Series Wound DC Motor or DC Series Motor

In electric motors, the series wound DC motor or DC series motor is one of the types of self-excited DC motor. The field winding of the series wound DC motor is internally connected to the armature winding and stands out for its robust starting torque and unique characteristics. Whereas in the shunt wound DC motor, the field winding is connected in parallel with the armature winding.

In this article, we will cover the topics of Construction, Voltage and current equation, Speed regulation and with the advantages, disadvantages and applications of series wound DC motor or DC Series Motor.

Table of Content

Series Wound DC Motor or DC series Motor
Construction
Voltage and Current Equation
Speed and Torque
Speed Regulation
Characteristics
Advantages and Disadvantages
Applications

What is a Series Wound DC Motor or DC series Motor?

It is one of the types of self-excited DC motors. In this motor, field winding is wired series with the armature winding which means the same current flows through both field and armature windings, creating a strong magnetic field proportional to the armature current. Mainly, it has two parts i.e., stator and rotor which meet electrically and magnetically. This configuration is what gives the series wound motor its defining characteristic – high starting torque.

Construction of a Series Wound DC Motor

The Components of the series wound DC motor include

Construction of Series wound Motor

Majorly, it is divided into three main categories.

Stator
Armature
Brushes

Stator

Frame: Unlike some motor designs that utilize a solid cast iron frame, series wound DC motors often employ a lighter and more economical approach. The stator frame can be constructed from rolled steel sheets laminated together. This lamination minimizes eddy currents, which are parasitic currents induced in the solid metal by the changing magnetic field. Eddy currents create unnecessary heat loss and reduce motor efficiency.

Field Windings: The field windings are the heart of the magnetic field generation. Traditionally, these windings are made from thick, round copper wires with high current carrying capacity. However, for some high-performance applications, alternative wire shapes like rectangular or square copper conductors might be used. These shapes can improve packing density within the limited space available for the windings, allowing for more turns and a stronger magnetic field.

Armature

Core: Similar to the stator frame, the armature core is often laminated from steel sheets to minimize eddy currents. However, the core design can be more intricate than a simple cylinder. Some series wound DC motors, particularly those designed for high torque, might employ slotted cores with skewed slots. By a slight change of angle to the axis of rotation helps to reduce cogging torque. Due to the interaction between the permanent magnets , unequal torque experienced by the motor shaft is called cogging torque.
Commutator: Mainly, It is used for armature windings to ensure correct current flow. Traditionally, commutator segments were made from solid copper. However, modern motors might utilize copper segments with silver brazing on the contact surfaces with the brushes. This brazing material offers superior wear resistance compared to pure copper, extending the commutator’s lifespan.
Armature Windings: It is typically made up of round copper wire, but for high-power applications, rectangular or square copper conductors might be used for improved packing density. Based on the motors design and performance, the winding pattern can be changed. Some motors might employ lap windings, while others utilize wave windings. These winding configurations influence factors like the number of poles generated, back EMF (electromotive force) produced, and overall motor behavior.

Brushes

Brushes are the sliding electrical contacts that transfer current to the commutator. It is made up of carbon or graphite. Brushes made from a combination of copper and graphite for high performance and for critical environments, brushes are used. Compared to pure carbon brushes, It improves current carrying capacity and reduces sparking. These brushes are also known as metal graphite brushes.

There is a another one which is responsible for rotation.

Shaft and Bearings: To handle the torque generated by the motor and transmit it to the load, the shaft is needed. It is made up of high strength steel. The shaft also extends outwards to connect the motor to the load it needs to drive. For the shaft rotation, bearings are supported. It can be ball bearings or roller bearings.

Voltage and Current Equation of a Series Wound DC Motor

Given Below is the Series Excited DC Motor

Series Excited DC Motor

The voltage equation for a series wound DC motor is similar to other DC motors and can be expressed as:

The basic voltage equation of the DC motor is

E = E _b + I _se R _se + I _a R _a

where,

E _b = Back EMF generated by motor

R _se = Series resistance

I _a = Armature current

R _a = Armature resistance

Since I _se = I _a , we can write the equation as

E = E _b + I _a (R _se + R _a )

This is the voltage equation of series wound DC motor.

For both the armature and the field winding receives the same current in series wound DC motor. Therefore, the total current is equal to the armature current and field current. This current equation is given by

I(total current) = I _a =I _f

The key difference lies in the relationship between current and magnetic field. In a series wound motor, the magnetic field strength is directly proportional to the armature current.

