1. Starter Motors

3. The magnetic forces within a DC electric motor
D.C. motors convert electrical energy into mechanical energy.
Let's see how the movement is produced. Between two pole shoes (permanent or electro-magnet) a magnetic field is created, with the lines of flux going from N to S. Also, When a current passes through a conductor, the lines of magnetic flux run around the conductor itself.
Two lines of magnetic flux will interact with each other. If we now move the conductor and place it within the pole shoe flux the two fields will interact.
With reference to Fig. 36, if the current is passed through the conductor flowing towards you, this produces a direction of flux as indicated by the arrows that will cause the conductor to move as indicated.
Magnetic lines of flux can be likened to taught elastic bands. If they're stretched they "pull back" to regain their original form. These lines of pole flux (Fig. 37) are doing exactly that, and if allowed, will push the conductor up out of their path.
If we introduce a second conductor, we will see that the lines of flux push A upwards and B downwards, causing the central shift to rotate.
That's how the basic DC electric motor works.
On some tractors, the starter solenoid is separate from the starter. Turning the key-start switch sends current to the solenoid windings. This moves the iron core and the electrical contacts attached to it. The contacts complete the circuit from the battery to the starter motor field coils.
The magnetic forces within a DC electric motor

4. 1. Rotor 2. Stator 3. Collector
DIRECT CURRENT ELECTRIC MOTOR
The electric motor is a machine that transforms electric energy into mechanical energy.
The dc motor (Fig. 52) has a fixed part that produces a magnetic field this is called the stator. The part free to rotate (item 1) is made up of windings placed in the slots of a cylinder of ferromagnetic material and is called the rotor.
When the windings of the rotor are crossed by current, the rotor turns. The rotation speed is a function of the voltage applied on the windings and of the load on the motor. The rotation direction of the rotor depends on the direction of the current that crosses the windings and therefore on the polarity of the voltage applied.
DC Electric Motor
1. Rotor Stator 3. Collector

5. Front housing
Brush pack
Solenoid
Commutator
Armature (rotor)
Field magnet housing
Output unit
End pack

6. The armature
The armature consists of an iron core with separate loops or windings around it longitudinally each end of each winding is connected to a separate segment of the commutator. The commutator is used to switch the current flow through the winding when the point where the loop is perpendicular to the magnetic field I.E. the north on the armature is attracted to the south on the body. If the current is switched the armature will carry on rotating in the same direction. Several loops are used to give a smooth uniform rotation and constant power.

7. Brush pack
The Brush pack supplies current to the commutator. In a large motor such as a starter motor there are four brushes working in pairs. The use of four brushes gives optimum current transfer and so optimum torque.
From the brush pack current is also supplied to the pole shoe windings or excitation windings. These increase the magnetism in the pole shoes and so increase the power of the motor. This can be wired in several ways depending on the application of the motor.
Shunt wound - the excitation winding is in parallel with the armature windings. Not really useful for a starter motor.
Series wound - the excitation winding is wired before the armature windings. Used for small starter motors.
Combined shunt and series wound - uses a combination of both and is used for large starter motors.

8. The starter solenoid does three things.
It starts to turn the motor slowly
It engages and disengages the drive pinion.
It supplies full power to the motor

9. STARTING CIRCUIT STARTING CIRCUIT
A main cable connects the battery to the starter solenoid. When the solenoid is energised, current flows from the switch to the starter motor (Fig. 34). It flows through four field coils, reaching the armature by means of two insulated brushes, and proceeds through the armature coils to earth by means of two earth brushes.
Passing through the field and armature coils, the current generates magnetic forces which oppose each other, causing the armature (motor shaft) to rotate.
The starter solenoid has two coils:
The hold on coil, connected directly to earth;
The closing coil, connected to earth via the starter motor windings.
With reference to Fig. 35, when the starter switch is on, current passes through the hold on coil and creates a magnetic field around the plunger. Current also passes through the closing coil, creating a second magnetic field, and proceeds into the starter motor, causing the motor to rotate slowly. The combined magnetic field created by the coils causes the plunger to move through the solenoid core. As the plunger is connected to the operating lever and pinion, it causes the pinion to mesh with the engine flywheel.
When the plunger moves its full stroke the solenoid contacts are closed allowing full power to the starter motor.
A starter motor must have a sufficient mass to dissipate the heat. As the heat is a form of dissipated energy, which is proportional to Rl2 (see Joule's law), and the higher the temperature is, the lower the current, we can only have heat dissipating problems in low temperature countries.

