LESSON

LESSON

2

LECTUREDIRECT CURRENT MOTORS

SUB-OBJECTIVE

At the end of this lesson, the trainee will be able to :

1.List and briefly describe major components of a typical DC motor.

2.State the function of and briefly describe the construction of the brush gear.

3.Define each of the following DC motor variables and describe general methods used for its control : -

a. Torque

b. Starting current

c. Speed

d. Rotation direction

4.Using labeled diagrams, describe connections for each of the following types of DC motors

a. Shunt

b. Series

c. Compound

5.Describe the following characteristics of each of the three types of DC motors:

a. Speed

b. Torque6.State the function of the starting rheostats and briefly describe their operation.

7.Describe construction and operation of a typical DC motor speed controller.

1.0INTRODUCTIONVery few DC motors are fitted in modern Power Plants, mainly due to the economic reasons of having to provide the additional sources of supply and control, over and above the standard AC systems.

However, some items of equipment are so essential to the safe and efficient operation of the plant that, DC motors are fitted to act as, a final safeguard in the event of a total loss of motive power due to failure of the AC systems.

These DC motors operate on a standby basis with the normal drive motor, an AC powered machine, and are normally never used, except in cases of emergency or when the machine is specifically started to prove its operational capability.

2.0DC MOTOR CONSTRUCTION (GENERAL)The main component parts of a DC motor are the stationary outer casing (yoke), the field or magnetizing assembly (poles), and the rotating prime mover (armature).

The mechanical features of the DC motors are similar to the AC machines, comprising motor central body, end shields, bearings and fan assemblies.

Figure. 2.1. shows the basic constructional outline and the path of the magnetic circuit.

Figure 2.1. - Basic construction of DC motor.

2.1Field ConstructionViewed externally, the casing of the DC motor is almost identical to the shell of a similar rated AC motor, with cooling fins running axially along the solid casting steel frame. Internal construction is somewhat different to that of an AC motor, the magnetic field assembly is built up of separate coils or poles.

The poles of a DC machine are either built up from laminated steel sheet (0.4 0.5mm thick) or are solidly cast in steel in the shape of the pole core. These pole assemblies are then mechanically attached to the yoke to form the outer section of the machine's magnetic path. In some highprecision machines, "liners" or thin steel plates are fitted between the pole base and the yoke, in order to adjust the air gap between thepole shoe and the armature.

Figure 2.2. is illustrative of the internal assembly for a typical small/mediumrating machine.

Figure. 2.2. DC Motor Yoke and Pole Assembly

The field coils are then fitted to the poles, the number of coils obviously being dependent on the number of poles and the type of motor assembly. Field coils are wound in similar fashion to AC motor stator winding coils, and in some DC motors the completed coil will be finally covered in insulating tape or fabric.

The diagram in Figure 2.3 shows an assembled field coil for a large motor, fitted to the pole shoe.

Figure 2.3 Completed Coil and Pole Shoe

2.2 Armature ConstructionConstruction of the armature ( rotor) of a DC motor follows the same basic mechanical principles as the rotor of an AC machine. The armature assembly is build up onto machined solid steel shaft. A typical shaft showing machined sections is shown in Figure 2.4.

Figure 2.4 - Machined armature shaft ( typical)The armature is then build up onto the shaft by pressing the punched armature laminations into position and locking the complete core sections by means of the slotted key way. An armature lamination with cooling air passage holes is shown in figure 2.5.

Figure 2.5 - Armature laminationThe armature is then wound with required winding formation and the ends of the winding coils are soldered to the appropriate commutator segment.

The commutator segments, which are manufactured from high quality copper, are assembled and fitted to the shaft in the manner shown in the diagrams in figure 2.6.

Figure 2.6. - Commutator assembly ( typical)

With the fitting of the internal cooling air fan and the bearings to the shaft, the armature assembly is completed, as shown in figure 2.7

Figure 2.7 Outline of armature assembly 2.3Brush GearThe main mechanical difference between the DC motor and AC induction machine is the brush gear assembly fitted to the DC units. The brush gear forms the means of electrically connecting DC supply from the stationary part to the rotating section, i.e. the armature. The minimum number of brush positions will correspond to the number of poles, a 4 pole machine will have a minimum of 4 brush assemblies, and so on.

