On this type of motor the field magnetism is produced by current flowing through a coil surrounding the magnetic circuit instead of by a magnet made from special material which, when once magnetised, retains this property, hence the term permanent magnet.

Wound field motors have been used in model locomotives for two reasons, firstly because when electrically driven models were first introduced permanent magnets were very much weaker than they are today, and secondly because some model manufacturers decided to use alternating current which, as already stated, made wound field series motors obligatory. The development of stronger permanent magnets, combined with the availability of cheap rectifiers to produce direct current, has made the wound field motor obsolescent although there are some collectors models still in service fitted with one.

The field coil can either be connected in shunt across the armature and the supply, Figure 1, or in series with the armature, Figure 2, the latter being by far the most common. Shunt wound motors will only operate on direct current but series wound ones will work on direct or alternating current and are often referred to as universal motors. Series connection is essential for alternating current because the current in the field and armature must reverse at the same instant. This is not the case with a shunt connection because the inductance of the field circuit causes its current to lag behind that in the armature.


Figure 1. Shunt wound motor.


Figure 2. Series wound motor.

The great disadvantage of a wound field motor is that normally it cannot be reversed by changing the polarity of the track supply. This is because doing so changes the direction of the current in both the armature and field and so does not alter the direction of rotation. To operate on AC, the relative direction of current in the armature and field windings must be altered by a switch on the locomotive as shown in Figures 3 and 4.


Figure 3. Reversing a shunt wound motor.


Figure 4. Reversing a series wound motor.

This can either be hand operated or a sequence reverser which operates each time the current is switched on. Neither method is an ideal solution, but when rectifiers were much more expensive than they are today they did allow a cheap supply system to be employed, 6 and 20 volt AC series motors being fairly widely used, particularly by train set manufacturers such as Hornby.

For operation on DC only, and as an alternative to using a reversing switch as described above, the availability of small diodes (which are electronic devices which pass current in one direction only) makes possible a very simple and cheap method of reversing a wound field motor by changing the polarity of the supply, but it is stressed that a motor so modified will not work on AC.

The method consists of supplying either the field winding or the armature through diodes connected in such a way that the current flows through the chosen winding in the same direction irrespective of the polarity of the supply. The diodes, which as a general rule should be able to carry at least 3 amps and withstand not less than 50 volts reverse voltage can be either four separate ones (Figure 5) or a single bridge rectifier (Figure 6). The latter will usually be the better arrangement. A number of suitable types small enough to be mounted between the frames of steam locomotives are readily available from electronic component dealers. There is no need to remove the original reversing switches, retention of which can be an advantage on three rail systems because it allows locomotives to run ‘nose to tail’, which would not normally be possible.


Figure 5. Reversing a shunt wound motor using individual diodes.


Figure 6. Reversing a shunt wound motor using a full wave rectifier.

This article was compiled by the Technical Committee for the Gauge O Guild Manual. It was adapted for the GOGWiki by Nick Baines.