Internal and External Earth of Starter Motors

To understand the internal and external earth of the starter, you must understand what the earth is first. The earth is a term of circuit, most commonly seen in the auto repair industry.

The path that current flows through is called circuit. The current starts from the positive pole and pass through the entire circuit, of course, there must be resistance in the circuit or there will be a short circuit. After the current passes through all the electrical appliances and back to the negative pole that’s a formed closed loop.

There are also positive pole and negative pole on the battery of the car. The current flows from the positive pole of the battery and pass all the appliances on the car to the negative pole which formed a closed loop to supply the electricity to all the electrical appliances, so that all the appliances on the car can work properly. And because the body of the car is made of metal which has electrical conductibility so when people design the car they usually use the body of the car as the negative pole to connect with the negative pole of the battery.

The internal earth of the starter means the armature of the starter is directly connected with the housing. Because the housing is installed on the frame of the car, the current directly flows through the frame and back to the negative pole of the battery to form a closed loop. The external earth is the armature of the starter insulates from the housing, it needs a terminal to connect the negative pole of the battery to form a closed loop.

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Material selection of Armature Sheet

The core of the armature is used to form the magnetic flux path and supports the armature windings. Generally stacked from silicon steel sheet.

The magnetic field polarity of the core has been changing because the armature turns. The rapid changes in the polarity will generate eddy current in the core making the current into excessive heat and consumption. The eddy current loss is inversely proportional to the resistance of the ferromagnetic material, which is proportional to the square of the thickness of the core steel sheet.

There is not only the eddy current loss in the core, but also the hysteresis. And the hysteresis loss is related to the hysteresis loop area of the ferromagnetic material. The smaller the hysteresis loop is, the smaller the hysteresis loss.

Silicon steel sheet is adding a small amount of silicon to the steel thus the resistivity is bigger and the hysteresis loop is narrower, so using the silicon steel plate to make the core can have a smaller eddy current loss and a hysteresis loss.

But in order to reduce the cost of armature production, the manufacturers in domestic will use cold-rolled plate or even scrap steel plate to produce the core of armature. So the quality of the armature won’t be hard to imagine.



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Material selection of armature shaft

The shaft is a key part of the armature so the quality of the material for the shaft also plays an important role in the whole product.

At present, the mainly used materials for the production of shaft in domestic are the following kind: 20CrMnTi, 20Cr/40Cr and #40/#45 steel.

Among them, 20CrMnTi is a vital material of gear which has a high mechanical properties. After carburizing, quenching and low temperature tempering, the hardness of the surface can reach 58-62HRC, the hardness of the core can reach 30-45HRC. Compare with the other materials, 20CrMnTi is also the most expensive one. The tensile strength and yield strength of 20Cr/40Cr are less than 20CrMnTi and they are always used for making products which has high hardness cores. 

As for #40/#45 steel, they are both high quality carbon steel, however the anti-fatigue strength and toughness are not as good as 20Cr/40Cr, also the cost of raw material for #40 and #45 steel is only half of 20Cr/40Cr, so the life of the gear won’t last long if you choose #40/#45 steel to produce it.

Coaxial (Sliding Gear) Starter Motor

Similar to the axial starter motor, the coaxial starter motor moves the pinion into engagement under reduced power and when it is fully meshed only then full power is applied. The main difference lies in the method of sliding the pinion into mesh with the ring gear. Instead of the whole armature assembly moving axially, in this type a solenoid mounted coaxially with the shaft in a housing moves only the pinion for engagement.

The main details of the circuit of a coaxial starting motor are shown in Fig. 15.30. The main terminal is connected directly to the battery and the terminal marked ‘sol’ is connected to the battery through the drivers starter switch. When this switch is operated, the two-stage solenoid is energized, which moves the pinion into mesh and also closes the first set of contacts, the second set is kept open by trip lever. During this period, a resistor limits the current to the main field, so the armature rotates slowly. Just before reaching the fully meshed position between the pinion and ring gear, a lever trips the second set of contacts. This action by-passes the resistor, and supplies full current to the main field so that the motor produces its maximum torque.


Fig. 15.31. Actions of coaxial starter.
A. At rest position. B. Cranking position.
C. Engaging position. D. Over-speed position.

