What is a DC motor?

A DC motor is an electromechanical device that converts electrical energy from electrical supply into mechanical energy to connected load by a shaft, by using direct current (DC) as the electrical energy.

DC motors are classified based on the connection between the field winding and armature winding.

There are two types of connections for DC motor, they are:

• Self-excited DC motor connection (field winding and armature winding are directly connected)
• Separately excited DC motor connection (field winding and armature winding are not directly connected)

Further self-excited DC motor is classified into two types of connection, they are:

• Series Connection (Series motor)
• Shunt Connection (DC shunt motor)

Compound Connection (DC compound motor) is simply the combination of these two connections.

Figure A shows DC shunt motor connection.

Figure B shows DC series motor connection.

Figure C shows compound motor connection.

Self-excited DC motor

In this DC motor type, the field winding and the armature windings are electrically connected. This is a single-supply motor.

Field winding

The field winding is the winding wound on the stator part; it is called field winding because this winding creates the magnetic field required for the rotation of the rotor. This field winding can be connected in series with armature winding, in this case, the magnetic field created is called a series field. If the field winding is connected in parallel/shunt with the armature windings, in this case, the magnetic field created is called the shunt field. When field winding is excited (connected to supply (DC in this case)) a magnetic field is generated inside the stator area, simultaneously creating magnetic poles.

Armature winding

The armature winding is the winding wound on the rotor part; this armature winding is responsible for the rotation of the rotor. When field winding is excited, thus creating magnetic field (maybe series field/shunt field) armature current in the armature winding experiences a torque and rotates the rotor part of DC motor. The field winding is connected to armature winding (in self-excited), this connection may be series, parallel, or combination.
Series connection gives us a DC series motor.
Parallel connection gives us DC shunt motor.
The combination of series and parallel gives DC compound motor.

Shunt wound DC motor/shunt motor

In the DC shunt motor, the field winding is connected parallel (shunt) to the armature winding. This motor is the most commonly used self-excited DC motor. The field coil is placed parallel (shunt) to the armature coil, producing a shunt field to the armature coil. In other words, the shunt field is the field created by the shunt coil placed parallel to the armature coil.

In this shunt connection type DC motor, current from the supply gets divided into two parts, one for field winding (let it be $I_f$) and other to armature winding (let it be $I_a$).

$I_f$: field Current.

$I_a$: Armature Current

$I_f$ creates magnetic field across the armature windings. Then $I_a$ is parallelly supplied to armature winding, thus experiencing a torque in the magnetic field. It results in the rotation of the rotor as a result, flux cutting action takes place. According to ‘Faraday's law of Electro Magnetic Induction’ an Electro Motive Force (EMF) is generated in the armature winding known as Back Electromotive force (Back EMF).

It is called Back Emf, because it opposes the cause (Lenz’s law).

Let Back EMF be denoted by $E_b$.

Let total current supplied be $I_t$.

According to Kirchhoff’s current law:

$I_t=I_f+I_a ......\left(1\right)$

Let the resistance of armature winding called as armature resistance denoted by $R_a$.

Then the supply voltage

 $E=E_b+\left(I_a\times R_a\right) ......\left(2\right)$

$I_f=\frac{E}{R_f} ......\left(3\right)$

Where $R_f$: field winding resistance.

As back emf is proportional to flux $\left(\varphi \right)$ and speed $\left(N\right)$ in RPM.

$$\begin{gathered} E_b\alpha \left(\varphi \right) \\ {E}_b\alpha \left(N\right) \end{gathered}$$

   $E_b=k\times N\times \varphi ......\left(4\right)$

By equation 2 and 4 we get,

$N=\frac{\left(E-I_a\times R_a\right)}{\left(k\times \varphi \right)} ......\left(5\right)$

Equation 5 describes that speed of the DC shunt motor can be controlled by controlling $E$ (supply voltage) and $\varphi$(flux).

Thus, speed control of the dc shunt motor can be achieved by controlling $I_a$ (armature current), $\varphi$(flux, in field winding) or $E$ (supply voltage).

Series wound DC motor/series motor

In the series wound DC motor, the field winding is connected in series (field coil is series-wound) with armature winding (having some resistance named armature resistance). This shows that only single current flows in the circuit, that is the whole armature current required for rotation flows from the field winding/field coil (whereas a part of armature current was flown in DC shunt motor).
The coil placed series to the armature is called a series-wound coil. This series-wound coil creates a series field, across the armature windings.

This series field is the field responsible for electromagnetic induction phenomenon occurred due to rotation.
In this circuit, only a single current flows through armature and field windings/field coils.

