Complete information on D.C and A.C Electric Motors

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A device for turning electrical energy into rotary mechanical energy. Electric motors are of two types:-

(1) D.C Motors

(2) A.C. Motors.

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(1) D.C. Motors:

D.C. Motors are very similar in construction to D.C. Generators. They work on the same fundamental principle of Electromagnetic induction, that a current carrying conductor experiences a force when placed in a magnetic field.

In D C motors field poles which are, excited by the D.C. current serves as the magnets and conductors are earned by armature on its periphery which is to revolve in its suitable bearings. Carbon brushes are fixed to the current armature coils.

The current generated in the armature coils is alternating in nature so to convert it into D.C. accumulator is provided. Cooling of the motor is done with the help of a ventilating fan provided on the shaft of the armature itself. The armature steel band is provided to keep the armature end turns m place against the centrifuge force.

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Types of D.C. Motors – Following are the types of D. C. Motors-

(A) Series Motor.

(B) Shunt Motor.

(C) Compound Wound Motor.

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This classification is based on the physical construction of the motors. The D.C. motors can also be classified by their following characteristics.

(1) Load-Speed Characteristic,

(i) Constant speed motor. In this type of motors speed remain constant for any load.

(ii) Adjustable speed motor. Speed may be changed on wide range but the speed may be kept constant under varying loads.

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(iii) Variable speed motors. Speed reduces as required torque increases.

(2) Load- Torque characteristic –

(A) Series Motors:

In the series motors the field winding is con­nected in series with the armature winding, the field and armature currents are the same and therefore the torque is proportional to the square of current. Hence at low speed when heavy current is drawn from the mains the torque is very-high.

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The speed of a series motor is uneasily proportional to the current under light load conditions; the motor tends to race up because of weak field.

It is for this reason a series motor should not be run on no load, as the speed will rise so high as to damage the armature. A series field winding consists of heavy conductors and has low resistance as it has to take the full load current.

Series motor is variable speed motor & used where high starting torque is required such as in cranes, hoists, elevators and locomotives.

2. Shunt Motors:

The shunt motor is essentially a constant speed motor and the speed may be adjusted. The motor is suitable for constant speed m/cs such as lathes, grinders etc. The speed regulation is secured by providing a rheostat in the field circuit. The field coils are of large number of turns of thin wire.

D.C. cumulative long shunt If. 6.43 D.C. cumulative long shunt compound motor.

(2) Type of A.C. Motors:

(A) Induction Motor:

How induction motor works?

In induction type Motor, the stator is made of high silicon steel stampings clamped together. Slots are cut in the stator in which windings are placed. In this stator a laminated cored rotor is placed. The rotor slots hold suitable windings which are so connected that it makes close circuits.

When a 3-phase supply is given to the stator, a rotating magnetic field is produced in the stator. This rotation field produces induced e.m.f.s in rotor windings which cause induced currents to circulate.

By Lenz’s law these induced currents tend to oppose the action, producing then and therefore they circulate in such a manner that a torque is produced in the rotor tending to cause it to follow the rotating field and thus reduce the relative motion which is producing the induced current.

There is no electrical connection to the rotor from the 3-phase supply and no other supply is needed. The currents in the rotor are set up entirely by the effect of electromagnetic induction from the stator, hence the name “induction motor”

Keep motor clean & cool

Motor should be varnished if necessary, keep motor property lubricated. Do not wait for motor to squirrel for lack of grand emit smoke from overload.

On the basis of rotor the induction motors can be divided into following four types:

(a) Single Squirrel cage induction motor.

(b) Wound rotor induction motor.

(c) Double squired cage induction motor.

(d) Squired cage deep bar induction motor.

The stator of these induction motors may be the same.

Principle of 3-phase induction motor:

The principle on which induction motor works is that whenever a short-circuited conductor is placed in a roating magnetic field, conductor turns to move.

Main parts of Induction motor:

1. Stator:

This is similar to that of an A.C. generator and is wound for two, four six poles, depending on the speed required. The core is laminated and is provided with slots, usually semi-enclosed.

2. Rotor:

This is built up of laminated iron strips. The winding is placed in slots which are either semi-enclosed or completely enclosed.

(A) Slip Ring Induction Motor:

Slip ring rotor has a three-phase winding which may be star connected. The ends of the phases are brought out to three slip rings on the rotor shaft and are insulated both from each other and from the rotor core.

