10-10-2012, 03:27 PM
Motors
3 PHASE MOTOR.ppt (Size: 3.22 MB / Downloads: 37)
Electric Motor Basic Principles
Interaction between magnetic field and current carrying wire produces a force
Opposite of a generator
Conventional (Brushed) DC Motors
Permanent magnets for outer stator
Rotating coils for inner rotor
Commutation performed with metal contact brushes and contacts designed to reverse the polarity of the rotor as it reaches horizontal
Conventional (Brushed) DC Motors
Common Applications:
Small/cheap devices such as toys, electric tooth brushes, small drills
Lab 3
Pros:
Cheap, simple
Easy to control - speed is governed by the voltage and torque by the current through the armature
Cons:
Mechanical brushes - electrical noise, arcing, sparking, friction, wear, inefficient, shorting
Brushless DC Motors
Essential difference - commutation is performed electronically with controller rather than mechanically with brushes
Brushless DC Motor Commutation
Commutation is performed electronically using a controller (e.g. HCS12 or logic circuit)
Similarity with stepper motor, but with less # poles
Needs rotor positional closed loop feedback: hall effect sensors, back EMF, photo transistors
Brushless DC Motors
Applications
CPU cooling fans
CD/DVD Players
Electric automobiles
Pros (compared to brushed DC)
Higher efficiency
Longer lifespan, low maintenance
Clean, fast, no sparking/issues with brushed contacts
Cons
Higher cost
More complex circuitry and requires a controller
AC Motors
Two main types of AC motor, Synchronous and Induction.
Synchronous motors supply power to both the rotor and the stator, where induction motors only supply power to the stator coils, and rely on induction to generate torque.
AC Induction Motors (3 Phase)
Use poly-phase (usually 3) AC current to create a rotating magnetic field on the stator
This induces a magnetic field on the rotor, which tries to follow stator - slipping required to produce torque
Workhorses of the industry - high powered applications
AC induction Motors
Induction motors only supply current to the stator, and rely on a second induced current in the rotor coils.
This requires a relative speed between the rotating magnetic field and the rotor. If the rotor somehow matches or exceeds the magnetic field speed, there is condition called slip.
Slip is required to produce torque, if there is no slip, there is no difference between the induced pole and the powered pole, and therefore no torque on the shaft.
Synchronous AC Motors
Current is applied to both the Rotor and the Stator.
This allows for precise control (stepper motors), but requires mechanical brushes or slip rings to supply DC current to the rotor.
There is no slip since the rotor does not rely on induction to produce torque.
Main features
The sequence of the applied pulses is directly related to the direction of motor shafts rotation.
The speed of the motor shafts rotation is directly related to the frequency of the input pulses.
The length of rotation is directly related to the number of input pulses applied.
Working principle
Stepper motors consist of a permanent magnet rotating shaft, called the rotor, and electromagnets on the stationary portion that surrounds the motor, called the stator.
When a phase winding of a stepper
motor is energized with current, a
magnetic flux is developed in the
stator. The direction of this flux is
determined by the “Right Hand
Rule”.
At position 1, the rotor is beginning at the upper electromagnet, which is currently active (has voltage applied to it).
To move the rotor clockwise (CW), the upper electromagnet is deactivated and the right electromagnet is activated, causing the rotor to move 90 degrees CW, aligning itself with the active magnet.
This process is repeated in the same manner at the south and west electromagnets until we once again reach the starting position.