Understanding Torque and Its Simplest Explanation

In the past few topics of AC Motor basics we have seen about general motor terms, different types of motor load and the Calculation of Synchronous speed from Poles. We all frequently use term called Torque in our day to day work life. But, do we really know the actual meaning of "Torque". We got many request from our readers to explain the term torque in simple words. So get a cup of coffee and sit back. Subscribe to Automation-Talk by Email.

What does Torque Signify?

In our industrial applications when we have to select the driving motor for any load we are always concerned about the horsepower, but believe me the real driving force is TORQUE and not horsepower. In the simplest word torque is the twisting/rotational force exerted by the drive on the load. Now let us take a simple example.

Do you remember that sometimes you try to open the lid or open a bottle but after so many tries it does not open? Do you know the reason why it doesn't open? Because we are unable to supply or apply the adequate torque to open the lid. The solution for this can be a) grit our teeth and try harder, b) use a rubber pad, or cloth, to increase the ability to transmit torque without slippage, or c) use a mechanical device to help multiply our torque producing capability. If still, it doesn't open then give it to some stronger person!!

Well, the main motive behind the above example was just to make u feel something about torque. We are always confused among HP and Torque. Now let us suppose that after so much try also the lid did not open and now we will wrap a cord around the lid and supply a force to the end of the cord through a scale. See the below diagram.

Example for Torque Calculation

Can we tell the torque required in this case? Yes, we can, the torque requirement will be the force as depicted on scale multiplied by the radius of the lid. For example, if the reading of force on the scale is 17 lbs( at the time of breakaway of lid) and the radius is 3 inches then the torque requirement would be:-

T = F * R = 17 * 3 = 51 lb.inches

Simplest Explanation of Torque

However, the above example gives a clear illustration of the torque but lets us discuss one more thing which is observed in industrial applications. If we recall the above example then we can derive one more term called "Stiction". Stiction can be defined as the amount of torque required to make a load lose and start the first revolution. Stiction is nothing but only the second name of the breakaway torque and frankly speaking for every industrial application the breakaway torque requirement of any load is greater than the continuous torque requirement for running the machine.


The amount of "stiction" present in a machine will be dependent on the characteristics of the machine as well as the type of bearings that are used on the moving parts. Below you can see a picture in which the breakaway torque requirement of different machines is shown.

So, we have got now a clear view of the torque and its characteristics, in the next few days we will try to see more different types of torques and detailed explanation of torque in a simple way. Till then you can subscribe to get all latest updates via Email.

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Motor Related Common Electrical Terms - Part 2

In the last section we studied about some of the frequently used motor electrical parameters in our day to day work. We have already understood about the different types of AMPS rating that a motor nameplate have, in this post we will try to understand about the design of motor that is what type of torque and speed curve motor have, we will also look in to efficiency, frame size, and frequency. Subscribe to Automation-Talk by Email.

What Does Design of AC Motor Indicates

There are generally four types of design A B C D that an AC Motor can have. The design of AC Motor just indicates the type of speed v/s torque curve that the motor will have. The speed v/s torque curve for all the designs of the motor is below.



The standard industrial motor will mostly have the design B, it has reasonable starting torque with average speed and starting current with a good performance. In the above curve, we see that design C has the high starting torque so this type of motor is used where we have to start the high loads. Design D is the so-called high slip motor which tends to have very high starting torque but has high slip RPM at full load torque. Design D motors are generally suited for press and elevator applications. Design A motors are not particularly specified but are used where the high pull out torque is required.


What does efficiency of AC Motor Means?


Efficiency is indicated on the nameplate of the motor and in simple terms, it means that it is the amount of input energy that is converted into work. We should always select a motor that has high efficiency.


Meaning of Frame Size in AC Motor

As everything in this world has some size, same is the case with the motors. As the motor HP increases or RPM decreased the frame size of the motor increases. NEMA (National Electrical Manufacturers Association) has prescribed standard shaft diameter for the particular frame size so as to maintain a standard among all the motor manufacturers in the world.

What does Motor Frequency Indicates

Frequency on the motor nameplate means that for which input frequency motor is designed to operate. Most common motors have operating frequency of either 50 or 60 Hz.

We have understood some of the more common electrical terms in this post. We will also cover other commonly used electrical motor parameters in the upcoming post. If u have any doubts at any point please feel free to write in the comment section.

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How to Find RPM of AC Motor From Poles

AC motor name plate have data from which we can get the information about motor. Generally an AC Motor name plate have the motor KW/HP rating, Current , Voltage and current rating.Design,efficiency and operating frequency are also indicated on the motor nameplate. Also we can find the number of poles written at motor name plate. Today we will see that what does poles means in reference to AC Motor.


What is Pole in AC Motor :-
Well pole indicates the number of magnetic poles that appear inside the motor when ac power is supplied to its power terminals. We should understand the poles always exist in set inside a motor i.e poles always are in even numbers.

How to Calculate RPM of Motor from Poles and Frequency :-
The synchronous speed of a motor is always calculated from the number of poles in conjugation with the frequency.
To calculate the RPM of a motor you just need to apply the below formula.
RPM = 120F/P
Where :-
F = Frequency
P = Number of poles

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Below is the picture in which RPM of motor having 2,4,6,8 poles at different operating frequency of 50/60 Hz is shown


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Understanding Frequently Used Motor Terms - Part 1

Motors are used in every industry and every automation engineer has to deal with motors while working in plants and industries. Most of us have to install and commission the VFD panels and have to feed the motor name plate data in the VFD. But most of the automation engineer are unaware of the meaning of different motor parameters. In this post we will see the meaning of some of the frequently used motor terms.
Frequently Occurring Motor Terms Meanings:-
We all know that the every motor has AMPS rating on its name plate , generally there are three types of AMPS that are related to motor viz full load amps, locked rotor amps and service factor amps. Let us understand the meaning of all these three terms.

