Showing posts with label AC Machine. Show all posts
Showing posts with label AC Machine. Show all posts

Sunday, January 17, 2016

January 17, 2016

What is meant by excitation?

"Self Excited vs Magneto". Licensed under PD-US via Wikipedia
Electric generators work on the principle of electromagnetic induction. The essential part of this principle is the magnetic field. The magnetic field is produced from a d.c. power source from an exciter that is part of the generator system. The main requirement for electricity generation as per the basic principle is a magnetic field. The generator while producing electricity also has to produce the excitation current at a constant voltage for the electrical system to work properly. Controlling the magnetic field controls the voltage output of the generator.
The rotor or field coils in a generator produce the magnetic flux that is essential to the production of the electric power. The rotor is rotating electromagnet that requires a d.c. electric power source to excite the magnetic field. This power comes from an exciter.
Exciter is a device that provides a magnetizing current for the electromagnets in a motor or generator. There are two types of exciter, static exciter and rotory exciter. Rotory exciter is an additional small generator mounted on shaft of main generator. It will supply d.c. voltage to the rotory poles through slip ring and brushes. If it is an a.c. exciter, output of a.c. exciter is rectified by rotating diodes and supply d.c. to main field poles.
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Friday, January 15, 2016

January 15, 2016

What is hunting in synchronous machine?/Phase Swinging/Surging

With the extensive use of synchronous machines, the importance of thoroughly investigating the influence of electrical constants such as resistance and reactance, on hunting is obvious. Hunting is a term used to designate the oscillations of the rotating parts of machines when they are accelerated or decelerated with respect to normal speed. It is essentially a mechanical phenomenon and produces pulsations in the current, voltage and power, due to the variations of angular velocity (due to irregularity of torque) or to the electrical operation of the machines; and if the oscillations exceed a certain amount the regulation of the machines becomes unstable and they fall out of step.

When a synchronous motor is loaded to a varying load, the rotor of the motor falls back by certain angle behind the revolving magnetic field. As the load on the motor is progressively increased, this angle also increases so as to produce necessary torque required to cope up with the load. If the load is suddenly decreased, the motor is immediately pulled up or advanced to a new value corresponding to new load. But in this process the rotor overshoot, hence it is again pulled back. In this way the rotor starts oscillating about its new position of equilibrium corresponding to a new load. If the time period of this oscillation happens to be equal to natural time period of the machine, the mechanical resonance is set up.
Hunting in synchronous machine
The amplitude of these oscillations is built up to a very large value and may eventually become so great that machine is thrown out of the synchronism. This oscillations of rotor about its equilibrium position due to change in load is called hunting. To stop the build up of these oscillations, damper winding are employed which consist of short copper bars embedded in the faces of field pole of the motor. The oscillations of rotor sets up eddy current in the damper winding which flow in such a way so as to suppress the oscillations.
Current variations during hunting
Figure above shows the variation in current during hunting.
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Friday, January 8, 2016

January 08, 2016

Back E.M.F.

What is Back e.m.f. ?

When the armature of a dc motor rotates under the influence of the driving torque, the armature conductor move through the mangetic field and hence e.m.f. is induced in them as in generator. The induced e.m.f. acts in opposite direction to the applied voltage V (by Lenz's Law) and is known as back e.m.f. or counter e.m.f. Eb. The value of back emf depends upon the speed of rotation of armature conductors.

Significance of Back e.m.f.

The presence of back e.m.f. makes the d.c. motor a self regulating machine i.e., it makes the motor to draw as much armature current as is just sufficient to develop the torque required by the load. The back e.m.f. is always less than applied voltage.
Let,
V = Applied voltage
Eb = Back e.m.f.
Ra = Resistance of armature conductor
Then the current in the armature conductor Ia at any instant is given by,
Ia = Net voltage/Resistance
=V-Eb/Ra
OR V = Eb + IaRa 
When the motor is running on no load small torque is required to overcome the friction and windage losses. Therefore, the armature current is Ia small and the back e.m.f. is nearly equal to the applied voltage. If the motor is suddenly loaded the first effect is to cause the armature to slow down. Therefore the speed at which the armature conductors move through the field is reduced and hence the back e.m.f. Eb falls. The decreased back e.m.f. Eb allows larger current to flow through the armature and larger current means increased driving torque. Thus, the driving torque increases as the motor slows down. The motor will stop slowing down when the armature current is just sufficient to produce the increased torque required by the load.

