Demonstrate construction and working of different types of starter, motor, its drive and switches

Demonstrate construction and working of different types of starter, motor, its drive and switches.

INTRODUCTION

The Internal combustion engines are not self starting and need to be rotated at a certain minimum speed in for 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 direct current motor which cranks the engine for starting. Cranking the engine means to rotate 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 series wound and designed to operate on large currents at low voltage. It must be capable of exerting a very high torque when starting and at low speeds. The armatures 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 current it draws; the slower it turns the more 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 a 12 volts starting motor to provide the power to rotate the crankshaft, especially in cold conditions. The torque produced is 2—4 N-m. The motor is powerful enough to turn the engine at a speed such that the carburetor supplies proper air -fuel mixture for starting.

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 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 current from the battery divides when it enters the motor, each branch leading to separate field winding. From the fields, current is led to the commutator of the 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.

Fig. also shows a starting motor with four field windings. It is used in large engine in order to develop more torque. It operates in the same manner as the two-winding type.

DRIVE ARRANGEMENTS

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 disengage 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 cranking motor requires only one fifteenth as much power as would an electric motor directly coupled to the crank. The armature may revolve at about 2000 to rpm when the cranking motor is operated and hence the flywheel will rotate as high as 200 rpm.

When the engine starts, its speed may 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 a very high speed. At this speed, the centrifugal force would cause the conductors and commutator segments to be thrown out of the armature damaging the motor. Hence, the pinion must be disengaged from the flywheel, after the engine has started. The automatic engagement and disengagement of the motor with the engine flywheel is obtained with the help of drive arrangement.

TYPES OF DRIVE ARRANGEMENTS

The arrangements may be of two types:

1. Inertia type

a. Bendix drive

b. Folo-Thru drive

2. Over Running Clutch OR Pre-engaged type

Bendix Drive

Fig. shows Bendix drive for starting motor. It is fastened in the armature shaft of the starting motor. The drive head is keyed to the end of the armature shaft. The pinion gear, having internal threads, is mounted on the threaded sleeve. just like a nut on a bolt. The sleeve is not connected directly to the shaft of the starting motor but uses it only as a bearing. A spring is attached to the drive head and also to the sleeve.

When the starting motor is at rest the pinion gear is not engaged with the flywheel. When the starting motor is switched on, the armature begins rotate. This causes the sleeve to rotate also, because the sleeve is fastened to the armature shaft through a spring.

The pinion because of its inertia of rest and its unbalanced weight turns very little, but it moves forward on the revolving volt, until it engages with the teeth of the flywheel. The slight turning of the pinion gear helps to engage it properly with the flywheel.

When the pinion gear strikes with the collar, it begins to turn with the sleeve, causing the flywheel to run with it. When the flywheel turns, the crankshaft also turns and th e engine starts. The spring between the armature shaft and the threaded sleeve takes the shock of the start.

After the engine has started, the opinion gear is turned by the engine much faster than when rotated by the starting motor. This causes the pinion gear to turn back on the threaded sleeve, making it disengaged with the flywheel.

Folo-Thru Drive

The Folo-thru drive is very similar to the Bendix drive. The difference is that the Folo-thru drive keeps the starting motor engaged with the flywheel until a predetermined engine speed is reach, but in bendix drive it is not so.

In the Folo-thru drive the threaded sleeve is attached to the armature shaft through a spiral spring. A pinion is mounted on the threaded sleeve. The pinion base has two small spring- loaded pins—a lock pin and an anti-drift pin. The anti-drift pin is similar to the lock pin, but has a stronger spring. The anti- drift pin rides on the anti-drift slope on the threaded sleeve and keeps the pinion from drifting into the ring gear when the starter is not in use. It imposes a fraction drag that holds the pinion in the disengaged position. The lock pin drops into a detent in the sleeve thread as the pinion moves out of the cranking position.

This holds the pinion from being disengaged with the flywheel during cranking. It prevents the pinion from being disengaged by a false start, during which the engine might fire few times and then die. The pinion is thus held in engaged position; and cranking continues until the engine really gets started.

Over-Running Clutch Drive

Fig. shows over-running clutch drive. The starter lever is linked to a starter pedal which extends into the driver's compartment and is operated by foot pressure. When the starter pedal is pressed, the shift ever compresses the drive shaft and spring which ultimately pushes the over-running clutch and pinion gear assembly toward the flywheel.

