by Tim Harris at the eMOG Pub


1.  In the classic vehicle dynamo, the field winding, which has small gauge wire since it requires little exciting current , is stationary whereas the armature (rotor) is the bit that supplies the heavy current and is, therefore, wound with thicker wire.  The max RPM of a dynamo is limited - by the choice of pulley size - so as not to throw off these heavy windings by centrifical force.  Usually, a dynamo will supply about 22amps at max RPM. Old cars were fitted with 30-0-30 scale ampmeters and the charge level
could be "seen".

2.  Secondly, the dynamo cannot be left connected to the battery at low RPM (tickover) as it would run itself like a motor.  The "control box", therefore, had both an electro-mechanical regulator and a relay (an
electro-mech switch) in it.  The former controlled the charge, the latter disconnected the dynamo from the battery when the output voltage was less than battery voltage.  Hence the ignition light would often come on at tickover in old cars

3.  The output of a dynamo is also limited by the brush gear on the commutator.  Unless the brushes are made larger, there is a finite current that can be interupted by the commutator before unacceptable arcing causes premature failure.


1.  In the vehicle alternator, the construction is effectively reversed from that of a dynamo.  Low current, high resistance, small gauge windings on  the rotor (rotating).  Heavy current, low resistance, large gauge windings in the stator (staionary).  Very much heavier (larger gauge) stator windings are possible on the alternator than in the armature of a dynamo.  As these winding don't move, the max RPM on the alternator is higher than a dynamo- circa 200% to 300% .  60amps and more at max RPM is also not unusual.  By dint of having a small pulley on the alternator allowing this higher max RPM, it will be spinning faster at "tick-over".  The alternator will, therefore, usually charge the battery at any engine RPM.  The "ignition light" rarely comes on at tickover on a modern car - except in "fault" conditions.

2.  The alternator is effectively stopped from taking current from the battery by the recitfier diodes connected between the former and latter. These allow current to flow only one way.  The charge control is done by varying the current into the rotor.  By the way, vehicle alternators are usually two or three phase and give relatively smoother DC after recitification.

3.  The rotor takes low currents through smooth slip-rings - not much arcing. As far as running a motor as a dynamo and visa versa.  Many DC motors and dynamos have the commutator segments or brushes offset relative to the windings.  In a motor, this causes it to run better in one direction than the other.  It also has the advantage that the brushes aren't swapping from one segment of the commutator to the next when the armature is taking or generating max current.   That means that not every motor makes a good dynamo and visa versa.

The other thing I see often - latest in Stan Brays new book from TEE - is a misconception about DC motors with wound fields (ie not permanent magnets). It is often said that by reducing the field current - ie inserting a resistor in the field circuit - the motor will run slower.  WRONG !  All other things being equal - ie the torgue required can be covered by the motor, the action is thus :-

The motor starts from rest and speeds up until the Current x (BatteryVoltage minus motor "back EMF") equals the required torque. (Please excuse an Engineer using the phrase "back EMF".  It helps to explain the workings, even if it's a false quantity.  FYI.  EMF = Electromotive Force and can be thought of as "voltage" if you like.)

Now, the "back EMF" is actually the same as the EMF from a generator.  By dint of the motor's armature rotating in a magnetic field it is acting as  a generator whose output is proprtional to speed x field strength.
Therefore, if we reduce the "field" by a series resistor, the motor MUST speed-up until the "Current x(Battery Voltage  minus motor "back EMF") equals the required torque" equation again holds true.
Some two speed windscreen wipers use this priciple.  Of course, the torque output of the motor is proportional to a number of factors.  So, if you take too much torque out of the motor, it will slow down and finally stall. Lowering the field strength reduces the torque and stalling can occur before
speed up if you "get the sums wrong".