If the load on the motor is suddenly decreased, the driving toque is momentarily in excess of the requirement so that armature is accelerated. As the armature speed increase and causes the armature current Ia to decrease as back e.m.f. increases too. The motor will stop accelerating when the armature current is just sufficient to produce the reduced torque required by the load.

It follows, therefore, that back e.m.f. in d.c. motor regulates the flow of armature current i.e., it automatically changes the armature current to meet the load requirement.
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Thursday, December 31, 2015

Necessity of Connecting Alternator in Parallel
December 31, 2015

Necessity of Connecting Alternator in Parallel

For high efficiency of operation, reliability, convenience and economy in maintenance and repair and possibility of additions to plant with the growth in load on the power station the alternators are put in parallel.
Alternator in parallel
Circuit diagram for connecting alternators in parallel
Photo Credit - www.allaboutcircuits.com

Continuity of supply:-

The continuity of supply is one of the important requirement of any electrical apparatus. If one alternator fails, the continuity of supply can be maintained through other units. This will ensure uninterrupted supply to consumer.

Efficiency:-

The load on the power system varies during whole day, being minimum during the late night hours. Single alternator operate most efficiently only when delivering full load, units can be added or put off depending upon the load requirement. This permit the efficient operation of the power system.

Maintenance and repair:-

It is often desirable to carry out routine maintenance and repair of one or more units. For this purpose, the desired unit can be shut down and the continuity of supply can be maintained through other units.

Load Growth:-

The load demand is increasing due to the increasing use of electrical energy. The load growth can be met by adding more units without disturbing the original installation.
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Wednesday, December 30, 2015

December 30, 2015

Star Delta Starter

If normal supply voltage is applied to the stationary motor then a very large initial current is taken by the stator for short while. This will lead to excess copper losses in the winding which will overheat the motor. This will produce line voltage drop, that in turn, will affect the operation of other electrical equipments connected to the same line and heavy starting current may damage the motor winding. In order to avoid these problem, a starter is used to start the induction motor safely.
Star-Delta Starter
Star-Delta Starter
The motor is designed to operate as delta connected motor under normal conditions. The motor starts as a star connected motor and the voltage per phase is VL/v3. The starting torque is reduced as it is directly proportional to square of stator voltage and there is jerk while switching from star to delta.

Start Mode

The 3 pole 6 way switch is kept in start mode. This will connect terminal R', Y' B' of stator winding to each other. This act as star point. The supply is connected to R,Y,B terminal of the stator winding. Thus in start mode the stator winding is connected to form the star.

Run Mode

The 3 pole 6 way switch is thrown in run position once the motor accelerate. This will connect the terminals in following manner:
R?B', Y?R', B?Y'
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Tuesday, December 29, 2015

December 29, 2015

Capacitor Start Capacitor Run Induction Motor

Capacitor Run Induction Motor
Capacitor Run Induction Motor-Representational Image 
We know that single phase induction motor is not self starting due to the absence of rotating magnetic field at starting. In order to produce rotating field there must be some phase difference. In capacitor start capacitor run induction motor we are using two winding, one main winding and other auxiliary winding. For producing the phase difference two capacitors are used since we know that in capacitor the current leads the applied voltage by some angle. The capacitor serves to shift the phase on one of the windings so that the voltage across the winding is at 90� from the other winding, thus making the capacitor run motor a truly two-phase machine at its rated load.
Capacitor start capacitor run induction motor
Capacitor start capacitor run induction motor
Instead of one if two capacitors are used in the auxiliary winding of the motor, one of them for starting and other for running, optimum starting and running performances can be obtained. Figure shows the schematic  diagram of the motor. The capacitor Cs is cut out when the motor reaches about 75% of the synchronous speed by centrifugal device. The capacitor C and auxiliary winding A retained in the circuit for running condition. A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be two + to + (or - to -) series connected polarized electrolytic capacitors. Such AC rated electrolytic capacitors have such high losses that they can only be used for intermittent duty (1 second on, 60 seconds off) like motor starting. A capacitor for motor running must not be of electrolytic construction, but a lower loss polymer type.

