It is well known that the the volt-ampere characteristics is the parameter identifying the linearity or non-linearity of the load.If the V-A ch.of the load is linear the load is linear and if not the load is nonlinear

If you consider the IM is running at no load or at standstill, then its linearity or non-linearity as a load will depend very much on how its magnetic circuit is saturated.The more saturated is the magnetic circuit ,the more nonlinear will be the IM. But if the Is running loaded the noway will behave as a linear load.

Correction of my last sentence. But if the IM is running driving a mechanical load ,then it will not by any means behave as a linear electrical load

Induction Motor on Load can be treated as Non Linear Load because

Torque T=K.s.E2^2.R2 / Sqrt (R2^2+(sX2)^2)

and also I.M. can be treated as a Rotating Transformer.

This can also be observed from the Circle Diagram from which we can see that the Current drawn on load moves along the circle.

Dr.P.S.

Thank you to Mr.Mahmoud Saleh ,Pisupati Subramanyam,Suhas S .at this point i have the doubt input about applied voltage is directly proposanal to torque.So it is a linear relation or not

The Torque in an Induction Motor is proportional to the square of the applied voltage.

Dr.P.S.

Induction motor itself is not a load . Load is what is connected to the motor shaft may be through a coupling . Every load gives you its specific torque-speed characteristics or should i say its torque requirements. From that curve you identify.

Induction Motor is an Electrical load. What is connected to Shaft by means of coupling will be Mechanical Load.

Well, to determine whether its linear or non linear load, one has to plot graph of V against I. But I am not able to understand why Torque factor came up in this discussion.

IM is always run at Rated Voltage. But using Auto-transformer if we run at different voltages, then currents will be proportional to the Voltage and lags because of inductance of the stator. So it should act as Linear Load upto the level of saturation.

I am not sure, please correct me if I am wrong.

when you connect a dc generator (lets say) to the shaft of induction motor, then we call loading the machine electrically (electrical load). The torque-speed characteristics are very important when it comes to identifying appropriate motor for the load. for example a paper roller , the most appropriate machine would be separately exited dc motor because of specific torque-speed requirements of the load. The discussion of torque comes because we always estimate our torque and the stator fluxes from the stator currents, so although its the current we are sensing but we design our controllers for torque. and like i said IM itself is not a load, IM works similar to a three phase transformer, now in power system would you call a three phase transformer load?? no.. so similarly its the case.

Suppose instead of coupling into generator, if a mechanical belt drive is loaded to IM shaft, then will it act as load?? Moreover, IM converts Electrical energy into Mechanical form... as we load IM mechanically, wont it act as a load??

Lets say we connect DC generator to a IM, we load the generator by some means. Then current is drawn by the DC Generator. This same current flows through the armature as well, which produces TORQUE in opposite direction as that of torque produced by IM. Anything which opposes the producing torque is a mechanical load isnt?? So is DC generator a mechanical Load to IM?

Firstly, and in general the IM has a highly coupled non linear internal electrical dynamic network resulting in a non-linear relationship between voltage and torque and current and torque as has been pointed out above. However through suitable external control such as Field Oriented or Vector control (FOC) it is possible to decouple the internal non-linear electrical dynamic network into a linear electrical dynamic network such that the electrical torque bears a linear relationship between the q-axis component of stator current and the electrical torque output just as in a dc machine. However, in terms of voltage it always remains as stated above.

Secondly in response to Sanath under FOC one is able to mechanically couple two IM and control one to run as motor and the other as a load or generator. In this way one may control the IM load such that it may have the characteristics of any mechanical load by controlling its q-axis stator current / load torque to replicate any type of mechanical load. This saves having to buy equipment such as fans, pumps etc as one can then control the load torque to duplicate the mechanical load characteristics of a fan, pump or any mechanical load whether linear or non-linear. Thus one can use either an AC-DC motor/generator set or a AC-ac motor/generator set.

