Dec 19, 2014

Characteristic of a DC Series Motor



circuit-diagram-of-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 Vs Armature Current:

armature-torque-vs-armature-current
We know that Ta ∝ ΦIa

Up to Magnetic Saturation Φ ∝ Ia, the armature torque Ta is directly proportional to square of the armature current (i.e.. Ta ∝ I2a). 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 Ta is directly proportional to the Ia. 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 (Ta∝ I2a) > Shunt Motor (Ta ∝ Ia).

Speed Vs. Armature Current Characteristic:

speed-vs-armature-current
The Series Motor Speed N is known as N ∝ (Eb/Φ).
We know that back EMF Eb = V-Ia(Ra+Rse).
As soon as the Ia increases, the back EMF Eb reduced due to drop in Ia(Ra+Rse)even though the flux increased. Still, Ia(Ra+Rse) 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:

speed-vs-armature-torque
The fig above shows the N/Ta 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.
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 Ta∝ I2a.
  • 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.


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