The animation shows how electric motors work and their operating behaviour. Series and shunt-wound motors are compared.
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Description
Electric motors convert electrical energy into mechanical energy. They consist of a stator (fixed part) and a rotor (moving part). Dynamic magnetic fields are generated in both the stator and the rotor. The rotor is set in motion by the interaction of the two magnetic fields.
In reality, two parameters of an electric motor can usually be changed directly: the speed and the torque. Speed and torque influence each other. If a drill is pressed against a workpiece, the torque increases and the speed decreases.
The relationship between speed and torque varies depending on the design of the motor. The motor type (series connection or shunt connection) has a major influence.
Series motor: Here, the armature and the excitation winding are connected in series. These motors offer a high starting torque and are often used in applications with variable loads, such as electric trains.
Shunt motor: The armature and the excitation winding are connected in parallel. These motors offer a constant speed and are often used in applications with a constant load, such as fans or pumps.
In principle, the torque of a motor is proportional to the armature current. The following formula applies to all electric motors:
\[ M = c \cdot \phi \cdot I_A \]
- M: torque
- c: motor constant
- Φ: magnetic flux
- IA: armature current
The parameter c is a constant specific to the respective motor. This constant indicates the motor’s performance capability and is a key parameter provided by the manufacturer. The flux Φ is a physical quantity that describes the magnetic field in the motor.
The internal resistance of a motor influences the power consumption and therefore the torque. A lower internal resistance leads to a higher current consumption and consequently to a higher torque.
The internal resistance depends not only on the conductor material (ohmic component) but can also change during operation (inductive component). The cause is back-EMF: when a conductor moves through a magnetic field, a back-voltage is induced. At low speeds this effect is small.
The following formula extends Ohm’s law to include the internal resistance of the voltage source:
\[ I_A = \frac{U – U_i}{R_A} \]
- IA: armature current
- U: terminal voltage
- Ui: back-EMF voltage
- RA: armature resistance
A series-wound motor is an electric motor in which the armature and the excitation winding are connected in series. As the speed increases, the back-EMF rises, which reduces the current and consequently the torque.
In contrast, in a shunt-wound motor, the armature and the excitation winding are connected in parallel. In a parallel circuit, the increase in resistance generally has less of an effect on the overall resistance of the circuit. This means that a shunt-wound motor still draws sufficient current even under heavy load.
The different characteristic curves lead to different areas of application. For a train, a slow start with high torque and low speed is desirable. For a pump, it may be important to maintain a constant speed even under load.
Neutral Zone
The animation also shows the dynamic magnetic field resulting from the combination of the stator and rotor magnetic fields.
If the torque of the motor increases (e.g. due to load), this results in an increase in the current in the rotor. This also increases the magnetic field of the rotor. The resulting magnetic field becomes more distorted.
The neutral zone in an electric motor, also known as the neutral line or zero point, is the area in the magnetic field of the motor in which the commutators do not induce any voltage. This area is crucial for the smooth operation of the motor.
When the brushes pass over the commutator segments in the neutral zone, the induced voltage is minimal. This reduces sparking, which can lead to wear of the brushes and the commutator.
The correct positioning of the brushes in the neutral zone ensures that the current flow through the windings is optimal. This maximizes the efficiency of the motor.
In the animation, the position of the neutral zone is marked by a dashed line.
Under load, the rotor’s magnetic field grows stronger and distorts the overall field. This shifts the neutral zone away from its original position. If the brushes remain fixed, they no longer sit in the true neutral zone. The result is increased sparking during commutation.
This sparking accelerates wear on both brushes and commutator segments. It also reduces the efficiency of the motor. In practice, motors with high load variation need careful brush positioning or compensating windings to counteract this shift.
Characteristic Curves
The characteristic curve of a motor shows the relationship between speed and torque. This relationship determines how the motor behaves under load.
For a series wound motor, the curve drops steeply. At low speed, the torque is high. As speed increases, the back EMF rises. This reduces the current through both the armature and the field winding, causing the torque to drop.
