why do dc series motor runaway when unloaded? books said there is an accelerating torque but where will it came from since as it speeds up, more CEMF is develop which makes less current in the field windings and the magnetic field becomes weaker that could result to have a less torque?

 

I'm not sure what you mean by runaway.

Perhaps you are referring to a situation in which you have two DC motors in series that are identical and equally loaded. As long as the motors remain equally loaded, they both would ideally turn at the same rotational speed.

If you increase the load on one while leaving the other motor's load unchanged, the other motor would speed up.

This phenomenon owes itself to the fact that as the first motor's load is increased then the current flowing in the loop is increased and thus the other motor receives an increase in power and so it speeds up.

hgmjr

 

the motor that I'm talking is not two motors. its a dc series motor (it is series because the field is in series with the armature) and runaway means it goes to a speed rapidly that sometimes they tear themselves to pieces before reaching final spped)
thanks again
ron

 

Sorry, my bad. I misinterpreted your use of the word series.

Perhaps one of our members can answer this one from direct experience with dc series motors.

In the meantime, I will see if I can find anything definitive out on the Web.

hgmjr

 

I think this reference - http://www.gizmology.net/motors.htm - gives a good explanation of why series-wound motors can overspeed to destruction.

 

If there is a significant varible load in series with this type of motor then destruction of the motor shouldn't occur.