COURSE NAME: ELECTRMECHANICAL ENERGY CONVERSION
COURSE CODE: EGYA 3001
INSTRUCTOR: PROFESSOR ADEL ELGAMMAL
BACHELOR OF APPLIED SCIENCE IN UTILITIES ENGINEERING
STUDENT NAME: LINLEY PILGRIM
STUDENT ID #: 50995
DATE SUBMITTED: 31st October 2018
TOC o “1-3” h z u 1.ABSTRACT PAGEREF _Toc527905320 h 32.INTRODUCTION PAGEREF _Toc527905321 h 43.PRINCIPLE OF OPERATION PAGEREF _Toc527905322 h 54.CONSTRUCTION PAGEREF _Toc527905323 h 64.1OUTER FRAME PAGEREF _Toc527905324 h 64.2STATOR CORE PAGEREF _Toc527905325 h 64.3STATOR WINDINGS PAGEREF _Toc527905326 h 64.4ARMATURE PAGEREF _Toc527905327 h 75.EQUIVALENT CIRCUIT PAGEREF _Toc527905328 h 86.EFFICIENCY PAGEREF _Toc527905329 h 96.1Constant Or Fixed Losses PAGEREF _Toc527905330 h 96.2Variable Losses PAGEREF _Toc527905331 h 97.SYNCHRONOUS SPEED PAGEREF _Toc527905332 h 108.SLIP PAGEREF _Toc527905333 h 109.TORQUE PAGEREF _Toc527905334 h 119.1Locked Rotor Or Starting Torque PAGEREF _Toc527905335 h 119.2Break-Down Torque PAGEREF _Toc527905336 h 119.3Pull-Up Torque PAGEREF _Toc527905337 h 119.4Full Load/Braking Torque PAGEREF _Toc527905338 h 1210.SPEED CONTROL PAGEREF _Toc527905339 h 1310.1STATOR SIDE PAGEREF _Toc527905340 h 1310.2ROTOR SIDE PAGEREF _Toc527905341 h 1411.CONCLUSION PAGEREF _Toc527905342 h 1512.REFERENCES PAGEREF _Toc527905343 h 16
ABSTRACTThe purpose of this report is to discuss the principles of operation, construction, equivalent circuit, efficiency, synchronous speed, slip, torque and speed control of the induction motor. The induction motor is an electromechanical device that takes an electrical current created by an induced magnetic field and uses it to produce a mechanical torque. The principles of operation of the induction motor is based on rotating magnetic fields which was first introduced by Francois Arago in 1824 and his experiment was further studied by the physicist Walter Baily who revealed the simple induction motor by using four electromagnets and by manually switching the circuit on and off. The induction motor has two primary electrical parts which are known as the stator (stationary part) and the rotor (rotating part).
The motor output divided by the input will give the efficiency. In synchronous and induction motors the speed is either at the same speed as the stator (synchronous) or slower than the stator (Induction) where in the induction motor, one will see that the magnetic field is changing constantly due to the rotor. The magnetic flux induces a current in the rotor like those found in transformers and base on Lenz’s Law the magnetic field that is produced will oppose the current change in the rotor. Slip is defined as the difference between the synchronous speed and the mechanical rotating operational speed. Torque is the measurement of a rotational force and is measured in Newton-meter(Nm). This is where the armature produces an output torque that is created by the electromagnetic induction which was subsequently induced by magnetic fields created by the alternating current.
The speed control of the motor is carried with the use of a variable frequency drive to vary the input frequency of the alternating current.
INTRODUCTIONThe common parts of the A.C. induction motor is the stator (stationary part) and the armature (rotating part). The invention of the induction motor was first introduced by Francois Arago in 1824. His experiment was proven by using a copper disc and a needle in the form of a circuit to show that a rotating magnetic field exist by manually switching the circuit on and off. The study of his invention went a step further when two other scientist named Charles Babbage and Charles Herschel decided to induce rotation within Arago’s copper disk by rotating a magnet shaped like a horseshoe under the disc. Michael Faraday then contributed towards this invention by using his experiment to show the effects of electromagnetism. In the year 1879, the physicist Walter Baily substituted the horseshoe magnet that was used in Babbage and Herschel’s experiment, with four electromagnets and by manually switching the circuit in an on and off position, he revealed a simple induction motor. This design is still currently used in todays’ society, however there has be continuous improvement to these induction motors in terms of design, dimensions, horsepower, phases.