The starting current is reduced by a rheostat connected to the rotor winding by means of brushes placed upon the slip rings. At starting, the rheostat is brought fully into circuit.

Hence rotor resistance is increased so we get high starting torque. As the motor picks up speed, the rheostat is gradually brought out of circuit until it is short-circuited at normal speed and so we get good running torque also.

Some of the slip ring motors are designed to have brush lifting and ring shorting devices.

(B) Wound Rotor Induction Motor

(C) Double Squirrel Cage Induction Motor:

This type of motor overcomes the disadvantage of a poor starting torque of one cage motor without resorting to wound rotors. In this type of motor rotor have two squirrel cage windings

A low resistance copper cage deeply embedded in iron i.e., deeply in slots. An iron or brass cage nearer the surfaces of the laminated iron core. On starting, the slip speed and hence frequency of induced current is high(may be equal to supply frequency, i.e., 50 c/s ).

The copper cage forms more inductive current, than the iron one as it is completely surrounded by iron. Thus the copper conductors accept only a small current and the motor is starting mainly by the high resistance iron or brass cage. Thus, it gives a higher starting torque.

As the motor gains speed, the frequency of rotor becomes less so that copper cage takes a greater proportion of the current. In this way motor running torque becomes good.

(D) Deep Bar Squirrel Cage Induction Motor:

In such a motor the rotor has deep slots. In these the depth to width ratio is 12 to 1.

At starting the leakage reactance of the conductor at the bottom of the slots is greater because they are linked by greater leakage fluxes, and current in the rotor winding is forced to flow in the top layers in the bars.

The current density in the top layers increase which amounts to a reduction in the cross section of the bars an increase in their effective resistance. As a result the torque increases and the starting current decreases. At normal speed a deep bar motor operates as an ordinary unit.

The Principle of Single-Phase Induction Motor

The principle of working of single phase is same as of 3-phase motor i.e. whenever a short-circuited conductor is placed in a rotating magnetic field, the conductor tends to move.

The single-phase induction motor is similar in construction to the two and three-phase types but the field produced by the stator current instead of rotating is fixed in space, its magnitude changing sinusoidal.

Such a field is equivalent to two field of equal magnitude rotating in opposite directions at equal speeds each being have the maximum value of the alternating field. So the single-phase motor is not self-starting. As the single phase induction motors are not self-started it can be started by any one way of the following methods:

a. Splitting one-phase in two-phase (Split phase motor)

b. Using capacitors (Capacitor Motor).

c. Universal Motors.

d. By shading the poles (Shaded pole motor).

e. By repulsion (Repulsion Motors).

(A) Split Phase Induction Motor:

It has two stator windings i.e. main winding and a starting winding.

The main winding is connected across the supply lines and has low resistance and a high inductance. The starting or auxiliary winding has high resistance and a low inductance and is physically displaced by 90° in the stator from the main winding.

After the motor has accelerated to 75 to 80 percent of its synchronous speed, a transfer or starting switch (centrifugal switch) opens its contacts to disconnect starting winding. Its function is to prevent the motor from drawing excessive currents from the line and also to protect the starting winding from damage due to heating.

(B) Capacitor Start Induction Motor:

This type of motor consists of a capacitor in series with the starting winding which is cut off by the centrifugal switch when the motor attains 75% of full speed. Rotor is squirrel cage wound and its starting torque from a rotating magnetic field produced by two stator windings physically displaced by 90°.

The main winding is connected directly across the line while the starting winding is connected to the line through a condenser, the current thus in each winding would be out of phase, thus motor has high torque. It is used for drilling machine, lathe machine, pump etc.

(c) Permanent capacitor Motor:

In this case there is no centrifugal switch. The starting winding is connected in series with a capacitor remains permanently in series even in running condition.

This type of motor is used only in ceiling and table fans, and where low torque is needed.

(D) Capacitor Start & Run Induction Motor:

This motor has two capacitors; one is switched out after starting and thereby leaving the other capacitor and the main winding in the circuit during running. These are used in refrigerators and compressors.

General Applications of Capacitor Start & Run Motors

These types of motors are suitable where constant speed is required under varying loads such as grinders, drilling machines, blowers, refrigerators and air conditioning apparatus, etc. Starting torque is good, operation is quite, power factor is high and the motor is free from radio interference.

