PDM Installation & Configuration: Starter and Alternator Control
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Starter and Alternator Control
10.36
| 00:00 | - There are a couple of items attached to almost every internal combustion engine powered performance car that form a critical part of the power distribution system to it's important that we've got a good understanding of how these devices operate. |
| 00:12 | These are the alternator and the starter motor. |
| 00:15 | In this module, we'll dive into their purpose and operation. |
| 00:18 | We've all probably got a pretty good idea about what a starter motor is and how they work but let's quickly cover off the basics just in case. |
| 00:25 | A starter motor is a powerful DC motor bolted to the side of the engine or gearbox that when activated will mesh its output, spring loaded gear called the pinion with another gear which is called the ring gear that is attached to the flywheel. |
| 00:40 | The starter motor then starts to spin, in turn spinning the internal combustion engine, allowing intake, compression, combustion and exhaust of the fuel and air, starting that combustion cycle. |
| 00:52 | At this point, the internal combustion engine is self sustaining and the starter motor disengages, pulling its pinion gear away from the ring gear. |
| 01:00 | When we're working practically with starter motors, there are a couple of things we need to keep in mind. |
| 01:04 | The first of these is that there will typically be a large cable directly connecting the battery positive terminal or the output of the battery isolator if one's fitted to the main power input stud on the starter motor itself. |
| 01:17 | It's not uncommon for a starter motor to draw a peak in rush current of 200 to 300 amps. |
| 01:23 | This is well above the supply capability of a PMU so there isn't a situation where the PMU will be supplying this power to the starter motor, it'll always come directly from the battery. |
| 01:35 | This needs to be via a large cable, usually 35 to 50 mm² and as it can be directly connected to the battery, it needs to be well insulated and mounted to make sure it can't be abraded or cut through accidentally and form a dead short to ground. |
| 01:50 | I'd also suggest rubber boots on the starter motor end of the cable, it's just very easy to slip with a spanner if you're working in this area and accidentally short out that cable. |
| 02:00 | The way the starter motor engages its pinion gear with the ring gear is with a solenoid. |
| 02:05 | When the solenoid is energised, it moves the spring loaded pinion gear into mesh with the ring gear as well as closing the contactors that connect the main power input stud to the starter motor brushes. |
| 02:18 | This energises the motor windings, the motor spins and starts the engine. |
| 02:22 | Once the solenoid is removed, the spring attached to the pinion gear pulls it back out of the way of the flywheel, also opening those starter motor contacts. |
| 02:32 | This solenoid can draw a significant current level, around 15 amps is not uncommon but this is absolutely something that we can supply from our PMU. |
| 02:41 | Doing this can allow us to work some logic into the starter motor control system. |
| 02:46 | For example, we can have a momentary push button that you push once, requesting that the PMU engage the starter motor. |
| 02:52 | The PMU can also be reading the engine RPM and if it sees that engine RPM is currently 0, it can engage the starter motor until the engine speed is above a value that means the engine must be started, typically around 500 RPM or some time out condition is met, 3 seconds is a pretty common value. |
| 03:09 | The PMU can also reject requests to engage the starter motor if it sees the engine RPM is above 0 when the button is pressed, avoiding inadvertent button presses trying to engage the starter onto an already starter engine. |
| 03:25 | With the starter motor operation and how we can control this from our PMU sorted, let's talk about the alternator. |
| 03:30 | This is an electrical generator bolted to the side of the engine and attached to the crankshaft pulley via a belt. |
| 03:36 | When the engine is running, it spins the alternator, creating the electrical energy that runs the system as well as recharging the starting battery. |
| 03:45 | Alternators work by passing current through electrical windings on the rotor, creating a magnetic field around the rotor. |
| 03:52 | This is attached to the pulley so when the engine spins, the rotor spins. |
| 03:57 | Incidentally this is why the rotor is called the rotor, because it's the part that rotates. |
| 04:02 | This of course means the magnetic field that has been created around that rotor, also spins. |
| 04:09 | This spinning magnetic field passes through windings attached to the stator which is the main alternator case and is called the stator because it's stationary. |
| 04:17 | This relative motion between the rotor's magnetic field and the stator windings, generates current in the stator windings giving us the electrical energy that we need to run the electronics in our vehicle. |
| 04:29 | Common automotive alternators have 3 connection points on them. |
| 04:32 | Usually called output field and sense. |
| 04:35 | The output connection is usually a threaded stud and this is where the current is pumped out of the alternator to the rest of the system. |
| 04:43 | The field wire is connected to the rotor inside the alternator and is used to establish the magnetic field around the rotor. |
| 04:51 | The sense pin is the input to the feedback control system that determines the amount of current that the alternator would output. |
| 04:58 | In a more modern setup you might find the alternator output is controlled by the vehicle ECU. |
| 05:03 | In these setups, the ECU will be monitoring the system voltage and will send a control signal to the alternator, adjusting its current output up or down to ensure the system voltage level is maintained within certain bounds. |
