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.
Aug 7, 2018
Understanding Torque and Its Simplest Explanation
Jul 4, 2018
Test Procedure of AC Induction Motor
AC Induction Motors are the heart of any type of Industries, for any work you require a Motor to do the work. Hence there must not be any issue in calling "AC Motors" as the heart of Industrial applications.
How to Test the AC Induction Motor
As we buy the AC Induction Motor from the market, it has been tested previously at the manufacturing time for all the defects, But still, we should also know and understand the testing procedure of AC Induction Motor. We will see step by step testing procedure here for AC Motor.
Magering:- Mager is used to test the Insulation of the Motor. But what is Mager??
Mager is actually a small DC Generator rotated by hand, it generates High Voltage and less Current. For the testing purpose,
upto 2000V it is hand operated and above 2000V it is mechanically operated.
The resulting outcome equation is as:- 1mohm / KV and 0.44mohm / 440V.
Drop Test:- Drop test is used to check the resistance of individual winding, and they should be equal for a more efficient AC Motor.
<> Absorption Coefficient Test:- It is same as Magering procedure. In this test procedure of AC Induction Motor, the Insulation value is measured for two different time duration one for t=15 sec and other for t=60sec.
If the Insulation Value for t=15 sec is less than 1 then AC Motor OK.
If the Insulation Value for t=15 sec is greater than 1 then AC Motor NOT OK.
Recommended Article: How to Change Servo Drive Parameter using Omron CX Drive
So It may be possible that moisture is present in the Windings of AC Induction Motor, To remove the Moisture from the Motor Windings, Raise the temperature of motor windings gradually so that all the moisture evaporates out. But one point to keep in Mind is that the rate of change of temperature of Motor Windings should not be greater 1 deg. celcius per 30 minutes.
After removing the moisture from the motor windings again perform the Absorption Coefficient Test and if this time Insulation Value for t=60sec is less than 1 then AC Motor OK. If you like this article, please Subscribe to Automation-Talk by Email and share this article with your friends.
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Dec 17, 2017
Different Types of Motor Load Meaning
While selecting the AC motor for an application, we always first lookout that what type of load will be driven by AC Motor i.e. constant Torque load or variable torque load. Almost all of the automation and plc engineers must have seen all such type of loads. Also while selecting the VFD for an AC motor we have to select the VFD according to the Load type. But do we actually know what this load type means?
Well if not, then in this post we will try to understand the different types of load.
What is Constant HorsePower Load?
The constant horsepower load is that type of load in which the torque requirement is more at low speed and as the speed increases the torque requirement decreases. This type of load examples can be found drill machines, lathe machines, and metal removing applications.
What is a Constant Load Torque?
In constant torque load, the torque requirement is always the same irrespective of the load speed. If we take the example of conveyor then the torque requirement of a conveyor is always same whether it is running at low speed or high speed.
What is Variable Type Load?
Variable torque load is a term which is used for loads that require low torque at low speeds and high torque at high speeds.
Examples of Variable type load are centrifugal pumps and fan. In the market, there are special VFD available for such type of applications that saves a lot of energy.
Hope that after reading this article, we now understood the different type of load meaning and next time we can easily identify the type of load that a motor is driving. Don't forget to subscribe to get all latest updates and also like our facebook page if you like Automation Talk.
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Nov 7, 2017
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.
Jul 22, 2012
How to Find RPM of AC Motor From Poles
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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Mar 8, 2011
Reduce AC Motor Heat & Noise Problem - Motor Maintenance
AC Motors are used widely in Industries for delivering mechanical power. But there should be proper maintenance of AC Motor for increasing its life and efficiency. Due to dust, wear and tear as time passes the efficiency of Motor tends to decrease, leading to many problems. In this article, we will see how to reduce the sound (noise) of AC Motor.
How To Solve Motor Heat and Sound Problem:-
There are many issues related to Motor problems. But we should remember one key point, i.e. the most important factor to reduce motor problems is schedule inspections and regular service. We will discuss more about Motor heat and noise problem in this post.
First and foremost, feel the motor nameplate and bearings for "heat and noise". If the Motor connections are loose then it could be also the reason for the motor noise.
There are 3 main reasons for Motor Heat generation.
1) Low Rating Motor:- It may be possible that Motor Selection is wrong or the starting torque required for the load is large than delivered by the motor.
2)Overloaded machine :- If the machined is driven overloaded then also heat generation will take place. The overloaded machine draws more current and thus more heat.
3) No Cooling Arrangement:- If the motor is installed in a dirty location or dusty environment, then the motor will have no cool air to flow around it and it will take hot air from surroundings and thus leading to more excessive heating.
The main problem which arises due to overheating is on the Insulation of Motor Windings. Heating beyond the insulation rating deteriorates Motor winding life.
