 
  • The point L is connected to the positive terminal of the DC supply, whereas the point A is connected to the armature winding of the DC motor as shown in fig. Below.
  • There are number of resistance connected in series with the armature winding to increase the resistance at the time of starting. 
  • A handle is also provided to decrease the resistance gradually with increase in speed. 
  • The no volt coil is provided to hold the handle in running condition. 
Note: -
The no volt coil is nothing but just an electromagnet which attract the handle towards it until the current flow through it. If the current discharged from the coil then the handle will come to its original position and motor will stop. 

  • The overload coil is also provided which is similar to the no volt coil, but the function is different. 
  • The overload coil will attract the metal contact towards it and de energized (short circuit) the no volt coil when motor takes over current. 
  • When the no volt coil is de enerzised the handle comes to its original positions and stop the motor. 

NOTE: -
In normal condition the attractive force of overload coil is not enough to attract the contact therefore it does not attract the contact in normal condition. 

Operating of two point starter: -

  • At the time of starting the handle is on the maximum value of resistance and slide gradually to reduce the value of the armature resistance. 
  • It reaches at maximum when motor picks up its full speed. 
  • The no volt coil attracts the handle and hold it so that it remains at the maximum value of resistance. 
  • The no volt coil release the handle when there is no current flow through the coil and in this situation the handle comes back to its original position so that it offers full resistance when motor starts again. 
  • In overload condition the overload relay operates and protect the motor.

2. Three point starter: -

  • Three point starter is used to protect the DC shunt motor or DC compound motor. 
  • As the name suggests it has mainly three point name as L (line), A (armature), and F (field) as shown in fig. Below.
  • The point L is connected to the positive terminal of the DC supply, A is connected to the armature winding of the DC motor and F is connected to the field winding of the DC motor. 
  • It has also a no volt coil which will attract the handle towards it and hold it in running condition. 
  • The overload coil is also provided to protect the motor from overloading. 
Z is F in fig. 

Operation of three point starter: -

  • The working of three point starter is simple to understand. When the switch S is turned ON the armature and field gets enerzised. 
  • The resistance of the starter reduces to zero gradually with the help of the handle. 
  • The no volt relay is connected in series with the field winding and hold the handle in running condition. 
  • The overload coil attracts the contact when over current flow through the motor which will de enerzised the no volt relay and stop the motor. 

3. Four point starter: -

  • The four point starter is also used for DC shunt and compound motor. 
  • As the name suggests it has four points name as L (line), F (field), A (armature) and N. 
  • The point L is connected to the positive terminal of DC supply, F connected to the field winding of the motor, A connected to the armature of a DC motor and N connected to the no volt coil. 
  • The construction of four point starter is similar to three point starter except that the four point starter has a fourth point which connects the no volt coil directly with the line through a current limiting resistor. 
  • In this case, there is no effect of field current in the no volt coil. It holds the handle by sufficient force until the current flow through it. And prevent the returning of the handle to the OFF position. 

Operation of four point starter: -

  • The operation of four point starter is similar to three point starter. 
  • The resistance of the armature circuit is decreases gradually up to zero with the help of the handle. 
  • The no volt coil holds the handle to its running position until the current flow through it. 
  • The overload coil operates when over current flow through the motor and stop the motor by attracting the contact towards it. 

Difference between three point starter and four point starter: -

  • There are three points in three point starter and four points in four point starter. 
  • The three point starters have three points L, F and A whereas the four point starters has four points L, F, A and N. 
  • In three point starters the no volt coil is connected in series with the filed winding whereas in four point starter the no volt coil is connected in parallel with the field winding. 

Conclusion: -

The DC motors take high inrush current at the time of starting which may be equal of 10 times of rated current of the motor and can damage it. So to avoid the damage we use starters with the DC motors. There are three types of starters used in DC motors known as two point, three point and four point starters. Each starter is use for the different type of DC motor. Like two point starter is used for DC series motor, three point starter and four point starters are used for DC shunt and DC compound motors.

If you have any doubt then tell me in a comment. 

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Thank you. 

Hope article helps you. 


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