# Characteristics of DC Series Motor

- Link: https://studyelectrical.com/2016/09/characteristics-of-dc-series-motor.html
- Published: 2016-09-20T01:48:00+00:00
- Author: 

![2016 09 30 22h25 34](https://4.bp.blogspot.com/-Su6SKrLz0sA/V-6Z9Rh73JI/AAAAAAAACB8/
VxraKzDvZ0EpwrmScKKj-PElFiil7Fj3gCLcB/s1600/2016-09-30_22h25_34.jpg)

In this post, we will look into the characteristics of a DC series motor. A DC series
motor is one [type of DC motor](https://studyelectrical.com/2014/12/types-of-dc-motors.html)
in which field winding is connected in series with the armature winding.

The performance of any motor can be judged from its characteristic curves known 
as motor characteristics. There are three characteristic curves that can be derived
from the [working of a DC motor](http://www.studyelectrical.com/2014/12/working-principle-of-dc-motor.html).

> [Read the Characteristics of Series wound DC Generator](http://www.studyelectrical.com/2014/08/characteristics-of-series-wound-dc-generator.html)

The characteristic curves of a DC series motor are those curves which show relationships
between the following quantities.

 1. **_Torque and armature current (__T___(a)_/_I_(a)__ characteristic)._** It is known
    as an electrical characteristic. 
 2. **Speed and armatur_e current (N_/_I_(a)__ characteristic)_**
 3. **_Speed and torque (N_/_T_(a)__ characteristic)_** It is also known as mechanical
    characteristic. It can be found from (1) and (2) above.

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

**_T___(a )_∝ Φ_I___(a  )_and **

**_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___(a )_and ∴ _T___(a)__( )∝ _I___(a)_^(2)**

At light loads, _I___(a)_ and hence Φ is small. But as _I___(a)_ increases, _T___(
a)_ increases as the square of

the current.  Hence, _T___(a)_/_I___(a)_ curve is a parabola as shown in the figure.

[⌊Ta/Ia Characteristic⌉⌊Ta/Ia Characteristic⌉[

Ta/Ia Characteristic

After saturation, Φ is almost independent of Ia hence _T___(a )_∝ _I___(a)_ only.
So the characteristic becomes a straight line. The shaft torque _T___(sh)_ is less
than armature torque due to stray losses. It is shown dotted in the figure. 

 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

Variations of speed can be deduced from the formula :

**_                                                                        _****
_N _∝ _E__(b)****/****Φ**

Change in **E_(b)**, for various load currents is small and hence may be neglected
for the time being. With increased _I___(a)_, Φ also increases. Hence, speed varies
inversely as armature current as shown in the figure below.

[⌊N/Ia Characteristic⌉⌊N/Ia Characteristic⌉[

N/Ia Characteristic

When the load is heavy, _I___(a)_ is large. Hence, speed is low (this decreases **
E_(b)** and allows more armature current to flow). 

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)

[⌊N/Ta Characteristic⌉⌊N/Ta Characteristic⌉[

N/Ta Characteristic

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

 

### Related posts:

No related posts.