| 05:16 | The form this control signal takes varies with manufacturer and if you're working with an ECU controlled alternator, you will want to talk to your ECU manufacturer about what will be required to control this system. |
| 05:30 | That's just a brief overview of how alternators work, it's actually a pretty intense subject and not really what this course is about. |
| 05:37 | But we do need to have some basics in place to determine how we're going to make the connections to the alternator and ensure its correct operation. |
| 05:43 | The first of these connections is the main output stud. |
| 05:47 | This is typically wired from the alternator to the isolator output connection, meaning that when the isolator is tripped, the alternator output is isolated from the battery. |
| 05:57 | The exception to this being with an old style manual example as outlined within the isolators course section. |
| 06:03 | If the system isn't using an isolator, this output is connected directly to the battery positive terminal via a fusible link, capable of carrying the maximum alternator output current. |
| 06:14 | The wiring for this connection in either case will be a large gauge cable, also capable of carrying the full alternator output current. |
| 06:22 | The second connection is the battery sense connection and this is the signal the alternator uses to determine how much current to supply the main output connection. |
| 06:31 | The alternator looks at the voltage on this pin and outputs more and more current, until this voltage reaches a pre determined level, typically between 13.8 and 14 volts. |
| 06:42 | This wire is connected to either the isolator output connection, if an isolator is used, or the battery positive when one isn't. |
| 06:51 | There's almost no current travelling along this wire so it's a smaller gauge as voltage drop across it isn't as much of a concern. |
| 06:59 | Now you've probably twigged to the fact that the main alternator output and the battery sense connect back exactly the same place. |
| 07:06 | So why don't we save ourselves the hassle of running the battery sense wire in the harness and just connect it to the alternator output stud at the alternator? As that's where it's connected to back at the battery or isolator anyway. |
| 07:16 | Well when the alternator is outputting a lot of current there will be a measurable volt drop across that main output wiring. |
| 07:24 | So if we connect the sense wire to the output stud at the alternator, this won't be taken into account by the charging system. |
| 07:31 | The sense wire will be reporting say 13.8 volts to the alternator but the battery might only be seeing 13 volts. |
| 07:38 | The last and most complicated connection is the field connection, also sometimes called the light connection. |
| 07:44 | This is where the rotor gets its current from in order to establish its magnetic field but only when the alternator is stationary. |
| 07:52 | Once the engine is started and the alternator is spinning, it'll generate current which self supplies the rotor within the alternator itself. |
| 08:00 | This is how the alternator charge light in many cars works. |
| 08:04 | When the ignition is initially turned on, the rotor is supplied current via the alternator warning light, causing this light to turn on. |
| 08:12 | Once the engine is started and the alternator is self supplying the rotor, current doesn't flow through that light anymore so it'll be turned off. |
| 08:20 | The complicated part of this connection is that it can both draw and supply power depending on whether the engine is running or not. |
| 08:28 | Where this can cause a problem is if you have some sort of switch in the system that you operate to give the PMU a turn on or wake up signal but that circuit also supplies power to the alternator rotor. |
| 08:42 | If the engine is running and you flick the switch off, it's possible that the alternator which is now self supplying its rotor, will then also supply that PMU turn on signal. |
| 08:53 | Luckily there's an easy solution to this and that is to supply the field connection on the alternator via either its own PMU output channel or one that supplies other engine related loads like ignition coils or injectors. |
| 09:06 | Separating it out like this will prevent any back feeding issues causing unexpected results if you try and turn the PMU off when the engine is running. |
| 09:13 | This does mean that the alternator field current needs to be accounted for in the PMU output channel configuration though. |
| 09:20 | As a general rule of thumb, this will be a maximum of 2 amps. |
| 09:24 | To recap what we've covered in this module, as it is a bit of a longer one, the starter is a high powered DC motor used to spin the internal combustion engine, enabling it to then start and sustain its full running cycle. |
| 09:36 | This motor has a high power draw and is wired either directly to the isolator output or to the battery positive terminal. |
| 09:45 | The alternator is the electrical power generation part of the system and is actually what our electrical system runs from when the internal combustion engine is operating. |
| 09:53 | There are 3 connections typically, the main output which we wire to the isolator output or battery positive if there's no isolator, using a large gauge cable. |
| 10:01 | This is sized and fused to handle the alternator maximum output current. |
| 10:07 | The battery sense wire which we wire to the same place as the main output wire but using a smaller cable as it doesn't handle any appreciable current. |
| 10:14 | And the rotor field wire which we power from the PMU output channel. |
| 10:19 | While these components are more peripheral to a PMU itself in that the PMU usually doesn't supply power to them directly, it's crucial that we have a good understanding of their basics when we're designing the power supply systems in our vehicles. |
