Characteristic of a DC Series Motor

The figure above shows the circuit diagram series motor. In series motor the current flows in the field winding and the armature are same. The armature current will increases whenever the mechanical load of the motor increase. As a result the flux will increase in the series motor when the armature current increases and vice versa. 

Armature Torque V_s Armature Current:

We know that T_a ∝ ΦI_a

Up to Magnetic Saturation Φ ∝ Ia, the armature torque Ta is directly proportional to square of the armature current (i.e.. T_a ∝ I²_a). If armature current is doubled over, then armature torque is almost increasing fourfold. Thus the armature torque vs. armature current curve up to magnetic saturation is a parabola, which is shown in the characteristic curve OA. On the other hand once the magnetic saturation is reached, the T_a is directly proportional to the I_a. As a result the armature torque vs. armature current magnetic saturation characteristic is a straight line, which is shown in the curve AB.

From this curve we understood that the starting torque of a DC series motor is high when compare to DC shunt motor.

                           (i.e.) Series Motor (T_a∝ I2_a) > Shunt Motor (T_a ∝ I_a).

• Characteristic of DC Series Generator

Speed Vs. Armature Current Characteristic:

The Series Motor Speed N is known as N ∝ (E_b/Φ).

We know that back EMF E_b = V-I_a(R_a+R_se).

As soon as the I_a increases, the back EMF E_b reduced due to drop in I_a(R_a+R_se)even though the flux increased. Still, I_a(R_a+R_se) is less in usual circumstance and might be vomited.

Therefore, N ∝ (1/ Φ). The speed vs. armature current characteristic follows the hyperbolic curve up to magnetic saturation {∝ (1/ Φ)}. Afterward the flux remains perpetual and so makes sure of the speed.

The figure above shows the circuit diagram series motor. In series motor the current flows in the field winding and the armature are same. The armature current will increases whenever the mechanical load of the motor increase. As a result the flux will increase in the series motor when the armature current increases and vice versa.

Speed vs. Armature Torque Characteristic:

The fig above shows the N/T_a characteristic curve of a Series motor. Form this curve it is evident that the series motor runs at low speed when it develops high torque and vice versa. This is due to the increase in armature torque need to increase the armature current and also the field current. As a result flux gets strengthened and drops the speed of the motor. If the speed gets increased the torque must be low.

•  DC Motor Voltage Equation and power Equation

From the above three characteristic of a series motor we conclude the three important points.

• The starting torque of a series motor is high since initially T_a∝ I²_a.
• The series motor runs at variable speed because it adjusts speed automatically when there is a change in load.
• During no load condition the armature current is very small and also the flux. As a result the speed of the series motor is very high. This is very unsafe to the machines, which may damage due to centrifugal forces set up in the rotational parts. Hence the series motor should not be started at no-load. Always the series motor should start with minimum load to keep the speed within limits.

CSV Dec 19, 2014

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