Role of Armature Current in DC Motors

[ranju]

In case of dc motors , w eknow that ..we have a supply which supplies a current which divides into armature & field current for shunt motor...& the field current generates flux which links the armature winding & emf is induced.. but here I want to know the role of armature current .! Armature current just increases the armature reaction which decrease the flux...but why then we want a larger value of armature current in motors..??
In generators obviously , armature current is responsible for field current which is further responsible for generating flux..! But here in case of motor I can't understand the role of armature current!

 

[jim hardy]

Armature current just increases the armature reaction which decrease the flux...

Where'd that come from?

The two simple formulas describing a DC motor are:

Counter-emf = K X Flux X RPM ,
where K for the motor is measured by reading its open circuit voltage with known excitation.
Torque in foot-pounds = same K X Flux X Armature Amps X 7.04

Armature reaction is a distortion of flux caused by armature current. That's why you measure K at zero armature amps. In most DC motors it's a necessary evil, but clever engineers of the 1920's figured out how to make use of it to provide better regulation. Model A Ford generators(1929 -1932} are a three brush machine, so are some modern windshield wiper motors.

The answer to you question is: "Armature amps make torque."

 

[jaus tail]

If you don't have armature current, the motor won't run. If the motor ran only with a stationary magnetic flux, then magnets would have iron objects revolving around them.

the armature current produces its flux. This flux interacts with field flux(N to S pole). The flux wants to move towards less crowded area. Currently the current flowing near the S-pole will produce flux that is rotating clockwise along the conductor. Now the magnet's flux is going from N-S. So below the armature conductor(near the S pole) the total flux is more while the armature flux is flowing in opposite direction to field flux above the conductor. The conductor wants to move towards less flux area(doesn't like to much crowd). Since the magnet cannot move, the armature conductor.

 

[Babadag]

I agree with jim hardy and jaus tail of course. I'll try to explain how the pole flux and armature reaction flux balance occurs.

The voltage supplied to the rotor windings is constant. The voltage drop in this winding-even at rated load-is negligible. So the emf has to be constant [more or less].V-Rrotor*Irotor=Emf

If the load [power or torque] increases the Irotor increase- P=V*I*efficiency.

As jim hardy explained Emf=k*flux*rpm. If flux decreases rpm has to increase. So if the armature reaction will reduce the flux then the motor speed will get up.

In shunt or separate excitation the ratio between the flux produced by poles and the armature reaction flux is elevated. Then, for a usual load, the result flux does not decrease much with the armature current rising and then, it will not require a rpm increasing in order to keep the efm constant.

In series motor the flux is directly proportional with the armature current since both fluxes increase and decrease in the same time.The resulted flux=k1*Irotor-k2*Irotor=k*Irotor. Then rpm=k3/Irotor

 

[jim hardy]

And that's why you never belt a series motor to its load.
When the belt breaks and Irotor drops, with Irotor being in denominator of Babadag's RPM formula -
what happens to speed?

An oversped motor can sling its armature conductors out of the slots into the airgap.