Electrical Machines : DC Machines

By Yash Bansal|Updated : May 18th, 2021

Electrical Machines : DC Machines

 

 

 

 

 

 

 

                                                                                                  

 

 

 

 

 

 

 

 

 

 

                                                                                                                                                                                                  

In this article, you will find the study notes on 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 as Generator.
  • If the conversion is from electrical to mechanical energy, the machine is called as 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 the 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 the Faraday’s law and Biot Savart’s law in the present study of the 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 the induction machines and a.c. commutator machines. The direction of the induced emf is decided next. This can be obtained by the application of the Lenz’s law and the law of interaction.

Law of induction-Generator action 

Law For Motoring Action

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.

  • 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 direction of rotation is made opposite. Similarly when the brush leads are considered, a forward lead given to a generator gives rise to 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 get increased when a pole tip has higher flux density. This increases the stress on the ‘mica’ (micanite) insulation used for the commutator, thus resulting in 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 percent more at full load compared to its no-load value.

Commutation 

  • The currents induced in armature conductors of a d.c. generator are 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 a N-pole and enter that of S-pole, the current in them is reversed. This reversal of current takes place along 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 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 short circuit or commutation period, then the commutation is ideal. If 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 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

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.

image027

Or

image028

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 motor
  • Series -parallel control method

 Speed Control Methods

Flux Control Method:

  • In this flux control method, speed of the motor is inversely proportional to the flux. Thus, by decreasing flux and speed can be increased vice versa.
  • To control the flux , he rheostat is added in series with the field winding will increase the speed (N), 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 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 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 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 very large variation in speed is required. In this method axillary machines along with 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 Vare 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 flux is kept constant, the speed changes linearly with VT.
  • As the terminal voltage is increased, the speed increases and vice versa.

Types of DC Machines:

  • The types of DC machine depends upon the excitation of 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.

image001

  • 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

Separately Excited DC Machine

As the name implies, the field coils are energized by a separate DC source. The armature and field winding are electrically separate from each other. 

image001

(a) Separately excited DC generator, and (b) Separately excited DC motor

Here, Ia = I, and Ra = Armature resistance.

  • For Generator

Ea = V + IaRa  

 or image002

  • For Motor

image003

Armature power = Pa = EaIa, Output = VIL, and Armature copper loss = I2a Ra

Shunt Wound DC Machine

  • The armature and field winding are connected in parallel.
  • A machine in which the field coils are connected in parallel with the armature is called a shunt machine.
  • The armature voltage and field voltage are the same.

image005

(a) Shunt wound DC generator, and (b) Shunt wound DC motor

Characteristics Equations:

image006

where, Pa = EaIa = Armature power (developed power),  I2sh Rsh= shunt field Cu loss, I2a Rsh = Armature Cu loss, and VIL= Power delivered.

Series Wound DC Machine

  • A DC machine in which the field coils are connected in series with the armature is called a series machine.
  • The field winding carries the same current as the armature winding.

image009

(a) DC series generator (b) DC series motor

  • A series wound motor is also called a universal motor. It is universal in the sense that it will run equally well using either an ac or a dc voltage source.

Characteristics Equations:

image010

where, Pa = EaIa = Armature power (developed power), VIL = Power delivered, and  I2se Rse= Series field Cu loss.

Compound Wound DC Machine

  • A DC machine, having both shunt and series fields is called a compound machine.
  • In a compound machine, the series field winding is connected in series with the armature, and the shunt field winding is connected in parallel.

 

Short-shunt compound DC Machine:

image012

Here, Figure (a) is Short-shunt compound DC generator, and (b) is Short-shunt compound DC motor.

  • For generator

image013

image014

image015

image016

image017

image018

image019

  • For motor

image020

image021

Similarly in motor,

where, Pa = Power developed, VIL = Power delivered, I2se Rse= Series field Cu loss, and I2sh Rsh= Shunt field Cu loss.

Two types of arrangements are possible in compound motors:

  • Cumulative compounding : If the magnetic fluxes produced by both series and shunt field windings are in the same direction (i.e., additive), the machine is called cumulative compound.
  • Differential compounding : If the two fluxes are in opposition, the machine is differential compound.

In both these types, the connection can be either short shunt or long shunt.

Long-shunt compound DC Machine:

image023

(a) Long-shunt DC generator (b) Long-shunt DC motor

 

  • For generator

image024

image025

image026

image021

or 

  • Similarly, in motor

image021

where, Pa = Developed power, and VIL = Delivered power.

Power Flow In DC Machines 

Here the case given to understand is for DC Motor

Posted by:

Yash BansalYash BansalMember since Nov 2018
Content Manager (GATE)
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