Electrical Machines (AC) - Three Phase Induction Motor (PDF)

28/01/2025 ELECTRICAL MACHINES (AC) THREE PHASE INDUCTION MOTOR (1) BY: DR. YAHYA TAREQ 1 THREE PHASE INDUCTION MOTOR The three-phase induction motors are the most widely u...

28/01/2025 ELECTRICAL MACHINES (AC) THREE PHASE INDUCTION MOTOR (1) BY: DR. YAHYA TAREQ 1 THREE PHASE INDUCTION MOTOR The three-phase induction motors are the most widely used electric motors industry. They run at essentially constant speed from no-load to full-load. a 3-phase induction motor has a stator and a rotor. The stator carries a 3-phase winding (called stator winding) while the rotor carries a short-circuited winding (called rotor winding). The induction motor may be considered to be a transformer with a rotating secondary and it can, therefore, be described as a “transformer type”a.c. machine in which electrical energy is converted into mechanical energy. Advantages (i) It has simple and rugged construction. (ii) It is relatively cheap. (iii) It requires little maintenance. (iv) It has high efficiency and reasonably good power factor. (v) It has self-starting torque. Disadvantages (i) It is essentially a constant speed motor and its speed cannot be changed easily. (ii) Its starting torque is inferior to d.c. shunt motor. 2 1  28/01/2025 THREE PHASE INDUCTION MOTOR Construction A 3-phase induction motor has two main parts (i) stator and (ii) rotor. The rotor is separated from the stator by a small air-gap which ranges from 0.4 mm to 4 mm, depending on the power of the motor. 1. Stator It consists of a steel frame which encloses a hollow, cylindrical core made up of thin laminations of silicon steel to reduce hysteresis and eddy current losses. Fig.1 stator winding 3 THREE PHASE INDUCTION MOTOR Construction 2. Rotor (i) Squirrel cage type (ii) Wound type (i) Squirrel cage type It consists of a laminated cylindrical core having Parallel slots on its outer periphery. One copper or aluminum bar is placed in each slot. All these bars are joined at each end by metal rings called end rings [See Fig. 2]. This forms a permanently short-circuited winding Which is indestructible? Those induction motors which employ squirrel cage rotor are called Squirrel cage induction motors. Fig.2 Squirrel cage 4 2  28/01/2025 THREE PHASE INDUCTION MOTOR (ii) Wound type It consists of a laminated cylindrical core and carries a 3-Phase winding, similar to the one on the stator [See Fig. (8.3)]. The rotor Winding is uniformly distributed in the slots and is usually star-connected. The open ends of the rotor winding are brought out and joined to three Insulated slip rings mounted on the rotor shaft with one brush resting on each slip ring. The three brushes are connected to a 3-phase star- connected rheostat as shown in Fig. (8.4). At starting, the external resistances are included in the rotor circuit to give a large starting torque. These resistances are gradually reduced to zero as the motor runs up to speed. The external resistances are used during starting period only. 5 THREE PHASE INDUCTION MOTOR Speed of rotating magnetic field The speed at which the rotating magnetic field revolves is called the Synchronous speed (Ns). four P poles, the rotating field makes one revolution in P/2 cycles of current. ! so Cycle of current = " ✕ revolutions of field. ! Or Cycle of current per second = ✕ revolutions of field per second. " Since revolutions per second is equal to the revolutions per minute (Ns ) divided by 60 and the number of cycles per second is the frequency f, ! ## &$' So f = " ✕ $% = ("% "#$ & Or Ns = ' 6 3  28/01/2025 THREE PHASE INDUCTION MOTOR Alternate Mathematical Analysis for Alternate Mathematical Analysis for Rotating Magnetic Field Rotating Magnetic Field To find the magnitude of the resultant flux, resolve each flux into horizontal and vertical components and then find their vector sum. 7 THREE PHASE INDUCTION MOTOR Principle of Operation Can be explained as under: (i) When 3-phase stator winding is energized from a 3-phase supply, a rotating magnetic field is set up which rotates round the stator at synchronous Speed Ns = (120 f/P). (ii) The rotating field passes through the air gap and cuts the rotor conductors, which as yet, are stationary. Due to the relative speed between the rotating flux and the stationary rotor, e.m.f.s are induced in the rotor conductors. Since the rotor circuit is short-circuited, currents start flowing in the rotor conductors. (iii) The current-carrying rotor conductors are placed in the magnetic field produced by the stator. Consequently, mechanical force acts on the rotor conductors. The sum of the mechanical forces on all the rotor conductors produces a torque which tends to move the rotor in the same direction as the rotating field. (iv) The fact that rotor is urged to follow the stator field (i.e., rotor moves in the direction of stator field) can be explained by Lenz’s law. According to this law, the direction of rotor currents will be such that they tend to Oppose the cause producing them. Now, the cause producing the rotor Currents is the relative speed between the rotating field and the stationary rotor conductors. Hence to reduce this relative speed, the rotor starts running in the same direction as that of stator field and tries to catch it 8 4  28/01/2025 THREE PHASE INDUCTION MOTOR Slip 9 THREE PHASE INDUCTION MOTOR Frequency of rotor current At standstill (i.e. when the rotor is stationary), the frequency of the rotor current is the same as the supply frequency (f). But when the rotor staters revolving, then the frequency depends upon the relative speed or slip-speed. If fr is the frequency of the rotor current, then %"& (! Ns ⎼ N = … (i) ! %"& ( Also Ns = …(ii) ! dividing (i) by (ii) we get )! *) ,# ,$ +" = , or 𝑠 = , or fr = sf (i) When the rotor is at standstill or stationary (i.e., s = 1), the frequency of rotor current is the same as that of supply frequency (fr = sf = 1 ✕ f = f). 10 5  28/01/2025 THREE PHASE INDUCTION MOTOR Example 1 slip-ring induction motor runs at 290 r.p.m. at full load, when connected to 50-Hz supply. Determine the number of poles and slip. Since N is 290 rpm; Ns has to be somewhere near it, NS = 300 rpm. 11 THREE PHASE INDUCTION MOTOR Example 2 A 3-_ induction motor is wound for 4 poles and is supplied from 50-Hz system. Calculate (i) the synchronous speed (ii) the rotor speed, when slip is 4% and (iii) rotor frequency when rotor runs at 600 rpm. 12 6