In the equation of torque, the rotor resistance, rotor inductive reactance and. During motorstarting, the voltage level at the motor terminals should be maintained. Starting of an induction motor a three phase induction motor is self starting. Design a is a generalpurpose design used for industrial motors. We are going to study the starting period of induction motor when its connected with different starter at the time of starting. Starting of 3 phase induction motors, high starting torque motors, speed control, rheostatic method, pole changing method, cascade control of speed, double cage induction motor, cogging an d crawling of induction motor, induction generator. Torque equation of an induction motor starting torque. Induction motor starting methods bright hub engineering. Starting methods for singlephase induction motor nptel. Introduction to motor starting analysis ced engineering. Induction motors are considered nowadays the most commonly used electrical machines, which are mainly used as electrical induction motors and sometimes as generators. For the starting pulse vibration torque impact of asynchronous motors on the mechanical. You should also make a dc measurement of stator winding linetoline resistance using a dmm. Higher maintenance of brushes and slip rings compared to squirrel cage motor.
The maximum generated torque ranges from 250% to 350% of the rated torque see figure 9 for torquespeed curve. Nov 18, 2015 induction motor torquespeed characteristics figure 6. This design exhibits normal torques and fullload slip of approximately 3 percent and can be used for many types of. The motor is mounted in a test rig that allows one to control the torque applied to the motor shaft and to measure both rpm and torque. The advantages of threephase ac induction motor are listed below. A high starting torque induction motor with simultaneously reduced starting current adapted to be associated with a drive having a high load torque on starting that includes a stator, a rotor, a first end ring fixed on said rotor, a second movable end ring associated with said rotor and having a lower electrical resistance than the first ring, and a centrifugal device associated with the. Wound rotor induction motors ac motors electronics. The characteristics of an induction motor, the speed, current, starting torque, maximum torque, the motor losses and efficiency can all be determined from an analysis of the equivalent circuit. We know that for the working of any electric motor whether its ac or dc motor, we require two fluxes, as the interaction of these two fluxes produced the required torque, which is desired parameter for any motor to. Induction motor the starting torque is low, typically 100% to 175% of the rated torque. Starting torque is the torque produced by induction motor when it starts. Wound rotor induction motors ac motors electronics textbook.
Torque equation of three phase induction motor electrical. Usually on a woundrotor induction motor, the external resistance is optimized for initial starting torque and then the resistor is shorted for optimal steady state operation. The developed torque define as ratio of the mechanical power developed by rotor to the mechanical angular velocity of the rotor. An induction motor consists essentially of two main parts. The motor draws high starting current, approximately 700% to 1,000% of the rated current. A high starting torque is more important for application or machines. A 230v, 6pole, 3f, 50hz, 15kw induction motor drives a constant torque load at rated frequency, rated voltage and rated kw output and has a speed of 980 rpm and an efficiency of 93%.
The reactance actually causes the induced voltage and current, and generated magnetic field to be out of phase with the stator field, generally lagging by about 90 degrees, so the magnetic interaction between the rotor and stator. The national electrical manufacturers association nema provides design letters to indicate the torque, slip, and starting characteristics of threephase induction motors they are as follows. The starting torque is obtained by putting s 1 in equation hence, it is clear from the above equation that the starting torque is proportional to the square of the stator applied voltage. Its starting torque is somewhat inferior to that of a d. A squirrel cage induction motor therefore draws a very high starting. During motor starting, the voltage level at the motor terminals should be maintained. Disadvantages of 3 phase induction motor induction motor has low inherent starting torque draw large starting currents, typically 68 x their full load values speeds not easily controlled as dc motors operate with a poor lagging power factor when lightly loaded induction motor components an induction motor has two main parts. Record all nameplate information for the induction motor. Starting of squirrel cage induction motors485 an induction motor can be regarded as a transformer with a small air gap in the magnetic circuit. Pdf improvement of start up torque of three phase induction motor. Methods of starting three phase induction motors as we know, once a supply is connected to a three phase induction motor a rotating magnetic field will be set up in the stator, this will link and cut the rotor bars which in turn will induce rotor currents and create a rotor field which will interact with the stator field and produce rotation.
