Types of Dc Motors and Dc Generator Essay

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MOTORS – convert electrical energy to mechanical energy

Motors and generators are the most frequent used electrical machines. ? Generators action can take place when and only when, there is a relative motion between conducting wires and magnetic lines of force. ? Electric motor is in operation when it is supplied with electrical energy and develops torque, that is, a tendency to produce rotation. ? In DC generator, the armature winding is mechanically rotated through the stationary magnetic fields created by the electromagnets or permanent magnets. In AC generator, the electromagnets or permanent magnets and their accompanying magnetic fields are rotated with respect to the stationary armature windings. Classification of single phase motors

  1. Shaded pole
  2. Reluctance
  3. Split-phase
  4. Repulsion
  5. Repulsion-start
  6. Repulsion-induction
  7. Series
  8. Synchronous

Classification of Poly phase Motors

  1. Induction Motor
  2. Commutator (Schrage Motor)
  3. Synchronous Commutation on DC Motor The function of the commutator and the brushes in DC motor and generator is to act as an inverter, that is to change direct current to alternating current.

DC Generator and DC Motor A dc generator and a dc motor are identical in structure and are interchangeable. Any dc motor can be used as a generator, and any dc generator can be used as motor with only a minor change in connections. When a conductor (wires of the armature) cuts the lines of force (magnetic field of the stator), a voltage is generated in that conductor (wires of the armature).

There are three factors or rules that govern the amount of voltage generated:

  1. The number of lines of force being cut.
  2. The speed at which the conductors are cutting the lines of force.
  3. The number of conductors cutting the lines of force.

DC MOTOR A dc motor is a machine that, when supplied with electric current, can be used for such mechanical work as driving pumps and running machine tools.

CLASSIFICATION OF DC MOTORS

1. Shunt motor – a comparatively high resistance field winding of many turns of fine wire is connected in parallel with the armature. From no load to full load, the speed of shunt motor will not change much, which means that it has a good speed regulation. Shunt motor controller: ? Over speed control – voltage control connected in series to control the speed of the motor. Full voltage applied to field and armature will produce slowest speed at full load.

Reduce field voltage reduces current flow and reduces lines of force, thus, less back EMF generated in the armature then more current to flow in the armature, increasing its magnetic power and so it will speed up. More speed and current will increase the motor horsepower. ? Under speed control – a controller placed in series with the armature to control the voltage. Less than full voltage applied to the armature will decrease the motor speed. The difference between back EMF and applied voltage to the armature is smaller.

The effect of the applied voltage to the armature become smaller and because of this less current will flow through it and the torque will decrease. The load will slow the armature, fewer lines of force being cut, back EMF decreases, more current to flow in the armature and increasing the torque until the sped again stabilized.

2. Series motor – an extremely low resistance field winding of very few turns of heavy wire is connected in series with the armature. It has high starting torque and a variable speed characteristic. As the speed of the series motor increases, the back EMF also increases, but the current flow decreases.

The current flows through the field poles so that the number of lines of force also decreases. When the lines of force decreases, so does the amount of back EMF. The back EMF of a series motor is never enough to limit the speed; only the load can limit the speed of a series motor. This motor does not have good speed regulation, but it is popular for high torque load.

3. Compound motor – a machine that is excited by a combination of shunt field connected in shunt with the armature and series field connected in series with the armature. The shunt field gives this motor good speed regulation, as a shunt motor has, but the series field gives it the ability to handle overloads well. The current through the series field and armature increases when an overload decreases the speed of the armature. This increases the motor’s power, and the overload does not reduce the speed as drastically as it would with a shunt motor. The compound motor can be controlled for over speed and under speed in the same way that the shunt motor can.

4.Permanent Magnet Motor – has permanent magnet fields and an armature that is similar to any DC motors armature (manufactured in fractional to the low integral horsepower sizes).

INTERPOLES OF DC MOTOR

Nearly all shunt and compound motors of ? hp or more have commutating poles or interpoles located between the main poles. This interpoles have one winding of heavy wire and are connected in series with the armature. The purpose of this interpole is to prevent sparking. The polarity of the interpoles depends on the polarity of the main poles aand the direction of the motor’s rotation. Rule for Interpole Polarity The polarity of an interpole in a motor is the same as the main pole behind it. This means that if motor viewed from the commutator end is rotating clockwise, the polarity of the interpole must be the same as that of the main pole that precedes it in the direction opposite to rotation.

REVERSING DC MOTORS

Direct current motors are reversed by changing the direction of current flow through the armature or through the field. To reverse a shunt–interpole motor, it is necessary to reverse the current flow through both the armature and the interpoles as a unit.

Reversing the armature leads without the interpole will cause the motor to have incorrect interpole polarity, which will make the motor run excessively hot and will produce sparking at the brushes.

DC GENERATOR GENERTATORS – convert mechanical energy to electrical energy Direct current generators are similar to dc motors in appearance and construction. They have the same type of armature and field poles and are generally identical. Three factors are needed to generate electricity:

  1. magnetic lines of force (flux)
  2. a conductor
  3. the cutting of the lines flux by the conductor

Three methods of producing the lines of force necessary in generating electricity:

  1. Use of permanent magnets.
  2. Excitation of the generator field coils with direct current from a battery or small generator (separate excitation).
  3. Excitation of the field coils by current from the armature (self-excitation).

