A dedicated website for electrical engineering student. It includes posts on basics of electrical engineering, electrical machines, power system, power electronics. It also has posts on various topics of electrical engineering.
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
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
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.
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
Advantages
It has high efficiency and good speed regulation.
It gives higher starting torque.
Lower starting current and are cheaper in cost.
They are more robust and are explosion proof since the risk of sparking is eliminated by the absence of slip ring and brushes.
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
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
Rotor drum is provided with slots. The stator is stationary which can be star connected or delta connected to the 3Φ AC supply through a switch as shown in figure below.
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 3Φ induction motor has 3Φ winding which is supplied by 3Φ alternating voltage and 3Φ 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.
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.
The fundamental torque rotating with synchronous speed
Fifth harmonics torque rotating at Ns/5
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.
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.