PRINCIPLE OF OPERATIONAn induction motor consists of two main components which are the armature (rotor) and the stator. The armature is the rotating device and the stator is the stationary device within the motor. The stator has field windings made up of coils which creates poles the alternating current is switched on. The AC induction motor has current supplied to only the stator field windings and the alternating flux is created when an alternating current is supplied to the stator. When this alternating current supply flows through the copper coils a rotational magnetic field (RMF) is produced and revolves at synchronous speed.
An induced electromotive force (emf) is created in the rotor conductors which is caused by the speed between stator rotational magnetic field and the rotor conductors. This produced current is because of short circuiting of the rotor conductors caused by the induced emf.
When the armature rotates, an alternating flux is produced due to the induced current. Now the stator flux is ahead of the armature flux and according to Lenz’s law the direction of induced armature current is such that it will tend to oppose the cause of its production. The result of the induced current in the rotor windings is due to the rotating stator magnetic field, to oppose the change in rotor-winding currents and the rotor will begin rotating in the direction of the rotating stator magnetic field. This rotor will then accelerate until the magnitude of induced rotor current and torque balances the applied mechanical load on the rotation of the rotor and as the speed of the rotor drops below synchronous speed, the rotation rate of the magnetic field in the rotor increases, inducing more current in the windings and creating more torque. The induction motor’s essential character is that it is created solely by induction instead of being separately excited as in synchronous or DC machines or being self-magnetized as in permanent magnet. CONSTRUCTIONSeveral key parts make up an induction motor however these are some of the main components to an induction motor.
OUTER FRAMEThe outer body of the induction motor is the part that houses the stator core as well as supports the other inner components of the machine such as the stator windings and the armature.
STATOR COREThe stator core of the induction motor is to alternate magnetic fields which in turn create hysteresis and eddy current losses. The stator uses a lamination construction which consist of special steel stampings insulated from each other with varnish or paper. This insulation is basically to keep down eddy current losses. The slots on its’ periphery, are punched in these steel stampings which are usually 0.3 to 0.5 mm thick.
STATOR WINDINGSThe windings are spread around in to slots in the stator and with the magnetic field having the same number of poles helps to optimize the field. When an AC current passes through it, a rotating magnetic field is generated. The number of poles determine the speed of the motor. The speed of the motor is obtained by the number of poles used wounded into the stator. More speed requires more poles and less speed requires less poles. The terminals from the windings are brought to the terminal box to be connected to the power supply.
ARMATUREThe armature is the rotating part of the motor and is supplied with current. When the current carrying armature is kept in a magnetic field it will experience a torque and start to rotate. The rotor (armature) is built of thin laminations as the same material as the stator. They are mounted on a cylindrical core then mounted directly on the shaft and they are laminated from each other. The armature’s also generates an electromotive force (EMF).
FIGURE 1: CONSTRUCTION OF INDUCTION MOTOR
EQUIVALENT CIRCUIT OF AN INDUCTION MOTOR
The equivalent circuit is a mathematical model used to display how an induction motor converts electrical input into a mechanical output. It is a single phase representation of a multi-phase inductor motor that is evaluated on steady state load conditions. In the diagram above for the induction motor the following variables represents the different parts that make up the circuit. The resistance in the stator winding is represented as R1; the magnetizing reactance caused by the winding that goes over the air gap is noted by Xm and core losses noted at Rc. The reactance of the stator leakage is represented by X1 and the resistance in the rotor is noted by R2 and X2 notes the reactance. X2 depends on the frequency of the rotor inductor e.m.f. The rotor begins to accelerate and the slip begins to decrease along with the induced e.m.f.
