Torque and Power


In an electrical motor, its driving force is not horsepower but torque. It can be described as a twisting force that initiates the running of a motor. This causes the torque of a motor to gain its operating speed from 0% to 100%.  In S.I system, its unit is depicted as ‘Nm, ’ and in case of imperial system, its unit is ‘lb ft.’

In an induction motor, production of torque is due to the interaction of stat or field and rotor.Both power and torque are explained via anequation.

9.1. Starting Torque

In an induction motor, a starting torque increases when there is a hike in the resistance of a rotor. But there is also an important fact that we require to keep in consideration that a motor requires a minimum torque to move from a stationary opposition.

Let us consider the start slip to be 1 which is denoted by ‘s’. So the equational expression for this starting torque can be acquire with the help of,

So, from this expression that is highlighted above, we can draw a conclusion,

9.3. Starting Torque of a Squirrel-Cage Motor

If compared to reactance, squirrel cage rotor’s resistance is small and fixed. The reactance of it is high, especially at the beginning. This is because rotor current’s frequency is similar to supply frequency when it is at a standstill. These motors are not good for the application in areas where the loads are heavy.

9.4. Starting torque of a slip-ring motor

Torque can be increased inside the slip ring motor by adding external resistance and improving the motor’s power factor. This is generally done in a rotor circuit which is linked with a star- connected rheostat. When the speed of motor starts increasing, slowly its rheostat resistance is cut out.

9.5. Power

Power can be described as the rate with which a work is done. It can also be relayed as the energy that is consumed per unit time. In this context, power can be taken as the main element which causes an electric motor’s output power to generate torque as the product.

It can be described with the help of a simple series circuit.

Its internal torque can be developed in per phase basis. So the equation is,

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