DC Motors Notes

INTRODUCTION

A motor is a mechanism by which electrical energy is converted into mechanical energy. Its operating principle is the reverse of a DC generator. When a coil, carrying current, is placed in a magnetic field, it experiences forces, which turn it about in a direction perpendicular to both the field and current. Thus the motor armature placed inside the magnetic field gets motion, converting electrical energy to mechanical.

DC MOTORS – Principle of operation

A motor is a mechanism by which electrical energy is converted into mechanical energy. Both in principle and design, a DC motor is the reverse of a DC generator.

A steady current is passed through the armature coil from the commutator and the brushes are so arranged as to reverse the current every half revolution. When a coil, carrying a current, is placed in a magnetic field, it experiences forces, which turn it about in a direction perpendicular to both the field and the current. Due to the rotating torque the motion of rotation will not be continuous, unless the direction of the current is reversed each half revolution with the help of a split ring commutator (in a 2-pole machine).

The electric motor is fundamentally similar to the primitive form of D.C. generator described earlier and is based on the fact that, if a "loop of wire". If it is supplied, through its commutator, with electric current from a battery or any other source of direct current (D.C.) supply, the loop will rotate.

If the brushes of the machine were connected to the terminals of a primary cell, instead of being connected to load R, the "loop of wire "would rotate. A greatly enhanced performance would be obtained by having an iron core on this loop, a further improvement would be to have many loops, another to have increased pole area, and a still further improvement would be obtained by having electromagnets instead of permanent magnets.

When used for traction, the direct current electric motor is usually of the series wound type, that is, the current, which passes through the armature also, passes through the field coils. The reason for this is that a motor having this particular type of winding has characteristics eminently desirable for traction work, its torque being proportional to the current flow, multiplied by the magnetic strength of the field system. The series wound motor is capable; therefore, of producing a high torque when the vehicle is started, and also has the advantage that as the load increases its speed drops.

The direct current traction motor can be considered as having the following major parts;

1. The electro-magnetic system consisting of the frame with pole pieces, the field winding's and brush gear.

2. The reduction gears between the armature shaft and the road wheels, together with the gear case, which protects the gear wheels and holds the gear lubricant.

3. The axle bearing where the traction motor frame rests on the axle of the vehicle, this arrangement maintains a constant.

4. The nose suspension arrangement prevents the frame of the motor from rotating round the axle of the vehicle, The nose is spring borne on a bogie cross member.

BACK EMF

Due to the rotation of the armature coil (i.e. a conductor) in the magnetic field, the motor works as a DC generator and induced e.m.f acts in the circuit, which opposes the current. This induced e.m.f is called back e.m.f.

TYPES OF D.C.MOTORS

Like DC generators, DC motors are also of 3 types-

(i) Series wound motor,

(ii) Shunt wound motor and

(iii) Compound wound motor.

SPEED EQUATION

We know that back e.m.f. is produced by the generator action of the motor

Hence back e.m.f. E = φZNP/60A, where symbols have their usual meanings.

Let V be the applied voltage and I_a and R_a is the armature circuit current and resistance respectively.

Then E =V-IaRa 

SPEED CONTROL OF DC MOTOR

We know that, N = (V-I_a R_a)/φ

From this formula it follows that the speed of a D.C. motor can be regulated by:

(i) varying the supply mains voltage V

(ii) Varying the voltage drop in the armature circuit I_a R_a

(iii) Varying the field flux

Methods

(ii) & (iii) are possible in any installation with constant supply voltage. But the first method is possible only in special installation; that permits the control of the supply voltage.

CHARACTERISTICS OF D.C.MOTORS

 There are three important characteristics of a D.C motor, which are given below: -

(Ref. Attached figures)

(i) Torque - Armature current characteristics

This shows the relation between mechanical torque developed and armature current.

(ii) Speed- Armature current characteristics

As the name indicates, it relates speed with armature current.

(iii) Speed - Torque characteristics

The characteristics curve gives the relation between speed and torque of a DC motor.

 

CONSTRUCTIONAL DETAILS OF DC MOTOR

INTER POLES

In addition to the main field coils of a motor being in series with the armature, there are also the coils of a smaller system of field magnets known as inter-poles. On generators with separately excited main fields, the inter-pole coils are in series with the armature.

