Separately Excited DC Motor

October 29, 2020

As we already know, a DC motor transforms electrical energy into mechanical energy, and it does under the Faraday and Lenz principle of electromagnetic reciprocity.

The great advantage of DC motors over AC motors is the ease of their speed and torque variation controls, which has made their application very important in sectors where wide limits of regulation of the rotation frequency are required.

Separately Excited DC Motor

It is a motor whose field circuit is powered by a separate constant voltage power source. Within the industrial applications of DC motors, this is the most widespread configuration.

The great advantage of the DC motor with independent excitation lies in the great depth of regulation of the rotation frequency and the mechanical torque, it also maintains high efficiency in the entire regulation band and can have mechanical characteristics that respond to special requirements.

In independently excited or shunt motors, the flux is approximately constant; an increase in torque is accompanied by an almost proportional increase in armature current.

The image shows the steady-state torque-speed curve of the DC motor with separate excitation.

Torque-speed Curve of the Separately Excited DC Motor

The scheme of an independent excitation motor is like the one in the following figure.

Separately Excited DC Motor Diagram
Separately Excited DC Motor Diagram

Clearly distinguishing two independent electrical circuits, the excitation one or inductor, and the armature, so that we can establish, according to Kirchhoff’s law, two electrical equations.

Where 2Ue is the voltage drop that occurs in the contact between the commutator strips and the brushes, it is quantified as a constant amount of 2 volts, and if the motor supply voltage is reasonably high, it is neglected when making calculations, since that a very significant error is not introduced.

The characteristic curves of the motor are usually two.

We represent the speed characteristic by how the rotational speed modifies as a function of the armature intensity while keeping the excitation intensity constant.

In independent excitation, the excitation intensity is always the same, and since a hundred from another supply, it does not change whatever happens in the inductor.

As the flux value is proportional to the independent excitation intensity, the following must be met:

As also in the inductor, it executes

Equating E ‘, and clearing

And the torque characteristic, where we represent the variation of the torque as a function of the armature current while the excitation current remains constant.

For what is right:

Curve graphics

Speed n=f(Ii) when Iex=constant                                                   Torque M=f(Ii) when Iex=constant

n=f(Ii) when Iex=constant, M=f(Ii) when Iex=constant