mod 3 rg 3.12 DC motors and generators

DC motors and generators are energy transfer devices. These machines can function as either motors or generators. DC motors and generators have the same basic construction, differing mainly in the direction of energy conversion. Both devices make use of the phenomenon of electromagnetic induction, where the motion of an electric charge in a magnetic field results in a force being exerted on the charge.

In motors this force is used to turn a rotor, in generators this force is used to drive an electric current around a circuit.

A current carrying conductor within a magnetic field experiences an electromagnetic force that is at

right angles to both the magnetic flux lines and the current.

electromagnetic force is strongest when

the current and flux are also at right angles to each other and falls to zero when the current and flux are parallel.

The DC motor is a machine that transfers energy from the electrical energy store of a power supply to the kinetic energy store of a

turning armature. The movement is caused by the motor effect.

Rather than moving a single wire in one direction (as shown above), an electric motor turns an armature, which is wrapped with many turns of wire, effectively a

rotating coil or inductor. This armature sits within a magnetic field created by either permanent magnets or electromagnets.

The armature is connected to a DC power source through a

split-ring commutator and a pair of brushes

The brushes provide a

continuous connection between the battery leads and the coil by brushing against the commutator ring as it turns. A current flows through the coil.

Applying Fleming’s Left Hand Rule to each side, it is clear that the left hand side of the coil will experience an upward force, while the right hand side of the coil will experience a downwards force. The forces are in

opposite directions because the current is flowing in opposite directions on either side, but the magnetic field is in the same direction on both sides.

The forces cause the armature to

rotate on its axis.

After a quarter of a turn, the splits in the commutator ring align with the brushes, causing

the coil to be momentarily disconnected from the supply.

Inertia carries the armature past this point and the circuit is then reconnected, but the direction of the current in the coil is now reversed. However, the coil is now upside-down so, relative to the magnets, the current is flowing in the same direction as previously, so the coil continues to rotate.

As the coil rotates, the commutator ring and brushes cause a change in polarity each half turn. As a result,

the current always flows in the same direction relative to the magnetic field, so the forces always drive the armature in the same direction.

The graphic below shows the construction of a DC generator. The part of the machine which remains stationary is known and the

‘stator’; the part which rotates is known as the ‘armature’ or ‘rotor’.

The stator consists of:

• A steel ring (yoke).

• Two main poles made of sheet metal with pole core and pole shoe.

• Excitation windings around each pole.

The purpose of the excitation winding is to

generate a stationary magnetic field in the stator.

Machines with a high output will usually also have

interpoles and compensating windings

The armature, also called the rotor, has three parts:

• The shaft, which is fixed to the laminated core of the rotor.

• The windings embedded in grooves in the armature.

• The commutator, which is fixed on the shaft.

In order to increase efficiency, DC generators (and motors) have several separate coils wound around

the armature, so that there is always one coil ideally situated in the magnetic field of the stator magnets.

The individual coils of the armature winding are connected to the commutator segments.

The commutator is made up of

several hard copper segments, separated from each other with mica (as shown in the graphic below). As the rotor turns, the brushes move across the commutator and the polarity of armature coils is reversed as they pass the neutral point.

A current is generated continuously through all three coils in each side of the circuit, except during the short time for which the brush contacts two commutator contacts simultaneously and shorts the coil. At this point the axis of the coil is parallel to the magnetic field and so, there is no EMF across the shorted coil. After each coil turns through the neutral point,

the current generated in it reverses and the whole process repeats.

As the armature turns, the commutator supplies current to the external circuit via a set of sliding contacts, known as brushes These brushes are generally made of

carbon, wear away over time and need to be replaced many times over the life of the generator.

In the DC machines used today, the main field is generated by electromagnets.

The main field winding is in

the stator. The magnetic flux path of the main field is completed by the laminated core of the armature. The armature core is laminated to minimise eddy currents.

Whether in a DC motor or DC generator, each current-carrying conductor in the armature generates a magnetic field, which runs

perpendicular to the magnetic field created by the stator magnets. This has implications for the operation of both motors and generators.

As the field of the armature crosses the main field, it is known as the

“armature cross field”. The higher the current flows in the armature, the stronger the cross field. The main field and the armature field overlap to form one total field.

