Difference Between Armature Torque and Shaft Torque

Difference Between Armature Torque and Shaft Torque
Photo by Erik Mclean

Torque is the tendency of an object to rotate, or if it is rotating, to continue rotating at the same speed or to slow down or speed up. The two types of torque most commonly discussed in electrical machines are armature and shaft.

Therefore it is useful to learn the difference between armature torque and shaft torque. Also, why each one has its relevance when examining a specific machine?

What is Armature Torque?

Armature torque is the rotational force that the armature of a motor creates. The armature is the part of the motor that turns, so this force is what makes the motor turn.

The amount of armature torque produced by a motor depends on several factors. This includes the strength of the magnetic field, the number of turns in the armature coil, and the speed at which the armature turns.

What is Shaft Torque?

Shaft torque is the rotational force that causes an object to rotate about an axis. It measures the turning or twisting force that can be applied to an object. The units of shaft torque are usually Newton-meters (Nm). 

One way to determine the shaft torque exerted on an object is by multiplying its radius times its angular velocity in radians per second (rps) divided by 2π.

So, for example, if you wanted to calculate the shear force in Newtons at the edge of a circle with a radius of 2 meters, rotating at one revolution per second, it would be 0.5π N/rps=1.25 Nm.

Since shaft torque is quite similar to armature torque, what is the difference between armature torque and shaft torque? Let’s take a look.

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Differences Between Armature Torque and Shaft Torque

Here is the core difference between armature and shaft torque

  • The torque created by the armature is influenced by how often it has been wound or unwound. In contrast, shaft torque does not change when it encounters different loads. 
  • Another difference is that armature torque is a current function, while shaft torque is a speed function.
  • Shaft torque is the rotational force that the armature creates. The armature is the rotating part of an electric motor. It is what the coils of wire are wrapped around to create a magnetic field. 
  • The armature torque is the force that is required to move the armature. It is affected by the strength of the magnetic field and the number of turns in the coil of wire. The shaft torque is affected by friction and the load on the shaft.
  • Additionally, armature torque is affected by the number of turns in the winding, while shaft torque is not. 
  • Armature torque tends to be constant, while shaft torque varies with speed.
  • Shaft torque requires more space than armature torque because you need room for belts and pulleys. 
  • Shaft torque will produce less heat and wear out faster, but armature torque produces more. 
  • Armature torque must be accounted for when designing motors because they require a lot of power and can cause big problems if they’re off balance. This makes them harder to design and less likely to break down over time. They require more maintenance simultaneously, though this doesn’t mean much for home appliances since most don’t get used often. In comparison, Shaft torque is advantageous in more industrial applications. This is because it has a better speed control range due to changing gears. 
  • It also allows for better handling of heavier loads because it can handle gearing ratios up to 20:1, whereas armature torque only goes up to 5:1. 11) depending on your specific application, one type may be better suited than another due to their efficiency differences. However, in many cases, it simply comes down to personal preference based on how each type works and performs under certain circumstances. 
  • Shaft torque is usually much easier for us homeowners without technical expertise or professional knowledge about mechanics because fewer parts are involved during installation. Armature torque is usually preferred by people who want something more durable or a cleaner look to their designs. 
  • If you’re looking for maximum performance, go with shaft torque; if you’re looking for longevity and durability, go with armature torque.  
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When to Use Each Type of Motor?

There are two types of torque in electric motors: armature and shaft. Armature torque is the force that rotates the armature, or the central part of the motor, while shaft torque is the force that turns the output shaft. So, which one should you use?

The answer is simple! You should choose a motor with the appropriate type of torque for your application. The higher the shaft torque on a motor, the more likely it will be able to power a heavy load (i.e., metal cutting).

On the other hand, an armature-torque type may be better suited for driving lighter loads such as pumps and fans.

Conclusion

The main difference between armature torque and shaft torque is that armature torque is produced by the interaction between the armature and the magnetic field.

In contrast, shaft torque is produced by the mechanical interaction between the rotating shaft and an external force.  

Conclusively, there are many differences between armature torque and shaft torque.  The amount of current used for each type of torque will vary based on the type of power source (AC vs. DC). 

If there is a problem with either one, it will be easier to find where it’s coming from. This is because each type has specific components.

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