9 Types of DC Motors: Working Principle and Uses

Summary

If you've ever wondered how electric vehicles smoothly accelerate from zero to highway speeds, or how industrial machinery maintains precise control over heavy loads, the answer often lies in one remarkable invention: the DC motor.

These electrical workhorses have been powering our world for over a century, from the smallest toy car to massive mining equipment.

Unlike their AC counterparts, DC motors offer exceptional speed control and high starting torque, making them indispensable in applications were precision and power matter most.

Today, as automation and electric mobility reshape our technological landscape, understanding the different types of DC motor becomes crucial for engineers, hobbyists, and anyone curious about the machinery that drives modern life. 

What are DC Motors?

A DC motor is an electrical machine that converts direct current electrical energy into mechanical rotational energy.

Unlike alternating current (AC) motors that rely on changing current direction, DC motors operate on the principle of electromagnetic induction using a constant current flow. 

The fundamental operation depends on the interaction between a magnetic field and current-carrying conductors, creating a force that produces rotational motion. 

The basic structure consists of a stator (stationary part) that provides the magnetic field, and a rotor (rotating part) called an armature that carries current-conducting coils.

When current flows through the armature windings within the magnetic field, it experiences a force according to Fleming's left-hand rule, causing rotation.

This rotation is maintained through a commutator and brush system in traditional designs, which reverses the current direction in the armature windings at appropriate intervals. 

DC motors excel in applications requiring variable speed control, high starting torque, and precise positioning. Their linear relationship between voltage and speed makes them ideal for servo systems, while their ability to provide maximum torque at zero speed suits heavy-duty applications perfectly. 

Classification of DC Motors 

The types of DC motor can be classified based on various criteria, including field excitation methods, commutation techniques, and construction design. Here are the nine primary types that cover the spectrum of DC motor application: 

1. Permanent Magnet DC Motors (PMDC) 

Permanent Magnet DC Motors represent one of the most efficient and compact designs in the DC motor family. These motors eliminate the need for field windings by using high-strength permanent magnets to create the required magnetic field.

This design makes them particularly popular in battery-powered applications where efficiency and size constraints are critical factors. 

Working Principle 

The working principle of PMDC motors relies on the interaction between permanent magnets and current-carrying conductors.

The permanent magnets, typically made of neodymium, ferrite, or samarium cobalt, are mounted on the stator to create a constant magnetic field. 

When DC current flows through the armature windings positioned within this magnetic field, each conductor experiences a force according to Fleming's left-hand rule. 

The force on each conductor is given by F = BIL, where B is the magnetic flux density, I is the current, and L is the length of the conductor.

Since the magnetic field from permanent magnets remains constant, the torque produced is directly proportional to the armature current.

The commutator and brush system reverses the current direction in the armature conductors at appropriate intervals, ensuring continuous rotation in the same direction.

This constant flux characteristic provides linear speed-torque relationships, making PMDC motors ideal for precise control applications. 

2. Separately Excited DC Motors 

Separately excited DC motors offer superior control flexibility by providing independent power supplies to the field and armature circuits.

This configuration allows engineers to optimize both field strength and armature current independently, making these motors highly versatile for applications requiring wide speed ranges and precise torque control. 

Working Principle 

The working principle involves two independent electrical circuits: the field circuit powered by a separate DC source and the armature circuit with its own power supply.

The field winding creates an electromagnet that generates the required magnetic field, while the armature carries the load current. Since these circuits are independent, the field current can be adjusted without affecting the armature current, and vice versa. 

The electromagnetic torque is proportional to the product of field flux and armature current (T ∝ Φ × Ia).

By controlling the field current, the flux can be varied, allowing field weakening operation where speed can be increased beyond base speed while maintaining constant power. 

The motor equation V = Eb + IaRa shows that back EMF (Eb) is proportional to field flux and speed. This relationship enables precise speed control through either armature voltage variation or field current adjustment, making separately excited motors ideal for applications like rolling mills and paper machines. 

3. Self-Excited Shunt DC Motors 

Shunt DC motors are renowned for their excellent speed regulation and stable performance under varying load conditions.

In these motors, the field winding connects in parallel with the armature circuit, receiving the same supply voltage.

This parallel connection ensures that field current remains relatively constant, providing consistent magnetic field strength regardless of load changes. 

Working Principle 

The working principle of shunt motors centers on the parallel connection between field and armature windings. The field winding, having high resistance and many turns, draws a small, nearly constant current from the supply voltage.

This creates a magnetic field of relatively constant strength. The armature current varies with load requirements while the field current remains stable. 

Since the field flux remains approximately constant, the back EMF is directly proportional to speed (Eb = kΦN, where k is a constant, Φ is flux, and N is speed).