Calculate i the new operating speed if there is a 10% drop in voltage. The rotor terminals are connected to three slip rings which turn with the. An induction motor can therefore be made without electrical connections to the rotor. This developed torque in the motor differs from the actual torque available at the terminals of the motor, which is almost equal to the friction. So, the equation of starting torque is easily obtained by simply putting the value of s 1 in the equation of torque of the three phase induction motor, maximum torque condition for three phase induction motor. It resembles a slipring induction motor and is started on resistance when it is up to nearly synchronous speed, d. At the time of starting, the motor slip is unity, and the starting current is very large. The vector diagram for the split phase induction motor is shown in the figure b. Normally, the starting current of an induction motor is six to eight time of full load current. The rotor will constantly be turning to align its magnetic field with that of the stator field. Starting pulse vibration torque analysis of aviation variable. Request pdf soft starting of induction motor with torque control this paper presents a simple technique based on statorflux estimation to control the electromagnetic torque of induction. One of the most common electrical motor used in most applications which is known as induction motor.
The magnetization current im flowing in the motors equivalent circuit fig. Due to the physical laws of inertia, where a body at rest tends to remain at rest, the amount of torque necessary to start a load starting torque is always much greater than the amount of torque required to maintain rotation of the load after it has achieved normal speed. The rotating force that a motor develops is called torque. But the rotor construction differs with respect to the type of motor. Speed is the resistance variable over 50% to 100% full speed. Therefore, the starting is obtained by putting the value of s 1 in the equation 6, we get. The torque is also known as electromagnetic torque. The torque developed by an asynchronous induction motor varies when the motor accelerates from zero to maximum operating speed. The resistance of the starting winding is increased by inserting extra resistance in series with it. Again, we know that the starting torque of an induction motor is proportional to the square of the voltage applied to the winding per phase. Thus a reduction of 10% in line voltage causes about 23% increase in cu losses. These devices should not be started on severe conditions as it may damage the windings because of the excess heat produced.
When started at of normal voltage the current is 2 a. Jan 11, 2018 the torque in the dc motor is related by the product of field flux and armature current whereas the torque in the induction motor is product of rotor flux, rotor current and rotor power factor. Hence it is not advisable to start directly motors of rating above 5 kw. The reduction in back emf increases the net voltage, since the supply voltage remains constant. Pdf minimization of starting torque and inrush current of. The stator of an induction motor consists of 3phase windings, which when connected to a 3phase supply creates a rotating magnetic field. Torque equation of an induction motor the developed torque or induced torque equation in a machine is defined as the torque generated by the electric to mechanical power conversion. Pdf this paper addresses a new singlephase starting technique of a three phase induction motor. Pdf on apr 30, 2019, suraj grover and others published minimization of starting torque and inrush current of induction motor by different starting methods. Any dc motor will experience a decrease in speed when a load is applied, which causes the back emf to decrease. Soft starting of induction motor with torque control. Low starting current compared to squirrel cage induction motor.
Torque equation of three phase induction motor electrical4u. For an induction motor, torque t is developed by v2, thus on 50% tapping, torque at starting is only 0. Torque equation of threephase induction motor electrical. The basic idea of an electric motor is to generate two magnetic fields.
The equation shows that star delta starter reduces the starting torque to onethird of that produced by dol starter. The rotor circuit resistance has significant impact on starting torque, the speed at which the breakdown torque occurs, and the slip during the normal running operation. The torque in the dc motor is related by the product of field flux and armature current whereas the torque in the induction motor is product of rotor flux, rotor current and rotor power factor. Jul 11, 2019 so, the equation of starting torque is easily obtained by simply putting the value of s 1 in the equation of torque of the three phase induction motor, the starting torque is also known as standstill torque. Requirements for minimum starting voltage can vary over a wide range, depending on the application. The motoring region of the induction machine torquespeed curve is the region of greatest interest.
This is mainly due to the simplicity of composition, low price, light weight, high. The power output in horsepower of the motor is defined as follows. In a split phase motor, the running winding should have a high resistance and low inductance b low resistance and high inductance c high resistance as well as high inductance d low resistance as well as low inductiance ans. Previously we shared the book on ac motors which explains all these properties briefly, this post is focused on the induction motor formulas and calculation examples. Split phase induction motor operation and characteristics. The 3phase set of currents, each of equal magnitude and with a phase. A wound rotor, has a 3phase winding similar to the stator winding.
Starting torque is the torque produced by induction motor when it is started. When the supply is connected to the stator of a threephase induction motor, a rotating magnetic field is produced, and the rotor starts rotating and the induction motor starts. This paper presents the starting time calculation for a squirrel cage induction motor. Starting of an induction motor starting methods circuit globe. This torque is also known as the electromagnetic torque. We know that at the start the rotor speed, n is zero.
