Electrical Machines Dcmotor Jun2015

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ELECTRICAL MACHINES-CHAPTER 2-

DC MOTOR

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Learning Objectives:

1. Understand the construction and principle operation of a DC motor

2. Apply the principle operation of DC motor.

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Electric Motor

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

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CONSTRUCTION

DC motors consist of one set of coils, called armature winding, inside another set of coils or a set of

permanent magnets, called the stator. Applying a voltage to the coils produces a torque in the armature, resulting in motion.

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Stator with with polesBrush

Rotor

Field• Stator: Stationary part of the machine.

The stator carries a field winding that is used to produce the required magneticfield by DC excitation. Often know as the field.• Rotor: The rotor is the rotating part of the machine. The rotor carries a distributed winding, and is the winding where the emf is induced. Also known as the armature.

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CONSTRUCTION

Stator

The stator is the stationary outside part of a motor.

The stator of a permanent magnet dc motor is composed of two or more permanent magnet pole pieces.

The magnetic field can alternatively be created by an electromagnet. In this case, a DC coil (field

winding) is wound around a magnetic material that forms part of the stator.

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CONSTRUCTION

Rotor

The rotor is the inner part which rotates.

The rotor is composed of windings (called armature windings) which are connected to the external

circuit through a mechanical commutator.

Both stator and rotor are made of ferromagnetic materials. The two are separated by air-gap.

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CONSTRUCTION

A winding is made up of series or parallel connection of coils.

Armature winding - The winding through which the voltage is applied or induced.

Field winding - The winding through which a current is passed to produce flux (for the electromagnet)

Windings are usually made of copper.

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CONSTRUCTION

• The rotor iron core is mounted on the shaft.

• Coils are placed in the slots.

• The end of the coils are bent and tied together to assure mechanical strength.

• Note the commutator mounted on the shaft. It consists of several copper segments, separated by insulation.

Commutator

Poles

Brushes

Rotor winding

Fan

Bearing

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DC MOTOR BASIC PRINCIPLES

Energy Conversion

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

current flowing in the conductor and the magnetic field will produce mechanical force (and therefore,

mechanical energy).

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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, a force 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.

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The force exerted upon the conductor can be expressed as follows : F = B x i x l (unit Newton)

where B is the density of the magnetic field, l is the length of conductor, and i the value of current flowing in the conductor. The direction of motion can be found using Fleming’s Left Hand Rule.

Fleming’s 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).

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Principle of operation

Consider a coil in a magnetic field of flux density B. When the two ends of the coil are connected across a DC voltage source, current I flows through it. A force is exerted on the coil as a result of the 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.

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In 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 = ) as shown. 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.

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Induced voltage in the armature winding of DC motor

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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 Lenz’s law. It results in the rotor slowing down. Eventually, the rotor slows just 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.

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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 The induced emf in a single coil, e = dc/dt

Since the flux linking the coil, c = Sin ωt

Induced voltage : e = ω Cos ωt

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Note that equation (induced voltage) gives the emf induced in one coil. As there are several coils wound all around the rotor, each with a different emf depending on the amount of flux change through it, the total emf can be obtained by summing up the individual emfs.

The total emf induced in the motor by several such coils wound on the rotor can be obtained by integrating equation, and expressed as:

where K is an armature constant, and is related to the geometry and magnetic properties of the motor, and ωm is the speed of rotation. The electrical power generated by the machine is given by:

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Generated EMF in a Real DC Machine

WhereZ = total number of conductors, P = total number of polesa = P for lap winding, a = 2 for wave winding, φ = flux,ω = speed in rad/s and n = speed in rpm.

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Counter EMF:

When the motor is running, internally generated

emf, (EG = EC) opposes the applied voltage, thus:

I=

V − E RA T C

A

Where: VT = terminal voltage, Ec = counter EMF, RA is the armature resistance and IA is the armature current

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The Relationship Between The Induced Emf And Torque

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Example 1

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Example 2

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Equivalent circuit

The equivalent circuit of DC Motors (and Generators) has two components:

•Armature circuit: It can be represented by a voltage source and a resistance connected in series (the armature resistance). The armature winding has a resistance, Ra.

•The field circuit: It is represented by a winding that generates the magnetic field and a resistance connected in series. The field winding has resistance Rf.

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•Shunt Motors

-Field and armature windings are connected in parallel.

•Series Motors

-Field and armature windings are connected in series.

•Compound Motors

-Has both shunt and series field so it combines features of series and shunt motors.

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Example 1

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Example 2

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Shunt Motor: Speed Control

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Shunt Motor : The effect of an open field circuit

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Compound Motor: Speed Control

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Torque-Speed Characteristics:

In order to effectively use a D.C. motor for an application, it is necessary to understand its characteristic curves.

For every motor, there is a specific Torque/Speed curve and Power curve.

The relation between torque and speed is important in choosing a DC motor for a particular application.

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DC Motor Speed Control

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DC Motor Starting

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DC Motor Efficiency

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It is important to mention that the total input and output power can be calculated in many different ways using the power flow diagram, depending on the information given. Also note that the torque developed inside the rotor is different from the final (output) torque supplied to the load due to rotational losses.

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Example 1

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Example 2

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Self Test 1

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Self Test 2

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Electrical Machines Dcmotor Jun2015