The torque equation T = kΦIa shows that torque is directly proportional to armature current when flux is constant. As load increases, armature current increases proportionally, but the speed decrease is minimal due to constant field flux.

This characteristic provides excellent speed regulation of DC motor, typically within 5% from no-load to full-load conditions, making shunt motors ideal for applications requiring consistent speed like machine tools and fans. 

4. Self-Excited Series DC Motors 

Series DC motors are the powerhouses of the DC motor family, designed to deliver maximum starting torque for heavy-duty applications.

These motors connect the field winding in series with the armature, ensuring that the same current flows through both components.

This configuration creates a unique relationship between torque and speed that makes series motors ideal for traction and lifting applications. 

Working Principle 

The working principle of series DC motors is based on the series connection of field and armature windings, creating a direct relationship between load current and magnetic field strength.

The field winding, having relatively few turns and low resistance, allows the full motor current to flow through it. This means that both field flux and armature current increase simultaneously with load. 

The torque equation T = kΦIa becomes T = k(kI)I = k₁I², showing that torque is proportional to the square of the current at low speeds when the magnetic circuit is unsaturated.

This relationship provides extremely high starting torque, often 4-5 times the rated torque. The speed equation N = (V - IaRa)/(kΦ) reveals that speed is inversely proportional to flux and current.

As load increases, current and flux both increase, causing speed to decrease significantly. This variable speed characteristic, while providing poor speed regulation, is advantageous for applications like electric vehicles and hoists where high torque at low speeds is essential. 

5. Compound DC Motors 

Compound DC motors combine the best characteristics of both series and shunt motors by incorporating both series and shunt field windings.

These motors offer a compromise between the high starting torque of series motors and the good speed regulation of shunt motors.

The compound configuration can be either cumulative or differential, depending on whether the magnetic fields aid or oppose each other. 

Working Principle 

The working principle of compound motors involves the interaction of two separate field windings: a shunt field connected in parallel with the armature and a series field connected in series with the armature.

In cumulative compound motors, both windings are connected to produce magnetic fields in the same direction, with the total flux being the sum of both field contributions. 

The series field provides additional flux proportional to load current, while the shunt field maintains a base level of magnetization.

As load increases, the series field strengthens, providing additional torque similar to a series motor, but the shunt field prevents excessive speed variation.

The torque equation becomes T = k(Φshunt + Φseries)Ia, where both flux components contribute to the total torque. 

In differential compound motors, the series and shunt fields oppose each other, resulting in flux that decreases with increasing load. This creates nearly constant speed characteristics but with reduced starting torque.

The working principle allows compound motors to provide good starting torque (better than shunt) with acceptable speed regulation (better than series), making them suitable for applications like elevators and rolling mills. 

6. Brushed DC Motor 

Brushed DC motors are the classic, time-tested design found in everything from toy cars to industrial cranes. Their appeal lies in a simple, low-cost construction that pairs permanent-magnet or wound-field stators with a rotating armature.

Because speed varies almost linearly with applied voltage, these motors are easy to control with nothing more than a variable-voltage supply, making them a popular choice where affordability and straightforward drive electronics matter more than absolute efficiency. 

Working Principle 

The working principle of brushed DC motors is based on the fundamental interaction between magnetic fields and current-carrying conductors, utilizing mechanical commutation to maintain continuous rotation.

The stator provides a stationary magnetic field through either permanent magnets or electromagnets, while the rotor (armature) consists of wire coils wound around an iron core and connected to copper commutator segments.

Carbon or graphite brushes maintain physical contact with the rotating commutator, allowing direct current to flow into the armature windings as the shaft turns. 

When current flows through the armature conductors positioned within the magnetic field, each conductor experiences a force according to Fleming's left-hand rule, where the force is proportional to the magnetic flux density, current magnitude, and conductor length.

This electromagnetic interaction creates torque that causes the rotor to spin. The key to continuous rotation lies in the mechanical commutation system, where the commutator segments and brushes work together to reverse the current direction in each armature coil every half-turn of the rotor. 

As the rotor rotates, the commutator mechanically switches the current direction in the armature windings at precisely the right moments to maintain unidirectional torque.

This automatic current reversal ensures that the magnetic forces always act in the same rotational direction, sustaining continuous motion without requiring external electronic switching circuits.

The motor's speed-torque characteristics show that torque is directly proportional to armature current, while speed decreases slightly with increasing load due to the back EMF that opposes the supply voltage.

Speed control is achieved through simple methods like varying the supply voltage or inserting series resistance, making brushed DC motors exceptionally user-friendly for basic applications despite their inherent limitations of brush wear, electrical noise, and maintenance requirements 

7. Brushless DC Motors (BLDC) 

Brushless DC motors represent the pinnacle of DC motor evolution, eliminating the mechanical limitations of traditional brush-commutator systems.

