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Starter Motor: Diagram, Parts, Working, Types & Uses [PDF]

Do you know that an engine and its parts won’t run without a starter motor? You should be aware, then. It is a major component in an engine that uses electrical power to start.

Since an engine cannot rotate independently, a starter motor is required to start the first cycle. It assists in starting the engines, which continue to run on their power.

Today, we’ll discuss the starting motor, its function, parts, diagram, types, and working principle, along with its bad symptoms and their possible solutions.

What is Starter Motor?

Definition of Starter Motor

A starter motor or starting motor, or cranking motor, is a direct current motor that cranks the engine for starting. Cranking the engine means rotating the crankshaft by applying torque on it so that the piston may get reciprocating motion.

The starting motor is mounted on the engine flywheel housing. It is a series wound and is made to run at low voltages with high currents. It must be able to exert a very high torque when starting and at low speeds.

The armature and fields are constructed of thick wire to reduce resistance and allow them to carry large currents without overheating. The faster it runs, the less current it draws. The slower it runs, the more torque it develops.

A motor used in passenger cars draws about 60 amperes when running at no load and about 600 amperes when cracking the engine slowly. This supply is only required for a short period of time.

On passenger cars, the starting motor voltage is typically 12 volts. Compression ignition engines may use 12 volts on starting motor to provide the power to rotate the crankshaft, especially in cold conditions. The torque produced is 1 to 2 kg/m.

Below, we’ll talk about its components and how they work.

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Parts of Starter Motor

The following are the parts of a starter motor that are required for its operation:

  1. Armature
  2. Commutator
  3. Solenoid
  4. Brushes
  5. Plunger
  6. Lever fork
  7. Pinion
  8. Field coils

#1 Armature

It is an electromagnet component used as a guide on the driveshaft or bearings. The armature contains a laminated soft iron core that has been wound or looped with numerous conductors.

#2 Commutator

It is the portion of the shaft that brushes run along to conduct electricity that is located at the back of the housing. The commutator is built from two plates mounted to the armature’s axle and connects the electromagnet’s coil.

#3 Solenoid

The solenoid has two wire coils wrapped around the core. This solenoid acts as a switch, connecting and disconnecting the electrical connection between the starter motor and the vehicle’s battery.

#4 Brushes

A series of brushes are used to transfer electrical current from a static conductor to a dynamic conductor. The brushes make contact with the commutator. Brushes conduct electricity as they rub the commutator.

#5 Plunger

In a starter motor, a plunger has the function of pushing forward so that the pinion can be engaged, which in turn starts the motor.

#6 Lever Fork

A lever fork is attached to a plunger, and when the lever fork is pushed forward together with the plunger, the pinion is engaged.

#7 Pinion

A pinion is a device that has a gear and springs inside. It engages immediately after starting the engine by extending the gear to the flywheel teeth.

#8 Field Coils

The field coils consist of two or more coils connected in series and are secured in the housing by screws. The battery provides power to these coils, turning them into an electromagnet that rotates the armature. As a result, a magnetic field develops around the armature.

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Starter Motor Construction

The construction of the starting motor is similar to that of the generator, but the winding brush terminals are heavier to deal with heavy currents. The brushes are made of low-resistance material, such as copper, instead of carbon, as in the generator case.

The main parts of the starting motor are the casing, armature, commutator, field winding brushes, poles, and terminals. The motor starts the engine using a drive mechanism that is located at the end of the armature shaft.

The starting motor use either two-field windings or four-field windings. Above figure shows a motor using two field windings. The current from the battery divides when it enters the motors, each branch leading to the separate field winding.

Current is led to the armature’s commutator from the fields through the two insulated brushes. The current in the armature simultaneously creates four poles adjacent to the four field poles to produce the attractive and repulsive forces that turn the armature.

The armature current is returned to the battery via the two grounded brushes. Above figure shows a starting motor with four field windings. It is used in large engines to develop more torque. It functions in a similar way to the two-winding type.

