# Motor Selection: Selecting a Motor based on Speed, Power, and Torque

In this article, we will discuss how to select an electric motor based on speed, power, and torque. Motor selection is one of the most challenging tasks. If you want to learn how to deal with this issue, please read this article.

## What is an Electric Motor?

An electric motor is an electromechanical energy conversion device that converts electrical energy into mechanical energy. A typical electric motor consists of two major parts: the stator and the rotor. The stator is a stationary part, and the rotor is the moving part of the motor.

An electric motor primarily drives mechanical loads such as a pump, fan, compressor, etc.

Based on different power supplies and construction, the electric motor can be classified into the following major types:

These motors are widely used in industrial and other applications to drive mechanical loads. An electric motor selected for a specific application is done based on speed, torque, and power.

Based on these three parameters, let us understand how to select an electric motor for a specific application.

## Motor Selection based on Speed

While selecting an electric motor for a particular application where the rotational speed is a critical parameter, we must follow the following procedure.

First, you must determine the rotational speed required for the given application. For example, there could be a situation that requires a variable speed motor whose speed can be easily changed in a desired range or a constant speed motor.

In the next step, you have to determine the critical characteristics of the load connected to the motor that may affect the performance of the motor—for example, the inertia of the load, friction coefficient, speed variation, etc.

After that, you must compute that application’s minimum and maximum speed values.

Finally, you have to pick an electric motor based on all the parameters determined in the above three steps. Here is a list of electric motors and their speed characteristics:

• DC MotorsDC Motors are considered motors with excellent and precise speed control. Therefore, if your load requirements are such that it requires variable speeds, then you should select a DC motor.
• Induction Motors – Induction motors are considered constant-speed motors. Hence, they are suitable for applications where speed variation is not required. However, the speed of induction motors can also be varied in a particular range using a variable frequency drive (VFD) device.
• Synchronous Motors – Synchronous motors are always constant-speed motors. Hence, they are suitable for constant-speed applications only.

## Motor Selection based on Torque

Sometimes, electric motors are selected for a particular application based on the torque characteristics of the load. For example, an electric train requires high initial torque due to high load inertia.

The following description guides you in selecting an electric motor based on torque requirements.

Firstly, determine the torque characteristics of the load to connect to the motor shaft. Also, determine the minimum and maximum torque for smooth motor operation. It is also suggested that you consider a safety margin of around 10% to 20% to prevent overload conditions due to load variations.

After these studies, pick a suitable electric motor meeting the requirements.

• DC Motors – DC Motors, particularly DC series motors, are suitable for high-starting torque applications. Also, they provide an efficient torque control. Therefore, if your application requires high-starting torque along with torque variation conditions, then DC motors can be the best selection.
• Induction Motors – Induction motors are suitable for applications requiring moderate starting torque with a constant speed. Hence, if your load requirements are such that speed variation is not critical and the torque required is not much higher, then induction motors can be the best selection.
• Synchronous Motors – Synchronous motors are known for their constant speed and high-torque capabilities. Hence, they are suitable for driving loads that require high torque at constant speed.

## Motor Selection based on Power

To produce the desired output, the power developed by the electric motor is a crucial factor.

To select an electric motor based on the power developed, we must determine the required power depending on the application and load. Along with power calculation, we must also determine the motor’s efficiency to reduce energy consumption and the operating cost.

Then, we select an electric motor that meets our power and efficiency criteria.

• DC Motors – DC Motors provides excellent control over the power output. Therefore, if we are selecting an electric motor where a variable power output is required, then DC motors can be the best choice.
• Induction Motors – Induction motors are generally designed to produce constant torque at a constant speed and, hence, constant power output. Therefore, induction motors are the best selection if our load requires constant running power.
• Synchronous Motors – Synchronous motors are known for developing constant power output. This is because they can run at constant speed and torque conditions. Hence, their output power is also constant. Therefore, synchronous motors are suitable for constant power-demanding applications.