What Does an Alternator Do?
Ellie Dyer-Brown, 6 months ago
5 min read
- Alternator
- How it works
FixMyCar discusses what an alternator is, what it does and how it works.
When people think about what’s powering their cars’ electrical systems, the battery gets most of the glory. But in reality, the alternator keeps things like the headlights, windscreen wipers and radio running.
So, what is an alternator, and how does it work? This guide explains everything you need to know.
Contents
How long should an alternator last?
What's the difference between an alternator and a generator?
What is an alternator?
The alternator is part of a car’s charging system. It looks like a small cylinder and is usually bolted to the engine. While the battery stores energy, the alternator supplies it by using electromagnetism to convert mechanical energy from the car's movement into electrical power.
If this sounds similar to a generator, that’s because it is. Alternators are like small generators; until recently, that’s what they were called.
Alternator components
Let’s take a closer look at the components of an alternator.
Rotor
The rotor is attached to a metal shaft that the drive belt spins. It is cylindrical and has a triangle pattern around the outside, known as triangle finger poles. These triangles are magnetic with alternating north and south orientations around the rotor. When the rotor is energised, it becomes an electromagnet.
Pulley
A pulley runs along the outside of the alternator, driven by a serpentine or drive belt. The belt is powered by the engine’s crankshaft pulley, which moves the alternator pulley. This movement allows the alternator to work by providing the mechanical energy that it converts into electrical power.
Voltage regulator
Too much voltage can damage electrical systems, so a voltage regulator controls the alternator’s output and the battery’s charging process.
Stator
The stator covers the spinning alternator rotor. Three copper wire windings are attached to the inside of the stator; as the magnetised rotor spins, an alternating current is created in the windings.
Rectifier
The stator creates an alternating charge (AC), but car batteries need a direct current (DC) charge. That’s where the rectifier comes in - it converts the alternating charge into direct power that can feed the battery. It does this using diodes (a semiconductor device) to redirect the AC charge into one direction of flow.
How does an alternator work?
The alternator has two roles: it keeps your car’s electrical systems working while the engine is on - including the headlights, windscreen wipers and dashboard instruments - and charges the battery. It achieves this by turning mechanical energy into electrical energy.
The engine powers a drive belt that rests on a pulley linked to the alternator. The pulley turns the alternator’s rotor shaft, spinning a set of magnets in a coil, which creates an alternating current in the stator. This current is sent into the rectifier, which converts it into DC energy, activating the car’s electrical systems.
How long should an alternator last?
The alternator should last the lifetime of a car. However, it can sometimes fail prematurely due to exposure to water, faulty parts, heat damage, frayed wires and general wear.
Signs of a faulty alternator
Common symptoms of a malfunctioning alternator include:
Slow or faulty electrical components
Check out this guide for more detailed advice about the signs of a faulty alternator.
How to test an alternator
The above symptoms suggest there could be a problem with the alternator, but other parts might be to blame. That’s why it’s a good idea to test your alternator using a voltmeter or multimeter before replacing it - the steps below explain how.
1. Set up the multimeter
The multimeter should be set to the correct unit measurement (DCV - direct current voltage).
2. Locate the battery terminals
The positive (+) terminal is usually red, and the negative (-) one is generally black. Connect the red wire of the multimeter to the positive terminal and the black one to the negative terminal. If the alternator works well, the multimeter should show a reading of around 13-14 volts.
3. Start the engine and check again
Once the engine is switched on, take another reading. The voltage should ideally be between 14.2 and 14.7 volts. A higher reading suggests the alternator is overcharging the battery; a lower one suggests it isn’t charging the battery enough.
4. Carry out a load test
Switch on the headlights, radio and other electrical components. The voltage reading should stay roughly within the same volt range.
Incorrect voltage readings during this process indicate a problem with the alternator or one of its components. The simplest solution is often to replace the alternator because a repair isn’t necessarily easier or cheaper.
What’s the difference between an alternator and a generator?
Alternators and generators are similar - they convert mechanical energy into electrical energy. However, an alternator always produces an alternating current, whereas a generator can create AC or DC.
They work slightly differently, too. A generator uses an alternating armature inside a stationary magnetic field, while an alternator uses a rotating magnetic field inside a stationary armature.
Can you jump-start a car with a bad alternator?
It’s possible to jump-start a car with a faulty alternator. However, doing so is only a temporary fix. The vehicle will stall as soon as the battery runs out of power.
There are other things to consider, too. A malfunctioning alternator might cause voltage irregularities and spikes, which could damage the electrical system, leading to a more extensive repair job. That’s why it’s best to fix the problem rather than opting for a short-term way around it.
How much is an alternator?
Alternators vary in price, typically between £100 and £ 400. That's how much you can expect to pay if you buy the part and fit it yourself. This guide discusses the cost of taking your car to a mechanic for an alternator replacement.
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