When it comes to the speed control of DC series motor, we could discuss it through the below methods: flux control method, armature-resistance control method.
Discussion about this approach, the flux generated by the DC series motor is changed. The three methods below would do a great clarification on the flux’s variation.
Through this approach, a variable resistance, also named field diverter, is connected in parallel with series field winding. Then the field gradually weakened by one part of the line current. Since N ∝ 1/ϕ , field flux also affects the speed. If the current reaches zero through the diverter, the motor’s usual speed ( the lowest speed) would be gained by this approach,that is diverter open circuited.
For gaining a lower speed, a variable resistance, also named armature diverter, is connected in parallel with the armature and to reduce the armature current. That would contribute to the increasing o f flux. So the speed decreases since N ∝ 1/ϕ.
Under this approach, to reduce the amount of the series field winding’s turns is the way to reduce the flux.
Fig (c) shows that field coil is tapped dividing number of turns. Thus through selecting different number of turns, we can select different value of Ø.
As the fig (d) shows, under this approach, several speeds can be gained by regrouping coils.
Under this approach, the direct connection in series between a variable resistance and the supply affects the voltage available across the armature and hence the speed falls.
Any speed lower than the normal speed could be gained through changing the variable resistance’s value. That changing the value to control the DC series motors’ speed is the most common approach.