The magnetic flux is directly proportional to armature current or supply current.

𝜙 = I _a = I _supply

In series wound DC motor, the magnetic flux produced is sufficient to produce torque, even with the minimum number of turns of coils.

Speed and Torque of a Series Wound DC Motor

The Series wound DC motor has a relationship between current and amount of torque is produced. The torque is directly proportional to current over a range of graph. As the armature current increases, the magnetic field strengthens, leading to a greater force between the field and the armature conductors, resulting in higher torque. The stronger magnetic field interacts with the current-carrying conductors in the armature, generating a more significant force between them. Series wound DC motor becomes extremely essential as starter motors for most industrial applications dealing in heavy mechanical load like huge cranes or large metal chunks etc. Series wound DC motors are operated for a very small time about only a few seconds, just for the purpose of starting.
By understanding this relationship between speed and torque is vital for selecting the right type of DC motor for your specific application.

Speed Regulation of a Series Wound DC Motor

Due to the self-regulating nature of a series wound motor, external methods are often needed to control its speed. Here are some common approaches:

Variable Resistor: Adding a variable resistor in series with the armature circuit allows for manual speed control by adjusting the current.
Solid-State Speed Controls: Electronic control circuits utilizing thyristors or transistors can offer more precise and automated speed regulation.

The Back EMF(E_b ) equation is given by

E_b = PΦNZ / 60A

Where,

P = Number of poles

A = Number of parallel paths

N = Speed of rotation(rpm)

Z = Total no of conductors in armature

Effect of load addition on DC Series Motor

Speed of rotation (N) = (E_b x 60A) / PΦZ

The speed of the motor is directly proportional to back EMF and inversely proportional to Magnetic flux(Φ).

However, the speed regulation of a series wound motor is poor. As the motor speeds up, the back EMF (E _b) increases, opposing the applied voltage (V). This reduces the current (I) flowing through the motor, consequently lowering the magnetic field strength and torque. This inherent characteristic makes it unsuitable for applications requiring precise speed control.

Characteristics of Series Wound DC Motors

High Starting Torque: Ideal for applications requiring rapid acceleration, like cranes, hoists, and electric locomotives.
Poor Speed Regulation: Speed decreases with increasing load due to the relationship between current and magnetic field.
High Current Draw: The series connection can lead to high armature currents, requiring careful design considerations.
Simple Construction: Series wound DC motor has simple design and low maintenance requirements are required.
Cost-effective: Due to simple design, series wound DC motor is more cost-effective.
Sparking: When brushes make contact with commutator, sparks are generated during the motor operation.

Advantages and Disadvantages of Series Wound DC Motors

Advantages of Series Wound DC Motors

High Starting Torque: Makes them suitable for applications requiring strong initial acceleration.
Simple Construction: Easier to manufacture and maintain compared to some other motor types.
Cost-Effective: Due to their relatively simple design, series wound motors can be more affordable.
High Speed: It can achieve relatively high speeds in applications where precise control isn’t necessary.
High Power Density: Due to their size, it can deliver a high amount of power.

Disadvantages of Series Wound DC Motors

Less efficient: They may not be the most efficient in terms of energy conversion.
High Current Draw: Can strain the power supply and require larger conductors.
Sparking: The brushes can generate sparks during the operation when contact with commutator that may pose a safety hazard in flammable environments.
High Current Draw: The series connection can lead to high armature currents, requiring careful design considerations.
Poor Speed Regulation: Inherent characteristic makes them unsuitable for applications requiring precise speed control.

Applications of Series Wound DC Model Motors

Electric Vehicles: Electric scooters, golf carts, and light rail vehicles due to their high starting torque.
Construction Equipment: Due to their ability to handle, Cranes, hoists, and winches utilizes series wound motors
Trains: Series wound motors make them a common choice for powering model train locomotives due to their cost effective and simplicity.
Power Tools: Grinders or uses series DC motor which provides high starting torque.
Elevators: Series wound DC motor also plays a role in elevators.

Conclusion

By the proper understanding of characteristics, advantages ,disadvantages, and applications of series wound DC motors, we can make informed decisions about their suitability for your specific needs. Their powerful starting torque makes them valuable assets in various situations, but careful consideration of their limitations, such as speed control and sparking, is essential.