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DC motor principles
DC motors consist of rotor-mounted windings (armature) and stationary windings (field poles). In all DC motors, except permanent magnet motors, current must be conducted to the armature windings by passing current through carbon brushes that slide over a set of copper surfaces called a commutator, which is mounted on the rotor.
The commutator bars are soldered to armature coils. The brush/commutator combination makes a sliding switch that energizes particular portions of the armature, based on the position of the rotor. This process creates north and south magnetic poles on the rotor that are attracted to or repelled by north and south poles on the stator, which are formed by passing direct current through the field windings. It's this magnetic attraction and repulsion that causes the rotor to rotate
The greatest advantage of DC motors may be speed control. Since speed is directly proportional to armature voltage and inversely proportional to the magnetic flux produced by the poles, adjusting the armature voltage and/or the field current will change the rotor speed.
Today, adjustable frequency drives can provide precise speed control for AC motors, but they do so at the expense of power quality, as the solid-state switching devices in the drives produce a rich harmonic spectrum. The DC motor has no adverse effects on power quality.
Major types of dc motors
§ Self excited dc motor
§ Series dc motor
§ Shunt dc motor
§ Compound dc motor
§ Separately excited dc motor
§ Permanent magnet dc motor
Series motors
Series motors connect the field windings in series with the armature.
Series motors lack good speed regulation, but are well-suited for high-torque loads like power tools and automobile starters because of their high torque production and compact size.
Shunt motors
Shunt motors use high-resistance field windings connected in parallel with the armature.
Varying the field resistance changes the motor speed. Shunt motors are prone to armature reaction, a distortion and weakening of the flux generated by the poles that results in commutation problems evidenced by sparking at the brushes. Installing additional poles, called interpoles, on the stator between the main poles wired in series with the armature reduces armature reaction.
Compound motors
In this motor the concept of the series and shunt designs are combined.
Separately Excited Motor
There is no direct connection between the armature and field winding resistance. DC field current is supplied by an independent source (such as battery or another generator or prime mover called an exciter)
Permanent Magnet motors
PMDC is a dc motor whose poles are made of permanent magnets.
Do not require external field circuit, no copper losses
No field winding, size smaller than other types dc motors
Disadvantage: cannot produce high flux density, lower induce voltage
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