(C) Universal Induction Motor:

The motor is designed to work on both A.C. and D.C. supply. In the universal motor stator winding is in series with motor winding. The stator is built up of laminations instead of a solid piece as in D.C. series motor. When working on A.C. the armature and field current have the same direction with respect to each other.

Hence, the motor works like ordinary series motor. The speed is above the synchronous speed and below the dangerous value whereas working on A.C. the load speed characteristic is similar to that of a D.C. series motor.

The motor is widely used in hair-driers, portable drills, blowers, sewing machines and kitchen appliances, etc. The direction of rotation of this can be changed by interchanging connections to the field with respect to armature as in a d.c. series motor. But speed should not be reversed, as it will give rise to heavy sparking.

(D) Shaded Pole Induction Motor:

It is a single-phase induction motor provided with an auxiliary short-circuited winding, also termed shading coils, consists of a low resistance copper ring embedded on one side of each stator pole and is used to provide the necessary torque. When current increases in the main coils, current is induced in the shading.

When the motor has attained a speed of about two third of synchronous speed a centrifugally operated mechanism will short circuits the commutator segments, thereby converting the armature into the equivalent of a squirrel cage winding- After this switching operation has been automatically performed, the motor runs as a single phase induction motor.

The speed-torque of the motor is that of the repulsion motor during the starting period and after the switching operation. It becomes similar to that, of the induction motor.

The speed of the motor can be varied by changing the brush position, i.e. by moving the rocker on the commutator,

(i) Plain Repulsion Induction Motor:

This is the simplest form. The single phases winding on the stator are fed from the A.C. mains. The rotor is provided with armature winding of D.C. motor type and is connected to a commutator and brushes as shown in Fig. 6.56. The brushes are short circuited together

The motor develops high starting torque, but its speed can be varied as load and direction of rotation can be reversed by moving the brushes with movable rocker. The speed can be controlled within the limits.

The motor is suitable for drives such as drilling M/c and fans etc.

(ii) Compensated Repulsion Induction Motor:

In compensated repulsion motors an additional compensating winding is fitted on the stator. In this motor an ordinary D.C. armature is placed in the stator having two or four poles wound for single-phase supply. The opposite brushes on the commutator are short-circuited through a thick wire.

The speed and load torque of this motor depends upon the angle between the field and brushes, i.e., by varying this angle speed can been controlled.

Properties of Repulsion Induction Motor –

(a) For a given brush setting characteristics are very similar to those of a D.C. series motor.

(b) These have a high starting torque with low starting current.

(c) Speed variation is obtained by shifting of brushes.

(d) Power taken output, so high efficiency is available at each speed and load.

(e) At starting the torque developed and the current taken depend upon the brush position.

(f) At full load speed torque is 1.25 times the full load torque. At higher speed torque is 3 or more times of full load.

(iii) Repulsion Start Induction Run Motor:

Repulsion motor gives high starting torque but on load torque fluctuates and induction motor gives low starting torque but good running torque on load. To take the combined action in one motor, repulsion start induction run motor is made.

In such motor stator, wound for single phase supply, is placed an armature having d.c. armature winding and a short-circuited device called necklace made of copper strips. Necklace works on the principle of centrifugal force.

When the supply is given to stator, this motor starts as repulsion motor giving high starting torque. At 75 to 80% of its speed the necklace short-circuits the armature commutator and motor works as an induction motor.

In some motors brush lifting device is also fitted which uplifts the brushes while the commutator is short-circuited. But it is not essential that brushes shall be lifted since it is clear that when commutator is short-circuited, the brushes have no effect.

Properties

Characteristics are as of series motor.

(a) Starting torque is 3 times the full load torque.

(b) As motor speeds up, the torque falls rapidly.

(c) Starting current 2 to 2.5 times the full load current.

(iii) Repulsion Induction Motor –

In such a motor the rotor has two sets of slots; upper slots contain armature winding, the lower slots squirrel cage winding. Starting is like a single-phase split phase motor having only one winding i.e., main winding on the stator when this is energised with A.C. to supply and rotor is at rest and in the motor the current induce on supply frequency.

As the cage winding is more embedded in iron so it has more reactance. So current taken is less while armature winding takes more current due to having less resistance. Hence, rotor starts as repulsion motor high starting torque.

When the motor speeds up, the motor works on slip frequency and the reactance of cage winding becomes less and motor also works as induction mote and it contributes the induction motor characteristic in repulsion motor action. Therefore, the total torque produced on motor torque + induction motor torque.