Figure 2.8 - Brush assembly details ( typical)The brush gear is insulated from the frame and is supported on brush rod assembly. A typical arrangement for a brush assembly shown above in Figure 2.8.

The brushes themselves are manufactured from high grade carbon and are selected on the basis of their compatibility with machine operation. Care must be taken when replacing brushes to use the correct grade of brushes as recommended by the manufacturer.

2.4OPERATING THEORYDC motors are selected on the basis of the load application. The requirement for the DC machines in the plant is for steady speed at constant load as encountered in lubricating oil systems. For this reason most DC motors found in the plant are the standard shunt wound type.

DC motors are rated in voltage, current, speed, and horsepower output.

2.4.1TORQUE

The rotating force produced by the interaction of the magnetic field of the armature ( rotor) and the field poles is known as torque. The greater the magnetic force of the shaft the higher the torque. Since torque is the force exerted upon a shaft, it is necessary to accurately define it as the product of the force in kilograms and the radius of the shaft in meters.

Torque of the motor depends on the magnetic strength of the field and of the armature. Since the armature flux depends on armature current, the torque increases in proportion to the armature current and the strength of magnetic field.

In motor work it is necessary to distinguish between the torque developed by motor when operating at its rated speed (Ifl) and the torque developed at the instant of starting(Ist). Certain types of motors have high running torque but poor starting torque.

2.4.2 STARTING CURRENT

The staring current of a DC motor is much higher than the current input while running freely at its rated speed. At the instant when power is applied, the armature is motionless and only a low armature circuit resistance limits the armature current. As the motor builds up to speed, the current input decreases until motor reaches its rated speed. At this point the armature current remains constant.

The mechanical load applied to the shaft will cause a reduction in speed and in counter emf. The voltage differential however, increases and causes an increased input current to the motor. Thus any increase in mechanical load must be accompanied by an increase in armature current.

Since the starting current may be many times greater than the rated current under full load, it is not permissible to start large DC motors by direct connection to the power. Starters for such motors generally limit the starting inrush current to 1.5 times full load current.

2.4.3 ROTATION

The rotation of a DC motor depends on the direction of the current in the field circuit and in the armature circuit. To reverse the direction of rotation it is necessary to reverse the current direction in either the field or the armature. Reversing power leads will not reverse direction of rotation as both windings are reversed.

2.4.4SPEED CONTROLThe speed of a DC motor can be changed by changing the voltage applied to the motor. This method is not used because the reduction of speed is accompanied by loss in torque.

DC motors are operated below normal speed by reducing the voltage applied to the armature. Resistors connected in series with the armature may be used for voltage reduction.

DC motors are operated above rated speed by reducing the strength of the field flux. A rheostat placed in the field circuit will vary the field circuit resistance, field current and in turn field flux. Reduction in flux reduces counter emf permitting the applied voltage to increase armature current.

Since motor speed increases with decrease in field flux, it is never safe to open field circuit of a motor in operation particularly when running freely without load. Some motors are protected against damaging over speed by disconnecting the motor from the power source if the field circuit opens.

3.0THE SHUNT MOTORThe suitability of a motor for a particular application is determined by two factors:

the variation in its speed with change in load

the variation in its torque with change in load

A shunt motor is essentially a constant speed device. If load is applied the motor tends to slow down, but the reduction in speed also re duces counter emf and results in an increase of armature current. This continues until the increased current produces enough torque to meet demands of the increased load. Thus shunt motor is in equilibrium because a change in load produces a change in armature current that adapts power input to the change in load. The basic circuit for a shunt motor is shown in figure 2.9.

Figure 2.9 - Basic shunt motor connection3.1Characteristics Dc Shunt MotorsA DC shunt motor has excellent speed control. For operation above rated speed a field rheostat is used to reduce the field current and field flux. For operation below rated speed, resistors are used to reduce voltage applied to the armature circuit.

Reversing the direction of current in either the field circuit or the armature circuit can change the direction of rotation.

