B - B - 3.18 - AC Motors PDF

Module 3 Electrical Fundamentals II Topic 3.18 – AC Motors  INTRODUCTION On completion of this topic you should be able to: 3.18.1 Describe the construction, operation and characteristics of the following AC motors:...

Module 3 Electrical Fundamentals II Topic 3.18 – AC Motors  INTRODUCTION On completion of this topic you should be able to: 3.18.1 Describe the construction, operation and characteristics of the following AC motors: Single phase and polyphase synchronous Single phase and polyphase induction 3.18.2 Describe methods for control of speed and direction of rotation in AC motors 3.18.3 Describe methods of producing a rotating field using the following: Capacitor Inductor Shaded or split pole 30-03-2024 Slide No. 2  AC MOTORS Advantages of AC motors: Wide availability of AC power Lower cost than DC motors Lower maintenance – Most AC motors do not use brushes and commutators Eliminates problem of dangerous sparking Well suited for constant-speed applications Speed is determined by frequency of AC voltage applied to motor. Motors are designed for use with either polyphase or single-phase power. 30-03-2024 Slide No. 3  AC MOTORS Induction motors: Most commonly used AC motor Single-phase or polyphase Rotors energised by induction Synchronous motors: Typically polyphase Constant speed Rotors energised with DC voltage 30-03-2024 Slide No. 4  THREE-PHASE ROTATING FIELDS Note individual windings for each phase. Three phases are tied together in a Y-connected stator – Dot indicates common point. Individual phase windings are equally spaced around stator – 120° apart. Use left-hand rule to determine electromagnetic polarity of poles at any instant. Current flows toward terminals for positive voltages, and away for negative. 30-03-2024 Slide No. 5  THREE-PHASE ROTATING FIELDS Rotating magnetic field – strong field which, in turn, is aided by weaker fields. When three Ø completes one full cycle – magnetic field has rotated through 360°. 30-03-2024 Slide No. 6  THREE-PHASE ROTATING FIELDS Assume a bar magnet (mounted free to Point 1 to 7 – bar magnet will follow stator rotate) in centre of the stator. field at same speed as rotating field. Assume bar magnet is aligned so its south This speed is known as synchronous pole is opposite large N of stator field. speed. 30-03-2024 Slide No. 7  SPEEDS OF AC MOTORS Magnetic field inside AC motor rotates at a speed related to frequency of supply. Field rotation speed is called the synchronous speed (Ns). Rotational speed of AC motor shaft is related to Ns. To determine speed: 𝑭𝒓𝒆𝒒 𝒙 𝟏𝟐𝟎 𝑵𝑺 = 𝑷 F –Frequency of AC power (Hz) P – Number of poles per phase wound into the motor Ns – Synchronous speed (rpm) 30-03-2024 Slide No. 8  SPEEDS OF AC MOTORS Some AC motors operate almost exactly Percentage difference between full-load at synchronous speed. speed and synchronous speed is called slip. These are called synchronous motors. Normal slip is around 5%, though it Asynchronous motors operate at can be much higher. something less than synchronous speed. Difference between synchronous & asynchronous motors is design, not Asynchronous motors are also known quality. as induction motors. 30-03-2024 Slide No. 9  INDUCTION MOTOR Probably the simplest and most rugged of Similar to induction between primary & all electric motors. secondary windings of a transformer. Most commonly used type of AC motor. Typically used to drive loads at fairly Has a rotor that is NOT connected to an constant speed. external source of voltage. Only two main components: AC voltages are induced in rotor circuit by stator and rotating magnetic field of stator. rotor. 30-03-2024 Slide No. 10  INDUCTION MOTOR – STATOR Stator contains a pattern of copper coils Note: there is NO direct electrical arranged in windings. connection between the stator and the rotor. As AC is passed through windings, a moving magnetic field is formed. This induces a current in the rotor, creating its own magnetic field. The interaction of these fields produces a torque on the rotor. 30-03-2024 Slide No. 11  INDUCTION MOTOR Induction rotor is made of a laminated Regardless of the type of rotor used, cylinder with slots in its surface. basic principle is the same. Two types of rotors for induction motors – Rotating magnetic field generated in both employ slotted laminated core: stator induces a magnetic field in rotor. Most common is the squirrel-cage rotor Two fields interact and cause rotor to turn. Other type contains actual wound coils placed in rotor slots – wound rotor. 