For speed-Torque characteristics

$Supply voltage=E=Voltage drop in field circuit +Voltage drop in the armature winding$

$E=E_b+I\times \left(R_f+R_a\right) ......\left(6\right)$

from equation (4) we get,

$E_b=k\times N\times \varphi$

Neglecting Saturation,

$\varphi =k_a\times I ......\left(7\right)$

Substituting (7) in (6)

$N=\frac{1}{k_s}\left(\frac{E}{I}-\left(R_f+R_a\right)\right) ......\left(8\right)$

The main difference between DC series motor and DC shunt motor is the speed of the dc series motor varies with variation in load, whereas in DC shunt motor it is a constant speed machine, does not vary with variation in output mechanical load.

DC compound motor

The dc compound motor is a special case of a DC motor. It is a combination of series-wound and shunt-wound field winding. In other words, in this dc compound motor, a series-wound coil is placed to armature windings and a shunt-wound coil is also placed across the armature windings.
This combination is done because to achieve two types of motors (that is series field DC motor and shunt field DC motor) in a single DC motor, to achieve high starting torque (series field dc motor) and better speed control (shunt field dc motor).
Following are the types of connections for a compound DC motor.

• Cumulative compound DC motor
• Differential compound DC motor
• Short-shunt
• Long -shunt

Speed Control of DC motors

Using a DC motor is not efficient in any case, controlling its speed with high accuracy and efficiency gives us more valuable usage of DC motors. Speed control of DC shunt motor (self-excited) can be done by controlling the supply voltage, part of supply current going to armature winding (armature current) control, controlling the part of field current (so that flux can be controlled). The speed of the DC shunt motor can be controlled, by varying field resistance using a rheostat connected in series with the field coil to increase the field resistance, which varies the field current, armature current. By varying/controlling field current and armature current we can control speed.

Speed control of series DC motor is not that accurate, because speed control of series motor (self-excited) depends on supply voltage only. So, by varying the load supply we can control the speed of self-excited series DC motors. Starting torque of the DC series motor is very high, but starting torque of the DC shunt motor is very low compared to the DC series motor. So, a shunt motor can operate at No-Load, whereas a series motor cannot work at No-Load.

Self-excited DC motor applications

Series motor

• Traction system
• Cranes
• Air compressors
• Vacuum cleaners
• Sewing machines

DC shunt motor

• Lathe machines
• Centrifugal pumps
• Fans
• Blowers
• Conveyers
• Lifts
• Spinning machines

The DC compound motor circuit is used generally in DC generators, for efficient power conversion. Especially long-shunt type of compound motors circuit is used for DC generators.

Context and applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelors in Science (Physics)
• Bachelors of Technology (Mechanical Engineering)
• Masters in Science (Physics)
• Masters of Technology (Mechanical Engineering)

Practice Problems

Q1. Which of the following is the alternative name for a DC shunt motor?

1. Constant flux motor
2. Constant Voltage motor
3. Constant current motor
4. Variable voltage motor

Correct Option- a

Explanation: As field winding is connected parallel to armature winding, field current for any variation in mechanical load remains the same for a constant power supply. So constant field flux is maintained in the circuit, for any mechanical load. It is called a constant flux motor.

Q2. In a shunt motor, if the armature resistance of armature windings is halved (maintaining shunt field constant) what happens to armature current?

1. Double
2. Halved
3. Remains Same
4. None

Correct Option- c

Explanation: In the question, it is said that constant torque, means for any changes in the circuit output torque is maintained the same. As torque is proportional to armature current, it does not change for any change in parameters in the circuit.

Q3. In which of the following cases series dc motors are required?

1. Load torque constant
2. Load torque varies frequently
3. Constant operating speed
4. High starting torque

Correct Option- d

Explanation: These motors are generally used where the starting torque required is high. As the no-load operation of dc series motor torque is high, it is used where starting torque required is high.

Q4. Which of the following is a common usage of DC shunt motor?

1. Cranes
2. Electric Traction
3. Elevators
4. Lathe Machines

Correct Option- d

Explanation: Lathe machines do not require any high starting torque; it requires smooth and accurate control of speed. This can be done by DC Shunt motor.

Q5. Which of the following tasks can be performed, for reversing the direction of the DC series motor shaft?

1. Interchanging supply terminals
2. Interchanging field terminals
3. Not possible
4. Cannot be determined

Correct Option- b

Explanation: Speed is inversely proportional to the field flux, if we reverse its direction, the motor rotates in the reverse direction.

Related concepts

• Braking of DC motor
• DC generator
• Permanent Magnet Direct Current Motor (PMDC)