# Speed never doubles?

1. Dec 28, 2007

### 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.

2. Dec 28, 2007

### Staff: Mentor

3. Dec 28, 2007

### 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.

4. Dec 28, 2007

### Staff: Mentor

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.

5. Dec 28, 2007

### 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 dont get the full energy transfered to the speed of your shaft that you put in

6. Dec 28, 2007

### Staff: Mentor

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

Last edited: Dec 28, 2007
7. Dec 29, 2007

### 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 :)