30-03-2024 Slide No. 12  SQUIRREL-CAGE ROTOR Construction: Number of conducting bars running parallel to motor axis Two conducting end rings Typically, conductors may be copper or aluminium Aluminium typically used in smaller motors Resembles a squirrel cage This type of motor is often called a squirrel-cage motor 30-03-2024 Slide No. 13  INDUCTION MOTOR – SLANTED ROTOR Slanting conductors in rotor ensures a smooth steady acceleration during starting. Varying physical design features of rotor can: increase their inductance give a lower starting current create a lower pull-out torque 30-03-2024 Slide No. 14  SQUIRREL-CAGE ROTOR No insulation between core and bars as only low voltages induced into rotor bars. Very small air gap between rotor and stator– necessary to obtain maximum field strength. 30-03-2024 Slide No. 15  WOUND ROTOR Typically only used on when starting requirements are particularly severe. Advantages – starting torque can be adjusted – speed of motor can be controlled. Wound rotor has three windings and three slip rings. Adjustments made by simultaneously varying three resistors (externally connected). Open circuit resistances and motor will not run. Vary resistances and motor varies in torque and speed. At speed, windings can be short circuited and motor will run as normal squirrel-cage. 30-03-2024 Slide No. 16  INDUCTION MOTOR Lenz's law – any induced EMF tries to Force tends to cancel the relative motion oppose the changing field that induces between the rotor and the stator field. it. The rotor, as a result, moves in the same In an induction motor, changing field is direction as the rotating stator field. the motion of the resultant stator field. A force is exerted on rotor by the induced EMF and the resultant magnetic field. 30-03-2024 Slide No. 17  INDUCTION MOTOR An induction motor cannot run at In order for relative motion to exist, rotor synchronous speed. must rotate slower than rotating field. If speeds were same, would be no relative Difference between speed of rotating stator motion between stator and rotor fields. field and rotor speed is called slip. Without relative motion there would be no The smaller the slip, the closer rotor speed induced voltage in the rotor. is to stator field speed. 30-03-2024 Slide No. 18  INDUCTION MOTOR Speed of the rotor depends upon the This EMF can increase only if magnetic field torque requirements of the load. cuts through rotor at a faster rate. The bigger the load, the stronger the To increase relative speed between field turning force needed to rotate the rotor. and rotor, the rotor must slow down. The turning force can increase only if the rotor-induced EMF increases. Therefore, induction motor turns slower for heavier loads than for lighter loads. 30-03-2024 Slide No. 19  INDUCTION MOTOR Therefore, slip is directly proportional to This is because the rotor windings have load on the motor. such a low resistance. Only a slight change in speed is necessary As a result, induction motors are called to produce the usual current changes constant-speed motors. required for normal changes in load. 30-03-2024 Slide No. 20  INDUCTION MOTORS – SLIP Common synchronous speeds for 50-hertz motors are: 3000, 1500, 1000, and 750 rpm, depending on No. of poles (2, 4, 6, 8 ) per phase. Common terminology – e.g. two pole motor (three Ø) has six physical poles. Rotor is never able to reach synchronous speed – if it did, NO torque would develop. 30-03-2024 Slide No. 21  INDUCTION MOTORS – SLIP Typically, the difference between these 2 speeds is not great. E.g. 2700 to 2900 rpm rotor speed expected from synchronous speed of 3000 rpm. The formula for determining the slip is: 𝑵𝑺 − 𝑵 𝑺% = 𝒙𝟏𝟎𝟎 𝑵𝑺 where: S% = percentage slip Ns = synchronous speed N = rotor speed 30-03-2024 Slide No. 22  SYNCHRONOUS MOTOR Characteristic of constant speed between no load and full load. May be designed as either single-phase or multiphase machines. Single phase also spins at synchronous speed. Discussion that follows is based on three-phase synchronous motors. 30-03-2024 Slide No. 23  SYNCHRONOUS MOTOR Three Ø AC power to stator causes a Results in a strong turning force on the rotating magnetic field to be set up around rotor shaft. rotor. Rotor therefore rotates in step with Rotor is energised with DC (it acts like a bar rotating magnetic field – synchronous magnet). speed. Strong rotating magnetic field attracts strong rotor field activated by DC. 30-03-2024 Slide No. 24  SYNCHRONOUS MOTOR Disadvantage of a synchronous motor is that Synchronous motor in its purest form it cannot be started from a standstill. has NO starting torque. When AC applied to stator, high-speed It has torque only when rotating at rotating magnetic field appears immediately. synchronous speed. Rotating field rushes past rotor poles so quickly, rotor has NO chance to get started. 