31-10-2012, 02:32 PM
Electrical Motors
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Electrical Motors are continuous actuators that convert electrical energy into mechanical energy in the form of a continuous angular rotation that can be used to rotate pumps, fans, compressors, wheels, etc. As well as rotary motors, linear motors are also available. There are basically three types of conventional electrical motor available: AC type Motors, DC type Motors and Stepper Motors.
A Typical Small DC Motor
AC Motors are generally used in high power single or multi-phase industrial applications were a constant rotational torque and speed is required to control large loads. In this tutorial on motors we will look only at simple light duty DC Motors and Stepper Motors which are used in many electronics, positional control, microprocessor, PIC and robotic circuits.
The DC Motor
The DC Motor or Direct Current Motor to give it its full title, is the most commonly used actuator for producing continuous movement and whose speed of rotation can easily be controlled, making them ideal for use in applications were speed control, servo type control, and/or positioning is required. A DC motor consists of two parts, a "Stator" which is the stationary part and a "Rotor" which is the rotating part. The result is that there are basically three types of DC Motor available.
• Brushed Motor - This type of motor produces a magnetic field in a wound rotor (the part that rotates) by passing an electrical current through a commutator and carbon brush assembly, hence the term "Brushed". The stators (the stationary part) magnetic field is produced by using either a wound stator field winding or by permanent magnets. Generally brushed DC motors are cheap, small and easily controlled.
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• Brushless Motor - This type of motor produce a magnetic field in the rotor by using permanent magnets attached to it and commutation is achieved electronically. They are generally smaller but more expensive than conventional brushed type DC motors because they use "Hall effect" switches in the stator to produce the required stator field rotational sequence but they have better torque/speed characteristics, are more efficient and have a longer operating life than equivalent brushed types.
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• Servo Motor - This type of motor is basically a brushed DC motor with some form of positional feedback control connected to the rotor shaft. They are connected to and controlled by a PWM type controller and are mainly used in positional control systems and radio controlled models.
Normal DC motors have almost linear characteristics with their speed of rotation being determined by the applied DC voltage and their output torque being determined by the current flowing through the motor windings. The speed of rotation of any DC motor can be varied from a few revolutions per minute (rpm) to many thousands of revolutions per minute making them suitable for electronic, automotive or robotic applications. By connecting them to gearboxes or gear-trains their output speed can be decreased while at the same time increasing the torque output of the motor at a high speed.
The "Brushed" DC Motor
A conventional brushed DC Motor consist basically of two parts, the stationary body of the motor called the Stator and the inner part which rotates producing the movement called the Rotor or "Armature" for DC machines.
The motors wound stator is an electromagnet which consists of electrical coils connected together in a circular configuration to produce a North-pole then a South-pole then a North-pole etc, type stationary magnetic field system (as opposed to AC machines whose stator field continually rotates with the applied frequency) with the current flowing within these field coils being known as the motor field current. The stators electromagnetic coils can be connected in series, parallel or both together (compound) with the armature. A series wound DC motor has the stator field windings connected in series with the armature while a shunt wound DC motor has the stator field windings connected in parallel with the armature as shown.
Series and Shunt Connected DC Motor
The rotor or armature of a DC machine consists of current carrying conductors connected together at one end to electrically isolated copper segments called the commutator. The commutator allows an electrical connection to be made via carbon brushes (hence the name "Brushed" motor) to an external power supply as the armature rotates. The magnetic field setup by the rotor tries to align itself with the stationary stator field causing the rotor to rotate on its axis, but can not align itself due to commutation delays. The rotational speed of the motor is dependent on the strength of the rotors magnetic field and the more voltage that is applied to the motor the faster the rotor will rotate. By varying this applied DC voltage the rotational speed of the motor can also be varied.