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Working of Starter Motor

The operation of a starter is simple, but most people are unaware of the secret behind motor cranking. When the ignition key or bottom is depressed, the transmission should be in the park or neutral position. In this instance, the battery voltage activates the solenoid by passing through the starter control circuit.

The solenoid powers the starter motor. This helps to move the starter gear forward so that it meshes with the engine flywheel. Since the flywheel is mounted on the engine’s crankshaft, when the starter motor turns, the flywheel also rotates with the crankshaft. When the engine starts, the system disengages from the flywheel.

Note: If your vehicle has an automatic transmission, you can only operate the starter motor in park or neutral. If you are driving a manual transmission, you must depress the clutch pedal.

Four field windings are mounted inside the starter motor and are internally connected to the housing. The field coils, and the armature (rotating parts) are connected in series by carbon brushes.

But remember that some starters use magnet fields instead of field coils. A small gear is connected to the armature’s front by an overrunning clutch.

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Types of Starter Motors

The following are the main types of starter motors:

  1. Direct drive starter motor (DD)
  2. Planetary gear (PLGR)
  3. Permanent magnet gear reduction (PMGR)
  4. Permanent magnet direct drive (PMDD)
  5. Off-set gear reduction (OSGR)
  6. Inertial starter

#1 Direct Drive Starter Motor (DD)

The most prevalent and traditional starter motors are direct drive. It has a variety of uses and unique construction, but it is a solenoid-operated unit. Its operation is still simple compared to other types.

When the ignition switch or key is turned on, the vehicle’s battery powers the solenoid. This moves the lever that controls the pinion gear by pushing the plunger. The engine flywheel then meshes with the pinion gear. Therefore, as the starter motor turns, the flywheel also turns, starting the engine’s combustion.

#2 Planetary Gear (PLGR)

These starter motor types have significantly eliminated the direct-drive types. Power is transferred between the armature and the pinion shaft by a permanent magnet. The armature spins faster and with more torque.

Planetary gear’s main function is to reduce gear, which further lowers the demand for a high current. At the end of the armature, there is a sun gear, and three plenary carrier gears are housed inside the stationary ring gear. Planetary gears can achieve great gear reductions because the ring gear holds the sun gear and outputs the carrier.

#3 Permanent Magnet Gear Reduction (PMGR)

The permanent magnet gear reduction is made to be lighter, simpler to construct, and less heat-generating. Instead of field coil starters, it has four to six magnet field assemblies.

It has three terminals on the 12V solenoid and is heavy-duty, requiring less current. Since there are no field coils, the current is sent from the commutator and brushes directly to the armature.

#4 Permanent Magnet Direct Drive (PMDD)

PMDD starter motors are similar to direct-drive starter motors in several ways. In the case of permanent direct magnet drive types, the field coil is swapped out for permanent magnets.

#5 Off-Set Gear Reduction (OSGR)

These starter motor types work at high speeds at low currents. They are simpler to assemble because they are lighter and more compact. Four-wheel-drive vehicles frequently use off-set gear starters because they boost cranking torque.

#6 Inertial Starter

An inertial starter is an electric type that can perform all the functions of all starters. When cranking, it performs flawlessly and even guarantees the safety of the motor part.

It is the best choice for breaking speed because it starts the engine quickly and powerfully. The starter’s weight minimally relates to its ability to generate torque.

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How Can You Test Starting Motor System?

If the starter motor isn’t working, you should check the battery’s charge level, terminals, and cables. When the key is turned to the START position, one of the signs of a weak battery is that the dash lights dim.

Testing the starter control circuit is usually the next step. When the key is in the START position, your trusted mechanic may begin by checking the battery voltage at the starter solenoid control terminal.

If there is no voltage, there is a problem with the starter control circuit, which includes the neutral safety switch, starter relay, ignition switch, and control wire.

