Varying a DC motor’s speed is simple, just vary the supply voltage. With advanced
electronics, and including a small tacho generator driven by the drive motor to signal
to the controls the actual motor speed, a speed range of 100:1 is easily possible.
This though is beyond the normal requirement of the home workshop.
There are numerous ways of varying the voltage, none of which can be covered in detail.
Typically, for a very small motor, it could be fed via the output from a small potentiometer
(variable resistance) with a fixed resistor to set the minimum speed, Sk 2.
At larger sizes, a variac (variable output transformer) could feed the motor via
a rectifier bridge, though a separate fixed supply would be required for the shunt
field, Sk 3. Be careful with this set up though as the motor is not isolated from
the mains with this type of transformer. A tapped transformer with a multi position
switch and again a rectifying bridge will give a range of fixed speeds, but again
a separate supply for the shunt field would be required.
An electronic controller would though be the ideal method, especially if with either
armature voltage feedback, or even better, tacho generator feedback. I have mentioned
feedback and some may like this explained. This feeds back to the controller a voltage
to represent the actual speed of the motor and enables the system to compensate when
the motor speed attempts to change due to changes in the load demanded.
It should be pointed out that for a given torque requirement the 1V difference between
supply and back emf, as suggested earlier, remains constant throughout the speed
range. Similarly the effect of load change, 1v to 2v in the example, also remains
the same. Whilst this is numerically the same in terms of rpm it is proportionally
much greater at lower speeds and as a result limits the speed range possible. Therefor,
attempting to power say a lathe spindle by this means would not be acceptable as
the speed would vary unacceptably at lower speeds as differing depths of cut and
feed rates were attempted. If using a controller with feedback then these problems
do not exist.
Varying the supply voltage to a series wound machine will also have the effect of
changing its speed, its poor speed holding though makes it a much less attractive
Returning to a shunt wound motor there is one other method of controlling its speed,
that is to vary its field strength by varying the supply voltage to the shunt field.
In the above example it was described how the back emf would increase until the difference
between it and the supply voltage was just sufficient to produce the required current.