As it runs perpendicular to the main field, the armature field displaces the neutral zone (imaginary zone in which no induced voltage is generated in the armature). This displacement depends on

the magnitude of the current. The higher the current flow, the greater the displacement of the neutral zone.

The impact of the armature cross field on the main field is referred to as

“armature reaction” This reaction displaces the neutral zone and distorts the main field.

If one considers a motor that is off-load, the main field is distributed symmetrically across the pole shoes. As the load on the motor increases, the current in the armature, and consequently the armature cross field, increases accordingly, causing a

displacement of the neutral zone by a small angle α. In motors the neutral zone is retarded (as shown) but in generators, with the current in the armature windings flowing in the opposite direction, it is advanced.

As the armature turns in the main field, there is some induction in the coils. Electromagnetic induction is highest

under the poles and zero between them.

In the neutral zone, there is an induction-free area; this is where

the carbon brushes, which supply current to the armature, need to be.

In the off-load condition, the coils meet the brushes in the neutral zone and there is no induced current flowing in the coil. As the load on the motor and the armature reaction increase,

the neutral zone moves. This enables the coils to meet the brushes outside the neutral zone, where there can be some induction in the coils, causing sparking at the commutator. This is known as “reactive sparking”.

Brush sparking causes high wear at both the commutator and the carbon brushes. To avoid this wear, the carbon brushes must be

adjusted according to the load, to ensure that they are always located in the neutral zone. However, this is not possible with constantly fluctuating loads. The use of interpoles can cancel out the displacement of the neutral zone.

Interpoles are small poles located between

the main poles

interpoles are connected in

series with the armature and, therefore, oppose the armature cross field with an interpole field of the same size

because of these interpoles, the neutral zone stays in the same place even under fluctuating load. The armature cross field is then cancelled out in the neutral zone. As the interpole field cannot cover the total armature cross field,

an additional compensating winding can be placed in the main poles.

Interpoles and compensating windings are connected in series. Both are supplied with current from the armature. This means that as the load on a motor increases, or the current generated by a generator increases,

so does the strength of the interpoles and compensating windings. By cancelling out the armature cross field, the brush adjustment in the neutral zone is not necessary.

DC generators produce an output voltage which is dependent on three main factors:

• Number of turns on the armature windings

• Rotational speed

• Excitation current in the stator windings

Increasing any of these will also increase the output voltage.

DC generators usually have a stated rotational speed at which they produce a particular output voltage. The current then depends on the

external load

Generally, the more current that is drawn from the generator the

lower the output voltage becomes.

The output voltage of a DC generator can be regulated by varying

excitation current in the stator windings, preventing changes in output voltage due to changes in the load current or rotational speed.

The direction of the output voltage, and therefore, the current flow from a DC generator, depends on

the direction of rotation.

DC generators usually have a direction of rotation marked on the

casing or the main shaft

It is very important to ensure that a DC generator is turning in the correct direction, or

the polarity of the circuit is reversed.

Power, torque (a rotational force), speed, and direction of rotation are important performance parameters for

DC motors

A regular and important specification, the rated Power Output PO is the product of

the motors rated torque T and rotational speed (w)

PO= T X (w)

Generally, output power is defined in terms of Horsepower (hp). To convert a calculated value of power from units of Watts (W) to units of hp, divide the power in watts by

746

Shaft rotation generates torque in the motor. This is the load the motor can generate or handle.

Torque is given in force-distance units (lb-ft, oz-in, N-m, etc.).

Torque specifications generally refer to the

stall torque and the continuous torque.

Stall torque is the at which the shaft speed is

zero, or the motor stalls.

Continuous torque is the maximum at

normal running conditions.

note that the Torque of a DC motor is proportional to

the armature current  this proportionality is expressed as the torque constant k

The following equation describes the relationships between torque and current.

T= k X I

I= T/k

This equation shows the importance of the torque constant. For a given torque, a high value of

limits the current to a low value. This is a measure of efficiency,

as lower current consumption means lower power (heat) dissipation.

the speed control via the armature voltage is used to calculate a speed that ranges from a

standstill to rated speed. This requires the field to have full excitation

A reduction in armature voltage and increase in load decreases the speed. In this mode of operation, the efficiency is hindered by

heat losses in the series-connected resistor. Starting from rest requires a starter that can be set in fine steps.

