Loads

 

Loads

Generally as common sense load means weight. It may be weight carried by human, animal or vehicle. Sometimes it refers to the quantum of work assigned to employee.  Teaching period is load for professor and syllabus is load on students.

Load on structure may be gravitational force of mass resting on it or force due to thrust or pull like wind pressure as on electric conductor and support. All these may cause strain in various parts of the structure. Strain may be basic like compressive, tensile and shearing or complex like torsional and bending. But in any case structure reacts by inducing opposing force known as stress to counter balance the load. Stress is proportional to strain up to elastic limit and strain is proportional to the load till elastic limit. In any case stress can be equal to and opposite to load but not more than the load. Material cannot induce stress beyond elastic limit. In such case uncontrolled strain results in failure.

Electric field intensity is load on dielectric (insulation). Insulating material have own dielectric strength. Insulation fails when electric field intensity (or potential gradient or dielectric stress) is more than dielectric strength of the insulation.

Load on machine is the resistance to its function. The statement appears to be unconvincing as all the machines are performing as per assigned duty. Actually the load resists but machine intensifies the effort to overcome it and perform. For this extra effort more energy is required by machines. So more loads mean more resistance that requires more effort and ultimately more energy. Therefore energy consumed by machine is related to load.

Here we scrutinize two types of machine loads, mechanical and electrical.

Mechanical Load

Mechanical load is a most common observed in routine life. No need to identify as mechanical load. But it termed as mechanical load to distinguish it from electrical load.

Function of the car is to move forward but friction with the road and wind thrust resists and restricts the motion. So these are the main loads of running car.  Similarly motor driving drill or lathe has load as friction of tool with the job. Motor driving elevator has to overcome resistance due to gravitational force. In short resistance to the motion i.e. friction is the load.

Friction belt is used in laboratory to simulate mechanical load on machine. Load on machine is regulated by adjusting tension of the belt that increases friction between pulley and belt.

Initially belt is loose without contact with pulley has no friction. It is no load condition. Subsequently tension of the belt is increased in steps. Each step friction between belt and pulley is increased that simulates increase in load on motor. As load increase the slip of the motor also increase and ultimately speed decrease.

Electrical Load

Any electricity sources generate EMF across its terminals and electrical load is connected to it. Here load is any gadget having internal circuit with some impedance Z through which current flows. This Z may be pure resistance as in lamp or resistance and inductance as in motor or resistance, inductance and capacitance as in fan.   

Lamp bank is used in laboratory as electrical load on source of electricity. Initially when no lamp is switched on, it represents no load condition. Then step by step lamps are switched on to increase load. Lamp has definite resistance. Switching more lamps means connecting more resistance in parallel. Resultant of parallel resistance is reduced resistance. It implies that more load means reduced load resistance (in general impedance).

Conclusion

Similarity

Motor produce torque that causes rotary motion but friction at job opposes it is the load.

Generator produce emf that cause current in the circuit but impedance in circuit opposes it is the load. 

Contradiction

Mechanical load is proportional to the friction.       Load  µ Friction.       More friction means more load.

Extreme low friction is the condition that does not restrict the movement at all.  It represents no load on machine.

Extreme high friction is the condition that totally blocks the movement.  It represents extreme high load on machine.

Electrical load is inversely proportional to impedance.   Load  µ 1/Impedance.   Less Impedance means more load.   

Extreme high impedance is no connection between the terminals.   It represents no load on machine.

Extreme low impedance is short circuit having zero impedance.  It represents extreme high load on machine.