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:
\[ M = c \cdot \phi \cdot I_A \]
The parameter c is a constant specific to the respective motor. The performance of the motor can be read from the constant. It is an important manufacturer specification. 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 counter-induction: when a conductor moves through a magnetic field, a counter-voltage is generated. At low speeds, this effect is minimal.
The following formula extends Ohm’s law to include the internal resistance of the voltage source:
\[ I_A = \frac{U – U_i}{R_A} \]
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 (high torque and low speed) is definitely 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 of the commutator meet the laminations 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.
Overview and download
| Title | Series and shunt-wound motors |
| Target group | Teachers and lecturers |
| Platforms | Microsoft® Windows® Apple® Macintosh® (version dependent) |
| Features | Full screen mode lossless zoom Large screens and projection screens supported |
| License | Freeware |
| Download | Contact us |
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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