Following are the three important characteristics of a DC motor
Performances of the motor can be judged from its characteristic curves. There are three types of characteristics of DC motors.
1. Torque and Armature Current Characteristics.
2. Speed and Armature Current Characteristics.
3. Speed and Torque Characteristics.
We will discuss each characteristic one by one.
Torque – Armature Current Characteristics
It is the graph plotted between the armature torque (τa) and the armature current (Ia) of a DC motor. It is also known as the electrical characteristics of the DC motor.
We know that according to the torque equation DC motor,
T ∝ Ⲫ * I………eq 1
where, T = Torque of DC Motor.
Ⲫ = Flux produced in DC Motor.
I = Armature Current.
Since the flux Ⲫ is nearly constant in the DC Shunt Motor, eq can be written as :
T ∝ I
As we can see from the above equation torque increases with the increase in armature current.
So, the torque-formature current Characteristic of a DC Shunt Motor is a straight line passing through the origin.
Torque -Current Characteristics of DC Shunt Motor
If the effect of the armature reaction is also considered, then the value of flux ⲫ decreases slightly with the increase in armature current, and therefore, at a higher value of Armature Current, the torque decreases slightly.
Speed – Armature Current Characteristics
It is the graph plotted between the speed (N) and the armature current (Ia) of a DC motor. This characteristic curve is mainly used for selecting a motor for a particular application.
The speed of the DC Motor is given as:
N ∝ (V – I*R)/Ⲫ
In the case of the DC Shunt Motor, flux is almost constant. So the above equation can be written as :
N ∝ (V – I*R)
So from the above equation, it is clear that the Speed – Armature Current Characteristics of a DC Shunt Motor is a straight line having a slight negative slope. That means the speed of the motor decreases slightly linearly with the increase in Armature current.
Speed – Current Characteristics of DC Shunt Motor
Since the voltage drop in armature at full load is very small as compared to the applied voltage, for all practical purposes the shunt motor is considered a constant-speed motor
The graph plotted between the speed (N) and the armature torque (τa) for a DC motor is known as the speed-torque characteristics. It is also known as the mechanical characteristics of a DC motor.
Torque – Speed Characteristics of DC Shunt motors can be derived from the above 2 characteristics, namely “Torque-Current Characteristics” and “Speed-Current Characteristics”.
When the load torque increases, then the armature current also increases and with the increase in armature current there is a slight decrease in speed. So, the resultant Torque-Speed Characteristics of the DC Shunt Motor are given below.
Torque-Speed Characteristics of DC Shunt Motor