The Separately Excited Generator When the field coils are connected to an outside source of electricity, the generator is known as a separately excited generator. When the armature rotates in the magnetic field, current is supplied to the load. The Self-excited Generator

Most generators use some of the current generated in the armature to supply excitation current to the fields. This type is called a self-excited generator. Building-up Process At standstill, the magnetic field is due only to residual magnetism of the field core and is very weak. When the armature rotates, the conductors cut this weak flux and generate a very low voltage that will excite the field coils slightly and create additional lines of force. Because the armature now turns in a stronger magnetic flux, it will generate higher voltage and cause more current to flow to the fields, which in turn will produce more lines of force.

This action continues until the field poles saturate magnetically. Three Types of Self-excited Generators

1. The Series Generator The connections are like those of a series motor with the load completing the circuit, and thus providing a current source. The armature, field and the load all are connected in series. If the load is disconnected from the generator terminals, the circuit through the generator will be open, and consequently no current can flow through the field coils, and no voltage will be generated. This is one of the characteristics of a series generator: The voltage at no load is zero, and it increases to a maximum at full load.

2. The Shunt Generator The field coils of the shunt generator are connected across the armature terminals. The field strength, therefore, is practically constant, regardless of load. However, as the load is increased, the terminal voltage will decreased because of an increased voltage drop within the armature. One characteristic of the shunt generator is therefore that a slight drop in voltage occurs as the load increased. The voltage at no load is maximum and decreases slightly as the load increased.

3. The Compound Generator This generator usually supplies constant voltage regardless of load, but its regulation can be varied by changing the number of turns in the series-field winding or by using a resistor across the series field to vary the current through it. By changing the number of turns in the series field, it is possible to obtain three kinds of compound generators:

  • a. Over-compound. If the turns in the series field are increased over the number necessary to give the same voltage output at all loads. This means that as the load is increased, the generated voltage increases. At no load, normal voltage is obtained, but as the load is increased to full load, the voltage rises approximately 5 percent.
  • b. Flat-compound. If the number of turns in the series field is decreased. In this generator, the voltage produced at full load is the same value as the voltage at no load.
  • c. Under-compound. If the turns in series winding is further decreased. In this type, the voltage at no load is normal. As the load is increased, the voltage drops considerably, until at full load it is approximately 20 percent below normal.

INTERPOLES OF DC GENERATOR

On all of the generators mentioned, interpoles are generally used. These are connected in series with the armature, as in dc motors. The polarity of the interpoles in a generator is, however, opposite to that in a motor. The rule is that the polarity of the interpoles in a generator is the same as the main pole ahead of it in the direction of rotation. Regulating the Generated Voltage To regulate the generated voltage, a field rheostat is inserted in the shunt-field circuit. This arrangement makes it possible to vary the current in the shunt field, which in turn varies the lines of force.

With full current in the field, the maximum voltage will obtained. As resistance is added, the generated voltage will fall. Connecting Compound Generators in Parallel To connect two generators in parallel, the voltage of each generator must be exactly the same. This can be regulated by means of a field resistance and is measured by a voltmeter. Line wires of the same polarity must be connected together. An equalizer connection, consisting of a wire that connects the series field of both generators in parallel, is necessary.

The reason for this equalizer connection is that if generator 1 (G1) runs slightly faster than does generator 2 (G2), it will generate more voltage; consequently, more current will flow through the series field and cause the output of G1 to exceed the output of G2. G1 will therefore assume more of the load, and G2, less. As the load on G2 decreases, more of the burden will be placed on G1 until it has taken the full load and G2 is running as a motor. If an equalizer is used, the excess current of G1 is divided between the series fields of both generators and prevents one from assuming more of the load than then other does. Each generator now has equal flux and therefore generates equal voltage. As a consequence, they share the load equally.

POWER LOSSES IN DYNAMOS LOSSSESAFFECTED BY

1. Rotational or stay power

  • a. Friction BearingSpeed changes BrushSpeed changes WindageSpeed changes
  • b. Armature Core HysterieisSpeed & flux changes Eddy CurrentSpeed &flux changes

2. Copper

  • a. Armature windingLoad
  • b. Inter-polesLoad
  • c. Series fieldLoad
  • d. Compensating windingLoad
  • e. Brush contactLoad f. Shunt field Terminal voltage changes

DC MOTORS

Mechanical Power Output of a DC Motor Hp = 2? NT / 33,000;Hp = 2? NT / 44,760 T = torque (ft-lb);T = torque (newton-meter)

Hp = mechanical power output (horsepower) N = speed of armature rotation (rpm) ? Percentage Speed Regulation (%NR) – rise in speed when the mechanical load of the motor is removed. %NR = [ (Nnl – Nfl) / Nfl ] x 100% ? Motor Efficiency = Poutput / Pinput Pout = Pin – Plosses Pin = Vs Im ;where: Vs – supply voltage Im – current drawn from supply %Efficiency = [ (Pin – Plosses) / Pin ] x 100% DC Generator ? Percent Voltage Regulation – percentage rise in terminal voltage when thee generator load is removed. %VR = [ (Vnl – Vfl) / Vfl ] x 100 % ? Power Loss = Pa + Pb + Pcore + Pf + Pfw + Pstray

Where: Pa – armature current loss Pb – brush contact loss Pcore – core loss Pf – field circuit loss Pfw – friction and windage loss Pstray – stray power loss ? Efficiency = Pout / Pin%Efficiency = [ Pl / (Pl + Ploss) ] x 100% Where: Pl – power delivered to the load or output power Pin – input power (Pl + Ploss) Ploss – power losses ? Maximum Efficiency – occurs only when the constant or rotational losses are made equal to the variable losses. Pv = Pa + Pf + Pb;Pk = Pfw + Pcore + Pstray %Max. Efficiency = [ Pl / (Pl + 2Pk) ] x 100% Where: Pv – variable losses Pk – constant losses

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