Properties:

(a)Its starting torque is 2.2 to 3 times full load torque.

(b)Its starting current is 3.5 times full load current.

(c) Power factor, is good=95% at full load.

(d) Since both windings are in operation effective resistance in parallel winding is less, so have high efficiency.

Uses:

The motor is used only when starting torque required is higher than the capacitor start split phase motor such as household refrigerators, air pumps, gasoline pumps, compressors etc,

Synchronous Motor:

It is actually an alternator runs as a motor It is not self starting and is necessary to run at synchronous speed by some ex e a, means as an alternator and then synchronized with the supply. The field first been excited by a separate D.C. source or by D.C. exciter mounted on the same shaft of machine.

When the motor runs in synchronism the mechanical power applied is withdrawn. The machine will then continue to run as a motor and take current from the supply as long as it keeps in step.

Polyphase A.C. supply is given to the stator winding and rotating magnetic field is produced. Rotor field is produced by D.C. excitation. The torque produced is proportional to the reaction of stator and rotor field. The motor runs at synchronous speed.

Electric Motors Maintenance:

There are four essential steps to proper motor maintenance:

(i) Performing regular and thorough inspections;

(ii) Keeping records of troubles and remedies;

(iii) Building an inventory of reserve units, parts, and subassemblies; and keeping spare units and renewal parts clean, dry, and in good condition.

Check belts for suitable slack & surface condition & check gear for signals uneven of wear examining all flexible couplings.

Chains should clear their housings & be inspected for each sprocket wear across the teath.

Weekly motor starting:

Should be tested to endues that the motor come up to proper speed each time power is applied Monthly inspector include windings brushes, commutation ball or roller bearings sleeve bearing enclosed gears & coupling & drives. ‘

Shunt Series & commutating motors:

Examine the windings for tightness:

Examine brushes in their holders for fit & free play & for brush spring pressure. Periodically the brush studs may have to be tightened to take up any slack caused by drying out of the washers. Brushes worn down to the rivets should be replaced and brush faces that have chipped toes & heads or head cracks should be replaced.

Commutator surfaces:

Check for roughness & scratches.

The risers should be clean.

Bearing housing should be inspected for leaks & the grease for ball or roller bearing should be drained & replaced.

Sleeve bearing:

Examine oil well for evidence of dirt or sludge flush with lights oil & the replace with Lubricant.

Enclosed gears:

Open the drain plugs & check oil flow for metal scale, sand or water. If oil condition is bad drain flushes & refill gear. Yearly inspection- winding air gaps & bearing.

Squirrel cage rotor:

Wound rotor mechanical condition & loads insulation of winding should be checked.

Waste packed and old bearing should be renewed if they become glazed or filled with metal. Rotor should be inspected for broken or loose bars & evidence of local heating. A thorough cleaning around the collector rims, washer & connec­tions. Mechanical condition – belt, chain.

Load rating should be taken at no load & at full load.

The riser should be clean.

Bearing housing should be inspected for leaks & the grease. Ball or roller bearing should be drained & replaced.

Advance notice of trouble is often provided by inspection. Careful inspections are much more valuable than careless routine inspections at more frequent internals. The frequency of inspections varies according to type of motor involved.

(i) Oil gauges on sleeve-bearing motors should be monitored and filled to the line on a weekly basis. Add oil to the bearing housing if needed, but only while the motor is stopped.

(ii) The ball or roller bearings of a motor should be inspected for evidence of vibration, unusual noise, or excessive temperature.

(iii) Weekly examinations of the brushes of a motor should check for evidence of sparking through several cycles of the motor’s operation.

(iv) In addition, examine the commutators or rings for burned spots or roughness. The surfaces of the commutator can be maintained with a “cleaning stick.”

(v) Air passages through the rotor punching also should be inspected weekly for clogs or foreign matter. With the machine stopped and the electricity off, a mild blowing or vacuuming sometimes is necessary. Wipe of dust with a dry cloth, and inspect the rotor and armature frame for any accumulation of water.

If oil or grease has worked its way up to the rotor or armature winding, it should be cleaned off in a well-ventilated room with carbon tetrachloride.

(vi) The mechanical condition of the motor should also be examined weekly.

A properly installed motor requires very little attention to keep it without any trouble in proper working stage. It should be kept clean, dry and properly lubricated at regular intervals. Regular inspection and testing pays trouble free continuous running of the motor at minimum repair cost.