The major benefit of doing this is that the energy utilised by the generator load may be recovered by connecting the DC bus of the motor / generator inverter system such that the only power drawn from the supply represents the losses in the system. In this way one can perform laboratory tests without having to consume large amounts of energy or have a large power supply in the laboratory. I have designed and implemented such AC test bed systems and they have worked very well.

Regarding Sanath's question, if I.M is connected to a D.C. machine,the D.C. M / C can act as a D.C. Generator and a D.C. Generator has.to be supplied with Mechanical energy. The I.M supplies that Mechanical Power and the D.C.Generator acts as Mechanical Load on I.M . as expressed by him.

If the D.C Machine coupled to I.M is run as D.C.Motor then the I.M can be made to run as Induction Generator by running it above synchronous speed. This way Induction Generator acts as a Mechanical Load to D.C. Motor.

Loading I.M. through a Brake Drum and Belt ,we Load the I.M Mechanically and that is the way the Performance Characteristics of I.M are evaluated generally in College Laboratories.

Regarding the statement of Diana our understanding is that for an I.M. to run as Induction Generator it has to run above Synchronous speed. I.M. always runs below Synchronous speed.

Hence to couple two I.M.s mechanically is not possible for this purpose.

Of-course I.M can be analyzed with two axes Theory.

Dr.P.S.

Dear Pisupati your comments are limited to an uncontrolled IM which runs off a fixed frequency, fixed voltage supply. Two-axis theory is not just used to analyze the IM it is and has been used to control the IM for some thirty years now and is able to decouple the IM electrical dynamic structure into a linear dynamic structure similar to that of the DC machine.

Your statement that it is not possible is thus incorrect. One can get the exact same results as using a drum brake, except that it can be done at virtually any speed. This is because the torque of an IM is dependent on slip and not the speed of the motor and why modern VSD's are able to maintain full torque from standstill to syncronous speed.

see

http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=707805&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D707805

These days some are using real-time simulators and FPGA hardware in the loop systems to simulate IM's as well.

As Diana has rightly stated,with the advent of Power Electronic applications, to Machines, Direct axis Torque Control, Torque and Speed Control of I.M. have gone a long way and drastically changed since 1986.

I.M can be made to act as motor or Induction Generator.

As the speed requirements for the two machines I.M and I.G. are different -one below Synchronous speed and the other above synchronous speed, my statement that both cannot be mechanically coupled was made in the context of discussing two coupled machines with one acting as motor and the other acting as Generator.

The referenced paper describes the hardware requirements for a user friendly, easy to use test bed system to evaluate the efficiency of variable speed drives under variable load conditions. The test bed will allow variable speed drives to be loaded with any desired load characteristic to enable measurement and comparison of the savings that can be achieved when replacing fixed speed drives with variable speed drives as seen from the abstract.I have to go through the full paper .

Dr.P.S.

The only way the IM in the test bed may be loaded is if the other acts as a generator. As both run at the same speed and in the same direction the motor has to develop a positve slip (positive driving or motoring torque) and the the other a negative slip (negative or re-generative torque).

The error you are making is assuming the speeds must be different. As they are mechanically and stiffly coupled they both run at the same speed but the motor synchronous frequency is Fmotor = Speed + Slip whereas that of the generator is Fgen = Speed - Slip.

Thus by causing the generator to load the motor it causes a drop in speed and the speed controller will increase the frequency to counter the drop in speed by developing a motoring torque and automatically raise the supply frequency until the necessary counter torque is developed by increasing the slip. In this mode the drive motor is speed controlled whereas the load motor is just torque controlled and only has a current loop whose reference may be fixed or speed related.

Dear Prof.Diana,

Your contention seems to be quite interesting as it is different from the known facts mentioned by me earlier.

Your statement " As they are mechanically and stiffly coupled they both run at the same speed but the motor synchronous frequency is Fmotor = Speed + Slip whereas that of the generator is Fgen = Speed - Slip" means that the entire set has to run at Synchronous speed of zero slip frequency which is not normally comprehensible.