The inter-poles are smaller than the main poles of either a generator or motor, but are the same length and positioned alternatively with the main poles. In a generator the polarity of an inter-pole is the same as the main pole ahead of it according to the rotational direction of the armature. The polarity of an inter-pole in a motor is the same as the main pole proceeding it. An electrical machine with no inter-poles would have some magnetically neutral regions between its pole-pieces. When a coil of the armature reaches a position during its rotation in the neutral region, its connections are short-circuited with the connection of the armature coil in advance, because in this position the commutator brushes will be in contact with both of their corresponding commutator segments. The purpose of the inter-poles, being situated in the neutral regions, is to induce a current in the short-circuited armature wingdings, which flows in the same direction as the current, which will flow when it has left the neutral region.

The use of inter-poles also serves to prevent the distortion of the main field of the generator by the reaction of the armature field, and thereby prevents the induction of Electromotive forces into coil sides, which are being short- circuited by the brushes.

In small machines the need for inter-poles is not important but on large generators and motors the net effect of the inter-poles is to improve the commutation. Ideally there should be no sparking of the brushes on the commutator surface, although this is often difficult to achieve in practice.

HEATING AND COOLING

Every electrical machine is a power (or energy) conversion device. During these power conversion some of the energy is wasted. In electrical machines the loss in energy occurs in electrical circuits and in portions of magnetic circuits also. There are also frictional losses in the dynamic parts of the machines. These losses are converted in the form of heat energy, which increases, or tends to increase the temperature of iron and copper above that of the ambient temperature, which in turn effects the winding insulation. In addition to the effect it has on the insulation, an excessive temperature rise may also adversely influence the mechanical operating conditions of a given machine part. Thus, for example the original dimension of the commutator may change. Solder between the commutator and winding may get washed out. So to avoid all these, it is very essential to provide a cooling system on machines.

In most cases, the cooling is done by air currents. The cooling of machines by air streams is called ventilation. The ventilation employed depends on the environmental conditions of the place where the machine is to operate.

According to the method of ventilation employed, the following types of machines are distinguished: -

(i) Machines with natural ventilation.

(ii) Machines with internal self-ventilation.

(iii) Machines with external self-ventilation.

(iv) Machines with independent ventilation.

Enclosures have got the direct bearing with the ventilation. The following are the main types of enclosures: -

(i) Open pedestal

Rotor and stator windings are in free contact with the surroundings.

(ii) Open end Bracket

Rotors and stator windings are in contact with surrounding through openings.

(iii) Protected (formerly called semi-enclosed)

Openings in the frame are protected with wire, perforated covers etc.

(iv) Drip proof

Opening so constructed that no solid or liquid particles falling at an angle greater than 150 will enter the machine.

(v) Splash proof

Similar to drip proof but the angle of approach is 100^O from vertical.

(vi) Duct or pipe ventilated

Air for ventilation enters and emerges through a pipe through the openings.

(vii) Totally enclosed

Exchange of air throughout side and inside of the machine is prohibited.

(viii) Water proof

The machine is totally enclosed so as to exclude water applied as a stream as specified.

(ix) Flame proof

It is designed normally for mines.

(x) Resistant

Machine is so constructed, that it will not be harmed easily by moisture fume, alkali etc.

(XI) Submersible

So constructed that it will work when submerged in water under specified condition of pressure and time.

RATINGS

There are three types of ratings as specified.

(i) Continuous Rating: This is an output, which a machine delivers continuously without exceeding the permissible temperature. It can deliver 25% overload for two hours.

(ii)Continuous maximum Rating: This is similar to continuous rating but not allowing overload.

(iii) Short time ratings: This is an output which a machine can deliver for a specified period (say 1 hr 1/2 hr, 1/4 hr etc) without exceeding the maximum temperature rise limit.

 

SUMMARY

Information regarding operating principle, construction, characteristic and selection of carbon brushes for DC motors  have been given in this unit. These information will help in maintaining the motors to ensure reliability and their trouble free functioning. Sketches and diagrams have been included in this unit to understand the unit with more practical and systematic approach.

SELF-ASSESSMENT EXERCISES

1.     Describe and mention the speed equation of a dc motor.

2.     What are the different types of cooling arrangements of DC motors?

3.     What do you mean by rating of a motor? What are the types of ratings?