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Lesson Objectives : Describe the construction and properties of various types of alternating current motor Explain the principle of operation of various types of a.c motor Perform a basic connection and test for ac motor 6.1 Introduction The major parts of all electrical power generated by ac, so most motors are designed for ac operation. Ac motors can duplicate the operation of dc motors and also are less troublesome to operate. Dc machines encounter difficulties from the action of commutation, which involves brushes, brush holders, neutral planes, etc. However, ac motors usually operate well over a very narrow speed range. Ac motors are particularly well suited for constant-speed applications, since the speed is determined by the frequency of the ac applied to the motor terminal. It is also manufactured with variable speed characteristics within certain limits. Ac motor can be designed to operate from a single-phase or a multi-phase ac supply. Whether the motor is single phase or multi phase, it operates on the same principle: that the ac applied to the motor generates a rotating magnetic field, and this rotating magnetic field causes the rotor of the motor to turn. 6.2 Types of motor Generally ac motors are classified into two basic types: 1. The synchronous motor 2. The induction motor Figure 6.1 Types of ac motor Electric motors and generators whose speeds are exactly synchronized with the line frequency are called synchronous machines. The synchronous motor is an ac generator operated as a motor. In it, ac is applied to the stator and dc is applied to the rotor. The induction motor differs from the synchronous motor in that its rotor is not connected to any source of power, but is powered by magnetic induction. The ac series motor, which is widely used for some appliances and small tools, is a modified version of the dc series motor. It has the advantage of readily adjustable speed, and also be used in applications where the dc series motor is used. Between the two basic types of ac motors mentioned, the induction motor is more commonly used. 6.3 Concept of rotating field Figure 6.2 Three-phase stator winding to which three-phase ac is applied The windings are physically spaced 120o apart and are connected in delta. The two windings in each phase are wound in the same direction. At any instant the magnetic field generated by one particular phase depends on the current through that phase. If the current is zero, the magnetic field is zero. If the current is a maximum, the magnetic field is a maximum. Since the currents in the three windings are 120o out of phase, the magnetic fields generated will also be 120o out of phase. Then, the three magnetic fields that exist at any instant will combine to produce one field, which acts on the rotor. Figure 6.3 Waveform of the three-phase ac current applied to the stator windings These waveforms are 120o out of phase with each other. Actually the waveform can represent either the three alternating magnetic fields generated by the three phases, or the currents in the phases. At point 1: Waveform C is positive and waveform B is negative. The current flow flows in opposite direction through phases B and C. The polarity is shown that B1 is a north pole and B is a south pole, and C is a north pole and C1 is a south pole. No current flowing through phase A, its magnetic field is zero. The magnetic fields leaving poles B1 and C will move toward the nearest south poles C1 and B as shown. The magnetic fields of B and C are equal in amplitude; the resultant magnetic field will lie between two fields and will have the direction shown. At point 2: 60o later, the input current waveforms to phases A and B are equal and opposite, and waveform C is zero. This change in current value per phase causes the flux to shift 60o in a clockwise direction. The resultant magnetic field has rotate through 60o as well. At point 3: Waveform B is zero, and the resultant magnetic field has rotated through another 60o. Following this procedure for one complete cycle (corresponding from point 1 to 7) will show that the magnetic field rotates 360o, or one complete revolution, in a clockwise direction. The conclusion is that the application of three-phase ac to three winding symmetrically spaced around a stator causes a rotating magnetic field to be generated. Two-phase system will also generate a rotating magnetic field. In fact, any number of phases will generate a rotating field. However, single phase system will not start, therefore, special arrangements are necessary in single-phase ac motors to make them operate properly. 6.4 Speed of the rotating magnetic field The field rotates 1 cycle for 1 current cycle. If the current were supplied from a 60-Hz source, the field would rotate 60 times per second or 3,600 times per minute. However if the number of stator coils were doubled (a 4-pole machine), the field would rotate half as fast. A 2-pole machine operating at 60Hz has speed of 3,600 rpm and a 4-pole machine has a speed of 1,800 rpm. For a given frequency output, the prime mover speed went down in proportion to the number of poles. To obtain lower speeds it is necessary to increase the number of poles on the stator. This speed is referred to as the synchronous speed of the motor. It can be determined by using the following formula; N = 120F/P 6-1 Where N = synchronous speed (rev/min) F = Frequency of the supply current (Hz) P = Number of poles on the stator The revolving field provides the driving force for most ac motors; hence, within narrow limits most ac motors are constant-speed devices. Example 6.1 What is the synchronous speed of a 12-pole, 60 Hz, squirrel cage induction motor? 