DC permanent magnet motor
The video about DC motors
Special thanks to:
Nathan Bruer, Christoph and Jon Ferran for proof reading the video script!
You can get the source code of the OpenGL program used to create the animation sequences in the column download.
Energy converterElectric motors are machines that convert electrical energy into mechanical energy (kinetic or potential energy). The motors described in this and the subsequent chapters operate through the interaction between an electric motor's magnetic field and winding currents to generate force within the motor.
Magnetic field of a solenoidFigure 1:
Similar to a bar magnet, a magnetic field with its poles at the ends of the coil is produced as soon as a current flows through the wire. In a metal wire, the positively charged atomic nuclei are held in a fixed position, and the electrons are free to move, carrying their charge from one place to another. However the direction the electric current is conventionally defined as the direction of the positive charge flow. Positive charges are entering the wire at the positive terminal of the voltage source, while they are leaving the solenoid at the negative terminal.
While moving along the wound wire, the charges flow counterclockwise when looking towards that side of the coil where the magnetic north pole is located.
A second rule for the magnetic north pole of an electromagnet is the right-hand rule:
If you wrap your right hand around the wire of the solenoid with your fingers in the direction of conventional current, your thumb points towards the magnetic north pole.
Damage to the insulation may reduce the strength of the magnetic field since the electrical flow might skip one or more windings. This type of damage in an electromagnet is known as a turn-to-turn fault.
Working principleFigure 5:
In the arrangement shown here, repulsive forces are acting between permanent magnets and solenoid on both ends of the electromagnet resulting in a torque pointing clockwise.
A rotary switch reverses the current direction of the electromagnet periodically in a DC electric motor. That switch is named commutator. The switching contacts running to the supply voltage are named brushes. Early machines used brushes made from strands of copper wire to contact the surface of the commutator. Even if modern rotating machines almost exclusively use solid carbon contacts, they are still named brushes.
In this picture, the dot marked end of the solenoid wire is connected to the positive terminal of the supply voltage through the upper brush (1b). The circle marked end is connected to the lower brush and so to the negative terminal. The magnetic north pole of the solenoid points to the top left of the arrangement, same as the wire end marked by the dot. In the magnetic field of the permanent magnets, a torque pointing clockwise is produced.
After a rotation for 180 degrees, the dot marked end is connected to the negative terminal of the supply voltage. The current flows from the wire end connected to the positive terminal and marked with a circle through the coil and finally to the negative terminal at the dot marked end. The positive and negative terminals at the solenoid are swapped in comparison to the initial state. Accordingly the magnetic north pole points now into the direction of the wire end marked by a circle. Because the electromagnet has rotated 180 degrees from it's initial position, once again the north pole is located at the top left of the arrangement - same as in the upper drawing.
At least three electromagnets arranged with an angle of 120 degrees between nearby coils are needed to built an electric motor spinning continuously. At the commutator, each end of each coil is connected to one end of the nearby coils. The commutator switches from one electromagnet to the next each 60 degrees. For a short interval, two coils are energized simultaneously. As you can see, the torque produced by both coils points clockwise in the drawing.
DC motor with four coils and permanent magnetsFigure 1: (Start animation.)
Elements of a DC motor with permanent magnets:
(1) Motor housing
(2) Permanent magnet, magnetic north pole inside
(3) Permanent magnet, magnetic south pole inside
(1) + (2) + (3) Stator
(4) Pivot axis
(8) Sliding contacts (brushes)
Inside of this electric motor, magnetic fields are produced by rotating electromagnets. The coils are exposed to the magnetic field of permanent magnets whereby a torque is created. At the drawing, four coils (two pairs) are arranged with an angle of 90 degrees between two nearby coils. The rotatable coils are called rotor or armature. Soft iron material is used as core material of the coils to boost the produced magnetic field. The motor housing and the permanent magnets form the stator. That pair of coils arranged on the vertical axis is arranged perpendicularly to the field lines of the permanent magnets and it is connected to the DC power source through the commutator. At the drawing above the magnetic south pole is at the top, the north pole at the bottom. The south pole of the right magnet and the north pole of the left magnet are pointing to that pair of coils, thus the upper half of the rotor is pulled to the left, the lower half to the right. The resulting torque acts counterclockwise. Following the torque the rotor moves counterclockwise. The rotation would stop as soon as the coils point in parallel to the magnetic field of the permanent magnets. To keep the torque pointing counterclockwise, a second pair of coils is installed at the rotor being perpendicularly to the first one. Before the first pair of coils is in parallel to the field of the permanent magnets, the current passing the coil is cut, while the second coil gets energized through the commutator. The direction of windings of the second pair of coils is identical to that of the first pair, hence the resulting torque points counterclockwise, too. Before the second pair of coils points in parallel to the field of the permanent magnets, it is cut from the power supply and the first pair of coils is energized again. Now, the polarity of the pair of coils differs from the initial state, but the motor as been rotating for 180 degrees, thus the magnetic south pole is at the top and the magnetic north pole at the bottom same a in the initial state. The resulting torque still points counterclockwise.
Besides mechanical stress, voltage peaks occur at the brushes. As mentioned in the chapter about self-induction, a voltage is induced by a coil whenever the current passing the device is altered. The induced voltage is higher, the faster the variation of current. The current passing the coils is abruptly cut off by the commutator, thus the induced voltage is very high. That leads to the formation of sparks at the commutator. Those sparks cause noticeable damage of the commutator and the brushes over time.
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