In case of a generator, generated emf has to supply armature resistance drop and remaining part is available across the load as a terminal voltage. But in case of d.c. motor, supply voltage V has to overcome back emf E_{b} which is opposing V and also various drops as armature resistance drop I_{a}R_{a}, brush drop etc.

Hence the voltage equation of a d.c. motor can be written as,

Brush drop

Neglecting the brush drop, the generalized voltage equation is,

In case of Motor

In case of Generator

In case of motor, the back emf is always less than the supply voltage. But is very small hence under normal running conditions, the difference between back emf and the supply voltage is very small. The net voltage across the armature is the difference between the supply voltage and back emf which decides the armature current. Hence from the voltage equation we can write,

Power Equation of a D.C. Motor

The voltage equation of a d.c. motor is given by,

Multiplying both sides of the above equation by we get,

This equation is called power equation of a d.c. motor.

= Net electrical power input to the armature measured in watts.

= Power loss due to the resistance of the armature called armature copper loss.

So difference between and i.e. input – losses gives the output power.

So is called electrical equivalent of gross mechanical power developed by the armature. This is denoted as

Gross mechanical power developed in the armature = Power input to the armature – Armature copper loss

Electrical Engineering (EE) Student at Malaviya National Institute of Technology (MNIT), Jaipur
|| Jai Shree Radhey || Jai Shree Shyam || Jai Siya Ram ||
View all posts by Ashok Saini