Introduction To Starter Motor
Internal combustion engines are not self-starting and need to be rotated at a certain minimum speed in order for t the engine to commence running by the fuel supply. This is the function of the starting motor.
The starting motor or the cranking motor is the direct current motor which cranks the engine for starting. Cranking the engine means to rotate the crankshaft or layshaft by applying torque on it so that the piston can get reciprocating motion. The starter motor is mounted on the engine flywheel housing. It is series wound and it operates on large currents at low voltage. It must be capable of exerting a very high torque when starting and at low speeds. The amateurs and fields are built with thick wire to keep the resistance low and to enable them to carry large currents without overheating. The faster it turns, the less amount of current it draws and the slower it turns, the large torque it develops. A motor used in passenger car draws about 60 amperes when running at no load, about 600 amperes when cranking the engine slowly.
This supply is needed for a few seconds only. The starting motor voltage is generally 12 volts on passenger cars. Compression ignition engines may use 12 volts starting the motor to provide the power to rotate the crankshaft, especially in cold conditions. The torque produced is 2-4 N-m. The starter motor is powerful enough to turn the engine at a speed such that the carburettor supplies proper air-fuel mixture for starting.
STARTER MOTOR CONSTRUCTION
The construction of the starting motor is similar to that of the generator, but the windings and 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 case of the generator. The main parts of the starting motor are casing, armature, commutator, field winding, brushes, poles and terminals. A drive mechanism is provided at the end of the armature shaft, by means of which the motor starts the engine.
The starting motor uses either two field windings or four field windings. The figure shows a motor using two field windings. The current from the battery divides when it enters the motor, each branch leading to the separate field winding. From the fields, the current is led to the commutator of armature through the two insulated brushes. The current in the armature creates simultaneously four poles that adjacent to the four field poles to produce the attractive and repulsive forces that turn the armature. The armature current returns to the battery through the two grounded brushes.
The below figure shows a starter motor with four field windings. It is used in large engines to develop more torque. It operates in the same manner as the two-winding type.
Series Wound Motor
Starter Motor Operation
Watch the video below to clearly understand the starter motor concept.