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I was studying electromagnetism and I tried to link with working of DC motors.

For the speed control of DC motor - The Flux of DC motor is reduced to increase the speed above rated speed.

I understood that as we reduce the flux density(B) due to field coils the back emf produced in the armature coil reduces as per faradays law. So Armature current will increase as per KCL applied in the armature circuit.

Ea= V - Ia*Ra

This increase in Armature current produces a further increase in magnetic flux density due to the armature current.

Thus overall Magnetic flux density in the air gap increase. This results for the increase in Magnetic torque on the armature winding due to Lorentz force equation and thus the motor's angular acceleration increases and thus the speed increases.

Is this understanding the right one? Please comment on whether this understanding the right one or any deeper understanding is available.

What happens if I Consider the armature reaction?

I also learnt that Flux weakening is commonly used to achieve motor speeds above rated speed.

As the increase in speed is indirectly achieved my increase in armature current Doesn't this make the current in the armature coil go beyond nominal value or rated value?

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I understood that as we reduce the flux density(B) due to field coils the back emf produced in the armature coil reduces as per faradays law. So Armature current will increase as per KCL applied in the armature circuit. Ea= V - Ia*Ra

OK so far.

This results for the increase in Magnetic torque on the armature winding due to Lorentz force equation and thus the motor's angular acceleration increases and thus the speed increases.

The increase in torque is a product of the flux and the increased armature current.

Thus overall Magnetic flux density in the air gap increase.

No - the flux generated by the armature conductors is tangential to the airgap. It's net effect is to increase the flux at one side of the pole, and decrease it at the other, but the sum of the flux doesn't increase, and due to the non-linearity of the magnetization curve of the lamination steel, may decrease a little. Armature reaction is the term that describes this distortion. enter image description here

As the increase in speed is indirectly achieved my increase in armature current Doesn't this make the current in the armature coil go beyond nominal value or rated value?

If you are trying to produce the same torque with the weakened field, then yes, the current will exceed the rated value. Normally the motor will operate at a constant power condition under field weakening, and the maximum torque available will fall proportionally as speed rises, and the armature current will not change much.

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  • \$\begingroup\$ The increase in torque is a product of the flux and the increased armature current So won't reducing Flux reduce the torque? Why is the influence of Current overrules the Reduce in flux density? @Phil G \$\endgroup\$ – VKJ Jul 17 at 14:56
  • \$\begingroup\$ It's net effect is to increase the flux at one side of the pole, and decrease it at the other This implies torque is varying spacially? @Phil G \$\endgroup\$ – VKJ Jul 17 at 14:57
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    \$\begingroup\$ Reducing flux reduces the torque, and as you point out, that reduces the emf, causing the current to rise - by a greater proportion than the change in flux, since the Ia*Ra term should be substantially smaller than the emf. Torque generated is a sum of the forces on the armature, but yes, some of the airgap is 'working harder'. Field weakening when you have a load that will increase with speed, or present a constant torque will cause the current to rise, and probably cause overheating. It's useful with a load that falls off with speed. \$\endgroup\$ – Phil G Jul 17 at 15:12
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    \$\begingroup\$ Examples of loads that require less torque at higher speeds: 1. A center-driven, constant-tension winder requires higher torque when the spool is empty and lower torque as the spool fills due to constant tension acting on a larger radius. 2. A vehicle traction motor allows higher speed on a level surface or when the cargo load is light. 3. Machine tools are usually driven at a higher speed and lower torque when the cutting bit is small. \$\endgroup\$ – Charles Cowie Jul 17 at 15:20
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    \$\begingroup\$ Ia * V is the amount of electrical power that is getting converted to mechanical power. Ia * Ra is lost as heat, so for a motor that has reasonable efficiency, the Ia*Ra term should be small, maybe 25% or less. \$\endgroup\$ – Phil G Jul 17 at 15:42

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