Many years ago, the magazine 'Model Engineer' published an article on building a model loco using a car dynamo as a motor. The practise has died out in UK, but, we are told, persists in Australia. Dynamos are designed as - dynamos. They are not designed as motors and cannot give best efficiency as a motor! Nevertheless our controllers will drive a car dynamo used as a motor. You could use an NCC, a 2QD or a 1QD series controller but since you can easily reverse the motor by reversing the field connections there is not so much of advantage in using the NCC series so the 2QD series may seem better suited.
Although there are problems in using dynamos as motors, we believe we have found the best solution to these. Unfortunately to get best performance requires a bit of trial and error and a little technical knowledge. The cheapest solutions often are not the easiest!
To explain the problems some points should be understood. The usually advocated method of controlling the dynamo is to apply 12v permanently to the armature and to vary the field. This works after a fashion but there are two snags: firstly there is no fine slow motor speed control: the motor starts with a jerk. Secondly, the dynamo's speed is increased by decreasing the field current (and/or voltage) but unfortunately the motor gets less efficient as the field is reduced so, as as the speed is increased the efficiency eventually drops so much that the motor stops completely - although it still draws a lot of current. This maximum speed point depends on the load and the battery voltage so it is unpredictable.
The 'correct' way to control the speed would be to treat the dynamo as if it was a field energised motor: i.e. apply a voltage permanently to the field and vary the armature. Efficiency is maximised and, to decrease the speed, you decrease the armature voltage (via the controller) as normal. The battery lasts a lot longer because of the improved efficiency and there is no maximum speed above which the dynamo stops. Unfortunately the dynamo was designed to be an efficient generator at slow speeds with 12v applied to the field. Used as a motor therefore it will run very slowly, so existing gearing would not be suitable: the magazine article used the drive from an old hand-drill as the gearing.
The best solution, if you have such gearing, is to control both armature and field simultaneously, decreasing the field voltage as the armature voltage in increased. This can be done using only one controller and it overcomes the disadvantages of separate armature or field control.
The controller works in effect by adjusting the motor's -ve output (the motor is connected between battery + and battery -). At zero speed M- is connected to B+ and at full speed it is connected to B-. In between M- varies between the two.
So, at, for instance, 1/3 full speed there will then be 4 volts across the armature (B+ to M-). It follows that there will then be 8 volts between M- and B- This voltage will mirror the armature voltage and is exactly what we want for the field.
The circuit shows the method. We illustrate using a reversing controller such as an NCC-60.
D1, D2 1N5401 or similar
for 12v, R1: omit (short circuit)
R2: 47 ohms 3 watt
for 24v, R1: 10 ohm 15 watt
R2: 100 ohm 5 watt
If using two dynamos from 24v, connect the armatures in parallel (this will double the top speed, compared with 12v) and the fields in series and use:
R1: omit (short circuit) R2: 100 ohm 5 watt
At zero speed the field is fed through R1 and R2 in parallel so (with a 12v supply) the full 12v is present across the field for starting. The dynamo we tested had a field resistance of 8 ohms, so with a 24v supply you will get about 11v with the values shown.
At full speed, M- is connected to B-, so D1 is reversed and the field is fed through the 47 ohm resistor only, giving about 1.75 volts on the field for full speed
The values aren't too critical. Increasing R2 will increase the top speed but it also reduces the motor's efficiency. If R2 is too large the motor will simply stall at high speeds, as explained above.
The field winding must of course be separated from the armature. If you don't want to use a reversing controller, the direction can be changed by connecting the field via a standard DPCO reversing switch (wiring is explained in the FAQ sheet). Beware of changing the field when the motor is running: the armature then tries to draw a large current (which will be safely limited by the 2QD) which will cause a mechanical jolt.
The diagram shows the layout of a suitable dynamo field board that can be used with the 2QD. R1A and R1B are in parallel, R2A and R2B are in series, but R2B may be linked out, enabling the board to be configured for any voltage. Sorry, 4QD cannot supply this board or the components.
If you use an 'ignition' switch on the motor speed control, when this is off the field will still draw current, as it will have a full 12v present across it. You would need a relay to open the power to the field connection.