4 Point Starter - Construction, Working & Disadvantage

     In a recent article, we have seen the construction and working principle of a three-point starter. The disadvantage of a three-point starter is that due to the series connection of field winding with the no-volt coil (NVC). In order to regulate the speed of the motor above rated speed, the field current is reduced with the help of an external field regulator. This causes the field excitation to weakness and speed can be regulated.

     In this process due to the series connection of NVC, the current through NVC also reduces to such an extent that NVC cannot hold the starter handle in the ON position. Thereby, the handle released by the NVC disconnecting the motor from the supply when it is not desired. This problem can be conquered by connecting the NVC across the field winding which gives rise to a new arrangement of starter called 'Four-point Starter'.

Construction of Four-point Starter :

     A four-point starter protects armature by limiting heavy currents at the time of starting a dc shunt or compound motor. Most of the construction part of a four-point starter is similar to that of a 3-point starter. A little different to that of a 3-point starter, that makes a 4-point starter is by directly connecting NVC across the supply through protective resistance. The connection with this arrangement with a dc shunt motor is shown below.

4 Point Starter

     In the above figure we can see that instead of connecting NVC with the field winding, it is connected separately across the supply that makes a parallel arrangement with the field winding. This creates an additional terminal L' that connects NVC with the supply. This arrangement brings out the starter with some functional difference and four terminals compared to a 3-point starter thus called 'Four-point Starter'. The four terminals of the four-point starter are,
  • Terminal-L ( line or positive terminal ),
  • Terminal-A ( armature terminal ),
  • Terminal-F ( field terminal ),
  • Terminal-L' ( no-volt coil terminal ).

     The positive terminal is connected to the over-load release ( OLR is used for overload protection as seen in 3-point starter construction ). The other end of OLR is connected to the starter handle which is movable over series of resistors terminals ( 1, 2, 3, 4, 5 as shown in the above figure ). An arc made up of copper or brass is placed below the resistors which supply current to the field winding and no-volt coil. The arc maintains contact with the current carrying handle that moves over it.

Working of Four-point Starter :

     When the motor is connected across the supply by moving the starter handle to the starting resistor terminal. The current from the terminal L flow through the OLR ( over-load release ) and then to the starter handle. Now the current from the handle is divided into three different paths.
  • A part of the supply current flows to the armature ( terminal-A ) through series of starter resistors,
  • A part of the supply current flows through field winding ( terminal-F ) of the motor, and
  • The remaining part of the supply current flows through the NVC ( terminal-L' ).

     Here, the point to be noted that, change in field current doesn't affect the electromagnetic pull force on the handle by the NVC coil. Because the NVC circuit (  NVC current path ) is independent of the field circuit ( field current path ). Due to this independency, a protective resistance is connected in series with the NVC to regulate current flow, when the field circuit is accidentally kept open while starting the motor. Therefore, the starting current of the motor is regulated and any change in the field excitation doesn't affect the NVC coil thereby holding the starter handle in ON position throughout the operation of motor.

Disadvantage of Four-point Starter :

     The only drawback of a four-point starter is that, in the case of a three-point starter. If the field circuit gets opened, the current in the field and NVC becomes zero due to the series connection. This tends to decreases the holding force of the NVC with the handle below the spring restoring force. Thus spring pulls back the handle to the OFF position and the motor gets disconnected.

     But, due to independent connections of field and NVC circuits in a four-point starter. Under such field failure, there will be no change in the NVC and it continues holding the handle in the same position. Due to some residual flux, the motor tries to run at dangerously high speeds.

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