Project Report Dc Motor

8/10/2019 Project Report Dc Motor

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Introducion:


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Theory:

Almost every mechanical movement that we see around us is accomplished by an electric

motor. Electric machines are a means of converting energy. Motors take electrical energy

and produce mechanical energy. Electric motors are used to power hundreds of devices weuse in everyday life. Motors come in various sizes. Huge motors that can take loads of

1000s of Horsepower are typically used inthe industry. Some examples of large motor

applications include elevators, electric trains, hoists, and heavy metal rolling mills. Examples

of small motor applications include motors used in automobiles, robots, hand power tools

and food blenders. Micro-machines are electric machines with parts the size of red blood

cells, and find many applications in medicine.

Electric motors are broadly classified into two different categories: DC (Direct Current) and

AC (Alternating Current). Within these categories are numerous types, each offering unique

abilities that suit them well for specific applications. In most cases, regardless of type,electric motors consist of a stator (stationary field) and a rotor (the rotating field or

armature) and operate through the interaction of magnetic flux and electric current to

produce rotational speed and torque. DC motors are distinguished by their ability to operate

from direct current.

There are different kinds of D.C. motors, but they all work on the same principles. In this

chapter, we will study their basic principle of operation and their characteristics. Its

important to understand motor characteristics so we can choose the right one for our

application requirement. The learning objectives for this chapter are listed below.

Electromechanical Energy Conversion:

An electromechanical energy conversion device is essentially a medium of transfer between

an input side and an output side. Three electrical machines (DC, induction and synchronous)

are used extensively for electromechanical energy conversion. Electromechanical energy

conversion occurs when there is a change in magnetic flux linking a coil, associated with

mechanical motion.

Electric Motor

The input is electrical energy (from the supply source), and the output is mechanical energy(to the load).

Electrical Electromechanical Mechanical

energy energy conversion device energy

Source Motor Load

Figure. 1

Electric Generator


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4.3. DC Motor Basic Principles

4.3.1 Energy Conversion

If electrical energy is supplied to a conductor lying perpendicular to a magnetic field, the

interaction ofcurrent flowing in the conductor and the magnetic field will produce mechanical force (and

therefore,

mechanical energy).

4.3.2 Value of Mechanical Force

There are two conditions which are necessary to produce a force on the conductor. The

conductor must

be carrying current, and must be within a magnetic field. When these two conditions exist, aforce will be

applied to the conductor, which will attempt to move the conductor in a direction

perpendicular to the magnetic

field. This is the basic theory by which all DC motors operate.

The force exerted upon the conductor can be expressed as follows.

F = B i l Newton (1)

where B is the density of the magnetic field, l is the length of conductor, and i the value ofcurrent flowing in the

conductor. The direction of motion can be found using Flemings Left Hand Rule.

Figure 3: Flemings Left Hand Rule

The first finger points in the direction of the magnetic field (first - field), which goes from the

North pole to the

South pole. The second finger points in the direction of the current in the wire (second -

current). The thumb

then points in the direction the wire is thrust or pushed while in the magnetic field (thumb -

torque or thrust).

How much force will be created on a wire that is parallel to the magnetic field? 4 DC Motors

4.3.3 Principle of operation

Consider a coil in a magnetic field of flux density B (figure 4). When the two ends of the coil

are

connected across a DC voltage source, current I flows through it. A force is exerted on thecoil as a result of the


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interaction of magnetic field and electric current. The force on the two sides of the coil is

such that the coil starts

to move in the direction of force.

Figure 4: Torque production in a DC motorIn an actual DC motor, several such coils are wound on the rotor, all of which experience

force,

resulting in rotation. The greater the current in the wire, or the greater the magnetic field,

the faster the wire

moves because of the greater force created.

At the same time this torque is being produced, the conductors are moving in a magnetic

field. At

different positions, the flux linked with it changes, which causes an emf to be induced (e =

figure 5. This voltage is in opposition to the voltage that causes current flow through the

conductor and is

referred to as a counter-voltage or back emf.

Figure 5: Induced voltage in the armature winding of DC motor

The value of current flowing through the armature is dependent upon the difference

between the

applied voltage and this counter-voltage. The current due to this counter-voltage tends to

oppose the very cause

for its production according to Lenzs law. It results in the rotor slowing down. Eventually,

the rotor slows just

Induced emf

Flux DC Motors 5

enough so that the force created by the magnetic field (F = Bil) equals the load force applied

on the shaft. Then

the system moves at constant velocity.

4.3.4 Torque Developed

The equation for torque developed in a DC motor can be derived as follows.

