Starter Motors and Circuits
Starting System Circuits
The starter circuit is very simple in comparison with most other circuits on the modern vehicle. The voltage drop in the main supply wires is the problem with the system which is to be overcome. This problem is because of the high current required by the starter particularly under adverse starting conditions such as very low temperature. The starter is usually operated by a spring loaded key switch, which also controls the ignition and accessories. The supply from the key switch, through a relay in many cases, causes the starter solenoid to operate and this in turn, controls the heavy current through a set of contacts. In some cases an extra terminal
on the starter solenoid provides an output while cranking, usually used to bypass a dropping resistor on the ignition or fuel pump circuits. Figure 15.6 illustrates basic circuit for the starting system.
Fig. 15.6 Basic starting circuit. A. Inertia starter circuit. B. Pre-engaged starter circuit.
For a light vehicle engine, a typical cranking current is around 150 A, which may increase to the order of 500 A to provide the initial stalled torque. A maximum voltage drop of only 0.5 V is generally allowed between the battery and starter when the latter is operating. Using Ohm’s law a maximum allowed circuit resistance can be calculated as 2.5 mfi for a 12 V supply. This is a worst situation and generally lower resistance values are used. The selection of suitable conductors in the starter circuit is highly important.
Principle of Operation of Starter Motors
When a conductor is placed in a magnetic field and a current flows through it a force is created, which acts on the conductor relative to the field. The magnitude of this force is directly proportional to the magnetic field strength, the length of the conductor in the field and the current flowing in the conductor.
Since a single conductor has no practical use in a DC motor, the conductor is shaped into a loop or many loops to form the armature. A many segment commutator is in contact with brushes through which the supply current flows to the commutator. The force on the conductor is created through the interaction of the main magnetic field and the field induced round the conductor. In earlier designs of starter motor used for light vehicles the main field was created by heavy duty series windings wound round soft iron pole shoes called electromagnet. With improvements in magnetic technology, the electromagnet is now largely replaced by permanent magnet, which
resulted in a smaller and lighter construction. The strength of the magnetic field created around the conductors in the armature is determined by the value of the current flowing.
Most starter motors use a four pole four brush system, so that four field poles concentrate the magnetic field in four areas (Fig. 15.7). The magnetism is created in one of three ways such as permanent magnets, series field windings or series-parallel field windings. The circuits of the latter two methods are illustrated in Fig. 15.8. The series-parallel field windings can be constructed
Fig. 15.7. Four pole magnetic field.
with a lower resistance, so that the current and hence torque of the motor can be increased. Four brushes, made of mixture of copper and carbon as is the case with most motor or generator brushes, are used to carry the heavy current. But the brushes for starter have a higher copper content to reduce electrical losses. Some old but typical field coils with attached brushes are shown in Fig. 15.9 and the field windings on the right of the figure are known as wave wound.
Fig. 15.8. Series and series I parallel field circuits.
Fig. 15.9. Typical field coils and brushes (Lucas).
The armature uses a segmented copper commutator and heavy duty copper windings. The windings on a motor armature can be wound in two ways, known as lap winding and wave winding. Figure 15.10 illustrates the difference between these two methods. Since the wave winding provides most appropriate torque and speed characteristic for a four pole system, starter motors generally use this technique.
A starter also incorporates some method of engaging with, and releasing from, the vehicle’s flywheel ring gear. This is achieved with light vehicles starters either by inertia type engagement or by pre-engagement.
Calculation of Speed, Torque, Power and Efficiency of Starter Motor
To understand the forces acting on a starter motor, a single conducting wire in a magnetic field is considered first. The force on a single conductor in a magnetic field is given by the formula:
Fig. 15.10. Typical lap and wave wound armature circuits
Fleming’s left hand rule indicates the direction of the force (the conductor is at 90 degrees to the field). The stalled torque of a motor with a number of armature windings can be calculated as follows :
This equation gives only stalled or lock torque because, when a motor is running, a ‘back emf is generated in the armature windings. This opposes the applied voltage and hence reduces the current flowing in the armature winding. With a series wound starter motor this also reduces the field strength B. The armature current in a motor can be calculated as follows:
where, / = armature current, A
V = applied voltage, V
R = resistance of the armature, O
and e = total back emf, V.
