14-07-2012, 04:29 PM
DC MOTOR
DC Motor.ppt (Size: 4.86 MB / Downloads: 41)
A Simple Electromagnet
A Nail with a Coil of Wire
Q – How do we set up a magnet?
A – The battery feeds current through the coil of wire. Current in the coil of wire produces a magnetic field (as long as the battery is connected).
Right Hand Rule # 3
The Right Hand Rule is used to determine the direction of the force when the direction of the current and the direction of the magnetic field are known.
Motor Basics
Motors convert electric energy to mechanical motion.
Either an AC or DC electrical energy source serves as the input to the motor.
Motors are powered by electricity, but rely on principles of magnetism to produce mechanical motion.
The result is mechanical motion of the output shaft, that is a rotation about or a translation along the shaft, provided the load carried by the shaft does not exceed the maximum load the motor is designed to carry.
Choosing a Motor
There are numerous ways to design a motor, thus there are many different types of motors.
The type of motor chosen for an application depends on the characteristics needed in that application.
These include:
How fast you want the object to move,
The weight, size of the object to be moved,
The cost and size of the motor,
The accuracy of position or speed control needed.
Motor Parameters
The level of performance a motor can provide is described by its parameters. These include:
Rated Speed
Speed measured in shaft revolutions per minute (RPM), a way to specify how fast the motor turns.
Torque
Rotational force produced around a given point, due to a force applied at a radius from that point
Torque-Speed performance of a motor
Types of Motors
The different types of motors possess different operating characteristics.
Heavy Industrial applications: AC motors
Mobile robotics & hobby robots: dc motor, dc servo motor, and stepper motors
Brief overview of the operation characteristics of:
AC motors
DC motors
DC servo motors
Stepper motors
DC Electric Motors
Electric Motors or Motors convert electrical energy to mechanical motion
Motors are powered by a source of electricity – either AC or DC.
DC Electric Motors use Direct Current (DC) sources of electricity:
Batteries
DC Power supply
Principle of How Motors Work:
Electrical current flowing in a loop of wire will produce a magnetic field across the loop.
When this loop is surrounded by the field of another magnet, the loop will turn, producing a force (called torque) that results in mechanical motion.
DC Motor Operation
In a dc motor, the stator poles are supplied by dc excitation current, which produces a dc magnetic field.
The rotor is supplied by dc current through the brushes, commutator and coils.
The interaction of the magnetic field and rotor current generates a force that drives the motor
The magnetic field lines enter into the rotor from the north pole (N) and exit toward the south pole (S).
The poles generate a magnetic field that is perpendicular to the current carrying conductors.
The interaction between the field and the current produces a Lorentz force,
The force is perpendicular to both the magnetic field and conductor
The generated force turns the rotor until the coil reaches the neutral point between the poles.
At this point, the magnetic field becomes practically zero together with the force.
However, inertia drives the motor beyond the neutral zone where the direction of the magnetic field reverses.
To avoid the reversal of the force direction, the commutator changes the current direction, which maintains the counterclockwise rotation.
Before reaching the neutral zone, the current enters in segment 1 and exits from segment 2,
Therefore, current enters the coil end at slot a and exits from slot b during this stage.
After passing the neutral zone, the current enters segment 2 and exits from segment 1,
This reverses the current direction through the rotor coil, when the coil passes the neutral zone.
The result of this current reversal is the maintenance of the rotation.
How the Commutator Works
As the rotor turns, the commutator terminals also turn and continuously reverse polarity of the current it gets from the stationary brushes attached to the battery.
Controlling Motor Direction
To change the direction of rotation:
Simply switch the polarity of the battery leads going to the motor (that is, switch the + and – battery leads)
DC Motors – Components
Field pole
North pole and south pole
Receive electricity to formmagnetic field
Armature
Cylinder between the poles
Electromagnet when current goes through
Linked to drive shaft to drive the load
Commutator
Overturns current direction in armature
DC motors
As the rotor is rotating, back emf (Ea) will be produced, the faster the rotor turn the higher Ea and the smaller Ia.
The starting current of motors will be much higher then the rating current.