What is DC Machine?

DC Machine

A DC Machine converts electrical energy into mechanical energy or mechanical energy into electrical energy.  The DC motor converts electrical energy into mechanical energy , and the DC generator converts mechanical energy into electrical energy.

DC  motor is widely used in the industrial applications where high starting torque and good speed regulation is desired. The DC motor has better speed regulation and high starting torque in comparison with the AC motor.

The construction of the DC motor and generator is almost similar. The generator is generally housed in the closed room and generator may be of open construction type. However, the DC motor is used at the site in the plant which may have dust and heat. Therefore, the DC motor enclosure is  dust proof , fire proof, as per the applications.

The DC generator is used for the application where large DC power is required. The applications are electrolysis, electroplating etc.

Construction of Electrical DC Machines

The DC motor and DC generator have mainly two parts; stator and rotor. In dc motor stator is the stationary part and the rotor is rotating part. The rotor rotates in the air gap.The field winding is housed in the stator slots. The field winding produces the required magnetic field for motor operation. The armature winding is housed in the rotor. When DC voltage is fed to armature winding under magnetic field produced by field winding induce the voltage in the armature winding.

The small rating DC generator has field winding or permanent magnet on stationary part and voltage is taken out from the rotating part. In a large rating DC generator the field coils are housed on the rotor and the armature winding is housed in the stator part.

Equivalent Circuit of a DC Machines 

The equivalent circuit of the DC generator can be represented by three series-connected elements E, Ra and Vb.

The equivalent circuit of the DC motor  is as given below.

Where,

Ra = Armature Resistance

eb  = Back EMF

V = Applied voltage ( In case of DC Motor)

     = Generated voltage in case of DC generator 

What is Back EMF in a DC Motor?

When the DC voltage is applied to the armature, the voltage is produced across the armature winding which oppose the flow of armature current. The voltage produced across the armature is known as counter or back EMF.

The back EMF in the DC motor is expressed by the following mathematical expression.

Eb= ΦNZ/60 *P/A

Where,

Φ= Flux /Pole

N = Armature Speed

Z = Total number of armature conductor

A = Number of parallel paths in the armature winding

The back EMF is proportional to the speed of the motor.The back EMF governs the armature current and thus the back EMF maintains the speed and torque of the motor. The back EMF regulates the armature current and hence maintains the torque delivery to the load.

Ia=(V-Eb)/Ra

If the motor is loaded the speed get reduced. The reduction in the speed cause reduction in back EMF and the reduced EMF allow the motor to draw the more armature current and as a result the torque delivery of the motor(T= ΦIa)increase to meet the torque requirement of the load. When the load torque and the motor delivery torque requirement is meet the armature current gets reduced.

If the load is thrown off the speed of the motor gets increased and the motor torque is now much more than the load torque. With an increase in the speed the back EMF gets increased and the armature current gets reduced. The torque delivering of the motor automatically reduces.

Thus the back EMF maintains the armature current to deliver the torque as per the load requirement