Lecture 7 load torques

Components of load torque is presented.

1. Load Torques Prof. S. Y. Gadgune

2. Components of load torques The load torque can be divided in to following components : 1) Torque required to do useful mechanical work ( ) Nature of this torque depends upon particular application. It may be constant and independent of speed. It may be some function of speed, it may be time invariant or time variant, its nature may also change with the load’s mode of operation .  

3. Components of load torques 2) Friction Torque ( ) Friction will be present at the motor shaft and also in various parts of the load . is the equivalent value of various friction torques referred to the motor shaft. Its value at standstill is much higher than its value slightly above zero speed. Friction torque can also be resolved into three components: i ) Stiction ( Ts )- Friction at zero speed is called as stiction or static friction. In order for drive to start, the motor torque should at least exceed stiction . ii ) Viscous Friction ( Tv )- It varies linearly with speed and it is given by where, - viscous friction coefficient.  

4. Components of load torques iii) Coulomb Friction ( Tc )- It is independent of speed and it is constant. Since, is present only at standstill, It is not taken into account in the dynamic analysis.  

5. Components of load torques 3) Windage Torque ( ): When motor runs, wind generates a torque opposing the motion . This is known as windage torque . It is proportional to speed squared is given by Where C –constant Therefore total load torque + + + + +  

6. Components of load torques In many applications + is very small compared to and it is negligible compared to . In general + is approximate by updating the value of . + Therefore fundamental torque equation becomes,  

7. Nature of Load torques A low speed hoist is an example of a load where the torque is constant and independent of the speed. At low speeds, windage torque is negligible. Therefore, net torque is mainly due to gravity which is constant and independent of speed . There are drives where coulomb friction dominates over other torque components. Consequently, torque is independent of speed, e.g. paper mill drive . Fans, compressors, aeroplanes , centrifugal pumps, ship- propellors , coilers, high speed hoists, traction etc. are example of the case where load torque is a function of speed.

8. Nature of Load torques In fans, compressors and aero planes, the windage dominates, consequently, load torque is proportional to speed squared as shown in fig.(a). Windage is the opposition offered by air to the motion. Similar nature of Load Torques can be expected when the motion is opposed by any other fluid, e.g. by water in centrifugal pumps and ship- propellors , giving the same characteristic as shown in Fig .(a).

9. Nature of Load torques In a high speed hoist, viscous friction and windage also have appreciable magnitude, in addition to gravity, thus giving the speed-torque curve of Fig .(b).  Nature of speed-torque characteristic of a traction load when moving on a levelled ground is shown in Fig .(c ). Because of its heavy mass, the stiction is large. Near zero speed, net torque is mainly due to stiction . The stiction however disappears at a finite speed and then windage and viscous friction dominate. Because of large stiction and need for accelerating a heavy mass, the motor torque required for starting a train is much larger than what is required to run it at full speed. 

10. Nature of Load torques Torque in a coiler drive is also a function of speed. It is approximately hyperbolic in nature as shown in Fig .(d ). The developed power is nearly constant at all speeds .

11. Classification of load torques Load Torques can be broadly classified into two categories- 1) Active 2) Passive Load torques which have the potential to drive the motor under equilibrium condition are called Active Load Torques. Such load torques usually retain their sign when the direction of the drive rotation is changed. Torque(s) due to gravitational force, tension, compression and torsion, undergone by an elastic body, come under this category . Load torques which always oppose the motion and change their sign on the reversal of motion are called  Passive Load Torques . Such torques are due to friction, windage , cutting etc.

12. References G. K. Dubey , “Fundamentals of Electrical Drives”, Second Edition, Narosa Publishing House

13. Thank You