19-09-2012, 10:48 AM
Dynamics of Motor-Load Combination
Dynamics of Motor.ppt (Size: 502 KB / Downloads: 36)
Speed-Torque characteristics depends on,
For motor-winding configuration or characteristics of electric supply
For load-mechanical characteristics
Speed of the motor-load system,
Depends on both motor and load characteristics
Hence characteristics of load cannot be neglected when designing an effective electric drive system
BIDIRECTIONAL ELECTRIC DRIVE SYSTEMS
During steady state operation,
A force exerted by either part of the drive system is opposed by a force equal in magnitude and opposite in direction from the other
→based on Newton's 3rd law(For every action there is an equal and opposite reaction)
TYPES OF LOAD TORQUES
Load torque can be of two types
• Active load torque
act in the same direction irrespective of the direction of the drive.
e.g. gravitational force or deformation in elastic bodies.
• Passive load torque
changes with the change in the direction of motion of drive.
e. g. torques due to friction, due to shear and deformation of inelastic bodies
Motor torque ,speed in same direction
Machine consumes electrical power from source and delivers mechanical power to load
Electric machine operates as a motor
Motor torque, speed in opposite direction
Machine consumes mechanical power from load and delivers electric power to source
Electric machine operates as a generator
If fast response is required:
Converters use semiconductor elements which has low thermal capability.
Therefore, this current rating is chosen as equal to the maximum current that may be required to flow through motor.
If motor current=k times rated current, then to get fast response during transient, converter rating must be k times the motor rating.
It leads to increase in converter cost and hence the drive cost increases substantially.
In addition to this converter cost, additional thermal protection has to be given if motor current is allowed to be k times its rated current.