The force on one coil of wire F =i l x B Newton

Note that l and B are vector quantities


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Therefore the torque for a multi turn coil with an armature current of Ia:

current flowing inthe armature winding.

Note: Torque T is a function of force and the distance, equation (2) lumps all the constant

parameters (eg.

length, area and distance) in constant K.

The mechanical power generated is the product of the machine torque and the mechanical

It is interesting to note that the same DC machine can be used either as a motor or as a

generator, by reversing

the terminal connections.

Figure 6: Reversability of a DC machine

4.3.5 Induced Counter-voltage (Back emf):

Due to the rotation of this coil in the magnetic field, the flux linked with it changes at

different positions,

which causes an emf to be induced (refer to figure 5).

(a) Motor action (b) Generator action

Electrical

Power

input

Electrical

Power

Mechanical output

output


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Figure 7: DC Motor representation

4.3.7 Voltage Equation

Applying KVL in the armature circuit of Figure 7:

VT = Eb + IaRa (7)

where VT is voltage applied to the armature terminals of the motor and Ra is the resistance

of the armature

winding.

Note: The induced voltage is typically represented by symbol e (or E) and the terminal

voltage by v (or V).

At standstill, the motor speed is zero, therefore back emf is also zero. The armature current

at starting

is thus very large.

Applying KVL in the field circuit of Figure 7:

Vf = Rf If (8)

+

Vf

_

Lf

Rf

+

VT

_

If

Ia

m, T

Ra


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+

Eb

_

Field circuit Armature (rotor) circuit DC Motors 7

Where Vf is voltage applied to the field winding (to produce the magnetic field), Rf is the

resistance of the field

winding, and If is the current through the field winding.

How would the inductance of the field winding affect the motor operation under steady-

state?

4.3.8 Power Transfer Equation

We have earlier obtained the following relationship for torque developed in the motor

(from equation 2):

The developed power is the power converted to mechanical form, and is given by (from

equation 3):

This is the power delivered to the induced armature voltage (counter-voltage) and given by:

EbIa (electrical

per second) by

N can be written as r/min or rpm, both mean the same thing.

Noting that the flux in the machine is proportional to the current flowing in the field winding

compare induced voltages at two different speeds.

If the induced voltage at the first operating speed N1, and field winding current If1 is given

by:

and the induced voltage at the first operating speed N2, and field winding current If2 is

given by:

Then the induced voltages at these operating points can be compared as:


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This equation is useful in determining the speed of the DC motor at different operating

conditions.

2 ( ) 1

1 1

N E K K I b f f

2 ( ) 2

2 2

N E K K I b f f

2 2

1 1

2

1

I N

I N

E

E

f

f


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Thetorque of a motor is generated by a current carrying conductor in a magnetic field.

Theright hand rule states that if you point your right hand fingers along the direction of

current, I, and curl them towards the direction of the magnetic flux, B, the direction of force

is along the thumb.

Now, imagine a loop of wire with

some resistance is inserted between the two permanent magnets. The following diagrams

show how the motor turns:

Diagram showing how the motor works Relationship between the Torque and the angle the

loop made with the magnet.

You might be able to notice that the direction of rotation is changing every half cycle. To

keep it rotating in the same direction, we have to switch the current direction. The process

of switching current is called commutation. To switch the direction of curent, we have to

use brushesand commutators. Commutation can also be done electronically (Brushless


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motors) and a brushless motor usually has a longer life. The following diagram shows how

brushes and commutators work.

We could also have several commutators and loops. The total torque generated is the sum

of all the torques from each of the loops added.

Motor with several commutators and loops

So, the torque is proportional to the current through the windings,

T = kI where T is the torque, I is the current, and k is a constant

The wire coils have both a resistance, R, and an inductance, L. When the motor is turning,

the current is switching, causing a voltage,

V = L dI/dt


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This voltage is known as the back-emf(electromotive force), e.

If the angular velocuty of the motor is w, then e = kw(like a generator)

This voltage, e, is working against the voltage we apply across the terminals, and so,

(V- kw) = IR where I =T/R

which implies (V-kw) = (T/k) R

The maximum or stall torque is the torque at which w = 0 orT= kV/R, and

The stall or starting current, I = V/R

The no load speed, w= V/k, is the maximum speed the motor can run. Given a constant

voltage, the motor will settle at a constant speed, just like a terminal velocity.

If we plot w = V/k - (T/k^2)R, we can get the speed-torque curve:

Observation:

Parameters Observe value

Mass of weight=m

Distance covered=h

Voltage applied=V

Current flow=I

Radius of pulley=r


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Average Time taken=t

Gravitational acceleration=g

Calculation:


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Conclusion:

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Project Report Dc Motor