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on video How Electric Motors Work - Three Phase AC Induction Motors

 


Generality
Independently excited DC motors are still regularly found in machine rooms of older buildings. In general, they are part of a Ward-Leonard group which makes it easy to vary the speed of rotation.

Currently, from the Ward-Leonard group, only the DC motor is kept, which this time is associated with a static variable speed drive (electronic variable speed drive) whose technology is simpler and inexpensive while requiring little maintenance. and offering high performance in a very wide speed range (from 1 to 100%).

Principle of operation
The DC motor consists of:

the inductor or the stator,
armature or rotor,
commutator and brushes.
When the winding of a motor inductor is supplied with direct current, on the same principle as a permanent magnet motor (like the figure below), it creates a magnetic field (excitation flux) in the North direction -South.

A whorl capable of rotating on an axis of rotation is placed in the magnetic field. In addition, the two conductors forming the turn are each electrically connected to a half-collector and supplied with direct current via two wiper brushes.

According to Laplace's law (any conductor through which a current flows and placed in a magnetic field is subject to a force), the armature conductors placed on either side of the brush axis (neutral line) are subjected to equal forces F but in opposite directions by creating a motor torque: the armature starts to rotate!

If the brush-collector system were not present (single coil supplied with direct current), the coil would stop rotating in the vertical position on an axis commonly called the “neutral line”. The role of the brush-collector system is to switch the direction of the current in the two conductors when passing the neutral line. The current being reversed, the driving forces on the conductors are also reversed, thus making it possible to continue the rotation of the coil.
In practice, the turn is replaced by an armature (rotor) of very complex design on which are mounted windings (composed of a large number of turns) connected to a commutator “wedged” at the end of the shaft. In this configuration, the armature can be considered as one and the same winding similar to a single turn.

Features
The advantages and disadvantages of the DC motor are listed below:

(+)
accompanied by an electronic speed variator, it has a wide variation range (1 to 100% of the range),
precise torque regulation,
its independence from the network frequency makes it a motor with a wide field of application,
(-)

not very robust compared to the asynchronous machine,
significant investment and costly maintenance (maintenance of the commutator and brushes,
Reversible machine
In the operating mode of traction elevators, the DC winch can:

Sometimes operate as a motor when the cabin and counterweight system opposes the rotational movement (so-called “resistant” load); the motor draws energy from the grid.
Sometimes work as a generator when the same system tends to favor rotation (so-called “driving” load); the generator returns energy to the grid.
DC motor type
Depending on the application, the field and armature windings can be connected differently. We generally find:
Independently excited motors.




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