DC Machines

What is DC Motor ?

Electrical motors  are everywhere around us. Almost all the electro-mechanical movements we see around us are caused either by a AC or a DC motor. Here we will be exploring DC motors. This is a device that converts DC electrical energy to a mechanical energy

Principle of DC Motor

This DC or direct current motor works on the principal, when a current carrying conductor is placed in a magnetic field, it experiences a torque and has a tendency to move.
This is known as motoring action. If the direction of current in the wire is reversed, the direction of rotation also reverses. When magnetic field and electric field interact they produce a mechanical force, and based on that the working principle of DC motor is established.

The direction of rotation of a this motor is given by Fleming’s left hand rule, which states that if the index finger, middle finger and thumb of your left hand are extended mutually perpendicular to each other and if the index finger represents the direction of magnetic field, middle finger indicates the direction of current, then the thumb represents the direction in which force is experienced by the shaft of the DC motor.

Structurally and construction wise a direct current motor is exactly similar to a DC generator, but electrically it is just the opposite. Here we unlike a generator we supply electrical energy to the input port and derive mechanical energy from the output port. We can represent it by the block diagram shown below.

Here in a DC motor, the supply voltage E and current I is given to the electrical port or the input port and we derive the mechanical output i.e. torque T and speed ω from the mechanical port or output port.

The input and output port variables of the direct current motor are related by the parameter K.So from the picture above we can well understand that motor is just the opposite phenomena of a DC generator, and we can derive both motoring and generating operation from the same machine by simply reversing the ports.

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Types of DC Motors

Direct motors are named according to the connection o the field winding with the armature. There are 3 types:

1. Shunt wound DC motor
2. Series wound DC motor
3. Compound wound DC motor 

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 DC Shunt Motor

The shunt wound DC motor falls under the category of self excited DC motors, where the field windings are shunted to, or are connected in parallel to the armature winding of the motor, as its name is suggestive of. And for this reason both the armature winding and the field winding are exposed to the same supply voltage, though there are separate branches for the flow of armature current and the field current as shown in the figure of DC shunt motor below

Construction of a Shunt Wound DC Motor

Just that there is one distinguishable feature in its designing which can be explained by taking into consideration, the torque generated by the motor. To produce a high torque,

1. 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.
2. 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.

application of a DC shunt motor

As you can see in above image, even though output torque of DC shunt motor increases comparatively speed doesn’t fall much.

DC shunt motor is also called as constant speed motor.

In other words, if we assume that the supply voltage is constant then flux also becomes constant. At the rated speed the back emf also becomes nearly constant if the load is same.

The various applications of DC shunt motor are in

1. Lathe Machines,
2. Centrifugal Pumps,
3. Fans,

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DC Series Motor

A series wound DC motor like in the case of shunt wound DC motor or compound wound DC motor falls under the category of self-excited DC motors, and it gets its name from the fact that the field winding in this case is connected internally in series to the armature winding. Thus the field winding are exposed to the entire armature current unlike in the case of a shunt motor.

Construction of Series DC Motor

Construction wise a this motor is similar to any other types of DC motors in almost all aspects. It consists of all the fundamental components like the stator housing the field winding or the rotor carrying the armature conductors, and the other vital parts like the commutator or the brush segments all attached in the proper sequence as in the case of a generic DC motor.

Yet if we are to take a close look into the wiring of the field and armature coils of this DC motor, its clearly distinguishable from the other members of this type.
To understand that let us revert back into the above mentioned basic fact, that the this motor has field coil connected in series to the armature winding. For this reason relatively higher current flows through the field coils, and its designed accordingly as mentioned below.

The field coils of DC series motor are 1,wound with relatively fewer turns as the current through the field is its armature current and hence for required mmf less numbers of turns are required

2,The wire is heavier, as the diameter is considerable increased to provide minimum electrical resistance to the flow of full armature current.

3,In spite of the above mentioned differences, about having fewer coil turns the running of this DC motor remains unaffected, as the current through the field is reasonably high to produce a field strong enough for generating the required amount of torque. To understand that better lets look into the voltage and current equation of DC series motor.

Speed Regulation 

Some other purposes of DC series motors are i.e. cranes, machine tools , pumps blowers etc.

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DC Compound Motor

A compound wound DC motor or rather a DC compound motor falls under the category of self excited motors, and is made up of both series the field coils S₁ S₂ and shunt field coils F₁ F₂ connected to the armature winding as shown in the figure below.
Both the field coils provide for the required amount of magnetic flux, that links with the armature coil and brings about the torque necessary to facilitate rotation at desired speed.
As we can understand, a compound wound DC motor is basically formed by the amalgamation of a shunt wound DC motor and series wound DC motor to achieve the better off properties of both these types. Like a shunt wound DC motor is bestowed with an extremely efficient speed regulation characteristic, whereas the DC series motor has high starting torque

So the compound wound DC motor reaches a compromise in terms of both this features and has a good combination of proper speed regulation and high starting toque.
Though its staring torque is not as high as in case of DC motor, nor is its speed regulation as good as a shunt DC motor. Overall characteristics of DC shunt motor falls somewhere in between these 2 extreme limits.

Types of Compound Wound DC Motor

The compound wound DC motor can further be subdivided into 2 major types on the basis of its field winding connection with respect to the armature winding, and they are:

Long Shunt Compound Wound 

 DC Motor In case of long shunt compound wound DC motor, the shunt field winding is connected in parallel across the series combination of both the armature and series field coil, as shown in the diagram below.

Short Shunt Compound Wound DC Motor

In case of short shunt compound wound DC motor, the shunt field winding is connected in parallel across the armature winding only. And series field coil is exposed to the entire supply current, before being split up into armature and shunt field current as shown in the diagram below