It is known as electrical characteristic.*Torque and armature current (*/*T*_{a}*I*_{a}*characteristic).***Speed and armatur***e current (N*/*I*_{a}*characteristic)*It is also known as mechanical characteristic. It can be found from (1) and (2) above.*Speed and torque (N*/*T*_{a}*characteristic)*

While discussing dc motor characteristics, the following two relations should always be kept in mind :

T∝ Φ_{a }Iand_{a }

N∝E_{b}/Φ

### T_{a}/I_{a} Characteristic (Electrical)

We have seen that in series motor *T*_{a}_{ }∝ Φ*I** _{a}*. In a series motor, as field windings also carry the armature current, Φ ∝

*I*

_{a }up to the point of magnetic saturation. Hence, before saturation,

*T*

*∝ Φ*

_{a }*I*

*and ∴*

_{a }*T*

_{a}_{ }∝

*I*

_{a}^{2}

^{}At light loads,

*I*

*and hence Φ is small. But as*

_{a}*I*

*increases,*

_{a}*T*

*increases as the square of*

_{a}the current. Hence, *T** _{a}*/

*I*

*curve is a parabola as shown in figure.*

_{a}*T*

*∝*

_{a }*I*

*only. So the characteristic becomes a straight line. The shaft torque*

_{a}*T*

*is less than armature torque due to stray losses. It is shown dotted in the figure.*

_{sh}### N/I_{a }Characteristic

*N*∝*E*_{b}**/**

**Φ**

**E**, for various load currents is small and hence may be neglected for the time being. With increased

_{b}*I*

*, Φ also increases. Hence, speed varies inversely as armature current as shown in figure below.*

_{a}*I*

*is large. Hence, speed is low (this decreases*

_{a}**E**and allows more armature current to flow).

_{b}*I*

*falls to a small value, speed becomes dangerously high. Hence, a series motor should never be started without some mechanical (not belt-driven) load on it otherwise it may develop excessive speed and get damaged due to heavy centrifugal forces so produced.*

_{a}

### N/T_{a }Characteristic (Mechanical)

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