From the above equation it should be noted that, when a voltage is applied to the terminals of a motor, the armature current is reached its maximum immediately, since the back emf is zero. As the speed increases the back emf also increases, consequently the armature current decreases. This is the reason why the maximum torque is attended at zero revolutions, which is also an advantage for a starter motor.
For any DC machine the back emf is given by :
If the constants are removed, this formula provides the relationship between field flux, speed and back emf as: n a e/O
While considering the magnetic flux, it is necessary to differentiate between permanent magnet starters and those using excitation through windings. Permanent magnetism remains reasonably constant and its strength is determined from the construction and design of the magnet. Flux density (B) can be calculated as follows:
Pole shoes with windings are more complicated to analyze, since the flux density depends on the pole shoe material, the coil and the current flowing.
Referring to the Fig. 15.5 the starter required for this application can be known. The efficiency can be calculated as; efficiency = (power out/power in) x 100%.
The efficiency of most starter motors is around 60%. The main losses are iron losses, copper losses and mechanical losses. Iron losses are caused by hysteresis loss, which in turn is due to changes in magnetic flux, and also caused by induced eddy currents in the iron parts of the
motor. Copper losses, also called 2 R losses, are caused by the resistance of the windings. Mechanical losses include friction and windage (air) losses.
At an efficiency of 60%, a supply of about 1.7 kW is necessary for a one kW starter motor. From a nominal 12 V supply and allowing for battery voltage drop, a current of about 150 A is necessary to achieve this power. The calculations presented in this section are only for understanding how a starter system works.
DC Motor Characteristics
A motor can be designed with most suitable characteristics for a particular task. Figure 15.11 shows a comparison of the speed and torque characteristics for main types of DC motors.
Fig. 15.11. Speed and torque characteristics of DC motors.
The four main types of motor are based on shunt wound, series wound, compound wound and permanent magnet excitation. In shunt wound motors the field winding is connected in parallel with the armature (Fig. 15.12 A). The speed of this motor remains constant, independent of torque, due to the constant excitation of the fields.
Fig. 15.12. DC motors.
A. Shunt wound type. B. Series wound type.
C. Compound wound type. D. Permanent magnet type.
In series wound motors the field winding is connected in series with the armature (Fig. 15.12 B), so that the armature current passes through the fields. This enables the field windings to consist of only a few turns of heavy wire. During starting of this motor under load, the high initial current, due to low resistance and no back emf, produces a very strong magnetic field and consequently high initial torque. This characteristic allows the series wound motor to be an ideal starter motor.
The compound wound motor (Fig. 15.12 C) is a combination of shunt and series wound motors. The characteristics can vary depending on how the field windings are connected. This variation is due to whether the shunt winding is connected either across the armature or across the armature and series winding. Large starter motors are often compound wound and are operated in two stages. In the first stage, the shunt winding is connected in series with the armature, so that a low meshing torque is attained due to the resistance of the shunt winding. During starting when the pinion of the starter is fully in mesh with the ring gear a set of contacts causes the main supply to be passed through the series winding and armature, giving full torque.
The shunt winding is now connected in parallel with the armature so that it acts to limit the maximum speed of the motor. This is the second stage operation.
In permanent magnet motor, the field excitation is by permanent magnet and remains constant under all operating conditions. The characteristics of this type of motor are more or less similar to that of shunt wound motor. However, when this motor is used for vehicle starting, the drop in battery voltage tends to cause the motor to behave in a similar way to series wound machine. In some cases to enhance the higher speed and lower torque characteristic an intermediate transmission gearbox is used inside the starter motor. Permanent magnet motors are simple and smaller in size compared to the other three discussed.
Figure 15.13 illustrates the variation of the speed of the motor with load. This motor delivers maximum power at a mid range speed but maximum torque is at zero speed. Two sets of curves are often provided on the graph, one at 253 K and the other at 293 K. The development of very high speeds under no load conditions may damage this type of motor. When the motor is running offload, the high centrifugal forces on the armature may cause damage to the windings.
Fig. 15.13. Starter motor characteristic curve.