A DC shunt motor has a high torque at any speed. At start, a DC shunt motor will develop 150% of rated torque if the resistors are used in the starting mechanism. For a very short period it will develop 350% of full load torque if necessary.

The speed regulation of a shunt motor drops from 5% to 10% from no load to full load.

These characteristics are summarized in figure 2.10.

Figure 2.10 - DC shunt motor characteristics4.0THE DC SERIES MOTORThe DC series motor still finds extensive applications despite the wide use of AC for generation and transmission. This type of motor is used as a starter motor in modern automobiles and aircraft. They are also used as traction motors because of their ability to provide high torque with moderate increase in power at reduced speed.

The basic circuit of the series motor is shown in figure 2.11

Figure 2.11 - Basic series motor connection4.1Characteristics Dc Series MotorsA series motor will develop 500% of its full load torque at starting and is used for railroads, cranes, and other heavy starting load applications for this reason. In a series motor any increase in load brings an increase of current in both field and armature circuits. Since torque depends on interaction of these fluxes, the series motor will produce grater torque for the same increase in current but at a greater reduction in speed.

The speed regulation of a series motor is poor. A reduction of mechanical load causes a simultaneous reduction of current in both field and armature and allows for a greater increase in speed than in shunt machines. If the mechanical load is removed completely, the speed increases without limit and destruction of armature through centrifugal forces may occur. For this reason, series motors are permanently connected to their loads.

Varying the applied voltage controls the speed of a series motor. Controllers for this type of motor are usually designed to start, stop, reverse, and regulate the speed.

The direction of rotation may be obtained by changing the direction of current in either the field or armature.

Figure 2.12 shows typical; characteristics of the series motor.

Figure 2.12. - Operating characteristics of a series motor.5.0THE DC COMPOUND MOTORSCompound wound motors are used whenever it is necessary to obtain good speed regulation characteristics not obtainable with either a shunt or series motors. Most have a high starting torque and reasonable constant speed regulation under load.

The compound motor has a normal shunt winding and a series winding on each field pole as shown in fig. 2.13 . When the series winding is connected to aid the shunt winding, the machine is cumulative compound. If the series field opposes the shunt field, it is differential compound.

Figure 2.13 - Basic compound motor connection5.1Characteristics Dc Compound MotorsThe operating characteristics of a cumulative wound compound motor are a combination of the series and shunt motors. When a load is applied, the increasing current through the series winding increases the field flux and causes torque to be greater that it would be for a shunt motor.

However this flux increase causes the speed to decrease to a lower value than in a shunt motor. Unlike a series motor, the cumulative compound machine has no load speed inverse relation and will not build up to destructive speed if the load is removed.

Reducing the applied voltage controls the speed of a cumulative compound motor by use of resistors in the armature circuit.

Changing direction of the current in the armature only reverses the rotation.

5.2Differential Compounded MotorsWhen a motor is connected as a differential compound machine, the series field opposes the shunt filed so that shunt filed is decreased when the load is applied. As a result, the speed remains constant with an increase in load. Since the field is weakened with increase of the load this type of motor has a tendency to speed instability and thus is not often used.

6.0STARTING DC MOTORSTwo factors limit the current taken by a motor armature from a direct current source: -

the counter emf

the armature resistance

Since there is no counter emf at standstill, the current taken by the armature would be abnormally high. Therefore it is necessary to limit the armature current by means of an external resistor. Using a starting rheostat does this.

A typical starting rheostat connected to a shunt motor is shown in figure 2.14. Note that the starter has three terminals or connection points. Because it is housed in a box like structure, it is often called a three TERMINAL STATING BOX.

Figure 2.14. - Three terminal starting rheostatWhen the arm is moved to the first contact A, the armature, in series with all the starting resistors is connected across the source. The shunt field, in series with the holding coil is also connected across the source. The initial inrush current is limited to a safe value by the resistors. Further, the shunt field current is at maximum to provide good starting torque.

As the arm is moved to the right toward contacts B, the starting resistance is reduced and the motor accelerates to the rated speed.

The holding coil is connected in series with the shunt field to provide a no field release. If the shunt field should become opened, the motor speed would become dangerously high if the armature circuit remained connected across the source.

