AC Motors PDF

AC Motors (3.18) Learning Objectives 3.18.1.1 Describe the construction, operation and characteristics of AC synchronous motors (Level 2). 3.18.1.2 Describe the construction, operation and characteristics of AC induction motors (Level 2). 3.18.1.3 Describe the construction, operation and characteris...

AC Motors (3.18) Learning Objectives 3.18.1.1 Describe the construction, operation and characteristics of AC synchronous motors (Level 2). 3.18.1.2 Describe the construction, operation and characteristics of AC induction motors (Level 2). 3.18.1.3 Describe the construction, operation and characteristics of AC motors both single and polyphase (Level 2). 3.18.2.1 Describe methods of speed control (Level 2). 3.18.2.2 Describe methods of controlling direction of rotation (Level 2). 3.18.3.1 Describe methods of producing a rotating field: capacitor (Level 2). 3.18.3.2 Describe methods of producing a rotating field for an inductor (Level 2). 3.18.3.2 Describe methods of producing a rotating field: shaded or split pole (Level 2). 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 258 of 284 AC Motor Theory I Advantages of AC Motors There are advantages to the use of AC motors besides the wide availability of AC power. In general, AC motors cost less than DC motors. Most types of AC motors do not use brushes and commutators. This eliminates many problems of maintenance and wear. It also eliminates the problem of dangerous sparking. An AC motor is particularly well suited for constant-speed applications because its speed is determined by the frequency of the AC voltage applied to the motor terminals. Industry builds AC motors in different sizes, shapes and ratings for many different types of jobs. These motors are designed for use with either polyphase or single-phase power systems. Induction motors, single-phase or polyphase, whose rotors are energised by induction, are the most commonly used AC motors. Synchronous motors, for the purposes of this subject, may be considered polyphase motors of constant speed whose rotors are energised with DC voltage. Exploded view of an AC induction motor 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 259 of 284 Three-Phase Rotating Fields The three-phase induction motor also operates on the principle of a rotating magnetic field. The stator's windings can be connected to a three-phase AC input and have a resultant magnetic field that rotates. The upper and lower left figures in the image show the individual windings for each phase. The lower right figure shows how the three phases are tied together in a Y-connected stator. Aviation Australia Three-phase windings The dot in each diagram indicates the common point of the Y-connection. The individual phase windings are equally spaced around the stator. This places the windings 120° apart. Use the left-hand rule for determining the electromagnetic polarity of the poles at any given instant. In applying the rule to the coils in the image, consider that current flows towards the terminals for positive voltages, and away from the terminals for negative voltages. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 260 of 284 Three-Phase Rotation Analysis Three-phase rotation The results of this analysis are shown for voltage points 1 through 7. At point 1, the magnetic field in coils 1-1A is at a maximum with polarities as shown. At the same time, negative voltages are being felt in the 2-2A and 3-3A windings. These create weaker magnetic fields, which tend to aid the 1-1A field. At point 2, maximum negative voltage is being felt in the 3-3A windings. This creates a strong magnetic field which, in turn, is aided by the weaker fields in 1-1A and 2-2A. At each point of the rotation, it can be seen that the resultant magnetic field is rotating in a clockwise direction. When the three-phase voltage completes one full cycle (point 7), the magnetic field has rotated through 360°. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 261 of 284 Rotor Movement To explain rotor movement (using the diagram above), let us assume we can mount a bar magnet in the centre of the stator so that it is free to rotate. Let us also assume that the bar magnet is aligned so that its south pole is opposite the north pole of the stator field. You can see that this alignment is natural. Unlike poles attract, and the two fields are aligned so that they are attracting. Rotating from point 1 through point 7, the stator field rotates clockwise. The bar magnet, free to move, will follow the stator field because the attraction between the two fields continues to exist. A shaft running through the pivot point of the bar magnet would rotate at the same speed as the rotating field. This speed is known as synchronous speed. The shaft represents the shaft of an operating motor to which the load is attached. Rotor rotation This explanation is an oversimplification to show how a rotating field can cause mechanical rotation of a shaft. Such an arrangement using a permanent magnet would work, but it is not used due to limitations associated with a permanent magnet rotor. Practical motors use other methods, as we shall see in the next paragraphs. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 262 of 284 Speed of AC Motors The magnetic field inside an AC motor rotates at a speed related to the frequency of the current supply. This field rotation speed is called the synchronous speed (Ns), and the rotational speed of the motor shaft is related to it. f × 120 NS = P In the above equation, Ns is the synchronous speed in rpm, f is the frequency in Hz and P is the number of poles per phase. The per-phase element of this last parameter is crucial. A three-phase motor with one pair of poles per phase has six poles, but rotates at the same speed as a single-phase motor with one pair of poles (i.e. two poles). Failure to appreciate this fact will cause speed calculation values to be three times too small. Synchronous Motors Some AC motors operate almost exactly at the synchronous speed; these are called synchronous motors. Asynchronous Motors Asynchronous motors operate at something less than the synchronous speed. The percentage difference between the full-load speed and the synchronous speed is called slip. Normal slip is around 5%, though it can be much higher. The difference between synchronous motors and asynchronous motors is one of fundamental design, not build quality. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 263 of 284 AC Motor Speed Control Induction motors are practically fixed-speed devices. There are only two practical methods to change the rotation speed of AC induction motors: Use a frequency converter. Use a motor with separate windings for different speeds. In some applications, motors with dual-speed windings are used. For applications in which accurate speed control is needed, you need a frequency converter. Variable speed motor A frequency converter can run a three-phase AC motor at a wide speed range quite well (the performance of the motor is usually reduced outside its optimal operation speed). A frequency converter does not work with AC induction motors that are run from a single-phase power source because the operation of the required motor phase conversion capacitor is very frequency sensitive – it works as expected only at normal mains frequency. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 264 of 284 Direction of Rotation Direction of Rotation in Three-Phase Motors The direction of rotation depends upon the direction of the rotating field. If the direction is reversed, then the rotor will follow in the reverse direction. This can be achieved by reversing any two of the phase terminals. Reversing a three-phase motor Reversing a Split-Phase Motor With the split-phase motor, a switch can select Phase B to lag A, or the B stator winding can be switched to carry or lead Phase A and thus cause the motor to follow the leading current. This option is only available on some single-phase motors due to the physical location (90°) of the windings. Many motors’ start and run windings are physically different so therefore cannot be reversed. Reversing a split phase motor 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 265 of 284 Induction Motors The Induction Motor The induction motor is the most commonly used type of AC motor. Its simple, rugged construction is relatively inexpensive to manufacture. The induction motor has a rotor that is not connected to an external source of voltage. The induction motor derives its name from the fact that AC voltages are induced in the rotor circuit by the rotating magnetic field of the stator. In many ways, induction in this motor is similar to the induction between the primary and secondary windings of a transformer. Large motors and permanently mounted motors that drive loads at fairly constant speed are often induction motors. Examples are found in washing machines, refrigerator compressors, bench grinders and table saws. Induction motors are probably the simplest and most rugged of all electric motors. There are only two main components: the stator and the rotor. Stator windings 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 266 of 284 Induction Motor Stator The stator contains a pattern of copper coils arranged in windings. As alternating current is passed through the windings, a moving magnetic field is formed near the stator. This induces a current in the rotor, creating its own magnetic field. The interaction of these fields produces a torque on the rotor. Note that there is no direct electrical connection between the stator and the rotor. Induction Motor Rotor The induction rotor is made of a laminated cylinder with slots in its surface. The windings in these slots are one of two types. The most common is the squirrel-cage winding. This entire winding is made up of heavy copper bars connected together at each end by a metal ring made of copper or brass. No insulation is required between the core and the bars. This is because of the very low voltages generated in the rotor bars. The size of the air gap between the rotor bars and stator windings necessary to obtain the maximum field strength is small. The other type of winding contains actual coils placed in the rotor slots. The rotor is then called a wound rotor. Induction motor rotor Regardless of the type of rotor used, the basic principle is the same. The rotating magnetic field generated in the stator induces a magnetic field in the rotor. The two fields interact and cause the rotor to turn. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 267 of 284 Squirrel-Cage Rotor The squirrel-cage rotor is constructed of a number of conducting bars running parallel to the axis of the motor and two conducting rings on the ends. Typically, the conductors may be copper or aluminium. The assembly resembles a squirrel cage, thus giving this type of motor its name. The rotor bars, short-circuited at each end by a solid ring, are often made of copper strip welded to copper or brass rings, but for small to medium-sized motors they may be cast in one piece out of aluminium. © Jeppesen Squirrel-cage rotor The induction motor rotor is made of a laminated cylinder with slots in its surface. The windings in the slots are one of two types. The most commonly used is the "squirrel-cage" rotor. This rotor is made of heavy copper bars that are connected at each end by a metal ring made of copper or brass. No insulation is required between the core and the bars because of the low voltages induced into the rotor bars. The size of the air gap between the rotor bars and stator windings necessary to obtain the maximum field strength is small. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 268 of 284 Rotor construction Slanted Rotor The main purpose for slanting the conductors in the rotor is to ensure a smooth, steady acceleration during starting. Varying the physical design features of the rotor, for example, increases their inductance and gives a lower starting current, but at the same time creates a lower pull-out torque. Slanted rotor 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 269 of 284 Wound Rotor The wound rotor is typically used only on induction motors when the starting requirements are particularly severe. Its advantages are that the starting torque can be adjusted and the speed of the motor can be controlled over a wide range. Wound rotor Adjustments are readily made by simultaneously varying three resistors connected to the wound rotor via means of three slip rings. Open-circuit the resistances and the motor will not run. Vary the resistance and the motor varies in torque and speed. At speed, the windings can be short-circuited and the motor will run as a normal squirrel-cage motor. Note that increasing the resistance effectively lowers the speed of the motor at any given load. Aviation Australia Wound rotor schematic 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 270 of 284 Induction Motor Operation To obtain maximum interaction between fields, the rotor and stator air gap is very small. Refer Lenz's law: any induced EMF tries to oppose the changing field that induces it. In the case of an induction motor, the changing field is the motion of the resultant stator field. A force is exerted on the rotor by the induced EMF and the resultant magnetic field. This force tends to cancel the relative motion between the rotor and the stator field. The rotor, as a result, moves in the same direction as the rotating stator field. Rotating field stator An induction motor therefore cannot run at synchronous speed. It is impossible for the rotor of an induction motor to turn at the same speed as the rotating magnetic field. If the speeds were the same, there would be no relative motion between the stator and rotor fields; without relative motion, there would be no induced voltage in the rotor. In order for relative motion to exist between the two, the rotor must rotate slower than the rotating magnetic field. The difference between the speed of the rotating stator field and the rotor speed is called slip. The smaller the slip, the closer the rotor speed approaches the stator field speed. The speed of the rotor depends on the torque requirements of the load. The bigger the load, the stronger the turning force needed to rotate the rotor. The turning force can increase only if the rotor-induced EMF increases. This EMF can increase only if the magnetic field cuts through the rotor at a faster rate. To increase the relative speed between the field and rotor, the rotor must slow down. Therefore, the induction motor turns slower for heavier loads than for lighter loads. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 271 of 284 You can see from the previous statement that slip is directly proportional to the load on the motor. Actually, only a slight change in speed is necessary to produce the usual current changes required for normal changes in load. This is because the rotor windings have such a low resistance. As a result, induction motors are called constant-speed motors. Slip Common synchronous speeds for 50-Hz motors are 3000, 1500, 1000 and 750 rpm, depending on the number of poles per phase in the original design. In common terminology, e.g. a two-pole three-phase motor has six physical poles. As we have seen, the rotor is never able to reach synchronous speed. If it did, there would be no voltage induced in the rotor. No torque would be developed. The motor would not operate. The difference between rotor speed and synchronous speed is called slip. The difference between these two speeds is not great. For example, a rotor speed of 2700 to 2900 rpm can be expected from a synchronous speed of 3000 rpm. The formula for determining the slip is: S% = NS − N × 100 NS Where: S(%) is percentage slip NS is synchronous speed N is rotor speed. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 272 of 284 Synchronous Motors The Synchronous Electric Motor The construction of the synchronous motor is essentially the same as the construction of the salientpole alternator. In fact, such an alternator may be run as an AC motor. It is similar to the drawing below. Synchronous motor Synchronous motors have the characteristic of constant speed between no-load and full-load. They are often used to drive DC generators. Synchronous motors are designed in sizes up to thousands of horsepower. They may be designed as either single-phase or polyphase machines. The discussion that follows is based on a three-phase design. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 273 of 284 Synchronous Motor Operation To understand how the synchronous motor works, assume that the application of three-phase AC power to the stator causes a rotating magnetic field to be set up around the rotor. The rotor is energised with DC (it acts like a bar magnet). The strong rotating magnetic field attracts the strong rotor field activated by the DC. This results in a strong turning force on the rotor shaft. The rotor is therefore able to turn a load as it rotates in step with the rotating magnetic field. Rotating magnet A synchronous motor works this way once it is started. But a disadvantage of this type of motor is that it cannot be started from standstill by applying three-phase AC power to the stator. When AC is applied to the stator, the high-speed rotating magnetic field appears instantly. This rotating field rushes past the rotor poles so quickly that the rotor does not have a chance to get started. In effect, the rotor is repelled first in one direction and then in the other. A synchronous motor in its purest form has no starting torque. It has torque only when it is running at synchronous speed. A squirrel-cage winding is added to the rotor of a synchronous motor to cause it to start. The squirrel cage is shown as the outer part of the rotor. It is 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. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 274 of 284 Synchronous motor rotor A low voltage is induced in these shorted windings by the rotating three-phase 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 the stator. Because of the interaction, the rotor begins to turn, following the stator field; the motor starts. We will run into squirrel cages again in other applications, where they will be covered in more detail. To start a practical synchronous motor, the stator is energised, but the DC supply to the rotor field is not energised. The squirrel-cage windings bring the rotor to near-synchronous speed. At that point, the DC field is energised. This locks the rotor in step with the rotating stator field. Full torque is developed, and the load is driven. A mechanical switching device that operates on centrifugal force is often used to apply DC to the rotor as synchronous speed is reached. Synchronous motor rotor 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 275 of 284 The rotor is usually made with salient poles. When excited with DC, the rotor produces alternate north and south magnetic poles, which are attracted to those produced in the stator. The practical synchronous motor has the disadvantage of requiring a DC exciter voltage for the rotor. This voltage may be obtained either externally or internally, depending on the design of the motor. The synchronous motor, as its name implies, operates at exactly synchronous speed with no slip. The rotor is of a constant polarity (either a permanent magnet or an energised electromagnet) and the windings of the stator are wrapped in a way that produces a rotating magnetic field. Such motors provide very little torque at zero speed, and thus need some kind of separate starting apparatus. Often a squirrel-cage rotor is built into the main rotor. When the motor reaches a few percent of synchronous speed, the rotor is energised and the squirrel cage becomes ineffective. A synchronous motor will run at speed regardless of load variations up to a point called the pull-out torque. The maximum value of torque that a motor can develop without losing synchronism is called its pull-out torque. A load higher than this will pull the motor out of synchronism and cause it to stop. © Jeppesen Synchronous motor schematic The main difference between the synchronous motor and the induction motor is that the rotor of a synchronous motor travels at the same speed as the rotating magnetic field. This is possible because the magnetic field of the rotor is not induced. The rotor has either permanent magnets or DC excited currents, which are forced to lock into a certain position when confronted with another magnetic field. Thus the problem with slip and speed variation with varying loads is solved. However, this introduces new problems, such as bringing the motor up to speed when connected to a 50-Hz commercial line. With the extra expense of a starter mechanism, synchronous motors overcome this problem. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 276 of 284 Applications in which constant speed is necessary or two or more motors need to be in sync are ideal for the synchronous motor. Besides a direct commercial line power source, there are other options to obtain different varieties of control. As already known, the synchronous motor rotor is locked into step with the rotating magnetic field and must continue to operate at synchronous speed for all loads. Aviation Australia Torque angle During no-load conditions, the centre lines of a pole of the rotating magnetic field and the DC field pole coincide. As load is applied to the motor, there is a backward shift of the rotor pole relative to the stator pole. There is no change in speed. The angle between the rotor and stator poles is called the torque angle. If the mechanical load on the motor is increased to the point that the rotor is pulled out of synchronism, the motor will stop. The maximum value of torque that a motor can develop without losing synchronism is called its pull-out torque. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 277 of 284 AC Motor Theory II Star and Delta Windings A star-wound stator winding will draw less current than a delta-wound type. It is therefore common to use a star-delta arrangement to start large 3-Ø induction motors. This limits starting current on initial start (star) and then (at approx. 75%) switches over to delta at speed. Full voltage is then achieved across each winding. Star and delta windings For a 208-V star connection, voltage is effectively reduced to 58%, or 115 V. If the star connection has sufficient torque to run up to 75% or 80% of full load speed, then the motor can be connected in delta mode. When connected to delta configuration, the phase voltage increases by a ratio of √3, or 173%. The phase currents increase by the same ratio. The line current increases three times its value in star connection. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 278 of 284 Single-Phase Induction Motors There are probably more single-phase AC induction motors in use today than the total of all the other types put together. It is logical that the least expensive, lowest maintenance type of AC motor should be used most often. The single-phase AC induction motor fits that description. Single-phase induction motors Unlike in polyphase induction motors, the stator field in the single-phase motor does not rotate. Instead it simply alternates polarity between poles as the AC voltage changes polarity. Voltage is induced in the rotor as a result of magnetic induction, and a magnetic field is produced around the rotor. This field will always be in opposition to the stator field (Lenz's law applies). Single-phase windings If the rotor is rotated by some outside force (e.g. a twist of your hand), the push-pull along the line in view A, is disturbed. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 279 of 284 Look at the fields as shown in view B. At this instant the South Pole on the rotor is being attracted by the left-hand pole. The north rotor pole is being attracted to the right-hand pole. All of this isAs a result of the rotor being rotated 90° by the outside force, the pull between the two fields becomes a rotary force, turning the rotor towards magnetic correspondence with the stator. Because the two fields continuously alternate, they will never actually line up, and the rotor will continue to turn once started. It remains for us to learn practical methods of getting the rotor to start. There are several types of single-phase induction motors in use today. Basically they are identical except for the means of starting. In this topic we will discuss the split-phase and shaded-pole motors, so named because of the methods employed to get them started. Once they are up to operating speed, all single-phase induction motors operate the same way. Split-Phase Induction Motors One type of induction motor, which incorporates a starting device, is called a split-phase induction motor. Split-phase motors are designed to use inductance, capacitance or resistance to develop a starting torque. The principles are those you learned in your study of alternating current. Typically, the start winding is disconnected when the motor reaches 75% of its rated speed. Split-phase motor Capacitor-Start Split-Phase Motors The first type of split-phase induction motor that will be covered is the capacitor-start type. Capacitor start split-phase motor 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 280 of 284 The stator consists of the main winding and a