Torque- Speed Characteristics of Capacitor Start Capacitor Run Induction Motor

Capacitor-start/capacitor-run motors have moderate-to-high starting torque compared to other types of single-phase motors. Starting torque generally ranges from 200%-350% of normal full-load torque.
Torque- Speed Characteristic of Capacitor Start Capacitor Run Induction Motor
Torque- Speed Characteristic of Capacitor Start Capacitor Run Induction Motor
Their main advantage over the capacitor-start motor is their lower starting-torque that results in lower starting current.
Because of the lower starting current, they are generally used for most single-phase applications between 3-and-10 horsepower  due to their lower starting current. Common applications include: larger single-phase compressors, pumps, grinders, conveyors, and larger single-phase air-conditioning compressors.
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Sunday, December 27, 2015

December 27, 2015

Universal Motors

A universal motor is defined as a motor which may be operated on either d.c. or single phase a.c. supply at very high speed in the range of 3000 to 7000 rpm or so. It has starting torque and variable speed characteristics. Universal motor are designed for weak field to minimize commutation difficulties. The stator core is laminated and high resistance brushes are used.
Universal Motor
Universal Motor
Photo Credit www.woodgears.ca

Construction

The construction of universal motor is similar to that of DC Machine. The field poles are mounted on stator winding. Field winding is wound on the field poles. Both the stator field poles and armature is laminated to minimize the Eddy current losses while working on a.c. supply. The armature has straight or skewed slots and the commutator and brush arrangement is resting on it. 
Universal Motor
Universal Motor - Construction

Working

Such motors develop unidirectional torque regardless of whether they operate on A.C. or D.C. supply. The motor runs on the same principle as d.c. motor i.e., force between the main pole flux and the current carrying armature conductor. The universal motor produces the electric torque proportional to the quadrate of the supply current. Since the same current flows through the field winding and the armature, it follows that ac reversals from positive to negative, or from negative to positive, will simultaneously affect both the field flux polarity and the current direction through the armature. This means that the direction of the developed torque will remain positive, and rotation will continue in the same direction. Thus, a universal motor can run both on dc and ac. So the electric torque has the same torque direction at any current polarity and in addition at AC power. The starting torque of a universal motor is determined by the current that flows through the armature and field windings. Due to the inductive reactance of these windings the AC starting current will always be less than the DC starting current.Consequently, the starting torque on AC power will be lower than the starting torque on DC power. The characteristics of universal motor are very much similar to those of D.C. series motors, but the series motor develops less torque when operating from an A.C. supply than when working from an equivalent D.C. supply.

Torque-Speed Characteristics

Torque Speed Characteristics of Universal Motors
Torque speed characteristics of Universal Motor

Application

  1. Universal motors are used in most hand held power tools such as jigsaw, routers, drills and sanders.
  2. They also find their use in household appliances like vacuum cleaner, drink and food mixers and sewing machine. 
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Saturday, December 26, 2015

December 26, 2015

Double Cage Induction Motor

Double Cage Induction motor Stator Construction
Double Cage Induction motor Stator Construction

Construction

An induction motor with two cage rotor is used for high starting torque. The slotting arrangement for double cage induction motor is as shown in above figure. As the name indicate the double cage induction motor has two winding in rotor. The outer bars consists of rotor bars having low reactance and high resistance. On the other hand, the inner cage consists of rotor bars having high reactance and low resistance.

Working

At start the rotor frequency is high, the outer cage carries most of the current despite its high resistance. The inner cage has low reactance and is mostly ineffective. This gives high starting torque and low starting current. As the motor picks up the speed, the rotor frequency reduces and the inner cage carries most of the current. Under normal running condition, the outer cage and inner cage are in parallel giving low combined resistance and both the cages are active.
When the speed is normal frequency reduces and it is so small that the reactance of both the cages are practically negligible. Hence it has been made possible to construct a single machine which has high starting torque with reasonable starting current which maintains speed regulation and high efficiency.

Torque-Speed characteristics

Torque Speed Characteristic of Double Cage Induction Motor
Torque Speed Characteristic of Double Cage Induction Motor
Advantages
  1. It has high efficiency and good speed regulation.
  2. It gives higher starting torque.
  3. Lower starting current and are cheaper in cost.
  4. They are more robust and are explosion proof since the risk of sparking is eliminated by the absence of slip ring and brushes.
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Tuesday, December 22, 2015

December 22, 2015

Construction and Working of Induction Motor

Construction:-

An induction motor consists of two main parts
  1. Stator
  2. Rotor
The stationary frame is called stator and the rotating armature is called rotor. The stator of induction motor is similar to that of the synchronous motor or generator. It is made of large number of stampings. These stampings are slotted in order to receive the stator windings. Figure below shows the construction of Induction motor.
Induction Motor
Induction Motor

