Explain starting methods of 3 phase induction motor. Induction motor starting methods bright hub engineering. For an induction motor, torque t is developed by v2, thus on 50% tapping, torque at starting is only 0. Mechanical methods are impractical and, therefore the motor is started temporarily converting it. At the time of starting, the motor slip is unity, and the starting current is very large. Starting and control of threephase asynchronous motors eaton. Higher maintenance of brushes and slip rings compared to squirrel cage motor. Motor starting and its associated problems are wellknown to many people who have worked on large industrial processes. The 3phase set of currents, each of equal magnitude and with a phase. As the starting current of an induction motor can be 68 times the running current the dol starter is typically only used for motors with a rating of less than 5kw. Starting methods of single phase motor circuits with protection.
The limitations of the cage induction motor of basic standard design are low breakaway torque and high starting current, the former typically ranging from 0. Starting methods of a single phase induction motor circuit. Operation beyond the range 0 starting current of the motor assuming 120 v. However during starting period, their starting currents. The rotor will constantly be turning to align its magnetic field with that of the stator field.
Journal of ar and tomar, lectr lectro n st, electrical. In the electrical model, the voltage, v, and the slip, s, determine the rotor current. The maximum generated torque ranges from 250% to 350% of the rated torque see figure 9 for torquespeed curve. Fast acting fuses to offer overload protection, a protective device, depending on its application and the motors service factor sf, should be sized at 115% or less of motor fla for 1. For reason of economy, most houses, offices and also rural areas are supplied with single phase a. When the motor was started on full voltage, the starting current was approximately 47 a. After the first halfcycle the motor begins to rotate and the. A typical cage induction motor having standard performance will achieve a starting current in the order of 6 times full load current. Low starting current compared to squirrel cage induction motor. Induction motor the starting torque is low, typically 100% to 175% of the rated torque. Starting induction motors on isolated or weak systems is a highly dynamic process that can cause motor and load damage as well as electrical network fluctuations.
If an induction motor is directly switched on from the supply, it takes 5 to 7 times its full. A code letter table and corresponding factors can be found in na tional electrical manufacturer s association nema motor and. Starters used in lager industries, it is larger in size and expensive. Pdf induction motors ims are widely used in industry including it be an electrical or not. Starting of an induction motor a three phase induction motor is self starting. Wound rotor induction motors ac motors electronics. The speed of rotation of an induction motor is fairly constant, but it does vary somewhat with loading. Is your calculated value close to the observed value. This reduced, yet still largely exaggerated current, is sustained only briefly, as the motor quickly reaches full running speed, where current then subsides to. The importance of starting time lies in determining the duration of large current, which flows during the starting of an induction motor. While starting an induction motor is the biggest headache for maintenance engineers. Generally, it is known as the asynchronous motor because its running speed is less than its synchronous speed. 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. Starting torque starting current transient state smoothness of the starting simplicity and economics of starting.
During motorstarting, the voltage level at the motor terminals should be maintained. In an induction motor, the electric current in the rotor needed to produce torque is obtained via electromagnetic induction from the rotating magnetic field of the stator winding. At this point the initial starting current subsides, reducing to a level of current equal to 4 8 times the normal running current for that motor. It also involves its switchgears to control its starting inrush current.
Pdf minimization of starting torque and inrush current of. Im not sure about the full load current calculation as well. The rotor of an induction motor can be a squirrel cage rotor or wound. Good applications for splitphase motors include small. 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. However, it reduces substantially in a slipring motor due to the high impedance of the rotor circuit. At the time of starting, an induction motor draws high values of current motor is a constant impedance device during the starting condition, that are very close to the motors locked rotor value and remains at this value for the time required to start the motor. 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. Why does induction motor draw heavy current at starting. Wound rotor induction motors ac motors electronics textbook. This procedure helps control startup current, limiting the inrush current without requiring a high leakage inductance in the stator. Voltages can range from 80% or lower to 95% or higher. First of all let me explain basic constructional details of iminduction motor it consits of two parts stator and rotor, both have 3 phase winding housed in them. This is the reason why the lockedrotor withstand time is used as an allowable.
Now you would be knowing how transformer works they have a primary and seconda. In figure 3, we see the current has been reduced, as expected, by a factor of 3, down to about 16 a. Requirements for minimum starting voltage can vary over a wide range, depending on the application. Different types of starters for induction motor nptel. Induction motor seminar ppt, pdf report study mafia. Figure 3 shows the current, torque, and horsepower for the motor that is started with a ydelta starter. It is desired to install a 3phase cage induction motor restricting the maximum line current drawn from a 400 v 3phase supply to 120 a.
Therefore different starting of induction motor, methods such as star delta starter, auto transformer starter and other starting methods are employed in order to reduce the high starting currents of induction motor. Starting current, sometimes abbreviated as i start in electrical engineering, is a measure or rating of how much current required for a single or three phase ac motor to get. The summation of all torques acting on the motor shaft comprises the mechanical model. There are two basic reasons for the use of singlephase motors rather than 3phase motors. I calculated the full load current by dividing the power by the voltage pvi. Three phase induction motor provided with the different starter like stardelta starter, autotransformer starter and dol starter etc. Simulation of 3ph induction motor in matlab with direct. We do not require to give external starting torque for starting induction motor but the starter is provided with an induction motor for limiting starting current. 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. Typically k r is in the range of 1 to 5, and k x is in the range of 0.
Small three phase induction motors can be started directonline, which means that the rated supply is directly applied to the motor. Motor starting currents when an ac motor is energized, a high inrush current occurs. But the rotor construction differs with respect to the type of motor. Normally with motors beyond 5 hp, starters are provided.
