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How to Reduce DC Motor Speed?

2022-12-17 15:53:58

DC motors are widely used in various industries due to their ability to convert electrical energy into mechanical energy. Controlling the speed of a DC motor is crucial for optimizing its performance and achieving desired outcomes in different applications. In this article, we will explore the best and most reliable methods to decrease the speed of a DC motor. By understanding these techniques, you will be equipped with valuable knowledge that can enhance your career in the field of electrical engineering.

I. Understanding DC Motor Speed Control

Before delving into the methods of decreasing DC motor speed, it is essential to grasp the fundamental principles of DC motor operation and speed control. A DC motor operates on the principle that when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force, resulting in rotational motion. The speed of a DC motor can be controlled by manipulating the supply voltage, the armature voltage, or the field flux.

Principle of Speed Control

Simple DC Motor Voltage Equation

From the above figure, it can be seen that the voltage equation of a simple DC motor is:

V = Eb + IaRa

Among them, V is the supply voltage, Eb is the back electromotive force, Ia is the armature current, and Ra is the armature resistance.

The formula for calculating back electromotive force (Eb)

We know that the formula for calculating the back electromotive force (EMF) Eb is:

Eb = (PøNZ) / 60A

Among them, P is the pole number, A is the constant, Z is the number of conductors, N is the motor speed, and ø is the magnetic flux.

Relationship between motor speed and various parameters

Substituting the value of Eb into the voltage equation, we can obtain:


Equivalent to: N = K(V - IaRa)/ø

Where K is a constant.

This means:

  • The motor speed is directly proportional to the power supply voltage.
  • The motor speed is inversely proportional to the voltage drop across the armature.
  • According to the on-site discovery, the motor speed is inversely proportional to the magnetic flux.

How to control the speed of a DC motor?

From the above minutes, it can be concluded that the speed of a DC motor can be changed in three ways:

  1. By changing the power supply voltage
  2. By changing the magnetic flux and altering the current through the excitation winding.
  3. By changing the armature voltage and the armature resistance.

II. Decreasing DC Motor Speed: Effective Control Methods

There are several reliable techniques available to decrease the speed of a DC motor. Let's explore the most prominent ones:

Armature Controlled Resistance:

This method involves connecting a regulating resistance in series with the motor supply. By increasing the resistance, the voltage across the armature decreases, leading to a reduction in motor speed. Armature controlled resistance is commonly employed in applications where a persistent torque is required, such as cranes, trains, and vehicles.

Shunted Armature Control:

In this technique, a rheostat is connected in both series and shunt configuration with the armature. By varying the series rheostat, the voltage applied to the armature can be adjusted, while the shunt rheostat controls the excitation current. Shunted armature control allows for some degree of speed regulation, although it may not exceed the normal speed range. This method is cost-effective but can result in significant power losses.

Field Diverter Technique:

The field diverter technique involves shunting a portion of the motor current across the series field, thereby reducing the flux across the field winding. By decreasing the resistance of the diverter, the field current is reduced, resulting in a decrease in motor speed. This method is suitable for applications where speed needs to increase with decreasing loads.

Controlling of Tapped Field:

In this technique, the speed of a DC motor is increased by reducing the number of turns in the field winding. By decreasing the field winding turns, the flux is reduced, leading to an increase in motor speed. Controlling of tapped field is commonly used in electric tractions.

Varying Armature Voltage and Resistance:

By varying the armature voltage and resistance simultaneously, the speed of a DC motor can be effectively controlled. Decreasing the armature voltage and increasing the armature resistance contribute to reducing the motor speed. This method is suitable for applications where precise speed control is required.

III. Factors to Consider

While implementing speed control techniques for a DC motor, it is essential to consider certain factors:

  • Efficiency: Some speed control methods may result in power losses or reduced motor efficiency. It is crucial to evaluate the trade-off between speed control and overall system efficiency.
  • Load Characteristics: The speed control method should be selected based on the specific load requirements. Different applications may demand varying levels of torque and speed regulation.
  • Cost-Effectiveness: Consider the cost implications of implementing different speed control techniques. Some methods may be more cost-effective than others, depending on the application and budget constraints.


Controlling the speed of a DC motor is vital for optimizing its performance and achieving desired outcomes in various applications. By employing reliable and effective speed control methods such as armature controlled resistance, shunted armature control, field diverter technique, controlling of tapped field, and varying armature voltage and resistance, it is possible to decrease the speed of a DC motor accurately. Understanding these techniques and considering factors such as efficiency, load characteristics, and cost-effectiveness will help you use your device better.

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