Basics of DC Machine

DC machines can be divided into two types, namely DC motors and DC generators. Most DC machines are equivalent to AC machines as they contain AC current and AC voltage.

The output of DC machines is DC output as they convert AC voltage into DC voltage. This conversion of the mechanism is called commutator, hence these machines are also called commutated machines. DC machines are most commonly used in electric motors.

The main advantages of this machine include torque regulation and easy speed regulation. The application of DC machines is limited to trains, factories, and mines. For example, underground subway cars as well as trams can use DC motors. In the past, cars were designed with DC generators to charge the batteries.

1.What is a DC machine?

A DC motor is an electromechanical energy conversion device. The working principle of a DC motor is that when current flows through a coil in a magnetic field, the magnetic force produces torque, which causes the DC motor to rotate. DC motors are divided into two types, namely DC generators and DC motors.

The main function of a DC generator is to convert mechanical energy into DC electrical energy, while a DC motor converts DC electrical energy into mechanical energy. AC motors are frequently used in industrial applications to convert electrical energy into mechanical energy. However, DC motors are suitable where good speed regulation and adequate speed range are required, such as in power trading systems.

2.Construction of DC machine

The construction of a DC machine can be done using some basic components such as yoke, pole core and pole shoes, pole and field coils, armature core, armature windings (otherwise conductors), commutator, brushes, and bearings. Some of the components of a DC machine are discussed below.

1) Yoke

Another name for a yoke is a frame. The main function of a yoke in a machine is to provide mechanical support to the rods and protect the entire machine from moisture, dust, etc. The materials used for the yoke include cast iron, cast steel, or rolled steel.

2) Poles and Pole Cores

The poles of a DC machine are electromagnets, and the field winding is wound between the poles. Whenever the field winding is energized, the poles produce magnetic flux. The materials used for this are cast steel, cast iron, or pole cores. It can be made of annealed steel laminations to reduce the power drop caused by eddy currents.

3) Pole Shoe

The pole shoe in a DC machine is an extended part that is used to expand the area of ​​the pole. Due to this area, the magnetic flux can spread within the air gap and the excess magnetic flux can flow to the armature through the air gap. The material used to make the pole shoe is cast iron or cast steel, and annealed steel laminations are also used to reduce the power loss caused by eddy current.

4) Field Winding

Wherein, the winding is wound around the pole core area, called the field coil. Whenever current passes through the field winding, it causes the pole to generate an electromagnetic field, thereby generating the required magnetic flux. The material used for the field winding is copper.

5) Armature Core

The armature core has a large number of slots within its edges. The armature conductors are located within these slots. It provides a low reluctance path for the magnetic flux generated by the field winding. The material used for this core is a permeability low reluctance material such as iron or cast iron. Lamination is used to reduce the losses caused by eddy current.

6) Armature Winding

The armature winding can be formed by connecting the armature conductors. Whenever the armature winding rotates with the help of the prime mover, voltage as well as magnetic flux is induced in it. This winding is connected to the external circuit. The material used for this winding is a conductive material such as copper.

7) Commutator

The main function of the commutator in a DC machine is to collect the current from the armature conductor and provide the current to the load using brushes. It also provides unidirectional torque to the DC machine. The commutator can be constructed with a large number of hard-drawn copper sheets in the form of edges. The sheets in the commutator are protected by a thin layer of mica.

8) Brushes

The brushes in a DC machine collect the current from the commutator and provide it to the external load. The brushes wear out over time and therefore need to be checked frequently. The material used for the brushes is graphite or carbon and is rectangular in shape.

3.Types of DC Machines

The excitation of a DC machine is of two types, namely, independent excitation and self-excitation. In independent excitation type DC machines, the field coil is activated by a separate DC power supply. In self-excitation type DC machines, the current in the entire field winding is provided by the motor.

The main types of DC machines are divided into the following four types.

1. Separately excited DC motor
2. Shunt winding/shunt machine.
3. Series winding/series machine.
4. Compound wound/compound machine.

1) Separately excited

In a separately excited DC machine, a separate DC power supply is used to excite the field coil.