30-03-2024 Slide No. 25  SYNCHRONOUS MOTOR – ROTOR A squirrel-cage winding is added to rotor of a synchronous motor for starting. Squirrel cage is shown as outer part of rotor. So named because it is shaped and looks something like a turnable squirrel cage. Simply, the windings are heavy copper bars shorted together by copper rings. 30-03-2024 Slide No. 26  SYNCHRONOUS MOTOR – ROTOR A low voltage is induced in these shorted windings by rotating stator field. Because of the short circuit, a relatively large current flows in the squirrel cage. This causes a magnetic field that interacts with the rotating field of stator. Interaction causes rotor to turn, following stator field and motor starts rotating. Squirrel cage rotors will be covered in more detail shortly. 30-03-2024 Slide No. 27  SYNCHRONOUS MOTOR – ROTOR To start synchronous motor: AC supplied to stator, but DC is NOT YET supplied to rotor field Squirrel-cage windings bring rotor to near synchronous speed At that point, the DC field in rotor is energised This locks rotor in step with rotating stator field and full torque is developed Centrifugal switch typically used to apply DC to rotor at sufficient speed 30-03-2024 Slide No. 28  SYNCHRONOUS MOTOR – ROTOR Rotor is typically constructed with salient poles. When excited with DC – produces alternate north and south magnetic poles. These magnetic poles on rotor outer are attracted in rotating stator field. Has disadvantage of requiring a DC exciter voltage for rotor. DC may be obtained either externally or internally, depending on design of motor. 30-03-2024 Slide No. 29  SYNCHRONOUS MOTOR PRINCIPLE OF OPERATION Principle of magnetic attraction between two magnetic fields of opposite polarity. One field is that of the rotating stator and the other that of the rotor. 30-03-2024 Slide No. 30  SYNCHRONOUS MOTOR Operates at synchronous speed with no slip. Rotor has constant polarity (either permanent magnet or energised electromagnet). Will run at speed regardless of load variations up to a point called pull-out torque. Pull-out torque – max. value of torque a motor can develop without losing synchronism. A load higher than this will pull the motor out of synchronism and cause it to stop. Stator is basically same as induction motor stator. 30-03-2024 Slide No. 31  SYNCHRONOUS MOTOR No-load – centre lines of ‘stator pole of Pull-out torque – max. value of torque a rotating field’ and ‘rotor pole’ coincide. motor can develop without losing synchronism. With load – backward shift of rotor pole, relative to stator pole – NO change in A load higher than this will pull the motor speed. out of synchronism and cause it to stop. Angle between rotor and stator poles is called the torque angle. 30-03-2024 Slide No. 32  SYNCHRONOUS MOTOR Star wound stator winding will draw less In Star – voltage is effectively reduced to current than a delta wound type. 58% (or 240v for 415v)(torque at ⅓). Common to use a ‘star-delta’ arrangement In Delta – phase voltage increases by for starting of large three Ø induction 173% as does current – increased torque. motors. This limits starting current on initial start. Line current (in delta) increases three times its value in star connection. At approx. 75% speed, switches over to ‘delta’. 30-03-2024 Slide No. 33  SINGLE-PHASE INDUCTION MOTORS Probably more One Ø AC induction As a result of magnetic induction, motors in use today than all other types. magnetic field is produced around the rotor. Unlike polyphase induction motors, stator field in One Ø AC motor does not rotate. This field will always be in opposition to the stator field (Lenz's law applies). Instead it simply alternates polarity between poles as AC changes polarity. 30-03-2024 Slide No. 34  SINGLE-PHASE INDUCTION MOTORS If rotor is rotated by some outside force, the push-pull along the line is disturbed. At this instant, South pole on rotor is attracted by left-hand pole – North to RH pole. All of this is a result of the rotor being rotated 90° by outside force. The pull that now exists between two fields now becomes a rotary force. This turns the rotor toward magnetic correspondence with the stator. Because two fields continuously alternate, they will never actually line up. Rotor will continue to turn once started – But still require method to start rotor. 