10. Intermediate transmissions are fitted into a starter motor so that a smaller motor can be used. Through the epiciclic gearing greater torque can be produced. Not all starter motors are fitted with an intermediate transmission in this case drive is taken straight through to the drive pinion.

11. Before testing a starting problem, consider the following points.
Is the engine seized?
Is the battery ok and fully charged?
Are the safety start mechanisms working?
Visually check the starting circuit for frayed or broken wires or loose terminal connections.
STARTER SYSTEM TESTING
Problem: Engine does not turn over when key start is operated and all interlocks are in neutral.
For easier and rapid diagnosis and for most conclusive test results, it is recommended that a battery–starter tester (high rate discharge tester) incorporating a 0–20 volt voltmeter and a 0–1000 amp ammeter be used to diagnose starting system problems.
When using test equipment follow the manufacturers recommended test procedures. If test equipment is not available the “Starter Motor Circuit Current Draw” test should be performed, using a standard 0–20 volt voltmeter and 0–1000 amp ammeter. This test will prove/disprove the correct operation of the starter without removing it from the engine.
Before testing:
Check that the battery is fully charged.
Check the complete starting system wiring circuit for frayed or broken wires or loose terminal connections.

12. Current draw test STARTER MOTOR CIRCUIT CURRENT DRAW
Procedure (reference Fig. 4):
.Attach a suitable clamp meter, (1), with a 0–1000amp range, over the battery positive cable.
Connect the voltmeter (2) positive lead to the battery positive terminal and the voltmeter negative lead to the battery negative terminal.
Disconnect the wire from the fuel injection pump shut off solenoid (if fitted).
.Crank the engine while observing the voltmeter and ammeter readings. The voltage and current draw recorded will vary depending on what make or model of starter is fitted to the machine.
Results:
If the current draw is within specification the starting motor (4) is functioning correctly. If the voltage drops during the test proceed to ‘Starting System Circuit Resistance’.
If the current draw is greater than specified, check the circuit as outlined below. If the starting system circuit tests are satisfactory the starting motor is defective and must be disassembled to determine the cause.
If the current draw is less than specified, the starting motor is defective and must be disassembled to determine the cause.
Current draw test

13. Resistance testing the battery positive lead
STARTING CIRCUIT RESISTANCE TESTING (VOLTAGE DROP)
If there is an excessive current draw the circuit should be checked by recording voltage drops across the individual components in the circuit.
Battery Positive Cable:
.Connect the voltmeter positive lead to the battery positive terminal.
.Connect the voltmeter negative lead to the starting motor solenoid battery terminal.
.Crank the engine while observing the voltmeter reading. If the voltage exceeds 0.2 volts, check and tighten the cable connections. Recheck the voltage; if still excessive install a new cable.
Resistance testing the battery positive lead

14. Resistance testing the battery ground cable
Resistance testing the starter motor ground
Starting Motor Ground Connections:
.Connect the voltmeter positive lead to the starting motor frame.
.Connect the voltmeter negative lead to the engine block.
.Crank the engine while observing the voltmeter reading. If the voltmeter reading exceeds 0.2 volts check the ground connections between the starting motor flange and the rear engine plate.
Battery Ground Cable:
.Connect the voltmeter positive lead to the engine block.
.Connect the voltmeter negative lead to the battery negative terminal.
.Crank the engine while observing the voltmeter reading. If the reading exceeds 0.2 volts, check and tighten the ground cable connections. Recheck the voltage; if it is still excessive install a new ground cable.
Resistance testing the battery ground cable