Noise in Motors, however, does not cause any damage but still, we should take proper measures to reduce it. Noise is directly associated with Vibration. Loose bearings, poor lubrication, excessive friction are a few causes of motor noise problems. If there is misalignment in motor shaft then also motor will make the sound. However, the sound problem in a motor can be due to Mechanical imbalance or Electrical imbalance.
To check out whether the Sound generated by Motor is due to the Mechanical or electrical issue, first disconnect the Motor shaft from Load and check out the Motor Operation. If the Motor Sound stops then look out for mechanical imbalance at load side.
If even after removing the load sound is coming then Check for electrical Imbalance in motor supply, then make sure to tighten the Motor wires correctly.
Sound from electrical imbalance is due to un-even magnetic attraction between stator and rotor.
We will be discussing more about other major AC Motor problems and maintenance of AC Motor in our further posts. Till then you can subscribe to get all latest updates in your Inbox. Subscribe to Automation-Talk by Email.
Feb 28, 2011
Different type of Electrical Motors
Motors are the heart of any plant or industry. The origin of Motor was to convert electrical energy to mechanical energy. DC motor was the first one to be invented and was result of concepts of electromagnetism by Michael Faraday. Today DC Motors , AC Motors - Induction , Squirrel Cage , Servo and Stepper Motor are used widely. We will see in brief about all these different types of Motors used in Automation Industry.
Electrical Motors In Industries :-
1) AC Motor :- We have already discussed the history of AC Motor in our earlier post. There is two broad classification of AC motors first Induction Motor and second as Synchronous Motors.
AC Induction Motor can viewed as Transformer for easy understanding. When voltage is applied to primary of Transformer a current flows and as a result current is induced in secondary winding of transformer.
The primary can be considered as Stator and Secondary as Rotor of AC Induction Motor. The magnetic field set up in the Stator results in Induced Magnetic Field in Rotor and results in Motion. Working of AC Induction Motor discussed already.
Synchronous Motor is same as Induction motor but with different Rotor configuration. The rotor is constructed in such a way that the Rotor is able to rotate with exactly same speed as of stator field. There are two variants of Synchronous Motor - self excited and Directly excited.
2) DC Motors :- DC Motors have been used in Past over the AC motors for applications demanding Speed and Position Control . Until the Introduction of Power electronics the speed variation of DC Motor was easier. In DC Motors the field in stationary in stator and rotor field, is set up by passing current through a commutator and into the rotor assembly. There are different types of DC motors as Shunt Wound Motor, Series Wound Motor and Compound Wound Motor.Subscribe to Automation-Talk by Email
3) Stepper Motor :- As the name indicates Step Motors are used for Converting the Digital Inputs commands in to Analog motion with the help of motor controller electronics.
Stepper Motor Rotates in angular positions according to the digital pulses Inputs. There are different types of Stepper Motors namely as solenoid activated, variable reluctance,permanent magnet and synchronous inductor stepper Motor.
4) PMDC Motor :- PMDC stands for permanent magnet DC , and is mostly used in closed loop system with effective feedback system. In these Motors there is linear speed-torque curve due to permanent magnet which provide provide constant field flux at all speeds. This Motor generates high starting or acceleration torque with light weight and thus used for rapid positioning applications.
5) Servo Motor :- Servo Motor works on servo-mechanism , best for closed loop applications . A controller issue commands for position/speed/torque control of motor shaft. "Servo" is a very big word to understand and we will be discussing more about servo drives, controllers and closed loop system in details in our further posts.
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Feb 20, 2011
How to Find/Limit AC Motor Inrush Current
Inrush current problems while start up of the solid state equipment's as copier, computer and magnetic devices like motor, drives a large amount of current flows and this value of current is several times of steady state value and known as Inrush current. There is no actual known formula for calculation of this Inrush Current value but yes we can determine an approximate value.
How to find AC Motor Inrush Current
When a Motor is started or Transformer energizing takes place then the inrush current can cause a imbalance in the power circuit. However, this inrush current is for few seconds but we should take appropriate actions for this problem.
How to Limit Inrush Current Value
We will discuss the few major actions which can be adopted to limit the Inrush Current value.
1) Try to keep the Large Ratings Motor on a separate power supply, it will prevent the voltage sag problem.
2) Use Inrush Current Limiting Thermistor, its resistance decreases the temperature increases.
3) Use Series capacitors for distribution circuit of large Rating Motors.
The value of the Inrush Current at start Up can be found out using the Clamp Meter and noting down the measured Value and after that try to use the one of the above method to limit the value. After implementing the method just measure the Value of Inrush Current again and check whether it is reduced or not. Subscribe to Automation-Talk by Email for more articles.
Dec 23, 2010
Starting Procedure and Speed of a Synchronous Motor
This article will tell you how to start a Synchronous Motor in a industry and will give you the complete procedure how to start a synchronous motor. First of all we should know what is a Synchronous Motor. As stated by wikipedia a synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets at the same rate as the alternating current and resulting rotating magnetic field which drives it. Subscribe to Automation-Talk by Email .
