Aside from the obvious
difference of voltage source, the significant difference between AC and DC
motors is structural, and is directly related to voltage source.
Typically, DC motors
(and generators) are built with the field windings incorporated into poles
built into the inside of the stator housing, while the armature windings are
built into the rotor and connect to the outside through the commutator,
commutator slots and brushes.
On the other hand, AC
motors are built with a slotted stator that houses the armature windings and
the field windings are installed on the rotor. With synchronous motors, the
field windings are wired to slip rings and brushes to that the field windings
can be energized, but with induction motors, the rotor sis built with a squirrel
cage that is energized by the rotating magnetic field of the armature windings.
The reason the armature
windings of the AC motor are in the stator is primarily because of magnitude.
AC motors are built to handle more voltage, more current and more load than DC
motors, and that voltage and current works in the armature winding. If the armature
windings were built into the rotor, the rotor would become impractically large
and heavy as the windings became larger and heavier to handle the higher
voltage and current.
With the armature
windings in the stator, the AC motor can handle more voltage and current with
bigger and heavier wires without loading down the rotor with more conductor and
insulation, and can do more work with less weight and materials.
Here is an answer I
wrote on AC vs DC generators, and generators are basically motors that are
operated in reverse: fed mechanical energy to produce electrical energy. Maybe
it will give you some additional insight, or at least another perspective.
CONCLUSION
DC motors are usually
seen in applications where the motor speed needs to be externally controlled.
AC motors work best in applications where power performance is sought for
extended periods of time. All DC motors are single phase, but AC motors can be
single phase or three phase.
AC and DC motors use the
same principle of using an armature winding and magnetic field except with DC
motors, the armature rotates while the magnetic field doesn’t rotate. In AC
motors the armature does not rotate and the magnetic field continuously
rotates.
In some applications
today, DC electric motors are replaced by combining an AC electric motor with
an electronic speed controller, known as variable frequency drive. DC electric
motors are replaced with an AC electric motor and an electronic speed controller
because it is a more economical and less expensive solution.
DC electric motors have
many moving parts that are expensive to replace, and DC electric motor repair
is usually more expensive than using a new AC electric motor with an electronic controller.
So as far as maintenance
is concern, AC motors are used most as easy availability of AC supply DC motor
has constant maintenance problem.
DC
motor advantages:
1. The starting and
speed regulation performance is good, the speed regulation range is wide and
smooth, the overload capability is strong, and the electromagnetic interference
is small;
2. DC motor has good
starting characteristics and speed regulation characteristics;
3, DC motor torque is
relatively large
4, the maintenance is
relatively cheap;
5. DC of DC motor is
more energy-saving and environmentally friendly than AC.
DC
motor disadvantages:
1, DC motor
manufacturing is more expensive, there are carbon brushes;
2. Compared with
asynchronous motors, DC motors have complex structures, are inconvenient to use
and maintain, and require DC power supplies;
3. The complicated
structure limits the further reduction of the volume and weight of the DC
motor, especially the sliding contact between the brush and the commutator
causes mechanical wear and sparks, which makes the DC motor have many faults,
low reliability, short life and maintenance. The maintenance workload is large.
4. The reversing spark
not only causes the electric corrosion of the commutator, but also a radio
interference source, which will have harmful effects on the surrounding
electrical equipment. The larger the capacity of the motor and the higher the
speed, the more serious the problem. Therefore, the brush and commutator of the
ordinary DC motor limit the development of the DC motor to high speed and large
capacity.
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