OK off we go, but not with the Mather and Platt dynamos yet because I can't find the pics. We'll start in the middle and just chat and post stuff and see where it goes.
Today, I want to take a first look at ride motors and their speed controls. In the era we're considering, most rides were driven by 110V DC brush motors. Either one big motor driving the centre of a ride such as a galloper, which could be in the range of say 5-20hp, or a number of smaller motors in individual cars such as you might find on a juvenile train ride. These motors differ in detail but almost every motor and indeed generator that we are going to meet at the fair is of the same general configuration: A wound armature fed by brushes on a commutator revolves in a stationary magnetic field.
These days, many DC motors have permanent magnets to create the field but for much of the history of DC motors, there were no permanent magnets strong enough to make an efficient, compact motor, so the magnetic poles on the frame were wound with field coils to form electromagnets. There are various ways the field coils can be connected - in series with the armature, in parallel (shunt) and a combination of the two (compound). We'll begin with shunt winding (see attached diagram) as this is the simplest way to produce a motor that runs at a steady, controllable speed. The field is not necessarily connected directly in parallel with the armature as there can be control resistances in either circuit. What we mean by shunt-wound is that the armature and field are energised separately from the supply voltage, so that their currents are independent and can be controlled separately. Note, this different to most small brush motors in household appliances and power tools, etc, which are series-wound.
Time to introduce some essential characteristics of ideal, generalised DC motors:
- Torque is proportional to armature current and also proportional to field strength.
- Speed is proportional to armature voltage but inversely proportional to field strength.
If we want a shunt-wound motor to run at a steady speed and be able to deliver maximum torque if the ride needs it, we must supply it with:
- Full field current (this is normally limited by the resistance of the field winding itself, and is at maximum when connected directly to the rated supply voltage.)
- Armature voltage to suit the desired speed. Again, nornally the rated speed of the motor corresponds to the rated voltage across the armature.
- Whatever current the armature demands, to develop the required torque. The higher the mechanical load, the more current it will take.
Before this post gets too wordy, let's have some pics of drive motors. Here you can see the main motors on an octopus, ferris wheel, dive bomber and speedway ark, and for comparison the small motor on a juvenile ride. In the next post we'll look at the speed controls themselves.