General about DC motors//
Separate field excitation DC motors are still sometimes used for driving machines at variable speed. These motors are very easy to miniaturise, and essential for very low powers and low voltages.
They are also particularly suitable, up to high power levels (several megawatts), for speed variation with simple, uncomplicated electronic technologies for high performance levels (variation range commonly used from 1 to 100).
Their characteristics also enable accurate torque regulation, when operating as a motor or as a generator. Their nominal rotation speed, which is independent of the line supply frequency, is easy to adapt by design to suit all applications.
They are however less rugged than asynchronous motors and much more expensive, in terms of both hardware and maintenance costs, as they require regular servicing of the commutator and the brushes.
DC motor construction parts
A DC motor is composed of the following main parts:
Field coil or stator
This is a non-moving part of the magnetic circuit on which a winding is wound in order to produce a magnetic field. The electro-magnet that is created has a cylindrical cavity between its poles.
Armature or rotor
This is a cylinder of magnetic laminations that are insulated from one another and perpendicular to the axis of the cylinder. The armature is a moving part that rotates round its axis, and is separated from the field coil by an air gap. Conductors are evenly distributed around its outer surface.
Commutator and brushes
The commutator is integral with the armature. The brushes are fixed. They rub against the commutator and thus supply power to the armature conductors.
When the field coil is energised, it creates a magnetic field (excitation flux) in the air gap, in the direction of the radii of the armature. This magnetic field “enters” the armature from the North pole side of the field coil and “exits” the armature from the South pole side of the field coil.
When the armature is energised, currents pass through the conductors located under one field coil pole (on the same side of the brushes) in the same direction and are thus, according to Laplace’s law, subject to a force.
The conductors located under the other pole are subject to a force of the same intensity in the opposite direction. The two forces create a torque which causes the motor armature to rotate (see Figure 1).
Figure 1 – Production of torque in a DC motor
When the motor armature is powered by a DC or rectified voltage supply U, it produces back emf E whose value is:
E = U – RI
where RI represents the ohmic voltage drop in the armature.
The back emf E is linked to the speed and the excitation by the equation:
E = k ω Φ
- k is a constant specific to the motor
- ω is the angular speed
- Φ is the flux
This equation shows that at constant excitation the back emf E (proportional to ω) is an image of the speed.
The torque is linked to the field coil flux and the current in the armature by the equation:
T = k Φ I
If the flux is reduced, the torque decreases.
There are two methods for increasing the speed //
- Either increase the back emf E, and thus the supply voltage at constant excitation: this is known as “constant torque” operation.
- Or decrease the excitation flux, and thus the excitation current, while keeping the supply voltage constant: this is known as “reduced flux” or “constant power” operation. This operation requires the torque to decrease as the speed increases (see Figure 2 below). However, for high reduced flux ratios this operation requires specially adapted motors (mechanically and electrically) to overcome switching problems.
Figure 2 – Torque/speed curves for a separate field excitation motor
The operation of this type of device (DC motor) is reversible //
If the load opposes the rotation movement (the load is said to be resistive), the device provides a torque and operates as a motor.
If the load is such that it tends to make the device rotate (the load is said to be driving) or it opposes the slow-down (stopping phase of a load with a certain inertia), the device provides electrical energy and operates as a generator.
Various types of DC motor
Figure 3 – Diagrams of the various types of DC motor
Parallel excitation (separate or shunt)
The coils, armature and field coil are connected in parallel or supplied via two sources with different voltages in order to adapt to the characteristics of the machine (e.g. armature voltage 400 volts and field coil voltage 180 volts).
The direction of rotation is reversed by inverting one or other of the windings, generally by inverting the armature voltage due to the much lower time constants. Most bidirectional speed drives for DC motors operate in this way.
DC shunt motor (photo credit: edisontechcenter.org)
The design of this motor is similar to that of the separate field excitation motor. The field coil is connected in series to the armature coil, hence its name. The direction of rotation can be reversed by inverting the polarities of the armature or the field coil.
This motor is mainly used for traction, in particular on trucks supplied by battery packs. In railway traction the old TGV (French high-speed train) motor coaches used this type of motor. More recent coaches use asynchronous motors.
Compound wound (series-parallel excitation)
This technology combines the qualities of the series wound motor and the shunt wound motor. This motor has two windings per field coil pole. One is connected in parallel with the armature. A low current (low in relation to the working current) flows through it. The other is connected in series.
It is an added flux motor if the ampere turns of the two windings add their effects. Otherwise it is a negative flux motor. But this particular mounting method is very rarely used as it leads to unstable operation with high loads.
Compound wound DC motor (photo credit: lucas-nuelle.com)
Source Reference: Electrical Engineering Portal (EEP)