The actions of one type of Lucas CAV motor are presented in Fig. 15.31. This design uses four steel balls, which lock the pinion sleeve to the shaft to avoid premature rejection of the pinion during spasmodically firing of the engine. When the engine starts and operates on it own, over-speeding of the motor is prevented by the centrifugal action of a set of steel balls placed adjacent to the locking balls. When a pre-set speed is reached, the centrifugal force on the balls moves the locking collar outward and allows the pinion to disengage. A return spring positioned at the flywheel end of the armature shaft helps pinion disengagement and holds the pinion clear of the flywheel during running of the engine.

In Bosch sliding-gear motors the main solenoid is installed at the opposite end of the motor to the pinion. This solenoid holds the pinion in full engagement position until the driver releases the starter switch. Drive from the armature to the pinion is transmitted by a multi-disc clutch. This clutch limits the torque to be transmitted and hence prevents over speeding of the motor by releasing the plates and slipping when the engine is started. Some motors have a shunt field winding, which limits the no-load speed and others use a brake winding that comes into action when the driver releases the starter switch.




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Axial (Sliding Armature) Starter Motor

The main features of this type starter motor are its size and robust construction. For the engagement of the pinion to the flywheel ring gear, the complete armature assembly slides axially through the motor casing. A simplified construction of the starter is illustrated in Fig. 15.27. In the figure, the motor is shown in the rest position. The armature is held by a spring so that it is offset to the field poles. When the field is energized, the armature is pulled to the left and the pinion is slid into engagement with the ring gear.

Figure illustrates the electrical circuit of the starter motor, which uses three field windings. The main wind­ing as usual is of thick-section and low-resistance wind­ing, and is connected in series to the armature. The auxiliary winding is wound with thinner wire so that it has a relatively high resistance, and is also connected in series with the armature but in parallel with the main winding. The holding winding is also a high-resistance winding but is connected in parallel with the armature as well as with the other two windings.

A two-stage solenoid switch, mounted on the starter operates the Delco Remy starter, and is energized by the driver’s

Fig. 15.28. Circuit of axial motor.

^switch. When the switch is operated, the first pair of contacts closes but the second pair is held open by a pawl that engages in a slot in the trip lever. Once the pinion is near fully engaged with the ring gear, the pawl allows the second pair of contacts to close.

Figure 15.29 illustrates the operation of the motor. Figure 15.30A shows the first pair of contacts is closed, which energizes the auxiliary windings, holding windings and armature. The armature rotates slowly and moves axially so that it is central to the field poles. Simultaneously,

the pinion is slowly slid into mesh with the ring gear. When the pinion is on the verse of its full engagement, the release disc on the armature strikes the pawl so that the trip lever closes the second pair of contacts (Fig. 15.29B). Now current passes through the main windings, causing the motor to develop its full torque.

Fig 15-29. Action of axial motor. A. Closure of first contacts. B. Closure of second contacts.

As cranking speed rises, the current through the main and auxiliary windings decreases due to back emf generated by the rotating armature, especially when engine fires spasmodically but does not actually start. Now the magnetic strength in the main and auxiliary winding is insufficient to oppose the armature return spring and hold the pinion in full engagement. However, this is prevented by the holding winding as the current in this winding is not affected by the back-emf. Once the pinion has de-meshed and the armature has returned back, the momentum of the rotating mass tends to keep the armature rotating. This is however resisted by the ‘generator effect’ developed due to the interaction of the holding winding and the armature. This electrical reaction quickly brings the armature to rest. The pinion joins to the armature shaft through a small multi-plate clutch, which serves two functions.

(i) It slips when the torque applied to it exceeds a predetermined limiting value, so that the starter is safeguarded from damage if the engine backfires.
(ii) It disengages when the engine starts and tends to drive the pinion faster than the armature, so that the armature is prevented from damage by excessive speed.

Fig. 15.30. Circuit diagram of coaxial starter.


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How to Replace a Car's Starter Motor---Part 4​​​​​​​

4. Bottom line, there should be 9 to 10 volts at the Delco Remy starter motor hot post when cranking. Don't forget to put the car in neutral or park and block the wheels so you don't run yourself over.

5. Battery cables okay? Try jumping—with jumper cables—directly from the battery positive terminal to the starter motor's solenoid post. If the solenoid pulls in and the starter turns over the engine, you've got a wiring problem.