Full voltage is only applied when the motor picks up speed. When power supply for the stator is switched on, an alternating current flows through the stator winding. Maximum starting current the starting code letter on the nameplate of an ac induction motor can be used to estimate the maximum starting current of the motor. In small groups, investigate the dol starter shown. Starting methods of a single phase induction motor the single phase motor is not self starting and hence needs an auxiliary means or equipment to start the single phase induction motor. This will link and cut the rotor conductors which in turn will induce a current in the rotor conductors and. The motor draws high starting current, approximately 700% to 1,000% of the rated current. Starting of an induction motor starting methods circuit globe. The stator of an induction motor consists of 3phase windings, which when connected to a 3phase supply creates a rotating magnetic field. If the motor draws overcurrent, then the voltage developed across the secondary of the transformer t2 energizes the relay rl1 to trip the relays rl2 and rl3. The single phase induction motors are clas sified based. For this, we need to build a heavy starter or big autotransformer or vfd to start higher rating motors. Motor analysis block diagram induction motor starting can be analyzed using electrical, mechanical, and thermal models which interact as diagrammed in figure 1. An induction motor also known as an asynchronous motor is a commonly used ac electric motor.
Speed is the resistance variable over 50% to 100% full speed. Thus the single phase motor fails to develop starting torque. The proposed system is intended to offer soft start of the single phase induction motor using a pwm sinusoidal voltage while starting the motor. Introduction to motor starting analysis ced engineering. This is because a large amount of energy is required to magnetise the motor enough to overcome the inertia the system has at standstill. For reduction in the starting current, a lower voltage is applied to the stator, especially for the squirrel cage induction motors. Slip ring induction motor is made up from wound rotor and squirrel cage induction motor is made up from the squirrel cage rotor.
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. Technical note starting methods for ac motors reducing electrical and mechanical stress at startup the starting current of an ac motor can vary from 3 to 7 times the nominal current. Power quality events such as voltage sags and harmonics may occur in starting motors due to the inrush current. Pdf evaluation of starting current of induction motors using. Starting method for induction motors a 3phase induction motor is theoretically self starting. The larger the motor power, the more severe this phenomenon. Thus starting current taken from supply will only be 25% of the taken by dol starter. Starting methods of a single phase induction motor. The construction of stator of the induction motor is almost the same as other motors. Induction motor the induction motor is the motor most commonly used in agriculture. Singlephase induction motors ac motors electronics textbook. This post is a quick introduction to motor starting. Thus, at normal supply voltage, like in transformers, the initial current taken by the primary is very large for a short while. Starting methods of three phase induction motor electrical.
During motor starting, the voltage level at the motor terminals should be maintained. Reduce the starting current of an induction motor wednesday, august 26, 2015 during the transient condition associated with acceleration of an ac machine started as an inductive load, current is drawn from the source that can be several times the fullload rating of the machine winding. Motors have been in use for over 100 years, and during that time there has been relatively little change in how they function. This paper presents the starting time calculation for a squirrel cage induction motor.
When these motors are used together with a softstarter it is possible to reduce the starting current further when compared to motors with low starting torque. The dol starter switches the supply directly on to the contacts of the motor. Jan 11, 2018 such properties include a number of poles, speed, frequency, slip, starting current and rated horsepower. The basic idea of an electric motor is to generate two magnetic fields. In fact, an induction motor can be treated as a rotating transformer i. The starting of an induction motor does not relate to simple switching alone. Why induction motor takes high starting current electrical4u. Free download induction motor seminar ppt, pdf report. Squirrel cage wound rotor with slip rings the rotating magnetic field the basic idea of an electric motor is to generate two magnetic fields. Starting the induction motor is the most important and dangerous step, where the motor properties play a major role in the evaluation of all electrical motors, and these properties are defined by the following factors. Mechanical methods are impractical and, therefore the motor is started temporarily converting it into two phase motor. An induction motor is similar to a polyphase transformer whose secondary is short circuited. To determine the starting current use the following formula. How to calculate the starting current of an induction motor if i have the power in kilo watts and the voltage.
Such properties include a number of poles, speed, frequency, slip, starting current and rated horsepower. Therefore different starting of induction motor, methods such as star delta starter, auto transformer starter and ot. Motor guide february 2014 low voltage motors motor guide. Typical motor starting characteristics are shown in curve 1. But, as mentioned above, here, the starting current would be very large, usually 5 to 7 times the rated current. Pdf on apr 30, 2019, suraj grover and others published minimization of starting torque and inrush current of induction motor by different starting methods.
High starting and surge currents result when the full voltage is applied. A comparison of induction motor starting methods being. Starting current, sometimes abbreviated as i start in electrical engineering, is a measure or rating of how much current required for a single or three phase ac motor to get started. Unlike in dc motors, large current at starting is due to the absence of back emf. Starting current of induction motor is as high as 5 to 7 times the normal full load current. Mechanical damage is associated with the high starting current drawn by a ramping induction motor. Aug 26, 2015 reduce the starting current of an induction motor wednesday, august 26, 2015 during the transient condition associated with acceleration of an ac machine started as an inductive load, current is drawn from the source that can be several times the fullload rating of the machine winding. Typically, during the initial half cycle, the inrush current is often higher than 20 times the normal full load current. Starting methods of single phase motor circuits with. That is why such motors are known as induction motors. How an induction motor works by equations and physics. The induction motor is discussed in detail in this unit. An induction motor can be defined as the ac electric motor device in which a power source fed by the alternating current with the help of a primary winding. Electrical, mechanical, and thermal motor characteristics define the frame needed for effective motor protection running stage, starting stage, and lockedrotor conditions serve to determine the main motor parameters motor heating and thermal damage motor characteristics depend on both positiveand negativesequence current in the.
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