30-03-2024 Slide No. 35  SINGLE-PHASE INDUCTION MOTORS Several types of single Ø AC induction motors in use today. They operate in the same fashion except for the means of starting. Once up to operating speed, all single Ø AC induction motors operate the same. Various methods used for starting single Ø AC induction motors: split-phase – most common method by far shaded-pole 30-03-2024 Slide No. 36  SPLIT PHASE AC INDUCTION MOTOR One type of induction motor which incorporates a starting device. Designed to use inductance, capacitance, or resistance to develop starting torque. Typically, start winding is disconnected when motor reaches 75% of rated speed. 30-03-2024 Slide No. 37  CAPACITOR-START SPLIT PHASE Stator consists of main winding and a starting winding (auxiliary). Start winding in parallel with main winding & placed physically at right angles to it. A 90-degree electrical phase difference between two windings is obtained by connecting auxiliary winding in series with a capacitor and starting switch. In start winding circuit (with XC), current leads voltage by about 45°. In main winding (with XL) current lags voltage by about 45°. Currents in each winding are therefore 90° out of phase - as are magnetic fields. 30-03-2024 Slide No. 38  CAPACITOR-START SPLIT PHASE Effect is two windings act like a two Ø stator Start (auxiliary) winding is only a light & produce rotating field required for start. winding – motors not for starting heavy loads. At 75% speed, a centrifugal device (starting switch) removes start winding. Split-phase motors, therefore, come only Motor then runs as a plain single-phase in small sizes. induction motor. 30-03-2024 Slide No. 39  PERMANENT-SPLIT CAPACITOR MOTOR Capacitor of this motor is left in Performance and speed regulation series with start winding during can be tailored by appropriate normal operation. capacitor value. Starting torque is quite low, No centrifugal switch is required. roughly 40% of full-load. Used on low-inertia loads such as fans and blowers. 30-03-2024 Slide No. 40  PERMANENT-SPLIT CAPACITOR MOTOR Another type of split-phase induction motor is the resistance-start motor. Start winding positioned at right angles to main winding. Start winding is switched in and out of circuit (75%) as in capacitor-start motor. Electrical phase shift between currents in two windings is obtained by making impedance of windings unequal. Main winding has high inductance & low resistance (I lags V by large angle). Start winding has low inductance & high resistance (I lags V by smaller angle). Starting torque is not as great as it is in capacitor-start motor. 30-03-2024 Slide No. 41  SHADED-POLE INDUCTION MOTOR First effort in development of a self- starting, single phase motor. It has field poles that extend inward from motor housing. In addition, a portion of each pole is encircled with a heavy copper ring. Copper ring causes magnetic field through ringed portion of pole face to lag appreciably behind that of other half of the pole. Results in slight rotation in the field that is strong enough to cause rotation. Torque created is small but enough to start motor – overall, motor is not efficient. To reverse direction, shading ring placed on other pole face. 30-03-2024 Slide No. 42  SPEED CONTROL Induction motors are practically fixed Where accurate speed control is needed, speed devices. a frequency converter is used. The there are practically only 2 methods A frequency converter can run a 3 Ø AC to change their rotation speed: motor over wide speed range quite well. use a frequency converter (VSD – Frequency converters do NOT work with Variable Speed Drive), or 1 Ø AC induction motors. use a motor with separate windings for different speeds 30-03-2024 Slide No. 43  DIRECTION OF ROTATION – 3 PHASE Direction of rotation depends upon direction of the rotating field. If direction is reversed then rotor will follow in reverse direction. Can be achieved by reversing any two (2) phases. 30-03-2024 Slide No. 44  REVERSING A SPLIT-PHASE MOTOR With some split-phase motors, a switch can select direction of rotation. Phase b to lag a, or b to lead a – thus causes motor to follow leading current. Only available on some single phase motors. Many motors’ start and run windings are physically different. Centrifugal switch must also be considered, if used. 30-03-2024 Slide No. 45  INTRODUCTION Now that you have completed this topic you should be able to: 3.18.1 Describe the construction, operation and characteristics of the following AC motors: Single phase and polyphase synchronous Single phase and polyphase induction 3.18.2 Describe methods for control of speed and direction of rotation in AC motors 3.18.3 Describe methods of producing a rotating field using the following: Capacitor Inductor Shaded or split pole 30-03-2024 Slide No. 46  This concludes: Module 3 Electrical Fundamentals II Topic 3.18 – AC Motors