Construction of DC Motor

DC motor is an electromagnetic device which converts electrical energy into mechanical energy. DC power input is converted into mechanical power at the shaft motor. This article describes the construction of DC motor.

There is no difference in the construction of DC Motor and Generator. A DC Motor can also be used as DC generator without any constructional change. Similarly, a DC generator can also be used as DC Motor. They are broadly termed as DC machine.  Therefore, construction of DC Motor is well applicable for DC Generator. In this post, construction of DC Machine is detailed. 

Construction of DC Machine

A DC machine consists of two main components: Stator and Rotor. Stator is the stationary part whereas rotor is the rotating part. Stator of DC machine consists of Yoke, Field Winding, Interpoles, Compensating Winding, Brushes and End Cover. Rotor consists of armature core, armature winding, commutator and shaft. A labeled diagram of DC machine is shown below.

Each Stator components of DC machine is described below in detail.

Yoke: Yoke is the magnetic core of stator. It provides path for the pole flux Ø and carries half of it. Apart from this, it provides machenical support to the whole machine. The Yoke of DC machine is not laminated as it carries stationary flux and hence there is no eddy current. Iron core is used for the construction of Yoke for small DC machine whereas Steel is used for large DC machine.

Field Poles: Fiedl pole consists of pole core and pole shoe. The pole core is made from the cast steel. The pole shoe of DC machine is laminated and fixed to the pole core. These Filed Poles are welded or bolted to the Yoke.

Field Winding or Exciting Winding: The pole is excited by a winding wound around the pole core. This winding is called the Field Winding or Exciting Winding and made from copper. The number of turns and cross-sectional of filed winding depends on the type of DC machine as below:

• Large number of turns of small cross-sectional area is used for DC Shunt machine.
• For DC Series machine, small number of turns of large cross-sectional area is used.
• Both series and shunt field winding is applied for DC Compound machine.

Interpoles: Interpoles are fixed to the Yoke in between the main poles of DC machine. The interpole winding is made of copper and consists of few turns of thick wire. This winding is connected in series with the armature winding.

Compensating Winding: Theses windings are placed in the slots cut in the pole faces of DC machine. Compensating winding is also connected in series with the armature winding.

Brushes: Brushes are housed in the brush holder and connected to the end cover. It is made up of Carbon for small DC machine. For large DC machine, electrographite is used to make brushes.  A spring keeps the brushes pressed on the commutator surface.

Rotor components of DC machine are described below:

Armature Core: It is a magnetic core made of laminated silicon steel of thickness 0.30 to 0.50 mm to minimize the iron losses. The main purpose of armature core is to house the armature conductor in its slot and provide low reluctance path to magnetic flux Ø/2 as shown in the labeled diagram of DC machine.

Armature Winding: Armature winding is made from copper. It consists of large number of insulated coils having one or more than one turns. Theses coils are placed in the armature core slots and connected appropriately in series and parallel depending on the type of winding. There are basically two types of winding: Lap Winding and Wave Winding.

Commutator: It is a cylindrical structure made up of wedge shaped segments of high conductivity hard drawn copper. Hard drawn copper is used to reduce wear and tear of commutator surface. The segments are insulated from each other by 0.8 mm thick mica sheet.

Shaft: Shaft of DC Motor is coupled to the load to transfer mechanical power. For DC Generator, shaft is coupled to prime mover to convert mechanical input energy into electrical output. Armature core, bearing, commutator etc. are mounted on the Shaft.