battery light

[Vanny]

google motor

its basics of a motor, without the excitor you cant make juice. these days alternators are self exciting, unless there self generating kerrs types like on my car, the harder you brake the more charged you get.

[electrokid]

The rotor is the bit that rotates. The stator is the bit that remains stationary.

The alternator works by having a load of coils forming the stator - the rotor has one coil which produces a magnetic field - when that magnet is rotated close to the coils a current is produced in the stator coils.

Modern alternators are called 'self exciting' because there is always enough of a magnetic field remaining in the rotor for it to start generating current. Providing a current to the coil in the rotor is 'to excite' the alternator. The more current supplied to the rotor the greater the magnetic field it produces - and the greater the output from the stator coils will be.

The current supplied to the rotor comes from the regulator - the connections to the rotor are via the brushes to the slipring and the other end of the coil is connected to the rotor spindle which is therefore connected to 'ground' 'earth' or 'battery negative' however you want to think of it.

The regulator monitors the voltage at the alternator output (which is connected directly to the battery +) and if it is below the battery's 'full charge' voltage it sends power to the rotor to increase the rotor's magnetic field and hence the output of the alternator. This is an analogue control system - as the voltage gets closer to the 'full charge' condition the drive current - or excitment - is reduced accordingly.

It takes comparatively little current to produce the rotor's magnetic field - the electricity produced by the alternator is the result of the mechanical power applied to rotate the alternator shaft. The more power that is drawn from the alternator, the more mechanical power is lost. The battery is usually at its flattest immediately after starting - which is why a loose alternator belt will slip noisily at this time when the load on the belt is at its greatest. The conversion from mechanical power to electrical power is - 1 BHP = 746 Watts - but there are some losses - mechanical and electrical which mean it falls short of this figure.

The coils in the stator produce alternating current (AC) which is converted to direct current (DC) or 'rectified' which is why there are (usually 6) thumping great diodes in the electronics assembly.

[Geoffrey Gould]

Thank you for the explanation Electrokid , one thing that wasn't touched was the 'Ign light' or charging light where if I remember correctly is a 12V supply (+) to one side of the bulb from the ignition switch, the other side going to a small tag on the alternator which I thought was for excitation, apologies if I have got it wrong it has been a long time since I played with one.
I do remember a fault that used to rear it's head to cause confusion and that was worn out/high resistance contacts in the ignition switch that caused the ignition light to glow.
G.

[Mickey taker]

the only electrical connections are a female spade that gos on the regulator and a round earthing ring style connector that gos to the B+ terminal on the alternator,
I removed the origional alternator and put on the spare which caused the battery light to stay on all the time , fitted new brushes to the origional alternator and am now getting the red light on when changing gear i.e. clutch down and revs dropping if that makes sense , light go's out when engine is under load i.e. accelerating and seems to stay off at tickover now as well

[Geoffrey Gould]

Yep thats the one, big connection direct to battery plus and the small one to the ign. light. Easy check is to take the little lead off and short it to earth and turn the ignition on. DO NOT START. and the ign. light should be on and go off when the ignition switch is off.
Brushes do take a little time to bed in.
Any doubts, volt meter across battery , start engine and the voltage should be about 14.6 volts with a few revs on, headlamps,heater etc. on and the volts should stay almost the same, say 14 volts. minimum.
G.

[electrokid]

Thank you for the explanation Electrokid , one thing that wasn't touched was the 'Ign light' or charging light where if I remember correctly is a 12V supply (+) to one side of the bulb from the ignition switch, the other side going to a small tag on the alternator which I thought was for excitation,

My pleasure The small connector on the alternator is indeed just for the 'IGN' light. I think I'm right in saying that there is +12 applied to one side of the IGN bulb and the other end to the 4mm tag on the alternator. Part of the regulator circuit, that connection is taken to ground when there is no charge taking place thus driving the bulb. I'm sure there are many different implementations of the reg circuit - one I've seen has just 3 diodes to change the AC of the stator coils to a pulsed DC to sense whether there's charging going on.

There are usually 3 pairs of coils in the stator - providing a 3 phase output - hence the need for 6 diodes to rectify to DC.

During the change from dynamos to alternators there probably were versions without an onboard regulator - where the regulator was external as was the norm for dynamos - and there would indeed have been a small connection for the rotor 'excitement'.

I do remember a fault that used to rear it's head to cause confusion and that was worn out/high resistance contacts in the ignition switch that caused the ignition light to glow.

Haven't come across that one - but there are very many different failure modes. One of the more common ones is when one of the 6 main diodes fails so it's charging with just 2 of the 3 phases - depending on how the regulator is designed this can make the IGN lamp glow dimly - particularly at tickover. The only way to absolutely determine whether or not it's charging is to monitor the battery volts - which should be increasing with time after start - normally sttling at around 14 volts.

[Geoffrey Gould]

Hi Electrokid re. the glowing ign. light ( have yet to work out how to do quotes, as you may have guessed.) It was a Ford Zodiac somewhat earlier than your Granada. Great fun.
G.

[electrokid]

The Zodiac / Zephyr was for me an unfulfilled ambition - never mind. The electrics of that era and including the MK2 were usually ok but occasionally they were capable of doing really weird things.