# ac induction torque relation?



## wasabipixels (Apr 17, 2009)

Update:
Found a really instructional section on siemens website 
http://www.sea.siemens.com/step/default.html

reading the AC motor courses now and have gotten to "slip" which apparently is whats responsible for torque. Also interesting to find out a AC induction motor starts with a ton of EMF and as it increases speed this EMF is decreased. 


Ok so here are my findings for anyone interested (the original questions bellow)











Ok above is the same graph found on the pdf given in the example but slightly altered to add more info.

A new line has been added (flux) in purple. 
Flux = Voltage / Hz
Our example motor is rated at a max of 560v and 60hz so our Flux is = 9.33...

Now as long as the flux is 9.333 then we have 100% torque, however as soon as the flux starts to drop then the torque too will drop.
(note voltage and frequency added to graph for the flux line, also there really is no "max" frequency you can apply to the motor in our graph)



To the left of the flux line we see a purple highlighted area this is (for our example) the max torque section of the graph.

To the right where the flux line is no longer constant the red area represents constant HP. Before this area HP has been rising slowly with the rpm.




Now the torque drops off at a specific rate as highlighted by the weakening field formula pointing to the torque line.

it gives you a % of torque available for the input frequency. 

in our example anything over 60hz will cause a drop in torque. If we say we run the motor at 90hz, then the torque available would be about 44% or in our graph about 200Nm at around 6600rpm.



Now onto the rpm in relation to where the torque curve starts to drop off.

Slip is responsible for us having two torque curves, the max and cont.

This is illustrated here : http://www.sea.siemens.com/step/templates/lesson.mason?ac_drives:2:4:1

Basically slip is equal to the rotation of the magnetic field produced by the stator divided by the rotational field produced by the rotor.

if the slip was equal to 0 then the motor would stall. Basically if you could somehow speed up the rotation of the rotor, say with another motor, to match or exceed the rotational field of the stator then it would stall out. And thus produce 0 torque. 


Well that's it hope someone finds this interesting  enjoy.!
So to answer my initial question: increasing the voltage and current to maintain the flux ratio would make a motor maintain 100% torque into higher rpms. The only problem is you cant go over the voltage of the motor, so some type of modification would have to take place in order of it to work. (more research to come)




*Original post========================================================================
*preface: if anyone knows some good books that explain the physics behind ac induction motors please let me know 


Ok illustration for discussion
http://www.automation.siemens.com/ld/bahnen/daten/elfa/ds_1pv5138-4ws24.pdf


This is a AC induction motor and looking at it made me wonder, what is the relation of torque to how the motor is constructed. Primarily I was interested to learn what effects when the torque curve drops off, for the max in this example around 3k rpm.

Now the constant torque looked interesting because in ICE the torque band starts to drop at 5252 rpm if you talk half of this you get 2626 rpm which looks close to where the constant torque drops off.

Possible suspects (before finding research material):
_given a constant voltage and amp rating_


mechanical resistance
heat resistance
electrical resistance
electrical feed back?
windings or how the motor is made

just some quick thoughts

I will most likely end up reading more and update this post as I did my other if no one responds . . . looking forward to learning.


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## Dennis (Feb 25, 2008)

Keep in mind that the above graph is only valid when connected up to a controller that can control the torque current separately. An AC induction motor hooked up to mains power will have a vastly different torque curve based on the design of the rotor bars in the rotor. Three phase motors are classified into four torque curve designs called A,B,C, and D which cater to a particular load.


The best books that you will need to learn about motors are:

http://www.amazon.com/Machinery-Fun...d_bbs_2?ie=UTF8&s=books&qid=1241065670&sr=8-2

http://www.amazon.com/Electric-Mach...d_bbs_4?ie=UTF8&s=books&qid=1241065670&sr=8-4


You will need BOTH of these books because some information is left out of one the books that is not in another and vice versa. But before you even get these books you must have a very strong background in circuit theorems such as Thevenin's and you must understand all the basic AC and DC electrical laws.

So I suggest you start here:

http://www.amazon.com/Essentials-Ci...=sr_1_2?ie=UTF8&s=books&qid=1241066106&sr=8-2

I used this book in my EET studies in tech school. It is a very good book I think.


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## wasabipixels (Apr 17, 2009)

Currently I have the art of electronics (a classic but goody) I will look into the ones you have suggested, used of course 

I don't know where my research will take me . . . maybe I will get back into EE seriously and make my own controller  or just farther my knowledge, the latter being a given. 

Thanks


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## SimonRafferty (Apr 13, 2009)

Agree - Electric Machinery Fundamentals is a good book - keeps everything quite friendly and straightforward. I found Electric Machinery a bit heavy going though!

Can you suggest a good book that covers synthesizing three phase using PWM plus computerised control strategies for three phase?

When I did my degree - I don't think synthesized polyphase was anything more than a theory and certainly never got a mention! 

In answer to the original question:


> mechanical resistance
> heat resistance
> electrical resistance
> electrical feed back?
> windings or how the motor is made


I think these are potentially all factors. However, at least in the case of Siemens controllers, another one is that at low frequencies the sine wave is synthesized
at higher resolution than high frequencies. The 'clock' frequency is constant so the number of PWM pulses per wavelength is greater at low frequencies.

This manifests itself as a less pure sine wave (with more harmonics) the higher the speed. The harmonics do not contribute to the torque - just heat up the
windings.

Si


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