Cemf

[K2500]

What is CEMF in relation to DC motors, how does it affect motor speed? Also I'm reading that an open field winding in shunt wound motors will cause excesive speed. Ultimately destroying the motor, why is that? I'm thinking that less field current = lesser magnetic field+ slower motor. I don't get it. I'm just getting into my new motors book, and the company master(my answer man) is out for the next few months.

 

[rattus]

Counter Electromotive Force:

Counter Electromotive Force:

CEMF is an emf (voltage) generated in the armature of a DC motor which opposes the applied voltage thereby reducing the armature current to whatever level is required to satisfy the load requirements of the motor.

FYI, the generators used on older autos would try to spin if the regulator contacts stuck and the engine was not running. That is, a shunt motor can be used as a DC generator and vice-versa.

The explanation of the speed characteristics of DC motors is somewhat complicated, but suffice it to say that the speed of a shunt wound motor is inversely proportional to the current in the field winding.

A simple explanation is that lowering the field current lowers the shunt field and the armature speed increases in an effort to maintain the CEMF.

 

[K2500]

CEMF is an emf (voltage) generated in the armature of a DC motor which opposes the applied voltage thereby reducing the armature current to whatever level is required to satisfy the load requirements of the motor.

FYI, the generators used on older autos would try to spin if the regulator contacts stuck and the engine was not running. That is, a shunt motor can be used as a DC generator and vice-versa.

The explanation of the speed characteristics of DC motors is somewhat complicated, but suffice it to say that the speed of a shunt wound motor is inversely proportional to the current in the field winding.

A simple explanation is that lowering the field current lowers the shunt field and the armature speed increases in an effort to maintain the CEMF.

i see, the cemf is generated by the motor, that one word helped a lot there, in my head though, I see cemf as an opposing voltage of inverse polarity generated by the relative movement between windings, would that be correct?


I understand that a DC motor/generator are of the same basic construction and will proform interchangably.

The speed of a shunt wound motor is inversely proportional to the current in the field winding, so a decrease in armature current via series resitors would result in a decrease in motor speed. Will this affect the field winding current? I am imagining the resistors in series between field and armature.

 

[cadpoint]

well, Armature / Stator !

well, Armature / Stator !

... Will this affect the field winding current? I am imagining the resistors in series between field and armature.

No, I think you missed rattus statement, first sentence being DC, it All part of the that stator AC, armature of a DC.

The core becomes a something that comsumes a quatity of power then become a resistance (to say) to that avaiable power of the applied DC. Or its happy with it's charge and doesn't want to give it up ...
Or
The properties of an armature will become an EMF force that will work againest the total sum
after saturation with DC applied.

well I know what you meant... Sorry shooting from Memory.


rattusCEMF is an emf (voltage) generated in the armature of a DC motor which opposes the applied voltage thereby reducing the armature current to whatever level is required to satisfy the load requirements of the motor.

 

[rattus]

i see, the cemf is generated by the motor, that one word helped a lot there, in my head though, I see cemf as an opposing voltage of inverse polarity generated by the relative movement between windings, would that be correct?

You could say that, but I prefer to think of the field winding being fixed and producing a constant field which is cut by the rotating coils of the armature. Thus we have a changing flux through the coils which induces the CEMF.

The speed of a shunt wound motor is inversely proportional to the current in the field winding, so a decrease in armature current via series resitors would result in a decrease in motor speed. Will this affect the field winding current? I am imagining the resistors in series between field and armature.

I would not try to control the speed through the armature, but if you did, it should not affect the field current. This is the method used with series wound motors.

 

[Jraef]

If it helps, the "C" in CEMF is for "Counter" as in against.

Counter Electro Motive Force opposes the EMF of the rotating component.