basic question about motor

[kien123]

The torque-speed characteristic is
w = Va/ (k phi) - Ra / (k phi)^2 Tm

phi: flux
Tm: torque of the motor
w: speed

if phi = 0 , then w goes to infinity. It is not the case in reality. Then what is the physics behind this?

Thank you

 

[Marke]

This looks somewhat like a derivation of the DC motor equations and if the flux is reduced to zero, then the motor will certainly accelerate to destruction!!

Bet regards,

Mark Empson

http://www.lmphotonics.com

 

[abcd3286]

A series motor disconnected from load can (and have) oversped until it throws a winding or other fatal hemmorage.

The only load seen by the motor would be bearing and windage loss.
SO
basically the formula is true.

 

[aolalde]

Any DC motor will try to increase the speed to infinity when the magnetic field is lost and the armature circuit is connected to the voltage supply.

To me the speed (w) expression is:

w = ( Va – Ia Ra) / (k*phi)

Were:
w = angular speed
Va = Armature voltage
Ia = armature current
Ra = armature resistance
phi = flux per pole
k = motor winding constant.

 

[edison123]

kien123,

"It is not the case in reality. Then what is the physics behind this?"

It is the case in reality in dc motors with field loss. The physics is that such a motor will race and be 'physically' destroyed.

* Anyone who goes to see a psychiatrist ought to have his head examined *

 

[eemotor]

kien123,
If you'd like to find out more about what happens to a DC motor when field is removed and armature circuit is powered, take a look at thread181-27567 .

 

[eemotor]

Sorry, wrong link in the previous post. The correct on is thread237-97642

 

[electmech]

Edison,

Flux is dictated by load (which then dictates the gating angle in DC drives). When there is no magnetic flux per pole, the counter EMF in the armature is basically very small thus the armature windings pass the conutors from the line (the brushes) more and more quickly. This continues to perpetuate itself in smaller and smaller integrals causing the counter EMF value to become correspondingly smaller as well. As the angular velocity increases to accelerate the next armature winding into the area where it will conduct, this winding experiences less and less time drawing current due to its velocity...hence the problem continues since the counter EMF is not regulated by the field.

This is illustrated by considering your equation from a Tm standpoint as the following equation still applies:

HP = (Torque * Speed)/5252

In this scenario, the torque (which is directly proportional to the current draw, and thus phi) will continue to decrease as the speed increases. Eventually, you find a point where the mechanical stresses will cause the motor to fail.

The answer to your question is this: with no flux regulation to produce counter EMF the DC motor will continue to accelerate due to the fact that its polarity per pole/revolution is going to be only by a rapidly diminishing magnetic field that is induced on the coil as it passes the brushes.

Does that answer your question? Were you looking for a more mathematical explanation?

 

[RalphChristie]

Maybe addressed the wrong guy?

 

[electmech]

DOH!

 

[kien123]

Thank all of you very much.