EFFICIENCYThe efficiency of the induction motor is the ratio of mechanical output of the motor over the electrical input of the motor. The efficiency of induction motors can be represented by the following equation and is represented as a percentage.
Efficiency (?)= PoutPin= Pm-Mechanical Losses3VL ILCos?Where Pm= P2- Pc, where Pc = 3I22R2, I22R2 is the rotor current and resistance, Pc is the copper losses, and P2 is the remaining rotor input, and VL IL are the line voltage and current supplied to the stator, and as stated before Cos? is the power factor of the three phase induction motor. The higher the percentage is the more efficient the motor will be, and the smaller the percentage is the less efficient will the motor be.
The induction motors experiences several losses in an induction motor which can affect its’ overall efficiency. The losses can be ether fixed losses or variable losses.
Constant Or Fixed LossesIron or core losses
This is caused by the laminating of the core and areas are decreased by extension resistance increases.
This is caused by the friction in the bearings
Brush friction losses
This is due to the friction caused by the brushes rubbing against the rotor.
Variable LossesThis is a combination of losses in the rotor and stator due to the current flowing through them.
SYNCHRONOUS SPEEDIn synchronous and induction motors the speed is either at the same speed as the stator (synchronous) or slower than the stator (Induction). In the induction motor there is a magnetic field that is constantly changing due to the rotor. According to Lenz’s Law, the magnetic flux induces a current in the rotor and the magnetic field that is produced will oppose the current change in the rotor and this causes the magnetic field to move in the same direction as motion. The actual mechanic speed of the armature may be equivalent to the rotation of the magnetic fields in the stator, when this is so the motor is said to be synchronous. The synchronous speed is a function of the frequency of the electricity used which is typically 60Hz or 50Hz. To find the synchronous speed, ns, the following equation can be used, where p is the number of poles and f is the frequency.
ns=2fpSLIPSlip is the difference between the synchronous speed of the magnetic flux and the operating speed of the rotor all at the same frequency. The slip can be calculated by the formula and is given as a percentage where Ns represent synchronous speed, Nr is rotor speed and S is the slip.
s=ns-nrnsDue to the small resistance in the rotor windings, slip induces a large current which produces a large torque. Also, when the motor is in rotation, slip is at 100 % and the motor current is at max. The current is reduced as the rotor turns, the frequency also decreases, and this causes slip to decrease. Current produced in the rotor, is the corresponding velocity of the rotating stator flux and the rotor. Therefore, the rotor will attempt to match the speed of the stator rotating magnetic field.
TORQUEThe force created as the motor turns is known as the torque and this torque is a mechanical force produced by the rotor. When the motor is switched on and starts to accelerate from rest to operating speed, the level of torque varies. The torque is created due to electromagnetic induction from the stator windings or coil an induced force is created on loop causing rotation. A current flow is created in the rotor and the current in the rotor will create a magnetic field around the rotor. The magnetic field of the stator passes over the bars of the rotor resulting in an induced voltage in the bars which is represented by the equation: e.m.f.ind = (v x B) L.
This interaction of the magnetic field in the rotor and in the stator a torque will produce a torque which is represented by ind = kBr x Bs, where ind is the induced torque, Br is the rotor magnetic field and Bs id the stator magnetic field.
When a three phase AC current flows into the stator, a magnetic field will be generated in the stator and this magnetic field will have a rotational speed, given by:
Nsyn=120FePWhere Fe is the system frequency in hertz and P is the number of poles in the machine.
Standard torque of a motor can be categorized as the following: locked rotor or starting torque, break-down torque, pull-up torque and full-load braking torque.
Locked Rotor Or Starting TorqueLocked rotor or starting torque is the torque developed when the induction motor starts from rest. This is usually between 75 to 275% of the rated torque of the induction motor.
Break-Down TorqueThis is the highest available torque output from the induction motor before it starts to decrease due to acceleration to its working condition. This is usually ranged between 175% to 300% of the rated torque of the induction motor.
Pull-Up TorqueThis is the minimum torque produced in a motor as it runs from rest to full speed load. This type of torque occurs just before it reaches its’ break-down torque. The motor starts at high torque and the motor begins to accelerate this torque will start to decrease. This pull-up torque is 65 to 190% of the induction motor rated torque.