2) Shunt wound

In a shunt DC motor, the field coil is connected in parallel through the armature. Since the shunt field obtains the full output voltage of the generator, otherwise the motor supply voltage, it is usually made up of a large number of fine wires twisted together and carries a small amount of excitation current.

3) Series wound

In a series-excited DC motor, the field coil is connected in series through the armature. Since the series excitation winding obtains the armature current, and the armature current is large, the series excitation winding contains a small number of large cross-sectional area wires twisted together.

4) Compound wound

The compound motor includes both series winding and shunt winding. Each motor pole is wound twice. The series winding of the motor consists of a small number of strands of large cross-sectional area, while the shunt winding consists of several strands of fine wire.

There are two ways of connecting the compound machine. If the shunt field is connected in parallel with the armature only, the machine can be called a “short shunt compound machine”; if the shunt field is connected in parallel with both the armature and the series field, the machine can be called a “long shunt compound machine”.

4.EMF equation for DC machine

The DC motor emf can be defined as the voltage generated in the coil when the armature in the DC motor rotates. In a generator, the rotating emf can be called the generated emf, Er=Eg. In a motor, the rotating emf can be called the back emf or the reverse emf, Er=Eb.

Let Φ be the useful flux at each pole in Weber space

P is the number of poles
z is the total number of conductors in the armature
n is the armature speed of one revolution per second
A is the number of parallel wires on the armature between the opposite-sex brushes.
Z/A is the number of series armature conductors per parallel lane

Since the flux per pole is “Φ”, each conductor generates a flux “PΦ” in one rotation.

The voltage generated by each conductor = flux per revolution in WB/time required for a single rotation in seconds
Since n revolutions are completed in one second, 1 revolution will be completed in 1/n seconds. Therefore, the time for a single armature rotation is 1/n seconds.

The standard value of the voltage generated by each conductor

p Φ/1/n = np Φ volts

The voltage generated (E) can be determined by the number of series armature conductors, that is, the number of armature conductors in any single channel in the brush, and therefore, the total voltage generated

E = standard voltage per conductor x number of series conductors per lane

E = nPΦ x Z/A

The above formula is the electromotive force equation of a DC machine.

5.DC machine vs AC machine

The differences between AC machine and DC machine include the following points.

6.Losses of DC Machines

As we know, the main function of a DC machine is to convert mechanical energy into electrical energy. During this conversion process, the entire input power cannot be converted into output power because the power is lost in different forms.

The types of losses may vary from device to device. These losses reduce the efficiency of the device and the temperature increases. The energy losses of a DC motor can be divided into electrical or copper losses, core losses or iron losses, mechanical losses, brush losses, and stray load losses.

7.Advantages of DC Machines

The advantages of this machine are as follows.

1. DC machines such as DC motors have various advantages such as high starting torque, reversible, fast start and stop speed, and speed can be changed by voltage input
2. They are very easy to control and cheaper compared to air conditioners
3. Very good speed control
4. High torque
5. Seamless operation
6. No harmonics
7. Simple installation and maintenance

8.Applications of DC Machines

Currently, the generation of electrical energy can be carried out in large quantities in the form of alternating current (AC). Therefore, the use of DC machines like motors and generators is extremely limited as they are mainly used to provide excitation to small and medium AC generators. In industry, DC motors are used in different processes like welding, electrolysis, etc.

Usually, after the AC power is generated, it is turned into DC power with the help of a rectifier. Therefore, the DC generator is suppressed by the AC power supply, which is used in a variety of applications after rectification. DC machines are often used in variable speed drives and where there are drastic torque changes.

The application of DC machines as motors is divided into three types: series, shunt and compound excited, while the application of DC motors as generators is divided into separately excited generators, series generators and shunt generators.

So, this is all about DC machines. From the above information, we can finally conclude that DC motors are DC generators and DC motors.

DC generators are mainly used to provide DC power to DC machines in power stations. Whereas DC motors drive some equipment like lathes, fans, centrifugal pumps, printing presses, electric locomotives, hoists, cranes, conveyor belts, rolling mills, auto rickshaws, ice makers, etc.