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 starter 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 carburetor 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
The starter motor may be classified as a series wound, shunt wound, compound wound and separately excited. Series wound motors are used in automobiles for cranking the engine. A series-wound motor has its field coil in series with the armature so that all the current flows through the field as well as the armature.
The field winding consists of a few turns of thick copper wire on each pole and the windings on all the poles are connected in the series. The current in the field depends upon the load. The current is large with heavy load and small with a light load. The resistance of the field winding is purposely made low so that the loss of voltage and the field is low.
The speed of the starter motor varies with a change in load. It has high speed at light load and low speed at high load. The emergency is high with throughout a wide range of speed. They have the advantage of a strong starting torque, and hence they are used in automobiles for cranking the engine.
Starter Motor Operation
The starting motor is linked to the engine flywheel through a set of gears. A pinion gear is attached to the starter armature which drives a ring gear attached to the flywheel. The arrangement is so made that the two gears engage to crank the engine until it starts and then disengages automatically when the engine is running. The gear ratio is about 15: 1. The armature rotates 15 times to cause the flywheel to rotate once.
Thus, the starter motor requires only 1/15th as much power as would an electric motor directly coupled to the crank. The armature may revolve at about 2k to 3k rpm when the starter motor is operated and hence the flywheel will rotate as high as 200 rpm. When the engine starts, its speed will increase to about 3000 rpm. If the pinion is still in mesh with the flywheel, it will revolve the armature at about 4500 rpm, which is very high speed.
At this speed, the centrifugal force would cause the conductors and commutator segments to be thrown out of the armature and damaging the starter motor. Hence, the pinion gear must be disengaged from the flywheel teeth, after the engine has started. The automatic engagement and disengagement of the starter motor with the engine flywheel are obtained with the help of drive arrangement.