Starting Systems- Starter Motors

Starter Motor

The most important electric motor fitted to motor vehicles is used for cranking the engine, it is called the starter motor. the main parts of a starter motor are, the Armature, the Field windings, brushes and the solenoid.

Starter motors have to carry large currents neccessary for a high power output, this is why the there are a large number of conductors in the windings of the armature and field magnets, these windings are usually made from thick rectangular copper strips or bars. the number of conductors in each armature coil is often only one, but it is sometimes two. with such a short path for the current, and a large cross-sectional area in the conductors, the starter armature can handle large currents without excessive voltage drop.

since the current in a series circuit is the same in all parts of the circuit, the field windings carry the same current as the armature windings and should therefore not add any resistance to the circuit. field windings are made of insulated copper strip similar to that used in the armature windings. The brushes of a starter motor and the connecting leads must also be able to carry the same current.

The starter circuit, beginning at the live battery terminal goes,

·         Main insulated cable from the battery to the starter switch

·         Switch terminals and contacts

·         Cable from switch to starter

·         Starter terminal

·         Field winding

·         Insulated brushes in contact with the commutator

·         Armature windings with connections to the commutator

·         Earthed brushes

·         Earthed connection of the brush leads

·         Metal casing of the starter mounted on the engine

·         Engine

·         Heavy connecting cable from the engine to the frame

·         Frame

·         Battery earth strap with connections to frame and the earthed battery terminal

·         The battery itself.

Since a high resistance or a break in any part of the starter circuit will affect the operation of the starter, it is obvious that every part of the starter circuit must be maintained in a sound condition. This includes the battery and the terminal connections, as the large starting current must pass through the cells and the electrolyte of the battery.

There are several different arrangements of internal field windings and poles in a starter motor. One of the simplest is with a series winding, where the current is the same in every part of the circuit, both internally and externally. Another common winding arrangement is often referred to as a “series-parallel” starter, although the current travels through the field windings in two parallel paths, the field windings are in series with the armature windings and so that the starter is really series wound and has the characteristics of a series motor. In a series motor the field windings are in series with the armature windings so that when the armature current decreases at high armature speeds the field current, and the strength of the magnetic field also decreases. Thus the torque (turning effort) decreases at high armature speeds in the series motor but not in the shunt motor.

 The shunt wound circuit, where the field windings are connected in parallel (shunt) with the armature windings, which is direct to the battery. Regardless of the load on the armature, or its speed of rotation, the field current remains the same, since normal battery voltage is applied to the constant resistance of the field coils. Thus the strength of the magnetic field remains constant.

The compound-wound starter has both series and shunt-field windings; this is because engines with high compression ratios and large valve overlaps require a starting motor which can develop more torque at higher speeds than the series motor is capable of producing. In this type of starter, three or four field poles are series wound and the fourth one has a shunt winding consisting of many turns of fine wire. The purpose of the shunt winding is to maintain the magnetic field at a more constant strength than is possible in a series-wound motor. To understand how this is achieved the characteristics of both shunt and series-wound motors must be considered. When the armature of a motor rotates at speed, a generator effect occurs and the armature windings generate a voltage which opposes the voltage of the battery. The opposing voltage rises as the armature speed increases but never equals the voltage of the battery. The effect of this is to reduce the current drawn from the battery.

Most motors have 4 brushes: two insulated and two earthed. This reduces the amount of current carries by each brush to half the total starting current, and the arrangement is suited to the split-field connections. 

The last part of the starter system is the solenoid which receives a large electrical current from the battery and a small electrical current from the ignition switch.  The small electrical current from the ignition switch is sent to the starter solenoid when It has been turned on, and current then flows from the live starter terminal through the coil creating a magnetic field around it. At the same time current flows through the PULL IN winding to the M terminal via the starter motor through the field coils, through the brushes through the armature through the negative brushes to earth. This produces a strong magnetic field around the PULL IN winding and turns the starter motor slowly.

The plunger is now magnetised and is drawn up into the centre of the hollow coil, this movement connects the B terminal to the M terminal. At this point the PULL IN winding is turned off because of the equal voltage at each end of the winding. The heavy current can now flow directly from the battery to B via M to the starter motor.