Classification or Types of Motor 2 Overview of Three-Phase Induction Motor Induction motors are used worldwide in many residential, commercial, industrial, and utility applications. Induction Motors transform electrical energy into mechanical energy. It can be part of a pump or fan, or connected to some other form of mechanical equipment such as a conveyor, or mixer. Introduction • A induction machine can be used as e i t h e r a i n d u c t i o n g e n e r a t o r o r a induction motor. • Induction motors are popularly used in the industry • M a i n f e a t u r e s : c h e a p a n d l o w maintenance • Main disadvantages: speed control is not easy Induction Motor 5 Construction The three basic parts of an AC motor are the rotor, stator, and enclosure. The stator and the rotor are electrical circuits that perform as electromagnets. Construction Parts of Induction motor 8 The other parts, which are required to complete the induction motor, are: • Shaft for transmitting the torque to the load. This shaft is made up of steel. • Bearings for supporting the rotating shaft. • One of the problems with electrical motor is the production of heat during its rotation. To overcome this problem, we need a fan for cooling. • For receiving external electrical connection Terminal box is needed. • There is a small distance between rotor and stator which usually varies from 0.4 mm to 4 mm. Such a distance is called air gap. Constructio n An induction motor has two main parts 1. Stator 2. Rotor Stat or Bearings Frame Stator Core Rotor Stator winding Rotor winding Construction Enclosure ( ) The enclosure consists of a frame (or yoke) and two end brackets (or bearing housings). The stator is mounted inside the frame. Stator Rotor Air gap Construction (Stator construction) The stator is the The stator core is made up of . Stator laminations are forming a . Electromagnetism is the principle behind motor operation. , together with the steel core it surrounds, form an electromagnet. The stator windings are connected directly to the power source. Stator Frame 13 It is the outer most part of the three phase induction motor. Its main function is to support the stator core and the field winding. It acts as a covering and it provide protection and mechanical strength to all the inner parts of the induction motor. The frame is either made up of die cast or fabricated steel. Stator Core 14 The main function of the stator core is to carry the alternating flux. In order to reduce the eddy current loss, the stator core is laminated. These laminated types of structure are made up of stamping which is about 0.4 to 0.5 mm thick. All the stamping are stamped together to form stator core, which is then housed in stator frame. The stamping is generally made up of silicon steel, which helps to reduce the hysteresis loss occurring in motor. Stator Winding or Field Winding 16 The slots on the periphery of stator core of the three phase induction motor carries three phase windings This three phase winding is supplied by three phase ac supply. The three phases of the winding are connected either in star or delta depending upon which type of starting method is used. The winding wound on the stator of three phase induction motor is also called field winding and when this winding is excited by three phase ac supply it produces a rotating magnetic field. Stat or Rotor Types: 1. Squirrel-cage rotor 2. wound-rotor Rotor (Laminations) Construction (Rotor construction) Induction motor types: Rotor winding is composed of copper bars embedded in the rotor slots and shorted at both end by end rings Simple, low cost, robust, low maintenance Rotor winding is wound by wires. The winding terminals can be connected to external circuits through slip rings and brushes. Easy to control speed, more expensive. Squirrel-cage rotor Squirrel Cage Rotor Squirrel Cage Rotor 23 The rotor of the squirrel cage three phase induction motor is cylindrical in shape and have slots on its periphery. The slots are not made parallel to each other but are bit skewed (skewing is not shown in the figure of squirrel cadge rotor beside) as the skewing prevents magnetic locking of stator and rotor teeth and makes the working of motor more smooth and quieter. Squirrel Cage Rotor 24 The squirrel cage rotor consists of aluminum, brass or copper bars (copper bras rotor is shown in the figure beside). These aluminum, brass or copper bars are called rotor conductors and are placed in the slots on the periphery of the rotor. The rotor conductors are permanently shorted by the copper or aluminum rings called the end rings. In order to provide mechanical strength these rotor conductor are braced to the end ring and hence form a complete closed circuit resembling like a cage and hence got its name as "squirrel cage induction motor". Advantages