But the starting torque of an induction motor can be improved by increasing the resistance of. This will link and cut the rotor conductors which in turn will induce a current in the rotor conductors and. Such properties include a number of poles, speed, frequency, slip, starting current and rated horsepower. We are going to analysis starting current, torque and speed of induction motor when it is started with the help of. T po pullout, breakdown or maximum torque, obtainable over the entire speed range. The stator carries a 3phase winding called stator winding while the rotor carries a shortcircuited winding called rotor winding only the stator winding is fed from the 3phase supply.
A motor rating of 10 horse power, 415 volts with 1440 rpm and 4pole is selected for design problem and an equivalent circuit model is developed to find out the various performance parameters like efficiency, starting torque, torque. Voltages can range from 80% or lower to 95% or higher. The maximum starting torque does not depend upon rotor resistance. Journal of ar and tomar, lectr lectro n st, electrical. The starting torque is also known as standstill torque. Okay, since the induction motor is a transformer when starting at zero speed, there are reactances to deal with.
This is because the pullout torques by the two cages occur at different speeds. Maximum torque condition for threephase induction motor. Torque of a three phase induction motor is proportional to flux per stator pole, rotor current and the power factor of the rotor. Obviously, one can increase the torque by adding more loops in pairs until reaching a limit set by the ability to get rid of heat. An induction motors rotor can be either wound type or squirrelcage. Both winding are connected in parallel with the alternating supply. Lecture 6 three phase induction motors cairo university. The starting torque is 150 to 200 percent of the full load torque with a starting current of 6 to 8 times the full load current due to their low cost, splitphase induction motors are the most popular singlephase motors in the market since the starting winding is made of fine wire, the current density is high and the winding heats up quickly. Such motors would possess a low power factor and efficiency compared to an ordinary squirrel cage motor, because of the high. As the winding resistance of starting winding is high, the angle between voltage and current i s is small similarly the inductance of the running or main winding is high therefore the angle between i m and v is very large but less than 90 0 the starting torque in the split phase motor is directly.
The locked rotor torque or starting torque is the torque an electrical motor develops when starting at zero speed. This procedure helps control startup current, limiting the inrush current without requiring a high leakage inductance in the stator. How an induction motor works by equations and physics. So, the equation of starting torque is easily obtained by simply putting the value of s 1 in the equation of torque of the three phase induction motor, the starting torque is also known as standstill torque. Good applications for splitphase motors include small. Like any electric motor, a 3phase induction motor has a stator and a rotor. The starting winding has high resistance and low reactance in order to achieve good starting torque whereas the running winding has high reactance and low resistance. Singlephase induction motor, starting torque, main and auxiliary windings, starting methods, splitphase, capacitor type, motor with capacitor start run.
The construction of stator of the induction motor is almost the same as other motors. The importance of starting time lies in determining the duration of large current, which flows during the starting of an induction motor. Why is the starting torque of a single induction motor zero. Starting torque in the three phase induction motor. Why is the starting torque of a single induction motor. There are many methods in use to start 3phase induction motors.
Single phase induction motor, theory of operation double. The maximum generated torque ranges from 250% to 350% of the rated torque see figure 9 for torque speed curve. Pdf minimization of starting torque and inrush current. At synchronous speed, s 0 and hence developed torque td 0. Therefore, 3phase induction motors employ a starting method not to provide a starting torque at the rotor, but because of the following reasons. An induction motor or asynchronous motor is an ac electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. Slip ring induction motor is made up from wound rotor and squirrel cage induction motor is made up from the squirrel cage rotor. Designing of three phase squirrel cage induction motor for. Starters used in lager industries, it is larger in size and expensive. Single phase induction motors multiple choice questions. A 3phase, 6pole, 50hz induction motor has a peak torque of 6 nm and a starting torque of 3 mm when operating at full voltage. Motor torque, load torque and selection of motors 245 performance in such motors the pullout torque is normally less than the starting torque. Construction an induction motor consists essentially of two main parts.
43 1304 306 39 1284 1086 924 108 1155 585 1238 1066 971 133 262 661 226 1521 736 246 868 1427 288 419 701 1218 165 678 847 1464 1004 1188 846 1004 312 1267 867 230 111