How to Make a Rheostat – Part 1
In operating small motors there is as a rule no means provided for regulating their speed, and this often is quite a disadvantage, especially in the case of toy motors such as used on miniature electric locomotives.
Excerpt from the book: THE BOY MECHANIC – BOOK 2 1000 THINGS FOR BOYS TO DO – WITH 995 ILLUSTRATIONS PUBLISHED 1915, BY H. H. WINDSOR CHICAGO POPULAR MECHANICS CO. PUBLISHERS
The speed, of course, can be regulated by changing the number of cells of battery by means of a special switch, but then all the cells are not used the same amount and some of them may be completely exhausted before the others show any appreciable depreciation.
If a small transformer is used with a number of taps taken off the secondary winding, the voltage impressed upon the motor, and consequently the speed, can be changed by varying the amount of the secondary winding across which the motor is connected.
But in both these cases, there is no means of varying the speed gradually. This can, however, be accomplished by means of a small rheostat placed in series with the motor. The rheostat acts in an electrical circuit in just the same way a valve does in a hydraulic circuit.
It consists of a resistance, which can be easily varied in value, placed in the circuit connecting the motor with the source of electrical energy. A diagram of the rheostat is shown in Fig. 1, in which A represents the armature of the motor; B, the field; C, the rheostat, and D, the source of electrical energy.
When the handle E is in such a position that the maximum amount of resistance is in circuit there will be a minimum current through the field and armature of the motor, and its speed will be a minimum.
As the resistance of the rheostat is decreased, the current increases and the motor speeds up, reaching a maximum value when the resistance of the rheostat has been reduced to zero value. Such a rheostat may be used in combination with a special switch F., as shown in. Fig. 2.
The switch gives a means of varying the voltage and the rheostat takes care of the desired changes in speed occurring between those produced by the variations in voltage.