Question

Explain the various characteristics of DC motor.

Answer 1

Answer:

From the above two characteristics of DC series motor, it can be found that when speed is high, torque is low and vice versa. In case of DC shunt motors, we can assume the field flux ɸ to be constant. Though at heavy loads, ɸ decreases in a small amount due to increased armature reaction.

Generally, three characteristic curves are considered important for DC motorswhich are, (i) Torque vs. armature current, (ii) Speed vs. armature current and (iii) Speed vs. torque. These are explained below for each type of DC motor. These characteristics are determined by keeping the following two relations in mind.
Ta ∝ ɸ.Ia and N ∝ Eb/ɸ
These above equations can be studied at - emf and torque equation of dc machine. For a DC motor, magnitude of the back emf is given by the same emf equation of a dc generator i.e. Eb = PɸNZ / 60A. For a machine, P, Z and A are constant, therefore, N ∝ Eb/ɸ

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Answer 2

Generally, three characteristic curves are considered important for DC motors which are, (i) Torque vs. armature current, (ii) Speed vs. armature current and (iii) Speed vs. torque. These are explained below for each type of Dc motor.These characteristics are determined by keeping the following two relations in mind.
Ta ∝ ɸ.Ia and N ∝ Eb/ɸ
These above equations can be studied at - emf and torque equation. For a DC motor, magnitude of the back emf is given by the same emf equation of a dc generator i.e. Eb= PɸNZ / 60A. For a machine, P, Z and A are constant, therefore, N ∝ Eb/ɸ

Characteristics Of DC Series Motors

Torque Vs. Armature Current (Ta-Ia)This characteristic is also known as electrical characteristic. We know that torque is directly proportional to the product of armature current and field flux,Ta∝ ɸ.Ia. In DC series motors, field winding is connected in series with the armature, i.e. Ia= If. Therefore, before magnetic saturation of the field, flux ɸ is directly proportional to Ia. Hence, before magnetic saturation Ta α Ia2. Therefore, the Ta-Ia curve is parabola for smaller values of Ia.
After magnetic saturation of the field poles, flux ɸ is independent of armature current Ia. Therefore, the torque varies proportionally to Ia only, T ∝ Ia.Therefore, after magnetic saturation, Ta-Ia curve becomes a straight line.
The shaft torque (Tsh) is less than armature torque (Ta) due to stray losses Hence, the curve Tsh vs Ia lies slightly lower.

In DC series motors, (prior to magnetic saturation) torque increases as the square of armature current, these motors are used where high starting torque is required.