Phase swinging or Hunting(Synchronous Motor)

Topic is Hunting in synchronous motor.

The phenomenon of oscillation of the rotor about its final equilibrium position is called Hunting.

On the sudden application of load, the rotor search for its new equilibrium position and this process is known as Hunting.
The Hunting process occurs in a synchronous motor as well as in synchronous generators if an abrupt change in load occurs.

The steady state or stable operation of a synchronous motor is a condition of equilibrium. In it, the load torque is equal as well as opposite to the electromagnetic torque. The rotor of the motor runs at synchronous speed in the steady state condition, maintain a constant value of the torque angle δ. The equilibrium gets disturbed if a sudden change occurs in the load torque. Thus, a resulting torque takes place which changes the speed of the motor. It is given by the equation shown below.

Where J is the moment of inertia ,ωM is the angular velocity of the rotor in mechanical units.

The speed of the motor slows down temporarily, and the torque angle δ is sufficiently increased. This is done to restore the torque equilibrium and the synchronous speed when there is a sudden increase if the load torque.

The electromagnetic torque is given by the equation shown below

If the value of δ is increased, the electromagnetic torque is also increased. As a result, the motor is accelerated. As the rotor reaches the synchronous speed, the torque angle δ is larger than the required value. Here the rotor speed continues to increase beyond the synchronous speed.

As the rotor accelerates above synchronous speed, the torque angle δ decreases. The point where the motor torque becomes equal to the load torque, the equilibrium is not restored because now the rotor speed is greater than the synchronous speed. Therefore, the rotor continues to swing backwards and as a result, the torque angle goes on decreasing.

When the load angle δ becomes less than the required value, the mechanical load becomes greater than the developed power. Therefore, the motor starts to slow down. The load angle starts increasing again. Thus, the rotor starts to swing or oscillates around the synchronous speed.

The motor responds to a decreasing load torque by a temporary increase in speed and a reduction of the torque angle δ. Thus, the rotor swings and rotate around the synchronous speed. Thus, this process of rotation of the rotor speed equal or around the synchronous speed is known as Hunting. Since, during the rotor oscillation, the phase of the phasor Ef changes about phasor V. Thus, hunting is known as Phase Swinging.

Causes of hunting-
a. Sudden changes of load.
b. Faults were occurring in the    
    system which the generator
    supplies.
c. Sudden change in the field
    current.
d. Cyclic variations of the load
     torque.

Effects of Hunting-
a. It can lead to loss of synchronism.
b. It can cause variations of the    
    supply voltage producing
    undesirable lamp flicker.
c. The possibility of Resonance
    condition increases. If the
    frequency of the torque
    component becomes equal to that       of the transient oscillations of the
    synchronous machine, resonance       may take place.
d. Large mechanical stresses may  
    develop in the rotor shaft.
e. The machine losses increases and
    the temperature of the machine  
    rises.

Reduction of Hunting-
a. Use of damper windings
b. Uses of flywheels
c. The prime mover is provided with     a large and heavy flywheel. This         increases the inertia of the prime       mover and helps in maintaining         the rotor speed constant.
d. By designing synchronous
     machines with suitable
     synchronising power coefficients.

Closed loop Control system

Close loop Control system ::-

It's an automatic control system in which an operation, process, or mechanism is regulated by feedback

Control Characteristics of the system depends upon the output of the system. It is also termed as feedback control system. The control action is actuated by an error signal 'e', which is the difference between Input signal and output signal. The purpose of feedback is to reduce the error between the reference input and the system output.

Advantages of Closed loop :-

i) Accurate and Reliable 

ii) Reduce effect of parameter variations. 

iii) Reduce the effect of non-linearity.

Disadvantages of closed loop:-

i) System is Complex & Costly. 

ii) Reduce the Gain will negative feedback.

Open loop Control system

Open-loop system, referred to as non-feedback system, is a type of continuous control system in which the output has no influence or effect on the control action of the input signal. In other words, in an open-loop control system the output is neither measured nor “fed back” for comparison with the input. Therefore, an open-loop system is expected to faithfully follow its input command or set point regardless of the final result.

Also, an open-loop system has no knowledge of the output condition so cannot self-correct any errors it could make when the preset value drifts, even if this results in large deviations from the preset value.

Another disadvantage of open-loop systems is that they are poorly equipped to handle disturbances or changes in the conditions which may reduce its ability to complete the desired task. For example, the dryer door opens and heat is lost. The timing controller continues regardless for the full 30 minutes but the clothes are not heated or dried at the end of the drying process. This is because there is no information fed back to maintain a constant temperature.

Control characteristics of such systems are :-
Independent of output of the system. The output is neither measured nor feedback for comparison with the Input. Then we can define the main characteristics of an “Open-loop system” as being:

• • There is no comparison between actual and desired values.
• • An open-loop system has no self-regulation or control action over the output value.
• • Each input setting determines a fixed operating position for the controller.
• • Changes or disturbances in external conditions does not result in a direct output change    (unless the controller setting is altered manually).

Advantages of Open loop control system:-
i) Simple and Economic
ii) More Stable

Disadvantages of Open loop control system:-
i) Inaccurate
ii) Affected by system parameter variation and external noise.

Control system Introduction and types

CONTROL SYSTEM ::

                 

Control system is mean by which we can get the desired output.
Control is used to modify the behavior of a system so it behaves in a specific desirable way over time.
For example, we may want the speed of a car on the highway to remain as close as possible to 60 miles per hour
in spite of possible hills or adverse wind; or we may want an aircraft to follow a desired altitude, heading,
and velocity profile independent of wind gusts; or we may want the temperature and pressure in a reactor
vessel in a chemical process plant to be maintained at desired levels. All these are being accomplished today
by control methods and the above are examples of what automatic control systems are designed to do,
without human intervention. Control is used whenever quantities such as speed, altitude, temperature, or
voltage must be made to behave in some desirable way over time.
This section provides an introduction to control system design methods.

The Role of Control Theory :

To design a controller that makes a system behave in a desirable manner, we need a way to predict the behav-
ior of the quantities of interest over time, specifically how they change in response to different inputs.
Mathematical models are most oftenly used to predict future behavior, and control system design methodolo-
gies are based on such models. Understanding control theory requires engineers to be well versed in basic
mathematical concepts and skills, such as solving differential equations and using Laplace transform. The role
of control theory is to help us gain insight on how and why feedback control systems work and how to sys-
tematically deal with various design and analysis issues. Specifically, the following issues are of both practi-
cal importance and theoretical interest:
1. Stability and stability margins of closed-loop systems.
2. How fast and smooth the error between the output and the set point is driven to zero.
3. How well the control system handles unexpected external disturbances, sensor noises, and internal dynamic
changes

A control system is a system, which provides the desired response by controlling the output.
             Other words we change the system function in as per the requirements.

Their are two types of control system:-
i) Open loop system
ii) Close loop system