The thyristor-controlled rectifier provides a different and considerably more efficient method for speed variation. This rectifier can

generate a variable adjustable DC from the supply voltage. This is easy to achieve and almost without loss. However, the excitation winding must be connected to a constant operating voltage.

The diagram below shows that a series-connected resistance in the excitation circuit reduces the excitation current (field weakening). This allows the machine to be operated above its rated speed.However, the field weakening can only be applied to a certain degree, as

the commutator and the armature can be destroyed by centrifugal forces, the excitation current must never be switched off completely. This would cause the machine to infinitely increase its speed (i.e., race). The resistive stress grader method slightly reduces the torque of the machine.

A DC motor’s direction of rotation depends on the directions of the magnetic field and the current flow in the armature. Rotating either of these directions reverses the rotation of the motor. However, reversing both factors at the same time

will keep the motor rotating in the same direction. In practice, the field excitation voltage is reversed to change motor direction.

A motor is typically set up to rotate in a particular way for the job required. However, there are functions that require a change in direction of rotation, e.g., a drive motor for a gun turret or missile launcher. Each of these must be able to

move in both directions.

Remember, to reverse the direction, one must either

change the armature or magnetic field, but not both. In these applications, the proper connections are brought out to a reversing switch.

Series wound DC machines are used as motors. Series wound motors have a very high starting torque. They must never be operated without load because they can

race in no-load operation.

In a series wound machine, the armature winding is connected in series with the excitation winding . This means that the same current flows through

both windings

The current consumption of the series wound machine is particularly high during start-up and must be limited by

the starting resistors.

The series wound machine basically consists of

the stator with the excitation winding, the rotor with the armature winding, and the carbon brushes The commutator and carbon brushes supply current to the armature

The speed depends highly on the load in the series wound DC machine. A change of load results in

a change of current. By increasing the load, the current increases and the speed decreases

During the start-up, and at high loads, the series wound motor

consumes a high current. This process generates a high torque.

A series wound motor must never be operated off-load, as this would increase its speed until the armature is destroyed. In other words,

the armature races at no-load operation.

These characteristics are called series wound characteristics.

Series wound motors are applied as

car starters; in hoists, trains, trams, and electric vehicles; and for windscreen wiper motors or shut-off valves.

An operating series wound motor is also a generator. This means a voltage is induced in the armature winding of the running motor. This voltage opposes the applied voltage (terminal voltage). The terminal voltage and the speed are strongly dependent on the

load

If the current flows in the opposite direction, across the armature and the excitation winding, the series wound generator

reverses its polarity

The series wound generator can only be fully excited if

the total load current (rated current) flows.

Very high loads pose the risk of a short circuit.

The excitation and armature winding are connected in series. The load current I, at the same time, is the

excitation current IE and the armature current IA. The generator can only be fully excited if there is normal load current.

Due to this disadvantage of constant load, the field of application for the series wound generator is

highly limited

The series wound DC machine must be accelerated by a

variable resistor (starter). This ensures slow acceleration.

Nowadays, the starting-up armature voltage for DC machines is increasingly reduced with electronics i.e. with phase angle controls via thyristors. The advantage of this method is that

starting-up armature voltage can be controlled almost without losses

The DC shunt wound motor is the most commonly used DC motor (except in aircraft). It is used whenever

a constant speed at various loads is required, e.g., for machine tools, hoists, and pumps.

A shunt wound DC machine consists primarily of a:

• Stator with excitation windings

• Rotor the armature windings

• Set of carbon brushes

The commutator and carbon brushes supply the current to the armature. The excitation and the armature windings are connected in parallel to the same power supply.

The speed of the shunt wound motor is nearly independent from the load, i.e., the speed only drops slightly at load. motor reaches its maximum speed (which is stable) at no-load operation. These two characteristics are also referred to as

“shunt characteristics”

in a shunt wound motor The current consumption of the excitation winding is exactly the same at no-load operation as it is at full-load operation. The level of armature current depends on

the load on the motor; therefore, a change in load alters the armature current. The armature current is proportional to the torque (see load characteristics of a shunt wound motor).

