Showing posts with label Induction Motor. Show all posts
Showing posts with label Induction Motor. Show all posts

Sunday, December 30, 2018

December 30, 2018

Motor Starter, Applications and Advantages

If you an electrical engineering student then you must hear about the motor starter. As an electrical engineer, you should know all types of starter and their applications, advantages.

What is Motor Starter

An electrical motor starter is a device or can be called a circuit which is to be connected in series with the motor to reduce the starting current and maintain the starting speed of the motor and also to protect the motor from overload, short circuit faults.
An electrical motor starter can be used alone or with the Contactor, circuit breaker according to requirement.
We also can say a motor starter is a device which is used to start and stop a motor safely.
The main principle of a motor starter is that it reduces the terminal voltage and current of the motor during starting time.

DC Motor Starter Types, Applications and Advantages

DC Motor Starter, types of motor starter

The starter is used for DC motors is called DC Motor Starter. To safely start and stop the DC Motors DC Motor starters are used. DC Motor starter does not only start and stop the motor safely they also protect the motor from over voltage and short circuit faults. DC motor starters very simple in construction and they are cheaper than AC Motor Starter.
Why we need a Starter for starting of DC Motor
In DC motor there is no principle of induction exists. Back EMF is very important for running of a DC Motor. As the polarity of back emf is opposite to the supply voltage, therefore, the back emf can control the armature current.
When the motor is in off condition there is no back emf in the armature winding. And we know that the resistance of the armature winding also very low. Now if we connect the motor to the full supply voltage a huge amount of current will flow through the armature winding. It can be shown by the equation
Ia=(V-Eb)/Ra
You can see the back emf (Eb) is Zero and the armature resistance (Ra) is also very low so the value of armature current (Ia) will be very high.
This huge amount of armature current may causes,
  1. Burning of the armature winding
  2. Damage the commutator and brushes
  3. A large voltage fluctuation in supply voltage which can damage other equipment.
So for these reasons to reduce the starting current Starter is used with DC Motors.

Types DC Motor Starter

There are three types of DC Motor Starter,
  1. Two-point Starter
  2. Three-point Starter
  3. Four-point Starter

Applications of Two-point Starter

  1. Two-point starters are used with DC series motors.
  2. Two-point starters are used in railway to start or stop the rail.
  3. Two-point starters are used in cranes.

Advantages of Two-point Starter

  1. It protects the motor from drawing high starting current.
  2. It can give protection against overload and short circuit faults.
  3. It automatically gets off when the power supply is off.

Applications of Three-point Starter

  1. The three-point starter is used with DC Shunt motors
  2. Three-point starters are used in lathe factories
  3. Three-point starters are used in spinning mills

Advantages of Three-point Starter

  1. Three-point starter gives the protection to the motor from overload and short circuit faults.
  2. The three-point starter helps to speed control of the motor by adding some extra equipment.

Disadvantages of Three-point starter

The main disadvantage of the three-point starter is that it is not suitable for variable speed motors.

Application of Four-point Starter

  1. The four-point starter is used with mainly DC compound motors but it can be used with DC shunt motors.
  2. Four-point starters are used in press industries.
  3. Four-point starters are used in rolling mills

Advantages of Four-point Starter

  1. The four-point starter is suitable for variable speed motors.

AC Motor Starter Types, Applications and Advantages

AC Motor Starter, types of motor starter



The starter used for AC motors is called AC Motor Starter. Like DC motor AC motor also need the starter for safely starting. Induction motors are widely used in any industry. Induction motor need starter for safely staring. So we will discuss the starter of induction motors.

Why we need Starter for starting of Induction Motor

We know that the Induction Motor is acting as a transformer with the secondary winding short-circuited during off condition.

We know that, I1N1=I2N2

This equation is the same for the transformer and the Induction motor at standstill condition

Where I1= primary winding current or stator winding current

           N1 = no. of turns in primary winding or stator winding
           I2 = Secondary winding current or rotor winding current
           N2= no. of turns in secondary winding or rotor winding

So at standstill condition of the motor if we connect the motor directly to the full supply voltage a high-value emf will be induced in the rotor of the induction motor for the transformer action. This high-value emf will cause to flow a high current(I2) in the rotor winding. A huge amount of current also will be flow in the stator winding to balance the equation, I1N1=I2N2

This high current in stator winding may,

  1. Damage the stator winding.
  2. Create a large fluctuation in supply voltage which may damage others equipment connected in the same line.
For these reasons, to reduce the magnitude of supply voltage and current at starting period Starter is used with the Induction motor.

