Classification of load torques :
1.Active Load torques
2.Passice Load torques
Active Load Torques:
Load torques which have the potential to drive the motor under equilibrium conditions are called active load torques.
Load torques usually retain sign when the drive rotation is changed.
Active Load Torques
Load torques which always oppose the motion and change their sign on the reversal of motion are called passive load torques.
Torque due to friction cutting – Passive torque.
Components of load torques:
1.Friction Torque (TF)
The friction torque (TF) is the equivalent value of various friction torques referred to the motor shaft.
2.Windage Torque (Tw)
When a motor runs, the wind generates a torque opposing the motion . This is known as the winding torque.
3.Torque required to do useful mechanical work ( Tm)
Nature of the torque depands of type of load.
It may be constant and indeoendent of speed, Some function of speed, may be time invariant or time variant.
The nature of the torque may change with the change in the loads mode of operation.
Characteristics of different types of load:
In electric drives the driving equipment is an electric motor.
Selection of particular type of motor driving a m/c is the matching of speed-torque charal of the driven unit and that of the motor.
· Different types of loads exhibit different speed torque charal.
· Most of the industrial loads can be classified into the following 4 general categories:
1. Constant torque type load.
2. Torque proportional to speed (generator type load)
3. Torque proportional to square of the speed (fan type load)
4. Torque inversely proportional to speed (const power type load)
1.Constant Torque Characteristic :
The speed – torque characteristic of this type of load is given by T=K.
Working motor have each mechanical nature of work like shaping , cutting, grinding or sharing, require constant torque irrespective of speed. Similarly cranes during the hoisting.
Similarly cranes during the hoisting and conveyors handling constant weight of material / unit, time also exhibit this type of characteristics.
Torque proportional to speed:
Separately excuted dc generators connected to a constant resistance load, eddy current brakes and calendaring m/cs have a speed torque characteristics m/cs have a speed – torque characteristics given by T= Kw.
Torque propositional to square of the speed :
Load Torque Square of speed
Example : Fans , Rotary pumps , compressors , ship propellers. The speed – torque characteristics of this type of load is given by
Torque inversely propositional to speed:
· In such types of loads , torque is inversely proportional to speed or load power remains constant.
· Eq: Lathes, boring m/cs, milling m/cs , steel mill colier and electric traction load.
· This type of characteristics is given by
· Most of the load require extra effort at the time of starting to overcome static friction. In power application it is known as brake away torque and load control engineers call it “stiction” . Because of slition , the speed torque characteristics of the load is modified near to zero speed.