If a voltage is applied to a field winding and the armature is put into rotation, the machine generates a voltage and works as a generator.

The shunt wound DC machine can be operated as

a self-excited or separately excited generator.

In a separately excited shunt wound generator, the field winding is connected to an external power supply. The size of the exciting current determines the generated armature voltage. If there is a load on the armature,

this reduces the generated armature voltage slightly; however, it can be increased to the desired value by increasing the excitation current, as long as the load is in the permissible range.

In a self excited shunt wound generator, the field winding is connected in parallel to the armature. If the armature is driven from rest, the residual magnetism generates a low voltage in the iron core of the machine.

Because of this voltage,

a current flows in the excitation coils, increasing the strength of the magnetic field, and further increasing the current until a maximum is reached.

Self excitation requires an

existing residual magnetism, correct polarity of the excitation winding, and correct rotation direction of the machine.

A polarity reversal reduces the residual magnetism and prevents the machine from exciting itself. If there is a load, the voltage at the self excited generator

falls more than the voltage at the separately excited generator. The voltage, which heavily depends on the load, is readjusted by a field regulator.

Depending on the type of winding, the shunt series DC machine can either be used as a shunt wound DC machine or as a series wound DC machine. This means that shunt series machines can be used as motors or as generators.

A shunt series DC machine is used if

the starting torque of the shunt wound motor is too low, or the speed drop of the series wound motor is too high.

A motor with a shunt field and a series field is also referred to as a

“compound motor”

The shunt series DC machine is a combination of a shunt wound DC machine and a series wound DC machine. It has two excitation windings:

one connected in series with the armature (series winding); the other connected in parallel with the armature (shunt winding).

The shunt series DC machine consists primarily of:

• A stator with the two excitation windings.

• A rotor with an armature winding.

• Carbon brushes.

in a shunt series (compound) DC motor The commutator and carbon brushes supply current to the armature; thus, the windings are connected so that their excitation fields overlap each other. This overlaying can either

strengthen or weaken the field.

The operational characteristics of the shunt series DC machine can be influenced by

varying the size of the shunt winding and the series winding. Therefore, either the series behaviour or the shunt behaviour of the machine prevails.

At a no-load operation, the shunt series DC machine behaves like a

shunt wound motor, i.e., it does not race. Racing can only occur if both excitation fields fail, or if the two fields cancel each other out.

If the shunt series motor is on load, the speed does not

decrease as much as the series wound motor.

At low speeds, the shunt series motor generates a high-starting torque and behaves like

a series wound motor.

There are two types of electric starting systems for gas turbine aircraft:

direct cranking electrical systems and starter generator systems.

Direct cranking electric starting systems are used mostly on

small turbine engines, such as Auxiliary Power Units (APUs) and some small turboshaft engines.

Many gas turbine aircraft are equipped with starter generator systems which employ an

electric motor to start the internal combustion engine.

Starter generator starting systems are similar to direct cranking electrical systems, except that they

contain a second series of windings that allow them to switch from operating as a motor to operating as a generator after the engine has reached a self-sustaining speed. This saves both weight and space.

The starter generator is permanently engaged with the engine shaft through the necessary drive gears. By contrast, the direct cranking starter needs a way to disengage itself from the shaft

after the engine has started.

The starter generator unit is basically a shunt generator with an additional heavy series winding. This series winding is electrically connected to generate

a strong field and a resulting high torque for starting.

Starter generator units are economically desirable, as they perform the functions of starter and generator within a single unit. This minimises

the total weight of the starting system components and the number of spare parts required

The starter generator internal circuit has four field windings:

• A series field (C field)

• A shunt field

• A compensating field

• An interpole or commutating winding

When starting, the C field uses compensating and commutating windings. The unit is similar to a direct cranking starter, as all the windings used while starting are in series with the source. While acting as a starter, the unit

makes no practical use of its shunt field. Starting usually requires a source of 24 V and 1500 peak amperes.

When operating as a generator, the shunt field, compensating field, and commutating windings are used.

The C field is only used for starting.