of squirrel cage induction rotor 25 Its construction is very simple and rugged. As there are no brushes and slip ring, these motors requires less maintenance. Applications: Squirrel cage induction motor is used in lathes, drilling machine, fan, blower printing machines etc Construction (Rotor construction) Squirrel-Cage Rotor Wound-rotor(Slip Ring) Wound-rotor Motor 28 In this type of three phase induction motor the rotor is wound for the same number of poles as that of stator but it has less number of slots and has less turns per phase of a heavier conductor. The rotor also carries star or delta winding similar to that of stator winding. The rotor consists of numbers of slots and rotor winding are placed inside these slots. The three end terminals are connected together to form star connection. Wound-rotor Motor 29 Wound-rotor Motor 30 The three ends of three phase windings are permanently connected to these slip rings. The external resistance can be easily connected through the brushes and slip rings and hence used for speed control and improving the starting torque of three phase induction motor. The brushes are used to carry current to and from the rotor winding. Applications 31 Application of Slip Ring Induction Motor Slip ring induction motor are used where high starting torque is required i.e in hoists, cranes, elevator etc. Applications of Squirrel Cage Induction Rotor We use the squirrel cage induction motors in lathes, drilling machine, fan, blower printing machines, etc 32 Slip ring or phase wound Squirrel cage induction motor Induction motor Construction is complicated due to presence of slip ring and Construction is very simple brushes The rotor consists of rotor bars The rotor winding is similar to which are permanently shorted the stator winding with the help of end rings Since the rotor bars are We can easily add rotor permanently shorted, its not resistance by using slip ring and possible to add external brushes resistance Due to presence of external Staring torque is low and resistance high starting torque cannot be improved can be obtained Slip ring and brushes are Slip ring and brushes are present absent Difference between Slip Ring and Squirrel Cage Induction Motor 33 Slip ring or phase wound Squirrel cage induction motor Induction motor The construction is complicated The construction is simple and and the presence of brushes robust and it is cheap as and slip ring makes the motor compared to slip ring induction more costly motor This motor is rarely used only Due to its simple construction 10% industry uses slip ring and low cost. The squirrel cage induction motor induction motor is widely used Rotor copper losses are high Less rotor copper losses and and hence less efficiency hence high efficiency Speed control by rotor Speed control by rotor resistance method is not resistance method is possible possible Slip ring induction motor are Squirrel cage induction motor is used where high starting torque used in lathes, drilling machine, is required i.e in hoists, cranes, fan, blower printing machines elevator etc etc Rotating Magnetic Field When a 3 phase stator winding is to a 3 phase voltage supply, 3 phase current will , which also will 3 phase flux in the stator. These flux will rotate at a speed called a . The flux is called as Rotating magnetic Field Synchronous speed1: s2p0efed of rotating flux n = s p Where; p = is the number of poles, and f = the frequency of supply Rotating Magnetic Field 35 When we apply a three-phase supply to a three-phase distributed winding of a rotating machine, a rotating magnetic field is produced which rotates in synchronous speed. 36 The magnetic flux produced by the current in each phase can be represented by the equations given below. 37 38 Principle of operation Three phase windings of stator are connected to three phase supply, so three phase magnetic fluxes are produced. Due to combination of three phase fluxes rotating magnetic flux is generated. This rotating magnetic field cuts the rotor windings and produces an induced voltage in the rotor windings. Due to the fact that the rotor windings are short circuited, for both squirrel cage and wound-rotor, and induced current flows in the rotor windings. The rotor current produces another magnetic field. A torque is produced as a result of the interaction of those two magnetic fields t ind =kBR ´ Bs Where ind is the induced torque and BR and BS are the magnetic flux densities of the rotor and the stator respectively Induction motor speed At what speed will the IM run? Can the IM run at the synchronous speed, why? If rotor runs at the synchronous speed, which is the same speed of the rotating magnetic field, then the rotor will appear stationary to the rotating magnetic field and the rotating magnetic field will not cut the rotor.