Solenoid switch for the proper operation of the starter motor

1. Solenoid Switch
By Dhruv Upadhaya

2. Main Function
• It switches the high current flow required by the starter motor on and off,
and
• It engages the starter drive with the ring gear.

3. Circuit Diagram

4. Pulling Winding

5. Holding Winding

6. Basic Function
• One end of the solenoid has a moving soft iron plunger, which is connected to a
lever that moves the starter drive.
• The other end has an insulated cap with electrical connections to the solenoid
windings along with a set of high current contacts and a movable copper disc that
completes the cranking circuit when the solenoid plunger is drawn forward
• Once the cranking circuit is completed, current flows from the battery to the
starter fields and armature.

7. Operating Principle
• When the starter is activated, current passes through both starter windings. The magnetic fields
from both windings work together to attract the solenoid plunger toward the main starter
terminals in the solenoid cap.
• Plunger movement also operates the shift fork lever, thus engaging the drive pinion with the
flywheel ring gear.
• The plunger contacts a switching pin, which transfers the motion through a contact spring, closing
the main solenoid terminals.
• This allows a large current to flow from the battery through the starter motor windings, causing
armature and pinion rotation, and rotation of the engine crankshaft.
• However, closing the contacts does another very important task; it shorts to power the output wire
of the pulling winding, resulting in two sequential actions.
• First, the short to power means that the pulling winding has battery voltage applied to both the
input and the output of the winding, stopping the current flow through the pulling winding and
stopping that magnetic field.
• But the holding winding still has power from the control circuit, so it continues to hold the plunger
in place while the starter cranks.

8. Operating Principle
• Once the engine starts, the control circuit is deactivated, and now is when the
second action comes into play. The current stops flowing through the control
circuit to supply the input of the holding and pulling windings, but the output of
the pulling winding is still activated due to back emf, so current flows backwards
through the pulling winding and then forward through the holding winding.
• Since the current flows in the two windings are opposite to each other, the two
magnetic fields oppose each other and tend to cancel each other out, thereby
allowing the plunger return spring to retract the plunger. This disconnects the
power from the pulling and holding windings as well as the starter motor, causing
it to stop cranking the engine. As the return spring in the solenoid returns the
plunger, it also retracts the pinion gear to its rest position.