Understanding DC Motor Operation

[Lunat1c]

I've been studying the concept of operation of DC motors and there are a couple things which I'm having trouble with.

When the rotor is rotating, there will be "cutting" of flux and according to Faraday's law an emf is supposed to be generated such that it opposes the change in flux producing it. This speed emf helps keep the current constant. What I can't understand is how the rotor rotates if the current is kept constant and we're increasing the load. Isn't the current supposed to be allowed to increase so that we have a stronger field around the conductor and hence much more torque?

Any help will be greatly appreciated. Thanks!

 

[rootX]

As load changes, torque and speed changes to accommodate change in the load. The characteristics of the torque-speed curve depend on the type of the motor: series, shunt, compound. And yes, armature current changes as the emf developed changes which changes in response to the load change.

The speed-torque relationships can be developed utilizing:
P_converted (produced as output before mechanical losses) = E_a.I_a = torque*speed.
Relation between voltage and speed/flux
Relation between torque and current/flux

Consult (page 13 for conceptual understanding):
http://www3.sea.siemens.com/step/pdfs/dcd_1.pdf
or
for equations for different types of motors
http://ftc.org.my/EMD_tutorial/EMD%28tu1%29.pdf

 

[Lunat1c]

Thank you rootX. The links you provided are very informative

 

[Phrak]

What I can't understand is how the rotor rotates if the current is kept constant and we're increasing the load.

It doesn't. An increase in load will increase the current. The simplest configuration to consider is the DC motor where the field is either produced by perm magnets or where the voltage supplying the field current is constant.

For a given DC motor the current through the rotor is proportional to the current through the rotor. The angular velocity of the rotor is proportional the the voltage across the rotor when the resistance of the rotor is zero.

In the real world the resistance is not zero, and so current both falls across both the load and the rotor resistance taken in series.

DC motors are some of the most simplest animals in the electrical world. Each motor has a K factor that takes both voltage to rpm and amperage to load. We need a FAQ explaining this...over and over.