# Study notes on DC Machines-1 For Electrical Engineering Students

By Deepak Yadav|Updated : July 13th, 2023

DC machines play a crucial role in various applications, including electric power generation, motor drives, and traction systems. Aspiring electrical engineering students must acquire a solid understanding of the principles and operation of DC machines to effectively design, analyze, and maintain these devices. These study notes provide comprehensive coverage of DC Machines-1, encompassing topics such as construction, working principles, types of DC machines (generators and motors), characteristics, and basic performance analysis. Whether you are a student studying electrical engineering or a professional seeking to enhance your knowledge of DC machines, these study notes serve as a valuable resource to grasp the fundamentals and practical aspects of DC machines.

DC machines are essential components in electrical engineering, serving as reliable sources of electrical power or as efficient motors. Understanding the operation and characteristics of DC machines is crucial for electrical engineering students to comprehend the principles behind power generation, motor control, and energy conversion. This study guide focuses on DC Machines-1, providing comprehensive study notes that cover the construction, working principles, and basic analysis of DC generators and motors.

## DC Machines

• DC machine is a highly versatile and flexible machine.
• It can satisfy the demands of load requiring high starting, accelerating and retarding torques.
• If the conversion is from mechanical to electrical energy, the machine is called a Generator.
• If the conversion is from electrical to mechanical energy, the machine is called a Motor.

To understand, design and use these machines the following laws must be studied.

• Electric circuit laws - Kirchoff′s Laws
• Magnetic circuit law - Ampere′s Law
• Law of electromagnetic induction - Faraday′s Law
• Law of electromagnetic interaction -BiotSavart′s Law

Most of present-day machines have one or two electric circuits linking a common magnetic circuit. In subsequent discussions, the knowledge of electric and magnetic circuit laws is assumed. The attention is focused on Faraday’s law and Biot Savart’s law in the present study of electrical machines.

Application of Faraday's law according to electro-mechanical energy conversion results in the generation of both transformer and rotational emf to be present in the coil moving under a changing field. This principle is utilized in induction machines and a.c. commutator machines. The direction of the induced emf is decided next. This can be obtained by the application of Lenz’s law and the law of interaction.

## Law of induction-Generator action

The Law of Induction, specifically in the context of generator action, is a fundamental principle in electrical engineering that governs the production of electric currents through electromagnetic induction. According to this law, when a conductor moves within a magnetic field or a magnetic field varies relative to a conductor, an electromotive force (EMF) is induced in the conductor, resulting in the generation of electrical power. This principle forms the basis for the operation of generators, which are vital in the generation of electricity for various applications.

## Law For Motoring Action

Law For Motoring Action (LFMA) refers to the legal framework and regulations governing various aspects of motoring and road usage. It encompasses the laws and guidelines pertaining to road safety, vehicle operations, driver behaviour, traffic violations, and legal procedures related to motoring offences. LFMA plays a crucial role in ensuring the safety, order, and efficient functioning of road transportation systems, as well as safeguarding the rights and responsibilities of motorists and road users.

## Armature Reaction in DC Motor

The effect of magnetic field set up by armature current on the distribution of flux under main poles of a generator. The armature magnetic field has two effects:
(i) It Demagnetizes or weakens the main flux and
(ii) It cross-magnetises or distorts it.

• In armature reaction in motors for a given polarity of the field and sense of rotation, the motoring and generating modes differ only in the direction of the armature current. Alternatively, for a given sense of armature current, the direction of rotation would be opposite for the two modes. The leading and trailing edges of the poles change positions if the direction of rotation is made opposite. Similarly, when the brush leads are considered, a forward lead given to a generator gives rise to the weakening of the generator field but strengthens the motor field and vice-versa. Hence it is highly desirable, even in the case of non-reversing drives, to keep the brush position at the geometrical neutral axis if the machine goes through both motoring and generating modes.
• The second effect of the armature reaction in the case of motors as well as generators is that the induced emf in the coils under the pole tips gets increased when a pole tip has a higher flux density. This increases the stress on the ‘mica’ (moissanite) insulation used for the commutator, thus resulting in an increased chance of breakdown of these insulating sheets. To avoid this effect the flux density distribution under the poles must be prevented from getting distorted and peaky.
• The third effect of the armature reaction mmf distorting the flux density is that the armature teeth experience a heavy degree of saturation in this region. This increases the iron losses occurring in the armature in that region. The increase in iron loss could be as high as 50 per cent more at full load compared to its no-load value.