The three terminal starter is not suitable for use with a motor if it is necessary to control the speed. A four, (4) terminal starting rheostat is used to obtain the desired speed. This type of starter is shown in figure 2.15.

Figure 2.15. - A four terminal starting rheostatA motor is started with the four terminal rheostat in the same manner as with a three terminal starter. Varying resistance of the field rheostat, which is in series with the shunt field circuit, controls the speed of the motor.

6.1Speed ControllersIt is often necessary to vary the speed of a DC motor. For speed above normal rating, resistance can be inserted in the shunt field circuit. Below normal speeds can be obtained by inserting resistance in the armature circuit. Although separate controllers are available to get above and below rated speeds, one controller can also be used to obtain both functions as shown in figure 2.16 and 2.17.

One arm is used to make connections with two rows of contacts. The contacts in the lower row are relatively large and are connected to the armature circuit. The contacts of the upper row are smaller and are connected to shunt field circuit. Considerable current passes through the armature resistors when the motor is operating under a heavy load at a low speed. Therefore the armature resistors must be large size and provided with some form of cooling.

Figure 2.16. - Above and below normal speed controller (set for below normal speed)

Figure 2.17. - Above and below normal speed controller ( set for above normal speed)7.0STOPPING OR BRAKING DC MOTORSA DC motor will stop if it is disconnected from the supply. The time it takes to reach standstill will depend on its inertia, its friction and wind-age losses. If fast braking is required, then once the motor is disconnected from supply it is quickly reconnected to a resistor. The inertia generated is dissipated in the connected resistance.

8.0MAINTENANCEThe satisfactory performance of any rotating machine depends to a very large extent on an efficient method of periodical inspection. These inspections will detect any defect and will give warning to correct problem before extensive damage occurs. This will sharply reduce cost of repairs, extent the life of equipment and reduce down time.

Moisture, oil, dirt, grease, carbon or metallic dust etc. are well known causes of electrical breakdowns and therefore machines must be kept clean and dry, both inside and outside at all times. Cooling fins, ventilation holes, and ducts should be kept free of all deposits of dirt and fluff.

8.1Routine InspectionIt is not possible to specify the best interval between inspection and overhauls without experience of the actual operating conditions. The following procedures may be used as a guide until operating experience becomes available. A record should be kept of all faults, replacements and repairs.

a. After 500 operating hours

Examine the commutator. A good commutator should have a shiny skin, which is different from clean raw copper. A good skin should not be treated in any way except that the surface may be wiped with a lint free, moist cloth to remove any carbon dust or foreign material.

b. Every 3,000 hours or every 6 months

Examine the commutator as described previously.

Clean and inspect the brushgear and insulators, particularly on the brush studs.

Check that all bolts and nuts are tight.

8.3Troubleshooting And Repaira. If the motor fails to start when switch is turned ON check for : -

Open fuse

Dirty or clogged brushes

Open armature circuit

Open field circuit

Shorted or grounded field

Shorted armature or commutator

Worn bearing

Grounded brush holder

Overload

Defective controller

b. If the motor runs slowly, the trouble may be : -

Shorted armature or commutator

Worn bearings

Open armature coils

Brushes set off neutral

Overload

Wrong voltage

c. If the motor runs faster than name plate speed, check : -

Open shunt field circuit

Series motor running without a load

Shorted or grounded field

Differential connection in a compound motor

d. If the motor sparks excessively, check : -

Poor brush contact on the commutator

Dirty commutator

Open circuit on the armature

Wrong interpole polarity

Shorted or grounded field

Reverse armature leads

Wrong lead swing

Brushes set off neutral

Open field circuit

High or low bars

Unbalanced armature

e. If the motor is noisy in operation, the trouble may be : -

Worn bearing

High or low bars

Rough commutator

Unbalanced armature

f. If motor runs hot, check : -

Overload

Sparking - see above

Tight bearings

Shorted coils

Too much brush pressure

MOTOR OVERHAULING AND MAINTENANCEELECTRICAL MAINTENANCE

LESSON 2 PAGE 16ADVANCED COURSEELECTRICAL MAINTENANCEMOTOR OVERHAULING AND MAINTENANCE

ADVANCED COURSELESSON 2 PAGE 17