Harold Hall


Considering first the shunt wound motor. If one were to measure the resistance of the motor and using this and ohm's law to calculate the current taken, it would be found that the result was very much greater than anticipated for the motors rating. This is because another factor, back emf, comes into play to control the eventual current taken.


The initial current will though conform to the calculated value but as the motor commences to rotate it also acts as a generator, generating a voltage (back emf) whose polarity opposes that of the supplied voltage thereby limiting the current.


As the motor speed, and back emf, continue to increase, the current will fall until the current is just sufficient to provide the torque the load demands. Incidentally, with a constant field strength, torque developed is proportional to current. At this point the back emf is only just short of the applied voltage, typically say for a 12V motor a back emf in the region of 11V. As it is this difference in voltage, 1V in the example, that drives the current, any change in load and therefore current demanded, will require a change in back emf to make the change possible.


Let us consider that the torque demanded by the load doubles requiring twice the current. In this case the 1V difference will require to increase to 2V which can only be achieved by the back emf reducing to 10V. As back emf is proportional to speed, the speed of the motor will have to reduce by the same proportion. Actually, in an attempt to keep my figures simple the speed variation suggested, around 10%, is rather greater than is likely to occur in practice, especially at higher power ratings.


Workshop Data

A series motor is though much more difficult to describe and understand in detail, though the basic principles of back emf, etc., are still applicable. Whilst with the shunt motor the field strength is constant, in the case of the series motor it will vary with torque demanded. Therefor, twice the motor current will produce four times the torque as both the field, field-strength and armature, field-strength will increase by a factor of two. This produces the main advantage of the series motor, that is, it capable of a greater torque output for short periods, either whilst running or during start up. On the down side, speed variation due to load is greater than with a shunt wound motor.


I mentioned above that the initial current at start up is limited only by the resistance of the armature. As this becomes proportionately less as the motor size increases some form of limiter to give it a soft start becomes essential, In the early days this would have been a resistor in series that would be switched out of circuit after a few seconds or in some cases, a tapped resistor which would be switched out gradually At what power rating this becomes necessary I am not sure as my experience has been with much larger motors.


AC/DC commutator motors

Alternating current commutator motors work in exactly the same way as DC motors but when used on AC they benefit from a minor internal modification to their construction. For this reason, motors labelled for use on DC should not be used on AC, though if labelled for use on AC they should work reasonably well on DC. Some motors are though labelled as being suitable for both.