Full Load Amps :- As it is very clear from the name , it means the amount of current that motor can withdraw under the full load conditions or under the requirement of full torque.

Locked Rotor Amps :- This AMPS is also known as Starting Inrush current , it refers the amount of current that motor can take under the full voltage starting conditions. We all know that when a motor is connected in DOL then motor takes much more current then its rated current , so Locked Rotor Amps refer to that only.

Service Factor Amps :- We all know that every induction motor has some amount of service factor indicated on its name plate, if a motor has service factor of 20 then it means that the motor can handle load condition up to 120 or we can say that motor can have overload condition up to 20%. Therefor the service factor amperage is the amount of current that motor will draw when it is subjected to the over load conditions.

One other Motor term that is very less common is "CODE LETTER" - it is just the indication of the amount of the inrush current required by motor under full voltage starting.

After studying these motor terms we are now familiar with the common AMPS related words with motors. In the coming days we will discuss more about the other useful and frequently used motor terms , in case you have any doubt you can write to us in the comment section. And yes don't forget to G+ and FLike us!!!!
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How to Find AC/Induction Motor Load and Efficiency

Determining Motor full load and its efficiency has always been a quite hard task in industry . But after reading this we will definitely be expert in finding out any motor full load and its efficiency . Most of the motors are designed to run at 50% to 100% of load , the most efficiency of the motor is at 75% load.Thus a 20 HP motor will have its peak efficiency at 15 hp load.
How to find motor full Load ?

1)Calculate the three phase input power to motor using below formula -

Pi = V x I x PF x √3/1000

Where:
Pi = Three-phase power
V = RMS voltage, mean line-to-line of 3 phases
I = RMS current, mean of 3 phases
PF = Power factor as a decimal

2) Pir = hp * 0.7457 / ηfl

Where:
Pir = Input power at full-rated load in kW
hp = Nameplate rated horsepower
ηfl = Efficiency at full-rated load

3) Now to find load apply the below formula

Load = Pi/Pir * 100 %

Where:
Load = Output power as a % of rated power
Pi = Measured three-phase power in kW
Pir = Input power at full-rated load in kW

Now from the above method you can easily calculate motor load , if u can see the motor nameplate data then there is another usefull equation below .
How to get Load from motor nameplate

Load = I/Ir * V/Vr * 100 %

Where:
Load = Output power as a % of rated power
I = RMS current, mean of 3 phases
Ir = Nameplate rated current
V = RMS voltage, mean line-to-line of 3 phases
Vr = Nameplate rated voltage

To calculate motor load using slip method is use full when only operating speed measurements are available.The synchronous speed of an induction motor depends on the frequency of the power supply and on the number of poles for which the motor is wound. The higher the frequency, the faster a motor runs.Subscribe to Automation-Talk by Email. The more poles the motor has, the slower it runs.By using a tachometer to measure actual motor speed, it is possible to calculate motor loads.

Load = Slip/Ss - Sr * 100%

Where:
Load = Output power as a % of rated power
Slip = Synchronous speed - Measured speed in rpm
Ss = Synchronous speed in rpm
Sr = Nameplate full-load speed

Below is an example as how to find motor load using slip method .

Example: Slip Load Calculation
Given: Synchronous speed in rpm = 1800
Nameplate full load speed = 1750
Measured speed in rpm = 1770
Nameplate rated horsepower = 25 hp
Determine actual output horsepower.

Now applying the above equation we get 60 % load. So Actual output horsepower would be 60% x 25 hp = 15 hp.As most of the motors shaft are designed in such a way that tachometer can be connected to measure the speed so slip method is preferred widely to find load due to its simplicity .The accuracy of the slip method, however, is limited. The largest uncertainty relates to the 20% tolerance that NEMA allows manufacturers in their reporting of nameplate full-load speed.Thus a voltage correction factor can be inserted in to slip load method to get better results .

Load = Slip / (Ss- Sr )*(Vr / V)2 * 100 %

Where:
Load = Output power as a % of rated power
Slip = Synchronous speed - Measured speed in rpm
Ss = Synchronous speed in rpm
Sr = Nameplate full-load speed
V = RMS voltage, mean line to line of 3 phases
Vr = Nameplate rated voltage

Now after studying all above formulas to calculate motor load , i think calculating motor load now will not be a big task . Finding Motor efficiency is also required several times for many applications .Subscribe to Automation-Talk by Email.To find the motor efficiency , we will discuss here some use full formulas . How to calculate Motor efficiency .definition of energy efficiency is the ratio of its useful power output to its total power input and is usually expressed in percentage as shown in below equation.

η = 0.7457 x hp x Load/Pi

Where:
η = Efficiency as operated in %
Por = Nameplate rated horsepower
Load = Output power as a % of rated power
Pi = Three-phase power in kW

Below is a graphical representation of motor conversion of electrical energy in to mechanical energy .Motor losses are the difference between the input and output power. Once the motor efficiency has been determined and the input power is known, you can calculate output power.


Note :- Click on the picture for a clear view.

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