starting winding (auxiliary). The starting winding is connected in parallel with the main winding and is placed physically at right angles to it. A 90° electrical phase difference between the two windings is obtained by connecting the auxiliary winding in series with a capacitor and starting switch. When the motor is first energised, the starting switch is closed. This places the capacitor in series with the auxiliary winding. The capacitor is of such value that the auxiliary circuit is effectively a resistive-capacitive circuit (referred to as capacitive reactance and expressed as XC). In this circuit the current leads the line voltage by about 45° (because XC about equals R). The main winding has enough resistance-inductance (referred to as inductive reactance and expressed as XL) to cause the current to lag the line voltage by about 45° (because XL about equals R). The currents in each winding are therefore 90° out of phase, and so are the magnetic fields that are generated. The effect is that the two windings act like a two-phase stator and produce the rotating field required to start the motor. When nearly full speed is obtained (75%), a centrifugal device (the starting switch) cuts out the starting winding. The motor then runs as a plain single-phase induction motor. Since the auxiliary winding is only a light winding, the motor does not develop sufficient torque to start heavy loads. Split-phase motors, therefore, come only in small sizes. Due to their generally desirable characteristics, they are also used for many applications in which high starting torque may not be required. The capacitor start motor can usually be recognised by the bulbous protrusion on the frame where the starting capacitor is located. Permanent-Split Capacitor Motors The capacitor of this motor is left in series with the starting winding during normal operation. The starting torque is quite low, roughly 40% of full-load, so low-inertia loads such as fans and blowers make common applications. Running performance and speed regulation can be tailored by selecting an appropriate capacitor value. No centrifugal switch is required. © Aviation Australia Permanent split capacitor motor 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 281 of 284 Resistance-Start Motors Another type of split-phase induction motor is the resistance-start motor. This motor also has a starting winding in addition to the main winding. It is switched in and out of the circuit just as it was in the capacitor-start motor. The starting winding is positioned at right angles to the main winding. The electrical phase shift between the currents in the two windings is obtained by making the impedance of the windings unequal. Resistor start schematic The main winding has a high inductance and a low resistance. The current, therefore, lags the voltage by a large angle. The starting winding is designed to have a fairly low inductance and a high resistance. Here the current lags the voltage by a smaller angle. For example, suppose the current in the main winding lags the voltage by 70°. The current in the auxiliary winding lags the voltage by 40°. The currents are, therefore, out of phase by 30°. The magnetic fields are out of phase by the same amount. Although the ideal angular phase difference is 90° for maximum starting torque, the 30° phase difference still generates a rotating field. This supplies enough torque to start the motor. When the motor comes up to speed, a speedcontrolled switch disconnects the starting winding from the line, and the motor continues to run as an induction motor. The starting torque is not as great as it is in the capacitor-start. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 282 of 284 Shaded-Pole Induction Motors The first effort in the development of a self-starting, single-phase motor was the shaded-pole induction motor. It has field poles that extend inward from the motor housing. In addition, a portion of each pole is encircled with a heavy copper ring. Shaded-pole motor The presence of the copper ring causes the magnetic field through the ringed portion of the pole face to lag appreciably behind that of the other half of the pole. This results in a slight component of rotation in the field that is strong enough to cause rotation. Although the torque created by this field is small, it is enough to accelerate the rotor to its rated speed. © Jeppesen Shaded pole 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 283 of 284 The direction of rotation has to be reversed by altering the direction of the rotating magnetic field across the pole face. This is done by shifting the shading ring to the other side of the pole face. Some poles are fitted with slots on both sides for this purpose. The shaded pole motor is simple and inexpensive, but has low efficiency and a very low starting torque. Speed regulation is poor, and it must be fan-cooled during normal operation. Shaded-pole motors are thus used in shaft-mounted fans and blowers, small pumps, toys and intermittently used household items. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 284 of 284