If there is a battery voltage at the starter solenoid control terminal, but the starter motor does not work, then the starter motor is bad. It’s also important to check that the starter solenoid control terminal is connected properly.

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Bad Symptoms of Starter Motor

A starter motor inside your car is responsible for ensuring a smooth start, but it can be a nightmare if it starts to fail. It exhibits failure signs and symptoms like other important parts of your car.

Here are 5 typical symptoms of a failing starter motor so you can atone before it’s too late.

  1. Engine won’t start
  2. Starter cranks, but the engine is unpowered
  3. Grinding noises
  4. Smell or smoke while starting
  5. when you start, lights get dim
  6. Whirring noise

#1 Engine Won’t Start

When you turn up your key, and nothing happens, that is the typical sign of a failing starter motor. A burned-out or fused starter solenoid or motor is a common source of this problem.

In addition, a dead battery may be the reason for this. However, if nothing happens when you try to start the ignition, regardless of the possible cause, be sure to have your car thoroughly inspected.

#2 Starter Cranks, But the Engine is Unpowered

You may have noticed that many times when you turn on the ignition, the starter will start to turn, but the engine won’t start. In these conditions, the problem may have a mechanical basis because the flywheel’s gears may be acting strangely.

This indicates that the gear has either been stripped or has come into contact with the flywheel. Whatever the situation, a replacement for the starter is required as soon as possible.

#3 Grinding Noises

This problem can also arise from defective or worn starter gears that connect the starter to the flywheel. On the other hand, the starter motor itself can make a grinding sound. The issue cannot be resolved in either scenario.

The best action plan will be to address this symptom if you notice it because a grinding starter may damage the flywheel, which would be even more costly to repair if it is not changed.

#4 Smell or Smoke While Starting

The starter is powered by electricity. Consequently, the starter sometimes overheats due to excessive and continuous power supply, or the motor will not shut off even after the engine runs.

If this happens, it is typical to smell or see smoke coming from the engine compartment. This might be the result of an ignition switch issue or even a blown fuse.

#5 When You Start, Lights Get Dim

When you start your car, you may notice that the interior or dashboard lights start to dim. This is often an indication that the starter has failed. This occurs when the starter motor draws excess power due to a short circuit in its internal wiring, which drains other systems, such as the lights.

Even though problems with the starter motor are nearly impossible to avoid, it is better to replace a failing unit before it becomes a bigger problem.

#6 Whirring Noise

As the engine cranks, a whirring sound is made when the starter pinion gear slips. This results from the pinion gear’s improper flywheel engagement, preventing the engine from starting.

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Solutions for Starter Motor Issues

As a DIYer, you can try the following methods to fix starter motor issues:

#1 Taking a Closer Look Under The Hood

Examining the battery and battery cables of the vehicle to ensure they are in good condition. Possibly the problem is with the cables or a weak or dead battery.

#2 Press the Starter

Taping the starter motor’s body with a light spinner might also be helpful. To prevent damaging the component, it needs to be handled lightly.

#3 Transmission Adjustment

In an automatic gearbox, try shifting from park to neutral if the starter won’t turn on. When the car starts in neutral, there may be a technical issue preventing it from starting in the park.

#4 Checking The Fuel Gauge

There is a possibility that a vehicle will not be able to start with an empty tank in a modern combustion engine as a result of the sensor that is installed in it.


In conclusion, a starter motor is a crucial part of a combustion engine with many benefits over manual cranking. It increases the engine’s lifespan and fuel efficiency and is also useful, consistent, safe, and reliable. As a result, a starter motor in good condition is crucial for ensuring an engine runs smoothly and effectively.

That’s it. Thanks for reading. I hope I have covered everything about the “Starting Motor” It would be helpful if you could let me know if there was anything I missed or if you have any doubts about anything I wrote.

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About Yousef

He is a mechanical engineering student, he likes to write about engineering stuff and he is really interested in learning about new technology in machines.

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