The difference between the rotor speed (N) and the rotating magnetic flux speed (N s) is called slip. The traditional way to control wound-rotor-induction-motor speed is to increase slip by adding resistance in the rotor circuit. When the actual speed of the rotor is equal to the synchronous speed, the slip is zero. Induction motor is used for a variety of end use industries, such as residential, industrial, and commercial and is forecast to grow at a CAGR of 12%. If the rotor winding resistance per phase is 0.2 Ω, the external resistance is (A) 0.2 Ω (B) 0.3 Ω (C) 0.4 Ω (D) 0.5 Ω Source d'alimentation supplémentaire: Une source d’alimentation en courant continu supplémentaire est nécessaire pour faire initialement tourner le rotor près de la vitesse synchrone. A. 2. At the synchronous speed, s = 0, therefore, the torque is zero. shespuzzling. Induction motor is one of the most popular motors in the electric industry. Iron losses are considered to be the constant losses. Menu. 37.5 Hz. What is the frequency of the rotor current? At start b. Starting torque is the torque produced by induction motor when it starts. B. A 4 pole, 3 phase, 50 Hz star connected induction motor has a full load slip of 5%. Slip just means that the rotating field in the stator does … Caleçon: N'a pas de slip. Because when the slip is zero, both speeds are equal and there is no relative motion. Answer D 9. However, this slip speed cannot become zero because that would stop generation of the torque producing current itself. Low Slip Region. Induction motor works on the concept of mutual induction i.e. About. The potentiometer 52 and variable resistor 55 may be adjusted to vary the motor slip frequency versus motor speed characterstic as a function of the specific type of motor used, the inertia of the load to be moved and any other factors having relevance to the operation of the motor. Is it possible to […] An induction motor can’t development torque at synchronous speed because an induction motor can’t run at synchronous speed because if it runs at synchronous speed the relative speed between the rotor and stator R.M.F will be zero so the torque will be zero. Analysis of the torque speed curve of an induction machine shows that an induction motor will find a steady equilibrium operating point at a speed between pull-out and sychronous speed. 1. is incorrect ? C. 2. (d) unity. A. D. 3.3 Hz. (d) It is maximum at standstill. View Answer. It depends on. Definition: The slip in an induction motor is the difference between the main flux speed and their rotor speed. Frequency. We know that at the start the rotor speed, N is zero. (c) It is directly dependent on slip. On full load, its speed is observed to be 1410 r.p.m. In medium sized induction motors, the slip is generally around (a) 0.04% (b) 0.4% (c) 4% (d) 14% Ans: c. 7. So slip of induction motor can not be zero under any circumstances. For the induction motor, this condition will never happen. 15. Hence, the motor cannot run. Feb 24, 2011 #1 Can someone explain both conceptually and mathematically why the rotor in an induction motor will never catch up to the revolving stator field? 1.At zero in an induction motor a) Motor runs as a generator b) Motor does not run c) The motor runs an at synchronous speed d) Slip produced is zero Electricalunits.com. Variable losses. The symbol S represents the slip. As seen in Table 1, smaller and lower-speed motors are associated with higher relative slip. A. 1500 b. 0. REE Board Exam September 2002. The speed difference is due to "slip" which you can look up yourself. 1. (b) It is zero at synchronous speed. C. 1.5 Hz. 3. (c) synchronous motor. Then it starts generating. If the rotor speed and stator speed will equal where will no change is flux linking with the rotor, and hence no current flow through the rotor conductor, resulting no torque or force. Thread starter shespuzzling; Start date Feb 24, 2011; Search Forums; New Posts; S. Thread Starter . The speed of an induction motor is 1170 rpm. A. Voltage. A 3-phase slip-ring induction motor has rotor resistance 0.2 Ω per phase and standstill rotor leakage reactance 2 Ω per phase. For typical Class B motors, the design operating slip is below 0.05. In the induction motor rotor always rotates speed less than synchronous speed. 1. While slip ring induction motors (also known as wound-rotor motor) aren’t as popular as squirrel cage induction motors, they do have a few advantages. C. Both (a) and (b) D. None of these. The slip is defined as the ratio of difference of synchronous speed and actual rotor … 2.5 Hz. Le moteur à induction à courant alternatif est appelé moteur asynchrone. Therefore, If R 2 is constant, the torque becomes. In case of ideal motor having zero friction and zero losses, motor can rotate at synchronous speed, because motor need not develop any torque to sustain rotation. What you see is slip and that slip creates enough induction which leads to enough torque to overcome residual friction and losses when no proper mechanical load is connected. Related Questions on Electrical Machines Test Questions. La valeur de slip est zéro. Can an induction motor run at synchronous speed? Joined Aug 13, 2009 88. In a 3-phase induction motor which of the following statements regarding frequency of induced rotor e.m.f. C. 0.5. The speed is reduced by inserting an external resistance in the rotor circuit and the slip is 0.15. The slip of low-hp motors is higher than that of high-hp motors because rotor-winding resistance is greater in smaller motors. 