Types of AC Motor Starter

  1. Direct-on-line Starter or DOL Starter
  2. Stator resistance starter
  3. Autotransformer Starter
  4. Star-Delta Starter
  5. Rotor Resistance Starter

Applications of Direct-on-line Starter or DOL Starter

  1. DOL starters are used with up to 7.5 KW Induction Motors.
  2. DOL starter acts as normally ON/OFF switch so it is used where frequently start and stop of the motor is required.
  3. DOL starters are used in conveyor belt and compressor.

Advantages of Direct-on-line Starter or DOL Starter

  1. It is very simple in construction.
  2. It is cheaper than other starters
  3. It can give overload and short-circuit protection.

Disadvantages of Direct-on-line Starter or DOL Starter

  1. It can not be used with more than 7.5 kW motors.
  2. It can not reduce the magnitude of starting current of the motor acts only normal ON/OFF switch.

Applications of Stator Resistance Starter

  1. This types of starter are suitable for more than 7.5 KW rating motors.

Advantages of Stator Resistance Starter

  1. It is very simple in construction.
  2. It reduces the terminal voltage of the motor during the starting time hence the starting current also decreases.
  3. It is also a low-cost starter.

Disadvantages of Stator Resistance Starter

  1. It lowers the starting torque of the motor.
  2. It increases the accelerating time of the motor
  3. A huge amount of power loss in by the starting resistance.

Advantages of Auto-Transformer Starter

  1. Simple operation
  2. Very low power loss as it has no resistance
  3. Easy to control

Disadvantages of Auto-Transformer Starter

  1.  It lowers the starting torque of the motor
  2. There is a chance of sparking.

Applications of Star-Delta Starter

  1. It used in heavy industries for starting high KW motors.

Advantages of Star-Delta Starter

  1. Star-delta starter can be controlled automatically by using electronic circuits
  2. Low power loss
  3. Smooth operation
  4. Starting current is reduced to one-third of full load current

Disadvantages of Star-Delta Starter

  1. Low starting torque
  2. Six terminals motor required.

Application of Rotor Resistance starter

  1. These starters are used with slip-ring induction motors.

Advantages of Rotor Resistance starter

  1. It can not decrease starting torque.
  2. Smooth speed control is possible.

Solid State Starter or Soft Starter

Nowadays Solid State Starters are widely used instead of the magnetic starter. in industries. Solid state starter has no moving elements. These Starters uses the Silicon Controlled Rectifier for their operation.

Advantages of Solid State Starter or Soft Starter

  1. These starters eliminate the power distribution issues in industries.
  2. These starters can handle multiple motors at a time.
  3. These starters decrease the acceleration time of the motor
  4. Very low power loss.

Applications of Solid State Starter or Soft Starter

  1. These starters are used in industry for the operation of Conveyor belt motors.
  2. It used with blowers to increase the accel time.
  3. These Starters are used with pumps also.
0
By at No comments:
Labels: , , ,

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.
0
By at No comments:
Labels: ,

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.
0
By at No comments:
Labels: ,

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.


0
By at No comments:
Labels: ,

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.
    0
    By at No comments:
    Labels: , ,

    Tuesday, September 15, 2015

    September 15, 2015

    Torque Speed Characteristics Of Double Cage Induction Motor

    Torque speed characteristics of Double cage induction motor - image
    Torque speed characteristics of Double cage induction motor
    The main disadvantage of squirrel cage motor is poor starting torque  because of low rotor resistance .The starting torque could be increased by having a cage of  high resistance but then the motor will have poor efficiency under normal running conditions.The difficulty with squirrel cage induction  motor is that its cage is permanently short circuited ,so no external resistance can be introduced . Many efforts have been made to build a squirrel motor with high starting torque without scarifying its electrical efficiency under normal running conditions ,which cannot be possible easily .Two  independent cages on the rotor was tried and a motor called as double cage motor is invented .The outer cage consist of bare of high resistance where as the inner cage has low resistance .Hence outer cage has high resistance and low ratio reactance to reactance where as the inner cage has low resistance but being situated at deep in the rotor .has large ratio of reactance to reactance .Hence the outer cage develops maximum torque at starting while the inner cage about 15%  slip .At starting and at large slip values ,frequency of induced emf in the rotor is high ,so the reactance of inner cage and therefore its impedance are both high .Hence very little current flows in it . Most of starting current is confused to outer cage ,despite its high resistance .Hence the motor develops high starting torque due to high resistance outer cage .
                                                   As the speed increase the frequency of rotor emf decreases ,so that the reactance and hence the impedance of inner cage decreases and becomes very small under normal running conditions . Most of the current then flows through it and hence it develops the grater part of the motor torque .When speed is normal ,frequency of the rotor emf is so small that the reactance of both cages is practically negligible .The current is carried by two cages in parallel giving a low combined resistance .Hence it has been made possible to construct a  single machine which has a good starting torque with reasonable starting current and which maintain high efficiency with good speed regulation under operating conditions.      
    0
    By at No comments:
    Labels: ,