Characteristics of DC Motor


Characteristics of DC Motor

  • The armature winding is connected to a DC source. shunt field dc motor
  • The armature current interacts with the magnetic flux produced by the field coils, causing a torque on the rotor. This machine will rotate in the counterclockwise direction.
  • The field coils produce a north pole on the left and a south pole on the right. The current in the armature coil also produces a magnetic field, with a south pole at the top of the machine and a north pole at the bottom.
  • An attractive force forms between the poles on the armature and those on the field that will rotate the armature.
  • The force turning the armature is a function of the strength of the magnetic fields on the armature and the stator.
  • The amount of flux density is related to the current, so the torque should also be related to current.

The DC motor may have a shunt field, series field, or compound.

The shunt field may be separately excited or it may be excited from the same supply as the armature.

Characteristics of DC Shunt Motor

  1. Speed V/s. Armature Current (N – Ia)

For the motor, the armature current is into machine and the voltage equation is: Vt = Ea + IaRa

At Starting Ea is zero (back e.m.f = 0), so Ia = Vs / R , and since If Ra is very small then Ia is very big.

Consequently a DC motor should never be started at rated voltage and an external resistance must be added and acts as  emporarily starting resistance that is removed as soon as the armature has attained its normal speed.

and The armature voltage of a motor is: Ea= Vt – IaRa

Shunt Motor: Speed Regulation is very low (constant speed motor)

  • 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.
  • Back emf Eb decreases slightly more than ɸ, therefore, the speed decreases slightly.
  • Generally, the speed decreases only by 5 to 15% of full load speed. Therefore, a shunt motor can be assumed as a constant speed motor.
  • In speed vs. armature current characteristic in the following figure, the straight horizontal line represents the ideal characteristic and the actual characteristic is shown in fig.

                      2. Torque Vs. Armature Current (Ta-Ia)

  • We know that in a DC Motor Ta ∝ ΦIa. In this the flux Φ is continuous by ignoring the armature reaction, since the motor is working from a continual source voltage.
  • Therefore the curve drawn between torque Vs armature current is a straight line transitory through the origin which is shown in fig.
  • The shaft torque(Tsh) is a smaller amount than armature torque. From this curve it is proved that to start a heavy load very large current is requisite.
  • Hence the shunt DC motor should not be started at full load.
  • Since heavy starting load needs heavy starting current, shunt motor should never be started on a heavy load.

                     3. Speed Vs Armature Torque(Ta – N)

  • This curve is drawn between the speed of the motor and armature current with various amps. From the curve it is understood that the speed reduces when the load torque increases.
  • With the above three characteristic it is clearly understood that when the shunt motor runs from no load to full load there is slight change in speed.
  • Thus, it is essentially a constant speed motor. Since the armature torque is directly proportional to the armature current, the starting torque is not high.