1 % to 5 %. Q6. The slip corresponding to full load speed of th motor is called full load slip. Given below are the most important Induction Motor MCQ you should know. Menu. The torque equation of the induction motor is given below. When k 2 = k 1 /R 2. Induction motor being the critical component of industrial productivity their upkeep and trouble free operation is of immediate industrial need to minimize the down time costs. When machine is rotating at its maximum speed c. When it is rotating at half of the maximum speed d. Cannot be zero View Answer / Hide Answer. A. The slip at the standstill condition or starting is unity because the rotor speed at the starting is zero in the three phase induction motor. Practically motor operates in the slip range of 0.01 to 0.05 i.e. Determine its synchronous speed. A typical high efficiency induction motor needs approximately 0.75 to 1.0 Hz of rotor bar frequency to develop full torque. In squirrel cage induction motors, the rotor slots are usually given slight skew in order to (a) reduce windage losses (b) reduce eddy currents (c) reduce accumulation of dirt and dust (d) reduce magnetic hum Ans: d. 8. D. None of these. The slip of an Induction motor can never be zero.This is because. Consider each of these curves. (d) series motor. The induction motor never runs at synchronous speed. Slip of a induction motor will be zero, a. Electrical induction motors are designed for different applications regarding characteristics like breakaway torque, pull-up torque, slip and more - check NEMA A, B, C and D classification of electrical inductions motors. Using the equivalent circuit of the three-phase induction motor and some additional information about the mechanical and core loses, one could calculate the performance of the three-phase induction motor from no-load to full-load. When the speed is very near to synchronous speed. 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, B. That is to say, with a locked rotor condition (zero speed) the motor shaft would be developing rated torque when the rated (slip) frequency and rated stator magnetic field strength is circulating around the rotor. Over the years, the levels of demand for induction motors have increased due to the economic cost of induction motors also these motors are maintenance-free owing to the absence of brushes, commutators, and slip rings. Have slip donc la valeur de slip n'est pas égale à zéro. 55. The torque slip curve for an induction motor gives us the information about the variation of torque with the slip. B. Hi friends, in this article I am describing the importance of slip in induction motor and hoping you will find it informative and useful. D. 2.5. Home. 54. Example 1 : A 4 pole, 3 phase induction motor is supplied from Hz supply. Figure 1 showed a plot of induction motor torque vs speed from zero to synchronous speed. The speed will be the synchronous speed of the motor if slip becomes Zero. The motor is now loaded and will run at slightly reduced speed compared to no load speed. A 3-phase, 220 volt induction motor having a power factor of 0.8 has 3% losses. 1. The value of slip at full load varies from 6% in case of small motor and 2% in the large motor. B. Let's see how an induction motor works to understand the concept of slip. Figure 1 shows a set of generic curves of efficiency, current, power factor, and slip for an induction motor. (a) Its speed varies inversely as slip. (b) 0.1. Stator impedance may be neglected. (c) 0.5. Answer B You may also read • Why all the voltage levels are multiples of 11 kV? Copper losses in a rotating machine are. Induction Motor - Rotor Slip. Torque developed by induction motor is proportional to the slip. Mathematically, it is written as . In an induction motor, the rotor voltage is induced rather than physically connecting the wires. B. It is expressed by the percentage of synchronous speed. S = [ ( N s – 0 ) / N s ] S = 1 Why slip is never zero in an induction motor? Torque Slip Characteristics of Three Phase Induction Motor. When the rotor of 3 phase induction motor is blocked, the value of slip will be (a) zero. When the motor is spun like a generator, at some point the residual magnetism gives rise to an AC current in the rotor. 1.5. Why the slip is never zero in an induction motor? We can derive an equation from the equivalent circuit that allows the calculation of the torque for a given value of slip. REE Board Exam September 2002. Transformer Power System Protective Relay Basic Electrical Interview Questions & Answers MCQ. (b) wound rotor motor. Answer & Explanation. Therefore, no EMF induced in the rotor circuit and rotor current is zero. Then full load speed of the motor is a. 1.At zero slip in an induction motor a) Motor runs as a generator b) Motor does not run c) The motor runs an at synchronous speed d) Slip produced is zero Ans: (b) 2. Electrical Engineering MCQ1 Starters are used in induction motor2 The slip of induction motor at […] ANSWER: d. Cannot be zero . At synchronous speed, slip is zero and hence torque developed by the motor is zero. The slip is very low and (sX 20) 2 is negligible in comparison with R 2. Three phase slip ring induction motor is also known as (a) controlled motor. This curve is called the torque-speed characteristic curve. In an induction motor, what is the percent slip if the speed is 3,510? The major growth … In a 3-phase slip-ring induction motor, the slip at some speed is 0.05. At start, the slip of the induction motor is. View Answerr .
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