All about the marine-diesel-engine motor-starter

The Diesel Motor-Starter

The systems that make up a typical diesel motor-starter electrical system include the battery; the engine control panel; the wiring loom; the preheating system; the starter motor and solenoid; shut-down solenoids; instrument sensors and transducers, and the alternator. There is a basic sequence of electrical functions that take place when starting the engine. When the key switch is turned to , this closes the circuit to supply voltage to the control circuit, and generally initiates alarms.

When no alarms audible or visual occur, it indicates that no power is on. When the key switch is turned to the position, this manually or automatically energizes the heating glow plugs or heating elements. When the key switch is turned to or the engine button is pressed, voltage is then applied to the starter motor solenoid coil. The solenoid then pulls in to supply main starting circuit current through a set of contacts. The contacts when closed supply current to the starter motor positive terminal. This then turns the starter motor to start the engine.

Electric the Engine Starter-Motor

Essentially the electric starter consists of a DC motor, a solenoid, and a pinion engaging drive. The DC motor is typically series wound as it provides the high initial torque required to exceed the friction and inertia, (such as oil viscosity), and cylinder compression and accelerate the engine to a point where self-ignition temperatures and combustion starts (typically in the range 60-200 rpm depending on whether glow plugs are used). The starter motor torque is transmitted by the pinion and ring gear on the flywheel. The drive gear pinion has a reduction gear of around 15:1.

Solenoids. The solenoid is essentially a large high current relay that consists of coil and armature, moving and fixed contacts. The solenoid is mounted directly to the starting motor housing, which reduces cables and interconnections to a minimum. When the solenoid coil is energized by the starting circuit, the solenoid plunger is drawn into the energized core and this closes the main contacts to supply current to the starter motor. On some starters the solenoid also has a mechanical function. The solenoid activates a shift or engaging lever to slide the overrunning clutch along the shaft to mesh the pinion gear with the flywheel, and when engaged the motor-starter then turns the engine, so meshing occurs before starting

Starter Motors. The motor-starter consists of four poles shoes or magnets, and some use permanent magnets. The poles are fitted with an excitation winding which creates the magnetic field when current is applied. The rotating part called the armature also incorporates the commutator. The four carbon brushes provide the positive and negative power supply. There are four basic DC motor types in use and are based on connection of the field windings. The field windings are connected either in series or parallel with the armature windings

Shunt (Parallel) Wound Motors. The motor operates at a constant speed irrespective of loads applied to it. It is the most common motor used in industrial applications and is suited to applications where starting torque conditions are not excessive.

Permanent Magnet Excited Motors. The permanent magnet motor-starter offers the advantages of reduced weight, physical size and generates less heat than normal field type starters. Current is supplied via the brushes and commutator directly to the armature. Another feature is that a reduction gear is used, which allows faster speeds and increased torque.

Series Wound Motors. This type of DC motor has a speed characteristic where the speed varies according to the load applied, speed increases with load decrease.

Compound Motors (Series/Shunt Wound). This configuration is often used on large motor-starter. This combines the advantages of both shunt and series motors, and is used where high starting torques and constant speeds are required

Pinion Engaging Drives.

The pinion engaging drive is located within the end shield assembly of the motor-starter and consists of the pinion engaging drive and pinion, the overrunning clutch, the engagement lever or linkage and spring. When the motor operates the drive gear meshes with the ring gear or flywheel teeth to turn the engine, and then disengages after starting. The overrunning clutch has two important functions, the first is to transmit the power from the motor to the pinion, and the second, which is to stop the starter motor armature from over-speeding and being damaged when the engine starts. Pre-engaged motor-starters generally used a roller type clutch, while larger multi-plate types are used in the sliding gear motor-starter.

For more on the motor-starter and engine systems