1. Dynamic Conditions of a drive system
Operating point changes from one steady state to another,following change in variables.
Dynamic conditions exist during starting, braking, speed reversal. (changes in supply voltage, load, frequency)
Mechanical inertia, electrical inductance.
Required for control designs.

2. Stability of the drive Steady –state & Transient
Defn. for state of equilibrium
At constant speed, if the developed motor torque is equal to the sum of load torque.- S.O.E.
Factors influencing stability
1.J
2.L

3. S.O.E. S.O.E. can be Stable, unstable, neutral.
Stable – Returns to steady state after disturbance.
Unstable – Speed of the drive increases or decreases and comes to rest.
Neutral – Preserves its steady state at different speeds after coming out of
equilibrium.

4. Defn. for stability
The stability of the motor load combination is defined as the capacity of the system which enables it to develop forces to restore equilibrium after any small deviation therefrom.
Steady state stability – Changes take place slowly and effect of J &L is insignificant.

5. Transient stability – sudden and fast changes
Transient stability enables us to use the equipment rationally.
Enables to estimate the performance of the drive w.r.t its stability of motion.

6. Mathematical condition for steady state stability
Conditions can be obtained by solving differential equations of the motor along with the dynamic equation of motion.
Stability studies can be carried out using speed torque characteristics.

7. Final conclusion for conditions:
It can be seen that a decrease (increase) in speed must bring a greater change in motor torque than in the load torque so that the machine can be accelerated (retarded) under the influence of the motor torque to its original point of operation.

8. MULTIQUADRANT OPERATION OF A MOTOR DRIVE
Motor operates in two modes :
Motoring & Braking

9. The speed is assumed to be positive if the direction of rotation is anticlockwise or in such a way to cause an ‘upward’ or forward motion of the drive. For reversible drive positive direction of the speed can be assumed arbitrarily either clockwise or anticlockwise.
The motor torque is positive if it produces increase in speed in the positive sense. The load torque is assigned the positive sign when it is directed against the motor torque.
Plot of speed torque characteristics of the load/ motor for all four quadrant of operation is known as quadrantal diagram.

10. Four quadrant operation of the motor driving a hoist load

11. Motor is driving a hoist consisting of a cage with or without load, a rope wound on to a drum to hoist the cage and a balance weight of magnitude greater than that of the empty cage but less than that of the loaded cage.
The arrow in the figure indicates the actual directions of the motor torque, load torque and motion in four quadrants.
The load torque of the hoisting mechanism is of active type and assumed to be constant due to negligible friction and windage for low speed hoist.

12. Speed torque curve of the hoist is represented by vertical line passing through two quadrants. Loaded hoist characteristics in first and fourth and unloaded in second and third quadrants.
In the first quadrant the load torque acts in the opposite direction to that of rotation. Hence to drive the loaded hoist up, the motor developed torque must be in the direction of the rotation or must be positive. The power will also be positive so, this quadrant is known as ‘forward motoring quadrant

13. The hoisting up of the unloaded cage is represented in the second quadrant. As the counterweight is heavier than the empty cage, the speed at which hoist moves upwards may reach a very high value. To avoid this, the motor torque must act in the opposite direction of rotation or motor torque must be negative. The power will be negative though the speed is positive, so this quadrant is known as ‘forward braking quadrant’.

14. The third quadrant represents the downward motion of the empty cage
The third quadrant represents the downward motion of the empty cage. Downward journey will be opposed by torque due to counterweight and friction at the transmitting parts, move cage downwards the motor torque should must be in the direction of the rotation. Electric machine acts as a motor but in the reverse direction compared to first quadrant. The torque is negative as speed is increased I the negative direction, but the power is positive, this quadrant is known as ‘Reverse motoring quadrant’.

15. Fourth quadrant has the downward motion of the loaded cage
Fourth quadrant has the downward motion of the loaded cage. As loaded cage has more weight than the balanced weight to limit the speed of the motion, motor torque must have opposite polarity with respect to rotation and acts as a brake. The motor torque sign is positive, but as speed has negative direction; the power will be negative, this quadrant is designated as ‘Reverse braking quadrant’.

17. Classification of Loads
Continuous constant mode
Continuous variable type mode –Lathes, Conveyors
Pulsating load – Reciprocating pumps
Impact load – rolling mills
Short time intermittent loads -Cranes
Short time loads – battery charging

18. Modes of operation of electrical drive
Three modes
1.Steady state mode
2. Acceleration mode including starting
3.Deceleration mode including stopping

19. Steady state mode Occurs when motor torque equals load torque.
Change in speed is achieved by varying the steady state motor speed torque curve so that motor torque equals load torque.
Steady state operation is obtained by adding a mechanical brake that produces torque in a direction to oppose the motion.

20. Acceleration mode Occurs during transient operations
Motor torque is greater than load torque.
In crease in motor torque is accomplished by an increase in motor current.
For acceleration periods of long duration, care should be taken for current rating of motor.
Also, soft starting may be required.

21. Deceleration mode Motor torque is less than the load torque.
Also, mechanical brakes may be used to produce reqd. deceleration.

22. Requirement of a motor drive
Rating
Steady state & transient – motor should drive the load satisfactorily.
Motor selected should not have excess temperature rise when it is fully loaded.
Must withstand short time overloads.

23. Starting
Starting involves change in its state from rest to a steady state of rotation.
Starting currents will add to motor heating.

24. Methods of starting
Full voltgae starting