Lecture 7 load torques

Lecture 7 load torques

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  LOAD TORQUES Prof.S. Y. Gadgune
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  COMPONENTS OF LOADTORQUES The load torque 𝑇𝑙 can be divided in to following components: 1) Torque required to do useful mechanical work (𝑇𝐿) 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.
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  COMPONENTS OF LOADTORQUES 2) Friction Torque (𝑻𝑭) Friction will be present at the motor shaft and also in various parts of the load. 𝑇𝐹 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
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  COMPONENTS OF LOADTORQUES iii) Coulomb Friction (Tc)- It is independent of speed and it is constant. ∴ 𝑇𝐹 = 𝑇𝑆 + 𝑇𝑉 + 𝑇𝐶 Since, 𝑇𝑆 is present only at standstill, It is not taken into account in the dynamic analysis. ∴ 𝑇𝐹 = 𝑇𝑉 + 𝑇𝐶
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  COMPONENTS OF LOADTORQUES 3) Windage Torque (𝑇𝑊): 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 𝑇𝑊 = 𝐶𝑤𝑚 2 Where C –constant Therefore total load torque 𝑇𝑙 = 𝑇𝐿 + 𝑇𝐹 + 𝑇𝑊 = 𝑇𝐿+𝑇𝑉 + 𝑇𝐶+𝑇𝑊 ∴ 𝑇𝑙 = 𝑇𝐿+𝐵 𝑤𝑚+𝑇𝐶+𝐶𝑤𝑚 2
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  COMPONENTS OF LOADTORQUES In many applications (𝑇𝐶+𝐶𝑤𝑚 2 ) is very small compared to 𝐵 𝑤𝑚 and it is negligible compared to 𝑇𝐿. In general (𝑇𝐶+𝐶𝑤𝑚 2 ) is approximate by updating the value of 𝐵. ∴ 𝑇𝑙 = 𝑇𝐿+𝐵 𝑤𝑚 Therefore fundamental torque equation becomes, 𝑇 = 𝐽 𝑑𝑤𝑚 𝑑𝑡 + 𝑇𝐿+𝐵 𝑤𝑚
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  NATURE OF LOADTORQUES  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.
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  NATURE OF LOADTORQUES  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).
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  NATURE OF LOADTORQUES 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
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  NATURE OF LOADTORQUES 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.
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  CLASSIFICATION OF LOAD TORQUES LoadTorques 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.
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  REFERENCES  G. K.Dubey, “Fundamentals of Electrical Drives”, Second Edition, Narosa Publishing House