A Basic Discussion of Torque

Posted on: November 5th, 2013 by admin No Comments

A Basic discussion of Torque

 

First of all let me be up front about this – I am not an engineer (although I have owned a few pocket protectors in my day). But if there is one thing I’ve learned in over 45 years in the motor service industry it is this: You need to be able to speak on the same level as your customer or they won’t understand your explanation. One of my pet peeves is technicians (or business owners!) that speak in acronyms and buzz words, as if their audience had any idea what they were saying.

I have met some very well educated people that know a lot more than I do about a lot of subjects. However, by the time they finish impressing you with their knowledge level you still don’t know what the answer to your question was.

 

A customer asked me recently what the comparison was between the torque of a 20HP motor and a 15HP motor. The easiest answer is that…. it depends. Let’s see if we can come up with a relatively simple answer and leave the caveats standing in the wings to amuse the engineers.

Before we answer the question we need to confirm a number of issues and before we get into those issues we’ll take a look at a definition of torque. Many people mistakenly interchange the terms “torque”, “horsepower” and “power”. So let’s look at the definitions to discern the differences:

 

Torque – “A twisting force that tends to cause rotation”.

Horsepower – “A unit of power equal to 745.7 watts (electrical) or 33,000 foot-pounds per minute (mechanical)”.

Power – “The rate of doing work”. In electrical terms, this is measured in watts.

 

Given the three definitions we can say that electrical power and horsepower in an electrical system are interchangeable. They both are (or can be) measured in watts. In the case of an electric motor there is a direct mathematical relationship between horsepower and power – one horsepower is equal to 745.7 watts (typically rounded to 746 for ease of calculation). The European standard for measuring power output of an electric motor is, in fact, the watt. We prefer to measure power in horsepower because we like horses. Someone, many years ago, decided that his horse could lift 33,000 pounds one foot in one minute and declared it a standard of “one horsepower”. Someone in France heard about this and responded “Watt?”

The one thing we need to note about our definitions is that horsepower and power both include a time factor in their calculation. They are the measurement of power over a period of time. Torque, on the other hand, does not contain an element of time. This then tells us that torque measured over time is, indeed, horsepower.

The mathematical formula that defines the relationship is:

Torque = Horsepower x 5252

           RPM

where torque is measured in lb-ft and 5252 is a mathematical constant used to accurately convert the units involved in the calculation.

 

We don’t have to be a math major to see that, as horsepower increases, torque increases; as speed increases, torque decreases.

So now we can go back to the original question – what is the comparison between the torque of a 20HP motor and a 15HP motor?

In order to answer this question we need to make some basic assumptions, namely that other than horsepower all other factors remain the same. They won’t be*, but we’ll keep it simple. Therefore the relationship between the torque of a 20 HP motor and a 15HP motor of the same speed is simply: 20/15 or, put another way, you can expect approximately 33% more torque from a 20HP motor when compared to the output torque of a 15HP motor.

A rule of thumb used when calculating torque of an electric motor is that you can get approximately 3 lb.-ft of torque per horsepower from an 1800 rpm motor. From the formula we can see a higher speed motor will reduce the available torque, a lower speed motor will increase torque. It is important to understand these relationships before changing speeds. The resultant change in available torque may catch you off guard. You may not need an engineer, but it may be wise to talk to the motor experts at Gem State Electric (I did warn you that this was a basic discussion). We are always happy to assist you.

 

*other factors that may change will be the power sources ability to provide sufficient power to the system to run a higher horsepower motor, losses in the system when using a higher horsepower motor, the efficiency level of the two motors, inertia of the two rotors and even the rpm. The actual rpm that an electric motor runs at is dependent on its ability to produce the needed torque. Given the same load on both motors a larger motor will run at a (marginally) higher rpm. This change in speed may be negligible, but in certain situations it may not be. Call Gem State Electric at 208-344-5461 if you need to discuss your application.

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