Why cannot series DC motors self regulate its speed?
Considering that there is no difference in the relations when it comes to series DC motors, except that the torque is related to the square of the current, why wouldn't a series DC motor be able to self regulate its speed ?
The answer to why a series DC motor is not able to self regulate compared to a Shunt motor is a D.C. Shunt Motor has a high resistance field (excitation) winding and a D.C. Series Motor does not.
To expand on this.
1). Firstly your question is poorly defined you should clearly state what type of equipment you are referencing. There are numerous types of D.C. Motors and variations on how the excitation can be achieved.
2). Second your conclusion is somewhat off the mark.You have failed to consider a) type of excitation and B) the load variation. Thus by taking a simplistic view of how the series excited motor works with no relation to load you have created a completely false scenario and compared that to a fictional shunt motor that in your world has been built with the same physical characteristics of a series motor.
With a series excited d.c. motor the load determines the speed at a given supply voltage and with no load the speed will continue to increase until the motor destroys itself. With a shunt excited motor the excitation circuit inherently has resistance added into the circuit and except in exceptional circumstances will also have an additional resistance in circuit (usually variable for speed control). The inherent resistance comes from the smaller conductor size and increased number of turn used for the excitation coils, which you cannot have in a series excited motor. The resistance in the shunt excited motor winding's thus reduces the speed so that the motor cannot reach a speed where it destroys itself.
When load increases on shunt DC motor, the speed decreases resulting in a downfall in back-EMF. This in turn, causes the net voltage, and hence the current to increase causing an increase in torque and thereby increasing the speed. Thus, it achieves a constant speed.
3). Thirdly you have told only half the story
But when we start loading the d.c. shunt motor, this decreases the speed of armature. This results in fall in back EMF. This reduced back EMF causes a larger current to flow through the armature winding and larger current produces increased driving torque.
Thus, the motor produces larger driving torque as it slows down. So motor will start running on the speed at which armature current is just sufficient to produce the required torque by the load.
On the other hand, when the load on the motor is reduced, the speed of armature increases due to excess of driving torque. This increases the back EMF which results in decreased armature current and thus required torque is reached. therefore speed of the Armature cannot indefinitely accelerate if all load is removed due to the back EMF and inherent resistance in the winding's of the shunt coil.
In this way back EMF in a DC shunt motor regulates the flow of armature current according to the load requirement automatically.
Where does the back emf originate??? Why shunt field resistance is high in the case of dc shunt motor?
Back emf is proportional to the rate of change of flux in the field winding. Tutorials
rate of change of Flux in the field winding is proportional to the mmf in the winding.
mmf = current passing through the winding * no of turns in the winding
To attain the required mmf we can use more current and less turns ( or ) less current and more turns .
As the power loss in the field winding is considered it is....Power loss = Square of voltage / resistance
As the voltage is constant for a high resistance it is possible to have less losses. So we prefer a high shunt field resistance.
As the field resistance is high, current is less, so number of turns in the shunt field will be more to attain the required mmf.
Types of D.C. Motors
In a series DC motor the field is connected in series with the armature.The field is wound with a few turns of large wire because it must carry the full armature current.
In a shunt motor the field is connected in parallel (shunt) with the armature winding's and can be wired with thinner conductors as it does not take the the full armature current. It can also be easily reverse connected.
A characteristic of series motors is the motor develops a large amount of starting torque. However, speed varies widely between no load and full load. Series motors cannot be used where a constant speed is required under varying loads.
Additionally, the speed of a series motor with no load increases to the point where the motor can become damaged. Some load must always be connected to a series-connected motor.
Series-connected motors generally are not suitable for use on most variable speed drive applications.
The shunt-connected motor offers good speed regulation. The field winding can be separately excited or connected to the same source as the armature.
An advantage to a separately excited shunt field is the ability of a variable speed drive to provide independent control of the armature and field.
The shunt-connected motor offers simplified control for reversing. This is especially beneficial in regenerative drives.
Note When I refer to separately excited it means the excitation field supply is independent from the motor supply.