The performance of a DC motor can be judged by its speed-torque characteristics and a motor for any particular application can be selected easily. In this article, speed-torque characteristics are discussed in detail.
DC MOTOR CHARACTERISTICS
The performance of a d.c. motor under various condition can be judged by the following speed-torque characteristics:
(i). Torque – Armature current characteristics
(ii). Speed – Armature current characteristics
(iii). Speed-Torque characteristics
1.Torque and armature current characteristics: In case of a series motor, the series field winding is carrying the entire armature current. Flux produced is proportional to the armature current.
The torque in case of a dc series motor is proportional to the square of the armature current. This relation is parabolic in nature as shown in the figure.
As load increases, armature current increases and torque produced increases proportional to the square of the armature current up to a particular limitation.
As the whole Armature current passes through the series field, there is a property of an electromagnet called saturation, may occur. Saturation means although the current through the winding increases, the flux produced remains constant. Hence after the saturation, the characteristics take the form of the straight line as flux becomes constant.
These types of motors can produce high torque fora little amount of armature current hence series motor is suitable for the applications which demand high starting torque.
2.Speed- armature current characteristics:
From, the speed equation, we get
Now the values of Ra and Rsc are so small that the effect of change in Ia on speed overrides the effect of change in V – IaRa -IaRsc on the speed.
Hence in the speed of the equation, Eb ≈ V and can be assumed constant. So the speed equation reduces :
So speed – armature current characteristics is rectangular hyperbola as shown Figure:
3.Speed Torque Characteristics: In the case of series motors,
Hence, WE can write,
in this way as torque increases when load increases, the speed decreases. On no load, torque is very small and hence speed increases to dangerously high value.
CHARACTERISTICS OF DC SHUNT MOTOR
1.Torque and armature current characteristics:
In this case of a Shunt Motor,
For a constant value of Rsh and supply voltage, V>>Ish is also constant and hence flux is also constant.
The equation represents a straight line, passing through the origin as shown in the figure. Torque increases linearly with armature current. It’s seen that armature current is decided by the load, as load increases,>> Armature current increases >> Increasing the torque developed linearly.
To produce high starting torque, this type of motor requires a large value of armature current at the beginning. This may injury the motor, therefore, DC shunt motors will develop moderate beginning force and therefore appropriate for such applications wherever beginning force demand is moderate.
In this case of a Shunt Motor,
So as load increases >> The armature current increases and hence drop IaRa also increases.
Hence for constant supply voltage, V-IaRa decreases and hence speed reduces. But as Ra is very small, for change Ia in from no load to full load, the drop IaRa is very small, and hence drop in speed is also not significant from no load to full load.
In DC shunt motor >> Speed regulation is the order of 5 to 10 % from no load to full load >> That’s why d.c. shunt motor is also known as a constant drive motor.
3.Speed Torque Characteristics:
So from these 2 equations, we can conclude that Speed and Torque both have a linear relationship. This characteristic is samely to speed – Armature current characteristics.
This curve shows that the speed almost bone constant through torque changes from no load to full load conditions.