Characteristics of DC Motor

DC MOTOR CHARACTERISTICS

The performance of a DC motor can be judged from its characteristics curves known as motor characteristics. Following are the three important characteristics of a DC motor.

1. Torque and Armature current characteristic (T_a/I_a)

It is also known as electrical characteristic of the motor.

2. Speed and Armature current characteristic (N/I_a)

3. Speed and Torque characteristic (N/T_a)

It is also known as mechanical characteristic of the motor.

1. Characteristics of DC Series Motor

1. T_a/I_a characteristic

We know that T_a ∝  I_a

In this case, as field windings also carry the armature current.

Upto magnetic saturation, ϕ ∝ I_a so that T_a ∝I²_a.

After magnetic saturation, ϕ is constant, so that T_a ∝ I_a

Thus upto magnetic saturation, the armature torque is proportional to the square of armature current. If I_a increases, T_a increases as the square of the current. Hence T_a/I_a curve is a parabola as shown in Fig. 2.38. After magnetic saturation, torque is directly proportional to the armature current. Therefore, T_a/I_a curve after magnetic saturation is a straight line. (portion AB of the curve).

It may be seen that in the initial portion of the curve (up to magnetic saturation), T_a ∝ I²_a. This means that starting torque of a DC series motor will be very high as compared to a shunt motor..

The shaft torque T_sh is less than armature torque due to stray losses. It is shown dotted in the figures.

So we conclude, that where high starting torque is required for accelerating heavy masses quickly as in hoists and electric trains etc, series motors are used.

2. N/I_a Characteristic

The speed N of a series motor is given by.

N ∝ E_b/ϕ

where E_b = V ‒ I_a(R_a + R_se).

When I_a increases, the back emf, E_b decreases due to I_a(R_a+R_se) drop while the flux ϕ increases. However, I_a(R_a + R_se) drop is small under normal conditions and may neglected.

With increased I_a, ϕ also increases. Hence, speed varies inversely as armature current as shown in Fig. 2.39.

When Load is heavy, I_a is large and ϕ is high. Hence the is speed low. But when the load current falls and hence I_a falls to a small value, speed becomes dangerously high. Hence a series motor should never be started without some mechanical load on it. It should be noted that series motor is a variable speed motor.

3. N/T_a or Mechanical Characteristic

It is found from the above characteristic curves that when speed is high, torque is low and vice‒versa. The relationship between these two is shown in Fig. 2.40.

2. Characteristics of DC Shunt Motor

1. T_a/l_a Characteristic

We know that in a DC motor,

T_a ∝ l_a

Since the motor is operating with a constant supply voltage, flux ϕ is constant (neglecting armature reaction)

:. T_a ∝ l_a

Hence T_a/I_a characteristic curve is a straight line passing through the origin as shown clear in Fig. 2.42. The shaft torque (T_sh) is less than T_a and is shown by a dotted line. It is clear from the curve that a very large current is required to start a heavy load. Therefore, a shunt motor should not be started on (heavy) load.

2. N/I_a characteristic

The speed N of a DC motor is given by

N ∝ E_b / ϕ

The flux ϕ and back emf E_b in a shunt motor are almost constant under normal conditions. Therefore speed of a shunt motor will remain constant as the armature current varies as shown in Fig. 2.43.

But strictly speaking, both E_b (E_b = V ‒ I_aR_a) and ϕ decrease with increase in load due to the armature resistance drop and armature reaction respectively. However, E_b decreases slightly more than ϕ so that the speed of the motor decreases slightly with load as shown in dotted line.

3. N/T_a characteristic

The curve is obtained by plotting the values of N and T_a for various armature currents. It may be seen that speed falls some what as the load torque increases.

From their characteristic curve the following points can be observed.

(i) There is slight change in the speed of a shunt motor from no load to full load. Hence it is essentially a constant speed motor.

(ii) The starting torque is not high because T_a ∝ I_a.

3. Characteristics of Compound Motors

A compound motor has both series and shunt field winding, compound motors are of two types.

1. Cumulative ‒ Compound motors in which series field flux aids the shunt field flux.

2. Differential ‒ Compound motors in which series field flux opposes the shunt field flux.

This type of motor has the combination of characteristics of series and shunt motors. As the load is increased, the combined flux increases if cumulatively compounded and causes a torque greater. At the same time, the increase in flux causes the speed to fall more rapidly. The characteristic curves of a DC compound motors are shown in Fig. 2.46.