General:

(i) Attention for cleaning of ventilating system should be made to avoid dust and dirt deposit.

(ii) Remove any leakage of oil or grease.

(iii) It should be quite free from moisture and condensation.

Weekly:

(i) By blower remove all dust and dirt by air hose and clean the outside of the machine.

(ii) Any leakage of oil or grease should be removed. Replace fuse oil in case of ring in lubricating machines if needed.

(iii) Inspect the brushes, slip rings or commutator. Remove all dust and clean all contact surfaces.

(iv) Clean the ventilation fan and shield with blower.

Monthly:

A close watch by as experienced engineer may find out any suspicious sound. By strict observations many breakdown and emergency calls may be avoided. In case of need every month inspect the following?

(i) Measure the insulation resistance by merger. Do not put the motor to work unless the insulation resistance exceeds the value required.

(ii) If constructional features convict measure the radial air gaps of general bearing by filler gauge.

(iii) Remove oil dust from the external air circuit of motor fitted with dust extractor.

(iv) Even above cares taken an experience maintenance engineer should take care to the parts of motor dried down, may be avoided.

Annually:

Once in every year the motor should be completely dismantled and each and every part cleaned and inspected. Sometimes annual overhauling is not required or is not even possible. If it is hot or dirty condition, the observation interval may be shorter than a year.

Dismantling:

The actual dismantling is a forward affair and no difficulty is felt by. The ball and roller bearing are contained in cartridge houses. There is no need for these housings to be removed. For satisfactory operation and correct lubrication. Once in three years are normally enough to open any bearing cartridge.

If the ball bearings are not cartridge enclosed, it will be essential to expose them at the time of dismantling of motor. Then it should require essential treat­ment for cartridge closed bearing. Another point to dismantling is that is is nec­essary to remove any internal leads from the terminals, brush gear or both.

Both the leads on the terminals should be marked distinctly to avoid the difficulty in connecting the leads to proper place at the time of reassembling. Hammer blows should not be used. If it is possible to obtain the required movement by gradual means as drawing essential.

It must be employed to soft wood to deed the blows. Furthermore all driving and drawing apart must be carried out with respect to comparative weakness of cast iron under shock loading and bending condition.

General:

1. Dismantle without using any excessive force and without hammer blows if possible, do not open cartridge bearing housings.

2. Clean every part of dust, dirt, oil and grit, use a blower compressed-air force slip rings or commutators or brushes, and wash with petrol as necessary. Complete removal of foreign matter is essential.

3. Check all parts for damage or wear, and repair or replace as necessary. Measure insulation resistance and dry out if necessary until correct is obtained. Repair or replace any damaged windings.

5. Re-enamel or varnish all windings and internal parts except stator box and rotor outer iron surface, dry thoroughly.

6. Reassemble, without using any excessive force, makes sure that the M/c leads are on the correct terminals and that everything is well tightened.

7. Check insulation resistance.

8. Check the air gaps.

9. Put back to work after making all checks and applying all rules as for initial starting.

Cleaning:

All parts should be thoroughly cleaned after complete dismantling and the pressure, dirt, oil or grease should be removed, lately wiped off. Petrol can be use instead of paraffin; the petrol is not harmful to windings. Moreover any other cleaning liquid suggested by the manufacturer may be used for rapid and efficient cleaning.

Inspection:

After all the parts have been changed they should be examined well for any damage due to wear or any other reason. If any fault is observed, steps should be properly taken to rectify it.

Reassembling of winding:

While the inspection is done the winding of the machine should be checked. It should not be burnt out and insulation resistance should not be less than one specified. All exposed winding and insulation should be given good quality coating of varnish or enamel. After enameling it should be dried out.

If the insulation resistance is low the winding should be dried in order to remove any moisture.

Damaged Parts:

They should be immediately removed and new one is provided.

Repair:

Repair work should be done either with the help of manufacturers as they are having all the design data and specified tools and machines or with the supervision as per instructions of the manufacturer.

Putting back to work:

After the above processing’s over, the machine is put back to work. It should be assured that the drive is running freely and the brushes are well set on slip rings or on commutator at the correct position. It is necessary in case of maintenance register giving one or more pages for each machine and records therein all important inspections and maintenance works carried out from time to time.

These records shall show part performance normal insulation level, gap measurements, nature of repairs and time between previous repairs and other important information’s which should be helpful for good performance and maintenance.

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