DC motors are the motors that run on a direct current. The connection of the armature winding to the field winding of this type of motor is of a DC type. The motor can convert electrical power to mechanical power when used as a motor. And can convert mechanical power to electrical power, when used as a generator. The construction of the motor and generator are the same, due the motor has a good wide range of spend and speed regulation in the electric interactions. There are two types of DC motor, self-excited and separately excited.
The self-excited are classified into (3) three;
- Series wounded or series motor.
- shunt wounded or shunt motor.
- Compound wounded or compound motor.
The DC motor work with the principle of Michael faraday. When a current-carrying conductor place on a magnetic field. The conductor experiences mechanical stress.
There are so many applications of DC motor ranging from industrial and commercial. For instance, in many industries, DC motors are used in driving loads, and as lifters in buildings. Belt or gears are conducted to the motor to help drive the main load. The DC motors are different from AC motors perhaps all of them have their advantages or disadvantages of each other.
Separately excited motor.
This is one of the types of DC motor. Just as the name implies, they have separate DC sources that energies the field coil. To run this type of motor, two DC power sources are needed.
In this type of motor, the field current is supplied by the motor or generator itself with the current. Just like the name self-excited, meaning it supplies itself with current from the field. They are classified into three.
Shunt-wound DC motor.
The type of DC motor is most commonly used. The field coil and the armature coil are connected in parallel as shown in the figure below.
Driving the current, voltage, and power equations for shunt-wound of DC motor.
For the current equations.
Using KCL on junction A on the above figure.
The sum of current entering point A equals the sum of current leaving point A.
I = total current.
Ia = armature current.
Ish = shunt field current.
For the voltage equations.
By using KVL ( Kirchoff’s voltage law) for the field winding circuit.
But, for the armature winding circuit, the equation is;
For the power equations.
The power input = mechanical power created + the losses in the armature + the losses in the field.
Using Ia to multiply equation 3 ( Ia × eq3) we have
where VIa is the power supply to the armature of the shunt DC motor.
series wound motor
In this type of DC motor, the field coil connects in series with the armature coil. And the field coil got energies from the armature current. Where the loads are connected to the end terminals. Below is the diagram of the series wound motor.
on applying Kirchoff’s current law (KCL) on the circuit above, we will have;
Where the Ise is the series field current. while the Ia is the armature current.
For the voltage equations.
The voltage equation is gotten by applying Kirchoff’s voltage law(KVL) on the above circuit.
V = E+I(Ra + Rse)……(8)
For power equation.
the power is gotten from multiplying equation (2) by I.
Input power = mechanical power developed + losses on the filed + loses on the armature.
Comparing the two equations, we have ;
compound wound DC motor.
A compound wound DC motor is any DC motor with a combination of the series wound and shunt-wound together.
Above is the diagram of the compound wound motor. This motor is of two different types, the cumulative compound motor, and the differential compound motor.
In the cumulative compound, the flux produce by the two fields windings is in the same direction. It can be mathematically,
In the differential compound motor, the flux generated by the series field windings are opposite to the flux generated by the shunt field winding. Mathematically;
the direction of the flux produce in the field winding is indicated with negative and positive signs on the diagram.