These motors use electronic switching to control current flow in the stator windings, based on rotor position feedback from sensors.

This design dramatically improves efficiency, reduces maintenance requirements, and extends operational life compared to brushed motors. 

Working Principle 

The working principle of BLDC motors involves electronically controlled commutation instead of mechanical brushes. The stator contains three-phase windings that create a rotating magnetic field, while the rotor typically consists of permanent magnets.

Position sensors, usually Hall effect sensors or encoders, provide real-time feedback about the rotor's angular position to an electronic controller. 

The controller uses this position information to energize the appropriate stator windings at precise moments, creating a magnetic field that leads the rotor by approximately 90 electrical degrees.

This generates maximum torque by maintaining optimal alignment between stator and rotor magnetic fields. The switching sequence ensures that the magnetic field rotates smoothly, with the permanent magnet rotor following this rotating field. 

Since there are no brushes to create voltage drops or friction losses, BLDC motors achieve higher efficiency (typically 85-90% compared to 75-80% for brushed motors).

The electronic commutation allows precise control over timing and current, enabling optimal performance across various operating conditions.

Speed control is achieved through pulse width modulation (PWM) of the supply voltage, providing smooth and efficient operation with excellent speed regulation. 

8. Long Shunt DC Motor 

Long shunt DC motors represent a specific configuration of compound DC motors where the shunt field winding is connected in parallel with the entire motor circuit, including both the armature and series field windings.

This arrangement provides unique operating characteristics that combine the benefits of both series and shunt motor features.

Long shunt configuration is commonly used in applications requiring good starting torque with reasonable speed regulation, such as elevators and printing presses. 

Working Principle 

The working principle of long shunt DC motors involves the parallel connection of the shunt field winding across the entire motor circuit.

The shunt field receives voltage equal to the terminal voltage, while the same current flows through both the armature and series field windings in series.

This configuration means the shunt field current remains relatively constant and independent of the armature current. 

The total magnetic flux is the sum of flux produced by both field windings: Φtotal = Φshunt + Φseries. The shunt field provides a base level of magnetization that remains nearly constant, while the series field contributes additional flux proportional to the load current.

As load increases, the armature current increases, strengthening the series field and providing additional torque. 

The voltage equation becomes V = Eb + Ia(Ra + Rs), where Rs is the series field resistance. Since the shunt field voltage equals terminal voltage, it remains stable regardless of load changes.

This provides better speed regulation compared to pure series motors while maintaining higher starting torque than pure shunt motors.

The long shunt configuration typically offers slightly better speed regulation than short shunt motors because the shunt field voltage is less affected by the voltage drop across the series field resistance. 

9. Short Shunt DC Motor 

Short shunt DC motors are another variant of compound DC motors where the shunt field winding connects in parallel only with the armature, excluding the series field winding.

This configuration creates different electrical characteristics compared to long shunt motors while still providing the combined benefits of series and shunt motor features.

Short shunt motors are preferred in applications where slightly higher starting torque is more important than perfect speed regulation. 

Working Principle 

The working principle of short shunt DC motors is based on connecting the shunt field winding in parallel with only the armature circuit. The series field remains in series with both the armature and shunt field combination.

This means the shunt field voltage equals the voltage across the armature terminals, which is slightly lower than the terminal voltage due to the voltage drop across the series field resistance. 

The current relationships show that the total line current divides between the shunt field and the series combination of armature and series field.

The series field carries the same current as the armature, while the shunt field current depends on the voltage across the armature terminals: Vsh = V - Is×Rs, where Is is the series field current. 

As load increases, the voltage drop across the series field increases, reducing the voltage available to the shunt field. This causes the shunt field flux to decrease slightly with increasing load, while the series field flux increases significantly.

The net result is that the series field has a stronger influence on the motor's characteristics compared to long shunt configuration. 

This arrangement typically provides slightly higher starting torque because the series field effect is more pronounced, but the speed regulation is marginally poorer than long shunt motors.

The working principle makes short shunt motors suitable for applications requiring good starting performance, such as machine tools and small industrial drives where the load variations are moderate. 

 

 

Conclusion 

The nine types of DC motor discussed represent the evolution of electrical machine technology, each designed to meet specific application requirements.

From the simplicity of permanent magnet motors to the precision of stepper motors, and from the ruggedness of series motors to the efficiency of brushless designs, DC motors continue to play crucial roles in modern technology.

Understanding their working principles and applications helps in selecting the right motor for specific needs, whether in industrial automation, automotive systems, or consumer products.

As technology advances toward greater efficiency and environmental consciousness, DC motors, particularly brushless variants, will likely become even more prevalent in our electrified future.