DC Motor Characteristics, Characteristics of DC Series Motor, Characteristics of DC Shunt Motor, Characteristics of DC Compound Motor

Characteristics of DC Motor

Three Characteristic curves are Typically thought to be significant for DC motors, and they are, (i) Torque vs. armature current, (ii) Speed vs. armature current and (iii) Speed vs. torque. Each type of DC motor is covered in the sections that follow. By considering the next two relations, these Characteristics can be determined.
T_a ∝ ɸ.I_a and N ∝ E_b/ɸ
For a DC motor, magnitude of the back emf is given by the same emf equation of a dc generator i.e. E_b = PɸNZ / 60A. For a machine, P, Z and A are constant, therefore, N ∝ E_b/ɸ

Characteristics Of DC Series Motor

Torque Vs. Armature Current (T_a-I_a)

This DC Series Motor characteristic is also known as electrical characteristic. Here torque is directly proportional to the product of armature current and field flux, T_a ∝ ɸ.I_a. In DC series motors, field winding is connected in series with the armature, i.e. I_a = I_f. 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. So, the torque varies proportionally to Ia only, T ∝ Ia. And 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, torque increases as the square of armature current (prior to magnetic saturation), DC series motors are used where high starting torque is required.

Speed Vs. Armature Current (N-Ia)

We know the relation, N ∝ E_b/ɸ

Since the current change in the back emf is minimal for small loads, Eb may be disregarded. Speed is therefore inversely proportional to ɸ for small currents. We already know that while speed is inversely proportional to Ia, flux is directly proportional to Ia. As a result, when the armature current is very low, the speed increases significantly. A series motor should never be started without any mechanical load because of this. However, armature current Ia is high at high loads. Therefore, there is less speed, which lowers the back emf Eb. More armature current is permitted because Eb has decreased.

Speed Vs. Torque (N-Ta)

This characteristic is also called as mechanical characteristic. From the above two characteristics of DC series motor, it can be found that when speed is high, torque is low and vice versa.

Characteristics Of DC Shunt Motor

Torque Vs. Armature Current (Ta-Ia)

We can assume that the field flux ɸ is constant for DC shunt motors. Flux ɸ does, however, slightly decrease at high loads because of the increased armature reaction. We can state that torque is proportional to armature current by ignoring the flux ɸ change. Therefore, a dc shunt motor’s Ta-Ia characteristic will be a straight line through the origin.
Shunt motors shouldn’t ever be started on a heavy load because they require a high starting current when starting a heavy load.

Speed Vs. Armature Current (N-Ia)

As flux ɸ is assumed to be constant, we can say N ∝ Eb. But, as back emf is also almost constant, the speed should remain constant. But practically, ɸ as well as Eb decreases with increase in load. The speed decreases slightly as the back emf Eb decreases slightly more than the flux. Typically, the speed drops from full load speed by only 5 to 15%. A shunt motor can be thought of as a constant speed motor as a result. The ideal characteristics are represented by the straight horizontal line in the speed vs. armature current characteristic in the following figure, while the actual characteristic is depicted by the dotted line.

Characteristics Of DC Compound Motor

Series and shunt windings are both present in DC compound motors. A compound motor is said to be cumulatively compounded if the series and shunt windings are connected so that the series flux is in the same direction as the shunt flux. The motor is referred to as differentially compounded if the series flux is in the opposite direction of the shunt flux. The following describes the traits of both of these compound motors.

(a) Cumulative compound motor

When series characteristics are required but the load is likely to be completely removed, cumulative compound motors are used. The heavy load is handled by the series winding, while the shunt winding stops the motor from running at an unsafely high speed when the load is abruptly removed. These motors typically use a flywheel, which is used in rolling mills to apply sudden, transient loads.

(b) Differential compound motor

Because series flux opposes shunt flux in differential field motors, the total flux decreases as the load increases. As a result, the speed nearly remains constant or even slightly increases as the load increases (N ∝ Eb/ɸ). Although differential compound motors are rarely used, they do have a few uses in experimental and research work.