What are DC motors?

A DC motor is a type of electric motor which converts electric energy into mechanical energy. The forces that are produced by the magnetic field are what those common types of motors rely upon. Either electro-mechanical or electronic, all types of DC motors have some internal mechanism to change the current direction in part of the DC motors. As the DC motors could be powered from existing DC lighting power distribution systems, these motors were the first form of motors widely used. By changing the strength of the current in its field winding or by using a variable supply voltage, the speed of the DC motors can be controlled over a wide range.

The small DC motors are used in toys, tools, and appliances. Large DC motors are used in elevators, hoists, and propulsion of electric vehicles and also used in drives for steel rolling mills. There are two types of DC motors such as self-excited and separately excited DC motors. The self-excited DC motors are in turn classified into three types which are DC series motor, DC shunt motor, and DC compound motor.

DC series motor

In this article, we will discuss DC series motors. The main function of this motor is to produce mechanical energy using electrical energy. The working principle of this motor mainly depends on the electromagnetic law which states that in the region of the current-carrying conductor whenever a magnetic field is formed and cooperates with the field outside then the rotation motion can be generated. When the series motor gets started, it gives utmost motor speed and torque slowly with high motor speed.

A current-carrying conductor cooperates with the exterior magnetic field whenever the magnetic field is approximately formed and then a rotating motion is generated. The series motors can work both on DC and AC and AC series motors are called Universal motors. This motor uses an electromagnet as its stator to create its magnetic field. These motors are very similar in construction to the DC series motor but it is modified slightly. These motors have high starting torque and also can run at high speed and are compact and lightweight. In portable power tools and equipment, these motors are used.

Components used in DC series motor

The components of the DC series motors are the rotor (the armature), commutator, axle, stator, field windings, and brushes. The fixed component of the DC series motor is the stator and it is made up of two otherwise more electromagnetic pole parts. Allied to the commutator, the rotor has the armature and the windings on the core. Through a brush array allied to the commutator, the power source can be connected to the armature windings.

For rotating, the rotor consists of a central axle and due to the larger quantity of current, the field winding must be able to hold a high amount of current. Therefore, with the help of solid gauge wire, the motor windings can be fabricated. A huge number of twists is not permitted by this wire. Since a large amount of current flows through the windings and a sufficient amount of heat is dissipated across the field winding so it must be fabricated with solid copper bars.

DC series motor circuit explanation

The field, as well as the stator windings in this motor, are coupled in series by each other. The armature and series field current is equivalent accordingly. From the supply towards the field winding, a huge amount of current is supplied. Since these windings are thick and have few turns so these series field windings can carry this huge current.

The stator windings are formed with the help of the copper bars generally. The heat which is generated by the heavy flow of current is dissipated by the thick copper bars. Here the stator field winding S1-S2 are connected in series with the A1-A2 of rotating armature.

In DC series motors, the power is supplied in between one end of the field winding and one end of the armature. The current flows from terminals of the power supply through the series and armature windings whenever the voltage is applied. To the flow of this current, the large conductors present in the armature and field windings provide the only resistance. The resistance of these conductors is very low since they are so large.

Due to this reason, the motor draws a large amount of current from the supply. The coils reach saturation when the large current begins to flow across this field and armature windings, which results in the production of the strongest possible magnetic field. The greatest amount of torque is provided to the armature shafts due to the strength of these magnetic fields. With the maximum amount of power, the large torque causes the armature to rotate.

Speed control of DC series motor

The following two methods are employed to control the speed of the DC series motor:

1. Flux or field control method
2. Armature-resistance control method

Since the flux generated can be changed in the case of the armature-resistance speed control method so this method is frequently used.

Advantages of series wound DC motors

The main advantages of series wound DC motors are:

• Starting torque is vast
• Simple design and can be assembled easily
• Easy protection methods
• Very cost-effective

Disadvantages of series wound DC motors

The disadvantages are as follow:

• The speed regulation is very poor. With the increase in load the machine speed decreases.
• The DC series motor torque reduces sharply with the increase in speed.
• Before running the motor it always needs load. So these motors are not suitable for those applications where there is no load.

Context and Applications

This topic is significant in the professional exams for graduate, and postgraduate courses.

• Bachelors in Science in Electrical Engineering
• Masters in Science in Electrical Engineering

Practice Problems

Q1. In which motor the field winding and the armature winding is coupled in series?

1. DC shunt motor
2. DC series motor
3. PMDC motor
4. DC compound motor

Answer: Option b

Explanation: The field and the armature windings in DC series motors are coupled in series by each other.

Q2. What are AC series motors called?

1. PMDC
2. Shunt motor
3. Universal motors
4. None

Answer: Option c

Explanation: The series motors can work both on DC and AC. The AC series motors are called universal motors.

Q3. How many types of DC series motors speed control methods are there?

1. One type
2. Two types
3. Three types
4. Four types

Answer: Option b

Explanation: The two following methods are employed in order to control the speed of DC series motor: Flux or field control method and Armature-resistance control method.

Q4. What amount of current is carried by the field windings?

1. Low
2. Medium
3. Very high
4. None

Answer: Option c

Explanation: From the supply towards the field winding a huge amount of current is supplied.

Q5. How many types of DC motors are there?

1. 2 types
2. 3 types
3. 4 types
4. 5 types

Answer: Option a

Explanation: There are two types of DC motors such as self-excited and separately excited DC motors.

Related Concepts

• Ward Leonard control
• Torque and speed of a DC motor
• Armature controlled DC motor