The functions of various parts is as follows:-

  • Frame:- Its function is to provide mechanical support to the entire construction. The frame also contains the stator winding of Induction motor.
  • Air gap:- Air gap provides the space for the rotating magnetic field between the stator and rotor.
  • Fan:- The fan rotates with the rotor. Its function is to cool down the motor.
  • Slip rings:- The rotor winding terminals are permanently connected to the slip rings(in slip ring type induction motor). The slip rings are continuously in contact with the brushes which are pressed against slip rings. External connections from the brushes are brought out,
Figure below shows the rotor and stator of induction motor.
Stator and Rotor construction of Induction motor
Stator and Rotor construction of Induction motor
Rotor drum is provided with slots. The stator is stationary which can be star connected or delta connected to the 3F AC supply through a switch as shown in figure below. 
Stator winding connections
Stator winding connections
The function of stator winding is to produce a rotating magnetic field in the air gap between the stator and rotor. It should be known that the rotor is connected to any external supply. The current flows through the rotor due to principle of induction.

Working:-

The 3F induction motor has 3F winding which is supplied by 3F alternating voltage and 3F balanced current flow in the winding. The current produce magnetic flux which is constant in magnitude and rotating at synchronous speed. The rotating magnetic filed swift pass the rotor conductors which as yet are stationary. The relative velocity between the magnetic flux and stationary rotor conductors induces emf in the rotor conductors according to Faraday's Law of Electromagnetic Induction. The induced emf is directly proportional to the relative velocity between the magnetic flux and stationary rotor conductors. The frequency of induced emf is same as the supply frequency and the direction of induced emf is given by Fleming's Right Hand Rule.
Since the rotor conductor forms a closed circuit, rotor current produces which opposes the very cause producing it according to Lenz's Law. In this case, the cause which is producing the rotor current is relative velocity between the magnetic flux and stationary rotor conductors. Hence the rotor starts running in the same direction as rotating magnetic field and always try to catch up the the speed of rotating magnetic field.

Note:-For better understanding of the working of Induction motor, refer following videos.


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Wednesday, September 30, 2015

September 30, 2015

Cogging And Crawling of Induction Motor

Induction Motor - Image
Induction Motor
Photo Credit - www.wikipedia.com

Crawling

 It has been found that induction motor practically this squirrel cage type ,sometimes exhibit,a tendency to run stable at speeds as low as 1/7th of their synchronous speed .This is known as crawling of induction motor. This action is due to ,the fact that the AC winding of the stator produces a flux wave which is not pure sine wave .It is a complex wave consisting of a fundamental wave which revolves synchronously and odd harmonics like 3rd,5th,7th etc. which rotate either in the forward or backward direction at Ns/3, Ns/5, Ns/7 speed respectively. As a result in addition to the fundamental torque ,harmonics torques also developed whose synchronous speed for fundamental torque. For Example Ns/n, where N is the order of harmonics torque. Since the 3rd harmonics current are absent in a balanced three phase system ,they produce  no torque .Hence total motor torque has there components.
  1. The fundamental torque rotating with synchronous speed 
  2. Fifth harmonics torque rotating at Ns/5 
  3. Seventh harmonics torque having a speed of Ns/7
If we neglect all the higher  harmonics, the resultant torque can be taken as equal to the sum of the fundamental torque and the seventh harmonics torque. When this happen the motor will not accelerate  up-to its normal speed  but will remain running at a speed which is nearly equal to 1/7th of its full speed. This is referred as a crawling and  motor starts running with unwanted sound .

Cogging

The rotor of particularly squirrel cage  induction motor sometimes refuse to start at all particularly a  when the voltage is low. This happens of stator teeth is equal to the  number of  rotor teeth ,and therefore  due to the magnetic locking or cogging .It is found that the reluctance of magnetic paths  is minimum  when the stator and rotor teeth comes in front of each other, it is in such position of maximum reluctance that the stator tends to remain fixed thus causes serious trouble during starting.

This can be easily overcome by making number of rotor slots more than the number of stator slots and by giving slightly skew to the rotor slots (skewed).What is meant is to arrange the stack of rotor laminations so that the rotor slots are "skewed" or angled with respect to the axis of rotation. Constructing the rotor with skewed slots and providing more (or fewer) rotor slots than stator slots is the remedy for both cogging and crawling.
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