Why Does Speed of Shunt Motor Not Double When Current is Reduced By 50%?

[Tee]

hi all , if we reduce the normal exciting current of a practical shunt motor by 50% , the speed increases but it never doubles ... why?

Please if you know why , let me know because i have been searching for a long time on google and no useful output yet.

 

[berkeman]

Maybe check out the link to the explanation of the Harry Ward Leaonard speed control scheme for shunt-wound motors at this wikipedia.org article (near the bottom):

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

http://en.wikipedia.org/wiki/Harry_Ward_Leonard#The_Ward_Leonard_motor_control_system

It talks about varying the current to give speed control from zero to "full speed", but it may also address the speed doubling issue you are asking about.

 

[TheAnalogKid83]

can someone explain how decreasing current to a motor makes it speed up? This doesn't make sense to me cause at 0 current you would expect the motor to run at maximum speed and this would be a perpetual motion machine right? I'm confused, please explain more.

 

[berkeman]

I don't understand that either, and it may just be a typo or misunderstanding by the OP. Maybe just check out the articles that I linked to -- my impression from skimming them quickly is that your intuition is correct, and the speed is proportional to the current.

 

[TheAnalogKid83]

if this is the case, then I shouldn't have deleted my other post, and I would say you just have to consider inefficiencies and nonlinearities of the motor, and at different speeds you will lose energy to resistance and friction to where you don't get the full energy transferred to the speed of your shaft that you put in

 

[berkeman]

if this is the case, then I shouldn't have deleted my other post, and I would say you just have to consider inefficiencies and nonlinearities of the motor, and at different speeds you will lose energy to resistance and friction to where you don't get the full energy transferred to the speed of your shaft that you put in

I can undelete your previous post for you. Here it is:

let me take a shot in the dark, because i know nothing of a shunt motor, and say it may be because of inefficiencies of the motor. You will get energy loss in any system that basically saps from total energy transfer you would expect (friction and resistance are good examples), so although you are trying to double the speed, inherent, non ideal, frictions and resistances will take a little out of the energy you think would give you double the speed. Also you have to consider nonlinearities, because the real world is a nonlinear one, and the speed region you are getting into where it should be doubled may respond differently than at lower speeds and its parameters may change. So maybe google what I offered, and it may be what you're looking for. If i had time to google a shunt motor I would be able to help you a lot more. This is just something for you to work with, I don't want to send you on a wild goose chase.

 

[Tee]

thanks a lot TheAnalogKid83 , i actually thought of that after i started the thread , because the speed is a mechanical function and there are other factors affecting it other than the flux , such as friction like you said , and i just want to clarify that the current i was talking about is the excitation current or field current , this isn't the current you feed the motor with , it's the current needed to create the field that will interact with the armature current and thus forces the motor to run ,

the speed of the motor will increase as you reduce the field current (by any means , either by adding a variable resistance into the field circuit or by using a variable voltage source , the later is used with separately excited motors, or a rectifier circuit ,etc...) because the speed is inversely proportional to the flux which is produced by the field current

And therefore , as this relationship (speed-flux) isn't linear , the speed will never double , although the flux will .

Please correct me if i am wrong , thank you :)