Short Answer:

The speed-torque characteristic of a DC motor shows the relationship between its speed and the torque it produces. In simple terms, as the load torque increases, the motor speed decreases, and vice versa. This characteristic helps in understanding how a motor behaves under different load conditions.

Different types of DC motors have different speed-torque curves. For example, a DC series motor shows a sharp drop in speed with increasing torque, while a DC shunt motor maintains almost constant speed. These characteristics are very useful when selecting a motor for specific applications.

Detailed Explanation:

Speed-torque characteristic of a DC motor

The speed-torque characteristic of a DC motor explains how the speed of the motor changes as the torque or load changes. It is one of the most important performance aspects of any motor because it directly shows how the motor reacts to various loads.

Let’s understand this in a basic way:

• Torque is the twisting force that helps the motor to rotate and do work.
• Speed is how fast the motor shaft spins.
  The relationship between these two tells us whether the motor can handle certain loads and still work efficiently.

There are three main types of DC motors, and each has its own speed-torque behavior:

1. DC Series Motor:

In a series motor, the field winding is connected in series with the armature. This type of motor shows a high torque at low speed. When load increases, the motor draws more current, increasing both torque and magnetic field strength, but speed drops sharply. At no load, it can reach dangerously high speeds.

Characteristic:

• High starting torque
• Speed decreases rapidly as torque increases
• Used in cranes, elevators, electric trains

2. DC Shunt Motor:

In a shunt motor, the field winding is connected in parallel (shunt) with the armature. This motor maintains a nearly constant speed regardless of load because the field current remains almost the same.

Characteristic:

• Torque increases slightly as speed drops
• Almost constant speed under varying loads
• Used in fans, lathes, blowers

3. DC Compound Motor:

This motor combines the features of series and shunt motors. The torque-speed curve lies between the two types mentioned above. It provides good starting torque and better speed regulation.

Characteristic:

• Better load-handling than shunt motor
• Good starting torque
• Used in presses, conveyors, rolling mills

How Speed is Affected by Torque:

Speed of a DC motor depends on the back EMF, which itself depends on the speed. When torque (load) increases, more current flows in the armature to produce more torque. But this increases the voltage drop across the armature resistance, reducing back EMF and thus reducing speed.

Mathematical Relation:
Speed (N) is approximately given by:
N ∝ (V – Ia × Ra)/Φ
Where:

• V = Supply voltage
• Ia = Armature current
• Ra = Armature resistance
• Φ = Magnetic flux

So, if load increases → Ia increases → voltage drop increases → back EMF decreases → speed decreases.

Graphical Understanding:

Even without a graph, you can imagine:

• For series motors, the curve is steep—high torque causes sharp speed reduction.
• For shunt motors, the curve is flat—speed remains stable even with changing torque.
• For compound motors, it’s a combination curve.

These characteristics help engineers select the right motor depending on whether the application needs high torque at startup, constant speed, or a mix of both.

Conclusion:

The speed-torque characteristic of a DC motor explains how motor speed changes with applied load torque. Series motors give high torque but lose speed quickly under load. Shunt motors keep speed steady, while compound motors offer a balance. Understanding this characteristic helps in choosing the right motor for specific industrial and mechanical needs.