6. If the solenoid doesn't pull in and energize the armature, try jumping directly to the motor's armature post, bypassing the solenoid. If the armature spins, the problem is in the solenoid or its wiring.

7. Don't forget that some antitheft systems will still disable the starter even if the crooks hot-wire the ignition key. And when something goes wrong with that alarm, you're stranded. It gets worse—it's usually difficult or impossible to disconnect the alarm, in order to keep the car thieves from doing so. Be prepared for serious reading of the factory shop manual or, if your alarm is aftermarket, a return to the alarm installer.

No More Excuses.

Regardless, you've determined that the starter is fried. Time to get to it. You might get lucky, especially if you have an FWD vehicle, and be able to swap starters from above the car. If not, the starter usually lives in a really remote location well underneath the car, somewhere near the side of the transmission. Don't try to do this on a hot car—which might seem to be superfluous advice if you can't even get it started—but invariably the exhaust system is nearby, and burns hurt. Also, protective eyewear is de rigueur, because you'll be dropping flakes of rust and underhood dirt from directly above your head into your baby blues. While you're waiting for the car to cool off, chock the wheels and jack it up a foot or so. Ramps will work, but I prefer a pair of sturdy jackstands. On the other hand, I've also been forced to do this in the middle of a muddy field by scooping out a trench to lie in.

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How to Replace a Car's Starter Motor---Part 2

Saturday morning you give your battery and charging system a full investigation. Even swapping in the new battery from your other car doesn't help. Nope—the screeching has become the only symptom you can elicit. That banshee wail is the teeth on the starter motor's bendix gear clashing against the ring-gear teeth because it's not completely engaged.

The solenoid, or on some starters, just a threaded part of the armature, pushes the bendix gear forward an inch or so until it meshes with the ring gear, allowing the starter motor to spin the engine over until it starts. When the engine rpm exceeds the cranking speed, the bendix automatically retracts, preventing the engine from spinning the starter too fast.

At least that's how it works in theory. Starter motor failure is rarely caused by a blown or shorted motor itself—usually it's a problem with the bendix mechanism or the solenoid. And frankly, most people will never need to replace a starter motor for the life of their vehicle. Intrepid but underfunded Saturday Mechanics might actually dismantle a malfunctioning starter and repair it themselves. You can still find auto parts stores that can get you new bearings, brushes and bendix assemblies. Generally, I just exchange the old starter for a new or remanufactured one, because repairing one doesn't save much money. On the other hand, if you have a rare or hard-to-find starter it might be necessary to fix what you have. Usually, auto electric shops （Denso Starter Supplier) can rebuild or repair a starter with a bad armature, shorted field windings, bad brushes, a bad commutator, or even a bad solenoid if there is no alternative. Be prepared to wait a few days or more.

How to Replace a Car's Starter Motor---Part1

Replacing a starter motor is usually a straightforward but inconvenience job. Here's how it's done.

This gear-reduction starter is a lightweight drop-in replacement for the older starter that failed. By spinning the armature faster and reducing its output speed with a planetary gearbox, it's possible to get the same starter power in a smaller, lighter package.

Click. Click. Click. That's the noise your car makes when you twist the key. A few more clicks and now you've got a metallic screeching that makes all the dogs in the neighborhood start to bark. The dash lights are plenty bright, the headlights don't dim much when the key is twisted, but obviously something is wrong. With the clock ticking, you resort to that old standby: a jump-start. A few more screeches grating enough to make you cringe, and the engine finally spins merrily.

After work, it's the same story: Clunking and clicking, a few bars of the "Ballad of the Tortured Ring Gear," and you get to drive home instead of ride in the cab of the tow truck.

You've got a bad starter motor. Time to fix it before you're on the bus.

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Starter Motor, How they Work (Parts 5) – armature & commutator

The armature
This sits in a field housing (or body – yoke). There are two types of field housing; one with permanent magnets, the other with field coils which are wound around ‘pole shoes’. These pole shoes only become magnetised when a current flows around them.

Most field coil windings can be replaced - although they are often hard to remove because the pole shoe screws are normally very tight indeed. The permanent magnet version suffers more from un-stuck magnets (which can be stuck back) - but don't turn them round. Broken magnets are normally caused by being tapped with a hammer.

The commutator
Usually at the back of the armature is the commutator. The brushes bear directly on this to transfer the electrical current to and from the armature.