The starter switch is closed by the shift lever when the starter pedal is fully pressed. As soon as the starter switch is closed, the pinion gear will run and engage with the flywheel, thus starting the engine.

When the engine starts, the over-running clutch comes into action. The unit is so designed that, as the starting motor turns, the pinion is driven through the over-running clutch. But as soon as the engine starts, the pinion turns much faster than the starting motor, due to which it slips backward into the over-running clutch. When the starting switch is opened, the engaging lever releases the pinion from the flywheel.

Fig. shows the Construction of an over-running clutch. It consists of an outer shell, pinion and collar assembly. The outer shell has four hardened steel rollers fitted into four notches. The notches are not concentric, but ate smaller in the end opposite to the plunger springs. When the clutch shell is turned by the armature shaft, the rollers are wedged i n the notches to force the collar to run with the shell. Since the collar drive the pinion gear, this actions enables the armature to rotate the pinion, cracking the engine.

After the engine starts, it turns the pinion gear faster than the armature, so that the rollers are rotated into the larger sections of the notches, where they are free. This allows the pinion to over-run the remainder of the clutch. When the shift lever is released, a spring on the shift lever pulls the pinion back out of engagement.

STARTING MOTOR SWITCHES

Starting motor takes from 150 to 500 amperes while starting the engine and as such the switches operating it must be capable of taking this much current with a minimum of voltage drop due to contact resistance. Also the wiring carrying high current should not be very much longer than the minimum required.

The switches generally used for starting motors are:

1. Manual switch.

2. Solenoid switch.

3. Solenoid switch-cum-shaft.

4. Solenoid shift with relay.

Out of these the manual switch is almost obsolete now. The solenoid switch, with or without relay, is used with Bendix drives. For overrunning clutch type drives, apart from starting the motor itself, the mechanism for obtaining the engagement of pinion with the flywheel also has to be actuated. This is done with a solenoid switch-cum-shift, commonly referred to as solenoid shift only. To obtain more positive operation of the solenoid, a relay is sometimes used along With the solenoid shift. All these types of switches shall now be explained in detail.

MANUAL SWITCH

The construction is very simple. As the plunger is pressed the contacts are made. It may be operated either with hand or with foot. It is sometimes operated by means of accelerator lever also.

Solenoid Switch

It is also called magnetic switch. As the button is pressed, the current flows from the battery to the winding which produces a magnetic field resulting in the movement of the plunger, to close the switch connecting the battery and the starting motor. The advantage of solenoid switch (compared to the manual) is that the heavy current wiring length is reduced (which reduces the voltage drop in the starter circuit) and the driver has to operate only a push button or key switch (at the dashboard) which carries a nominal amount of current only. This driver’s switch is only of light duty type and its wiring is also thin and light.

The solenoid switch here has got one winding only. Some solenoid switches have two windings also, viz., the pull-in winding and the hold-in winding. Both the windings exert combined force to pull the armature so as to make the contacts of the switch when the pull-in winding is short-circuited and only hold-in winding remains to hold the contacts closed. The advantage of this type is that less current is drawn from the battery during the period of hold - in. Further only the leads from the battery to the solenoid terminal and from the other solenoid terminal to the starting motor need only be thick, other wires including the one going to the driver switch may be thin as explained above.

SOLENOID SWITCH CUM SHIFT

Solenoid switch cum-shift consists of a solenoid switch which also performs the function of actuating the shift lever in an overrunning clutch type of Dyer’s drive. The solenoid shift shown in Fig. 5.8 is similar in construction to the solenoid switch (Fig. 5.7), except that it has got a different plunger construction suited for operating the shift lever. Further this is of two -winding type. It is not necessary, however, that a solenoid shift must have always two windings.

The wiring diagram shown in Fig. 5.9 is for a fully automatic circuit for the starting motor. The relay used serves to control the solenoid currents. Further, completing the relay circuit through the generator provides protection against operating of the starter while the engine is running, because then the generator voltage is approximately equal to the battery voltage. As such no voltage is applied through the relay even when the switch contacts are closed. A vacuum switch is also sometimes employed in the circuit, which provides added protection against starting of the cranking motor while the engine is running.

Another safeguard has also been provided in the circuit. The neutral safety switch ensures that the circuit will not close unless the transmission of the vehicle is in neutral. The ignition switch is operated by the driver with the help of a key so that the starting motor circuit is closed a bit further than the key position for closing the ignition circuit.