Ten important things about DC Motors


Ten important things about DC Motors

1. DC motor is device which convert DC electrical energy into mechanical energy. Although a machine that converts DC power into mechanical power is known as DC motor and machine that converts AC power into mechanical power is known as AC motor.

2. DC motor works on the principle of Lorentz force i.e. when current carrying conductor placed in a magnetic field it will experience a mechanical force. Suppose, ‘L’ is length of conductor, ‘I’ current flowing through conductor, and ‘B’ is magnetic flux density, then magnitude of force is given as

 F = B×I×L

3. In DC Motors, the direction of rotation of rotor can be determined by Fleming’s left hand rule. According to this rule when thumb, fourth finger and middle finger of the left hand are stretched in such a way that all are mutually perpendicular to each other and if the fourth finger represent the direction of magnetic field, middle finger indicates the direction of current then thumb represents the direction of force.

4. DC Motors are generally of three types
– DC shunt Motors
– DC series Motors
– DC compound motors
In DC shunt motor, the field winding is connected in parallel to the armature winding. In DC series motor, the field winding is connected in series to the armature winding and the DC compound motor is the combination of the series motor and the shunt motor.

5. Current flow through the winding of a DC motor is not direct current, its an alternating current. No doubt The input current to DC motor is DC current but it is further converted into AC so that it can do useful work.

6. The device which is used to convert DC into AC in DC motor is called commutator. Actually it is the most important part of DC motor. It is responsible to produce the unidirectional torque in DC motor.

7. Construction of DC motor is similar to DC generator. Same machine can be used as DC motor as well as DC generator. It consists of two important parts one is rotating part and another is stator part. The stator  part of DC motor produce the main field flux and known as magnetic field system. The rotating part of DC motor is called armature.

8. The yoke of DC Motor is made of cast steel and the armature of DC motor is made of silicon steel. Both the cast steel and silicon Steels provides a part of low reluctance for the flux and having high permeability.  Although, cast steel is harder than silicon steel but silicon steel have better permeability and low reluctance.

9. There are three main types of losses in DC Motors
– copper losses
– magnetic losses
– mechanical losses
Copper loss is the loss of power in the form of heat when current flaws in any conductor. It is and I square are laws and also known as variable loss.
The magnetic losses consist of hysteresis losses and eddy current losses. These losses are also known as constant losses. Mechanical losses consists of frictional losses, windage losses etc.

10. The direction of rotation of DC Motors can be reversed by interchanging either armature winding or field winding. Most probably, a thought come in our mind that it can reverse simply by changing the supply terminals but it never works.

The direction of current flowing through the armature of DC motor is in reverse direction of induced emf.

Small DC motors (up to 5 kilowatt) usually have two poles and large/medium DC motor usually have four poles.

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