For a shunt wound motor, the curve is flat. The speed remains almost constant regardless of load. The field winding receives a constant voltage from the parallel connection, so only the armature current changes.
The back EMF depends on speed and magnetic flux:
\[ U_i = c \cdot \Phi \cdot n \]
- Ui: back EMF voltage
- c: motor constant
- Φ: magnetic flux
- n: speed
In a series motor, both current and flux decrease together as speed rises. This creates a steep drop in torque. In a shunt motor, the flux stays constant because the field winding voltage does not change. The torque decreases only in proportion to the armature current.
Direct Comparison
The following table summarizes the key differences between the two motor types:
| Property | Series Wound Motor | Shunt Wound Motor |
| Circuit | Armature and field winding in series | Armature and field winding in parallel |
| Starting torque | High | Moderate |
| Speed under load | Drops significantly | Remains almost constant |
| No load behaviour | Speed can rise dangerously (runaway) | Stable speed |
| Speed control | Via supply voltage or series resistance | Via field current or armature voltage |
| Typical applications | Trains, cranes, starter motors | Fans, pumps, machine tools |
Practical Applications
Series wound motors deliver high torque at low speeds. This makes them suitable for applications where heavy loads must be accelerated from standstill.
Examples include electric trains and trams, where the motor must move large masses. Cranes and hoists also use series motors, as the load varies and high starting force is needed. Starter motors in combustion engines work on the same principle: a brief burst of high torque to turn the engine.
Shunt wound motors maintain a constant speed under varying loads. Fans and centrifugal pumps require this, as fluctuations in speed would affect airflow or fluid pressure. Lathes and milling machines also benefit from constant speed, since the cutting process depends on uniform rotation. Conveyor systems use shunt motors to transport materials at a steady pace.
Historical Background
The history of the electric motor begins in 1821 with Michael Faraday. He demonstrated that a current carrying conductor rotates around a magnet. This was the first proof that electrical energy can produce continuous motion.
In 1834, Moritz Hermann von Jacobi built the first electric motor with usable mechanical power. His motor achieved about 15 watts. In 1838, he used an improved version to drive a boat on the Neva River in St. Petersburg, carrying 14 passengers.
The distinction between series and shunt wound motors emerged in the 1870s and 1880s. Engineers discovered that the wiring of the field winding determines the operating behaviour. Werner von Siemens and others used this insight to build motors for specific purposes.
From the 1880s onward, electric trams became widespread. Series wound motors proved ideal for this application due to their high starting torque. Cities like Berlin (1881), Vienna, and New York adopted electric traction within a few years. The shunt wound motor found its place in industrial drives where constant speed was required.
Modern Alternatives
Series and shunt wound motors are still in use today, but modern alternatives have replaced them in many areas.
Brushless DC motors (BLDC) use electronic commutation instead of mechanical brushes. This eliminates brush wear and sparking. BLDC motors are common in computer fans, drones, and electric vehicles.
Three phase asynchronous motors dominate industrial applications. They are robust, require little maintenance, and can be controlled precisely with frequency inverters. Most factory drives today use this type.
Permanent magnet synchronous motors offer high efficiency in a compact design. They are used in modern electric cars, where weight and efficiency matter.
Despite these alternatives, the series wound motor remains relevant in railway applications and power tools. The shunt wound motor principle lives on in separately excited DC motors used in precision applications. Understanding these basic motor types remains essential for grasping how modern drives work.
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Overview and Download
| Title | Series and shunt-wound motors |
| Target audience | Teachers and lecturers |
| Platforms | Microsoft® Windows® Apple® Macintosh® (version-dependent) |
| Features | Full-screen mode Lossless scaling Large screens and projectors supported |
| License | Freeware |
| Download | Contact |
Contributors
C. Hein, S. Rikowski
Source Information
- Bezier curves magnetic field: Created with ActionScript 3 class CubicBezier by Andy Woodruff. (http://www.cartogrammar.com/blog)
- Formulas: Generated with http://www.codecogs.com/latex/eqneditor.php
- Authoring tool (control elements supplied): Adobe Animate
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