Step by Step Starting Procedure of a Synchronous Motor
1.First main field winding i.e rotor winding is short circuited.
2.Now Reduce Voltage, with the help of Auto Transformer is applied across the stator terminals. At this position Motor starts up.
3. When it reaches a steady speed a weak D.C excitation is applied by removing the short circuit on the main field winding.
4.If excitation is sufficient than the machine will be pulled into Synchronization.
5. Now Full supply Voltage is Applied across stator terminal by cutting out the Auto Transformer.
Speed of a Synchronous Motor
Speed of Synchronous motor is given by product of 120 and frequency divided by No. of Poles in a Motor.
Ns=(120xF)/P
Where Ns= Synchronous Speed
F= Frequency
P= No/. Of Poles.
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Dec 7, 2010
How to Find AC/Induction Motor Load and Efficiency
How to find motor full Load ?
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 = 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
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.
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
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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Nov 22, 2010
History of AC Motors
Later on in the year 1885 Galileo Ferraris applied his great mind and roll back the concept and in turn in year 188 he published his research paper to the Royal Academy of Sciences in Turin.
And then started the what called Second Industrial Revolution in year 1888 when Tesla’s launched his motor and it made possible the efficient generation and long distance distribution of electrical energy by the use of alternating current transmission system. Before this all concept were based on non efficient homopolar motor.
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Tesla’s teacher professor Poeschel was not able to understand all of tesla’s ideas and intially suggested that it would be akin to build a perpetual motion machine on tesla’s suggestion of removing commmutators from a machine and operating the device on a rotary field of electromagnetic force . But Tesla was his own way and he did his work and later on Professor Poeschel realized that the induced rotating & pulsating electromagnetic field would force to rotate the rotor of the motor spinning and counter electromotive force generated will gradually bring the machine to stop . Later Tesla obtained the US patent 0,416,194 electric motor (December, 1889) . This classic alternating current motor was induction motor. Later Michail Osipovich Dolivo - Dobrovolsky invented three-phase “cage-rotor” in year 1890 and this type of motor is now used for the majority of commercial applications.
For more information about AC Induction Motor History keep reading here .
Nov 21, 2010
Working of AC Induction Motor
AC Motor are of two types, namely Induction motor and Synchronous motor. But still in majority of application Induction motor is used due to less maintenance cost and less wear and tear and good efficiency.
In Industrial application 3-phase induction motor are used but single phase motor are also available in market as they still find their use in some field.
To understand the working of Induction motor we first should know what is present inside the motor. Induction motor consists of stator and rotor and it works on the principal of Faraday law of electromagnetic induction. An electric motor turns due to this law only , where stator is a stationary magnet and rotor is rotating electro-magnet.
The basic working principal of AC induction motor is as follows:-
When a 3-phase power is supplied to the rotor a rotating and pulsating flux is produced inside stator and this in turn impose electromagnetic torque on the rotor and thus an electromagnetic locking is formed and the rotor turns .There is always a difference in between speed of rotor and speed of rotating flux and this difference is known as slip. In other words we can say that slip is necessary for an induction motor to rotate because if there will be no slip then there will be no magnetic interlocking and rotor will not rotate.
Slip = Ns – N r / Ns
Ns – is synchronous speed
In this way the Ac Induction Motor operates . In further sections we will see what are the different types of rotor and history of Induction Motors. Also we will put some light on construction of AC Induction motor.
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Nov 20, 2010
How to find AC Motor/Induction Motor Current,Synchronous Speed
To calculate motor parameters is very important for deciding the rating of motor and its suitability for the load. In this first section we will study the basics formula for a AC induction motor. Horsepower Formulas To Find Use Formula Example Given Find Solution HP HP = I X E X Eff. 240V, 20A, 85% Eff. HP HP = 240V x 20A x 85% I I = HP x 746 10HP, 240V, I I = 10HP x 746
1) To find out the synchronous speed apply the below formula
Ns= 120 x F/P
Where Ns is synchronous speed , F is frequency and P is number of poles of motor.
2) To find out the horse power of a motor when rpm and torque are known use the below formula.
HP = rpm x T (torque)/5252(constant)
This above formula is very use full if the data on the nameplate of motor is not available. We can find HP or KW by using above equation
1 HP = 746 watt.
3) There is also an another way to find rating of motor if the voltage and current , efficiency of motor are known
HP = I X E X Eff./746
This formula proves help full and give the accurate rating of motor.
Inversely from the above equation you can find current of motor also if all other parameters are known to you.
Below is a table which will depict an example as how to calculate.
Horsepower Formulas
746
746
HP=5.5
E X Eff x PF
90% Eff., 88% PF
240V x 90% x 88%
I = 39 A
In the next section we will discuss further about the more motor parameters and the equations to find them.
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