Full Load/Braking Torque454025110998000The motor requires a torque that will equal the rated power of the motor at full load speed. That is called the Full load/Braking Torque. In other words, the full-load torque is the torque required to produce the rated power of the induction motor at full speed.
FIGURE 3: TORQUE CURVE OF AN INDUCTION MOTOR
SPEED CONTROLThere are various methods of speed control of induction motor. In fact, these methods can be categorized based on the side i.e. stator or rotor side which is used for speed control and these methods are:
Stator Side Speed Control method:
Pole Changing MethodStator Voltage ControlVariable Voltage Variable Frequency Speed ControlRotor circuit side for speed control:
Cascade Operation of Induction Motor
E.m.f. Injection in Rotor Circuit
Rotor Circuit Resistance Control MethodSTATOR SIDEPole Changing Method
Pole changing method is a speed control system is where the number of pole pairs is changed by changing the field connection. This method is used mainly for cage motor only because the cage rotor automatically develops a number of poles, which is equal to the poles of the stator winding. The field winding in the induction motor is wounded on the stator while the armature winding is wounded on the rotor.
Stator Voltage Control
Stator Voltage Control of Induction Motor is a method for the speed control of induction motor by changing the stator terminal voltage. Stator terminal voltage can be changed one of the following ways, which are by either by adding a series resistor, series inductor or an auto transformer in stator circuit. Series resistance tends to be somewhat inefficient due to losses in the resistor. The modern way to acquire stator voltage control is by the use of solid state semiconductor devices such as a AC voltage controller or AC chopper.
Variable Voltage Variable Frequency Speed Control
In a variable voltage, variable frequency operation, any combination of voltage and frequency can be used to supply the motor once the operation stays within the parameters of the rated voltage and frequency.
ROTOR SIDE Cascade Operation Of Induction Motor
In this method of speed control of three phase induction motor, the two three-phase induction motors are connected on a common shaft and hence called cascaded motor.. One motor is the main and is of a slip ring type motor and the other is the auxiliary motor. Three phase power supply is connected to the stator of the main motor and the auxiliary motor supply comes from the slip frequency of the slip rings in the main motor.
E.M.F. Injection in Rotor Circuit
When Injecting induced e.m.f in the rotor circuit, the speed can be easily controlled. The speed control of three phase induction motor is achieved by adding resistance in rotor circuit, and the slip power is lost and the efficiency is somewhat reduced. The slip power loss can be recovered and supplied back to improve the overall efficiency of the three-phase induction motor by connecting an external source of e.m.f of slip frequency to the rotor circuit and this injected e.m.f will either oppose the rotor induced e.m.f or aids the rotor induced e.m.f. When it opposes the rotor induced e.m.f, the total rotor resistance subsequently increases and result in a decrease in the speed. When the injected e.m.f aids the main rotor e.m.f the total resistance decreases and the speed subsequently increases.
Rotor Circuit Resistance Control Method
By increasing rotor resistance, the torque subsequently decreases. When slip increases this will result in decrease of the rotor speed. Therefore the addition of more resistance in the rotor circuit will result in decrease of the three-phase induction motor speed. The main advantage of this method is the start torque increases with addition of external resistance. The disadvantages of this method are:
The speed above the normal value is not possible.
Large speed change requires a large value of resistance and this will cause losses and reduction in efficiency.
This method cannot be used for squirrel cage induction motor.
CONCLUSIONThe induction motor is an AC motor in which the electrical current in the rotor is produced by an electromagnetic induction from the magnetic field generated in the stator winding which produces a torque. In this report we discussed the principles of operation, construction, the equivalent circuit, its’ efficiency and synchronous speed, slip, torque and finally the speed control. We saw that from its inception, that the induction motor has improved in its design and operational functions using simple factors to achieve the desired end results. The induction motor is said to be one of the simplest and most cost effective operating systems which can be used anywhere and varies in many different sizes. The purpose of this report was to give an understanding and overview of the internal components and operation of the induction motor
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