I would like to supplement your statement that the only way of generating Torque on one Induction Machine to develop Torque at Synchronous speed is by Direct Torque Control Method.

The direct torque method performs very well even without speed sensors. However, the flux estimation is usually based on the integration of the motor phase voltages. Due to the inevitable errors in the voltage measurement and stator resistance estimate the integrals tend to become erroneous at low speed. Thus it is not possible to control the motor if the output frequency of the variable frequency drive is zero. However, by careful design of the control system it is possible to have the minimum frequency in the range 0.5 Hz to 1 Hz that is enough to make possible to start an induction motor with full torque from a standstill situation. A reversal of the rotation direction is possible too if the speed is passing through the zero range rapidly enough to prevent excessive flux estimate deviation.

If continuous operation at low speeds including zero frequency operation is required, a speed or position sensor can be added to the DTC system. With the sensor, high accuracy of the torque and speed control can be maintained in the whole speed range.

For the benefit of the Readers I would ask them to go through the Link:

http://en.wikipedia.org/wiki/Direct_torque_control

Direct Torque Control of I.M is one of the pet projects of many a student.

As I have gone through the abstract only and not the Full Paper referred by you I do not know the method followed in the Test Bed.

The abstract of the Paper referred by you is

"This paper describes the hardware requirements for a user friendly, easy to use test bed system to evaluate the efficiency of variable speed drives under variable load conditions. The test bed will allow variable speed drives to be loaded with any desired load characteristic to enable measurement and comparison of the savings that can be achieved when replacing fixed speed drives with variable speed drives"

Are you using this Test Bed in your University? If so kindly give some details of the Test Bed and the method followed therein.

This information will be helpful for Scholars as well.

Regards.

Dr.P.S.

I am really amazed how a simple question,

" An induction motor is a linear load or a nonlinear load? Then how to identify the non linear loads and liner loads in electrical applications?

What are the parameters used to identify the electrical nonlinear load?"

asked by Mr.Sivaraman Pandarinathan has led to such an in depth Discussion on Induction Motor and Induction Generator and Two I.Ms. coupled Mechanically !

Very many Thanks to him,Prof.Diana and other Contributors.

We are Grateful to Research Gate for providing such a wonderful platform.

Dr.P.S.

Please note that "synchronous speed" and "rotor speed" are not the same. Synchronous speed is normally the speed of the applied stator frequency whereas the rotor speed is the actual physical mechanical speed of the rotor. If any IM rotor runs at synchronous speed it is unable to develop torque as it requires slip to do this. Thus a two pole IM supplied with 50Hz has a synchronous speed of 3000 rpm and its rotor must run below this to be able to develop torque as you have mentioned previously.

Thus to develop rated torque at zero speed (standstill) the applied stator frequency must be greater than zero Hz. Thus whilst the IM rotor is at standstill the applied stator frequency (or synchronous speed) is not.

The same applies to develop torque in an IM at any other speed.

There seems to be a lot of confusion between us .My original statement was"As the speed requirements for the two machines I.M and I.G. are different -one below Synchronous speed and the other above synchronous speed, my statement that both cannot be mechanically coupled was made in the context of discussing two coupled machines with one acting as motor and the other acting as Generator. " was replied 20 hours ago or so .After lot of discussion your also came to same conclusions"The error you are making is assuming the speeds must be different. As they are mechanically and stiffly coupled they both run at the same speed but the motor synchronous frequency is Fmotor = Speed + Slip whereas that of the generator is Fgen = Speed - Slip" and also " If any IM rotor runs at synchronous speed it is unable to develop torque as it requires slip to do this." .

Slip frequency, EMF can also be used for speed control of IM and it is different from stator supply frequency.

As both of us have the same view there is no scope for any further misunderstanding .

Dr. P.S.

linear loads are those when subjected to sinusoidal voltage, they will draw sinusoidal current..(example RLC circuits).

nonlinear loads are those when subjected to sinusoidal voltage they will draw a non sinusoidal current (example PE switches, diodes SCR..etc)