### Commutation

• The currents induced in armature conductors of a d.c. the generator is alternating. These currents flow in one direction when armature conductors are under N-pole and in the opposite direction when they are under S-pole.
• As conductors pass out of the influence of an N-pole and enter that of an S-pole, the current in them is reversed. This reversal of current takes place along a magnetic neutral axis or brush axis i.e. when the brush spans and hence short circuits that particular coil undergoing reversal of current through it.
• This process by which current in the short-circuited coil is reversed while it crosses the M.N.A. is called commutation. The brief period during which coil remains short-circuited is known as the commutation period Tc.
• If the current reversal i.e.the change from +I to zero and then to −I is completed by the end of the short circuit or commutation period, then the commutation is ideal. If the current reversal is not complete by that time, then sparking is produced between the brush and the commutator which results in progressive damage to both.
• The brush width is equal to the width of one commutator segment and one mica insulation.

## Types of DC Machines

• The types of DC machine depends upon the excitation of the DC machine.
• The production of magnetic flux in the machine by circulating current in the field winding is called excitation.
• DC Machines can be classified according to the electrical connections of the armature winding and the field windings.

There are two methods of excitation namely, separate excitation and self-excitation.

• In separate excitation, the field coils are energised by a separate DC source. The terminals of the winding can be connected across the input voltage terminals or fed from a separate voltage source.
• In self-excitation, the current flowing through the field winding is supplied by the machine itself. The field winding can be connected either in series or in parallel with the armature winding

## Speed Control of DC Motors

The speed of a DC motor can be varied by varying flux, armature resistance or applied voltage. Different speed control methods for different DC shunt and series methods are there.

Or

Speed Control of Shunt Motors:

• Flux control method
• Armature and Rheostatic control method
• Voltage control method
1. Multiple voltage control
2. Ward Leonard system

Speed Control of Series Motors:

• Flux control method
1. Field diverter
2. Armature diverter
3. Trapped field control
4. Paralleling field coils
• Variable Resistance in series with the motor
• Series -parallel control method

## Speed Control Methods

• In this flux control method, the speed of the motor is inversely proportional to the flux. Thus, by decreasing flux and speed can be increased and vice versa.
• To control the flux, he rheostat is added in series with the field winding will increase the speed (N), and because of this flux will decrease.
• The field current is relatively small and hence I2R loss is decreased. This method is quite efficient.

In this method of speed control, Ra and VT remain fixed.

Therefore, from the equation
ωm α 1/φ
Assuming magnetic linearity, φ α If
Or,
ωm α 1/If
i.e., Speed can be controlled by varying field current If.

## Armature Control Method

• In the armature control method, the speed of the DC motor is directly proportional to the back emf (Eb) and Eb = V- IaRa.
• When supply voltage (V) and armature resistance Ra are kept constant, the Speed is directly proportional to armature current (Ia).
• If we add resistance in series with the armature, the armature current (Ia) decreases and hence speed decreases.

From the speed-torque characteristics equation, we know that

For a load of constant torque, if VT and φ are kept constant, as the armature resistance Ra is increased, speed decreases. As the actual resistance of the armature winding is fixed for a given motor, the overall resistance in the armature circuit can be increased by inserting an additional variable resistance in series with the armature.

## Voltage control method

• Multiple voltage control: In this method shunt field of the motor is connected to a fixed exciting voltage, but the armature is supplied with different voltages by connecting it across one of the several voltages with the help of a switch. The intermediate speeds can be obtained by adjusting the field regulator. This method is very rarely used.
• Ward-Leonard system: This system is used where a very large variation in speed is required. In this method axillary machines along with a DC motor whose speed is to be varied. The motor is supplied by a generator which is driven by a motor. Very sensitive and smooth speed control can be obtained by this system. Thus this method can be used in colliery winders, electric excavators, elevators and the main drives in steel mills.

This method is usually applicable to separately excited DC motors. In this method of speed control, Ra and Vt are kept constant.
In normal operation, the drop across the armature resistance is small compared to Eb and therefore: Eb α Vt

Since, Eb = KΦω
ωm is the Angular speed can be expressed as:

ωm = Vt/KΦ

• If the flux is kept constant, the speed changes linearly with VT.
• As the terminal voltage is increased, the speed increases and vice versa.

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## FAQs about Study notes on DC Machines-1 For Electrical Engineering Students

• The main components of a DC machine include the stator (field system), rotor (armature), commutator, brushes, and shaft. The stator produces a magnetic field, while the rotor carries the armature winding.

• A DC generator converts mechanical energy into electrical energy by rotating a conductor (armature) within a magnetic field. The relative motion induces an electromotive force (EMF) in the armature winding, generating a DC voltage output.

• The commutator in a DC machine serves as a mechanical rectifier, converting alternating current induced in the armature windings into direct current by reversing the connections as the armature rotates.

• Key characteristics of a DC motor include torque-speed relationship, starting torque, armature reaction, speed control methods, and efficiency. These characteristics determine the motor's performance, control, and suitability for specific applications.

• The speed of a DC motor can be controlled by varying the armature voltage, field flux, or both. Other methods include using external resistances, electronic speed controllers, and pulse-width modulation (PWM) techniques to regulate the motor speed.

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