# Characteristics of DC Series Motor

It is known as an 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** _{a }*∝ Φ

*I*

*and*

_{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** _{a }*∝ Φ

*I*

*and ∴*

_{a }*T*

_{a}_{ }∝

*I*

_{a}^{2}

At light loads, *I** _{a}* and hence Φ is small. But as

*I*

*increases,*

_{a}*T*

*increases as the square of*

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

*I*

*curve is a parabola as shown in the figure.*

_{a}After saturation, Φ is almost independent of Ia hence *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}So we conclude that (prior to magnetic saturation) on heavy loads, a series motor exerts a torque proportional to the square of armature current.

**Hence, in cases where huge starting torque is required for accelerating heavy masses quickly as in hoists and electric trains, etc., series motors are used.**

## 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 the figure below.*

_{a}When the load is heavy, *I** _{a}* is large. Hence, speed is low (this decreases

**E**and allows more armature current to flow).

_{b}But when load current and hence *I** _{a}* 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.

It should be noted that the series motor is a variable speed motor.

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

It is found from above that when speed is high, torque is low and vice-versa.

It is clear that series motor develops high torque at low speed and vice-versa.

It is because an increase in torque requires an increase in armature current, which is also the field current. The result is that flux is strengthened and hence the speed drops