- DC Shunt Motor Definition: A DC shunt motor is defined as a type of DC motor where the field windings are connected in parallel to the armature winding, allowing both to receive the same voltage.
- Constant Flux: The DC shunt motor is a constant flux motor because the field flux remains nearly constant due to the parallel connection of the windings.
- Self-Speed Regulation: DC shunt motors can self-regulate their speed when the load changes, maintaining a steady speed without external modifications.
- Torque and Speed Relationship: In a DC shunt motor, the torque is proportional to the armature current, which helps the motor adjust its speed when the load varies.
- Industrial Use: DC shunt motors are popular in industrial applications where constant speed operation is essential, thanks to their self-regulating speed feature.
A DC shunt motor (also called a shunt wound DC motor) is a self-excited DC motor whose field winding is in parallel with the armature winding. Both windings therefore take the same supply voltage. Armature and field current then flow in separate branches.
DC Shunt Motor Equations
The DC shunt motor equations start from supply voltage E and line current Itotal at the terminals.
In a DC shunt motor that line current splits into Ia in the armature winding of resistance Ra and Ish in the field winding of resistance Rsh. Both windings have the same voltage.
Those currents then satisfy
Substitute that Ia to obtain the usual voltage equation of a DC shunt motor.
With a constant supply while the motor is running, shunt field current is
Also Ish ∝ Φ
so field flux Φ is proportional to field current Ish
Field flux therefore stays nearly constant, so a shunt wound DC motor is called a constant-flux motor. Speed still droops a little with load because of armature resistance. More DC motor study is in the set of over 1000 electrical questions.
Construction of a Shunt Wound DC Motor
A dc shunt motor uses the same main parts as other DC motors, as in the figure below.
The DC shunt motor still needs useful torque. Armature current is therefore much larger than field current, and the field winding uses many turns so flux linkage stays adequate.
- The armature winding must be exposed to an amount of current that’s much higher than the field windings current, as the torque is proportional to the armature current.
- The field winding must be wound with many turns to increase the flux linkage, as flux linkage between the field and armature winding is also proportional to the torque.
Keeping these two above mentioned criterion in mind a DC shunt motor has been designed in a way, that the field winding possess much higher number of turns to increase net flux linkage and are lesser in diameter of conductor to increase resistance (reduce current flow) compared to the armature winding of the DC motor. And this is how a shunt wound DC motor is visibly distinguishable in static condition from the DC series motor (having thicker field coils) of the self excited type motor’s category.
Self-Speed Regulation of a Shunt Wound DC Motor
A DC shunt motor largely holds its speed when load is applied to the shaft. Speed does fall a little, then armature current and torque rise and recover most of that drop, without a separate speed controller. The steps below show that loop.
The sequence is:
- Initially considering the motor to be running under no load or lightly loaded condition at a speed of N rpm.
- On adding a load to the shaft, the motor does slow down initially, but this is where the concept of self regulation comes into the picture.
- At the very onset of load introduction to a shunt wound DC motor, the speed definitely reduces, and along with speed also reduces the back emf, Eb. Since Eb ∝ N, given by,
This can be graphically explained below.
- This reduction in the counter emf or the back emf Eb results in the increase of the net voltage. As net voltage Enet = E − Eb. Since supply voltage E remains constant.
- As a result of this increased amount of net voltage, the armature current increases and consequently the torque increases.
Since, Ia ∝ Τ given by
The change in armature current and torque on supplying load is graphically shown below.
- This increase in the amount of torque increases the speed and thus compensating for the speed loss on loading. Thus the final speed characteristic of a DC shunt motor, looks like.
A shunt wound DC motor is therefore treated as a constant-flux, nearly constant-speed machine for jobs that need fairly steady speed under changing load, not a flat speed line.