6.5 A synchronous motor A synchronous motor is so called because its rotor is synchronized with the rotating field set up by the stator. The construction of the synchronous motors is essentially the same as the construction of the salient-pole alternator. In fact, such an alternator may be run as an ac motor, see figure 6.4. Synchronous motors have the characteristic of constant speed between no load and full load. They are capable of correcting the low power factor of an inductive load when they are operated under certain conditions. Figure 6.4 A synchronous motor 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 multiphase machines. The discussion that follows is based on a three-phase design. To understand how the synchronous motor works, assume that the application of threephase ac power to the stator causes a rotating magnetic field to be set up around the rotor. The rotor is energized 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. It works this way once it's started. However, one of the disadvantages of a synchronous motor is that it cannot be started from a standstill by applying three-phase ac power to the stator. When ac is applied to the stator, a high-speed rotating magnetic field appears immediately. 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 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 type of 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 in figure 6.5. Figure 6.5 The squirrel cage rotor winding 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. 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. To start a practical synchronous motor, the stator is energized, but the dc supply to the rotor field is not energized. The squirrel-cage windings bring the rotor to near synchronous speed. At that point, the dc field is energized. 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. 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. 6.6 Induction Motor The induction motor is the most commonly used ac motor because of its simplicity, its robust construction, and its low manufacturing cost. These characteristics of the induction motor are due to the fact that the rotor is self-contained and usually not connected physically to the external source of voltage. The induction motor derives its name from the fact that ac currents are induced in the rotor circuit by the rotating magnetic field in the stator. The stator constructions of the induction motor of the synchronous motor are almost identical, but their rotors are different. The rotor of the induction motor is a laminated cylinder with slots in its surface. The windings in these slots are of two types. The most common is called a squirrel cage winding, which is made up of heavy copper bars connected together at either end by a metal ring made of copper or brass, see figure 6.6. No insulation is required between the core and the bars because of the very low voltages generated in these rotor bars. The air gap between the rotor and the stator is kept very small so as to obtain maximum field strength. The other type of winding contains coils placed in the rotor slots. This type of rotor is called a wound rotor as shown in figure 6.6. Figure 6.6 The rotors used in the induction motor Regardless of the type of rotor used, the basic principle of operation is the same. The rotating magnetic field generated in the stator induces an emf in the rotor. The induce current in the rotor circuit sets up a magnetic field. The two fields interact and cause the rotor to turn. Wound rotor motors often have slip rings connecting the windings to external resistances. The variable resistances provide a means for increasing rotor resistance during starting to give better starting characteristics. When the motor is up to speed, the windings are shorted and the operation is like a squirrel cage rotor. When ac is applied to the stator winding, a rotating magnetic field is generated. This rotating field cuts the bars of the rotor and induces a current in rotor, see figure 6.7 Figure 6.7 Inducing a field in a rotor The induced current will generate a magnetic field around the conductors of the rotor, which will try to line up with the stator field. However, since the stator field is rotating continuously, the rotor must always follow along behind it. From Lenz’s law, any induced current tries to oppose the changing field that induces it. The force exerted on the rotor by the reaction between the rotor and the stator fields will set about trying to cancel out the continuous motion of the stator field. That is to say, the rotor will move in the same direction as the stator field, and try to line up with it. In practice, it gets as close to the moving stator field as its weight and its load will allow. Dc motor and synchronous motor gets their armature current by means of conduction. The induction motor receives its rotor current by induction. 6.7 Induction motors slip The difference between the synchronous speed and the rotor speed is called the slip of the rotor and stated in revolutions per minute or percent. The percentage of slip calculated as follows: %S = (n1 – n2)/n1 X 100 6-2 Where: %S = percentage of slip n1 = synchronous speed, in revolutions per minute (rev/min) n2 = rotor speed, in revolutions per minute (rev/min) Slip in revolutions per minute is calculated as follows: S = n1 – n2 6-3 Where: S = slip, in revolution per minute (rev/min) n1 = synchronous speed, in revolutions per minute (rev/min) n2 = rotor speed, in revolutions per minute (rev/min) The rotor frequency is directly proportional to the slip, therefore, fr = Sfs 6-4 Where fr = rotor frequency S = slip percent fs = slip frequency Example 6.2 Calculate the percentage of slip and the rotor frequency of a 50 Hz, 8 pole motor operating at 840rev/min.