PIA Module 3 (Electrical Fundamentals) Sub Module 3.18 (AC

For Training Purpose Only ISO 9001:2008 Certified

CAA Approval No: HQCAA/2231/44/AW Dated: 11th Sept, 09

Module 3 – ELECTRICAL FUNDAMENTALS Category – Aerospace/Avionics Sub Module 3.18 – AC Motors

3.18 Rev. 00 Nov 2009

Training Centre

MODULE 3

Sub Module 3.18

AC MOTORS




3.18 - i Rev. 00 Nov 2009

Training Centre

Contents

INTRODUCTION --------------------------------------------------------------------- 1

PRODUCTION OF A ROTATING FIELD ------------------------------------------ 1

TYPES OF AC MOTOR -------------------------------------------------------------- 3

THE SYNCHRONOUS MOTOR ---------------------------------------------------- 3

INDUCTION MOTORS -------------------------------------------------------------- 4

TWO-PHASE INDUCTION MOTOR ---------------------------------------------- 6

SINGLE-PHASE INDUCTION MOTORS ------------------------------------------ 8

SHADED-POLE INDUCTION MOTOR ------------------------------------------- 11

HYSTERESIS MOTORS ------------------------------------------------------------- 13

SINGLE-PHASE COMMUTATOR MOTOR ------------------------------------- 13




3.18 - 1 Rev. 00 Nov 2009

Training Centre

INTRODUCTION The basic principles of magnetism and electromagnetic induction are the same for both ac and dc motors, but the application of the principles is different because of the rapid reversals of direction and changes in magnitude characteristic of alternating current. Certain characteristics of ac motors make most types more efficient than dc motors, therefore such motors are used commercially whenever possible. During recent years, ac power systems have been developed for large aircraft with the result that a much larger amount of electrical power is available on aircraft than would be available with dc systems of the same weight. Thus one of the main advantages of the ac power system is that it provides more power for less weight. PRODUCTION OF A ROTATING FIELD A rotating field may be produced by applying a three-phase supply to a three-phase stator. The field produced is of unvarying strength and its speed of rotation depends upon the frequency of the supply.

FIG 1 TYPICAL THREE-PHASE STATOR

Fig 1(a) shows a typical three-phase stator. The two windings in each phase (for example A and A1) are connected in series and are so wound that current flowing through the two windings produces a North pole at one of them and a South pole at the other. So, if a current is flowing in the A phase in the direction from the A to the A1 terminals, pole piece A becomes a North Pole and A1 a South pole. The three-phase stator is connected in delta, so that only three terminals, each common to two of the windings, are provided for the three-phase ac input. At any instant, the magnetic field generated by one particular phase is proportional to the current in that phase. Therefore, as the current alternates, so does the magnetic field. As the currents in all three phases are 120 out of phase with each other, then so must the magnetic fields be and the resultant magnetic field will be the vector sum of these three.




3.18 - 2 Rev. 00 Nov 2009

Training Centre

FIG 2 OUTPUT FROM THREE-PHASE STATOR

From earlier studies, it will be remembered that the flux path follows the line of least resistance and this can be clearly seen in Fig 2. At position 1, phase A of the input supply is inactive with both B and C phases providing an output. The two flux paths B-C and B1-C1 are the lines of least reluctance and magnetically form a single resultant axis with which any permeable material located within its sphere of influence would tend to align. Staying with position 1, the current in the A phase is zero, the current in the C phase is positive and flows in the direction C to

C1 and the current in the B phase is negative and flows in the direction B to B1. Equal currents therefore flow in opposite directions through the B and C windings and magnetic poles are established as shown in Fig 2. The shortest path for the magnetic lines of flux is such that the lines leave B1 (North Pole) and go to C1 (South Pole) with a similar result for C to B. Because the magnetic fields of the B and C phases are equal in amplitude (due to equal currents) the resultant field lies in the direction of the arrow. Moving on to position 2, where the supply cycle has advanced by 60, the current in C is now zero, A is positive and B is negative. The resultant magnetic field is produced in the same way as described for position 1, and the other positions show the conditions at intervals of 60. Thus, the magnetic field rotates one complete revolution (in a clockwise direction in this case) during one complete cycle of three-phase supply, so it is in time with, or synchronous with, the ac input. Example An input frequency of 50 Hz produces a field rotating at 50 revolutions per second or 3000 rpm.




3.18 - 3 Rev. 00 Nov 2009

Training Centre

TYPES OF AC MOTOR There are three principal types of ac motors. They are the commutator motor, the induction motor and the synchronous motor. THE SYNCHRONOUS MOTOR The ac generator, like the dc generator, is a reversible machine; if supplied with electrical energy, it runs as a motor. Thus synchronous motors have the same construction as rotating-field ac generators. The input alternating current is applied to the stator and the rotor carries the magnetic field windings which are supplied with dc from a separate source.

NOTE: The rotor may in theory (and practice) be either a permanent magnet or a wound rotor separately excited from a dc source.

If the rotor is energised with dc it acts like a bar magnet and will therefore try to line itself up with the magnetic field produced by the stator. In the synchronous motor the three-phase ac produces a rotating magnetic field, which causes the rotor to follow the field, (assuming that the motor is already running). The synchronous motor will not start of its own accord, because the rotating magnetic field moves too quickly to provide a starting force. The inertia of the rotor does not allow it to respond to the rapidly rotating field. It has to be started and run up to speed by another motor, usually a small induction motor.

When the speed of the driven rotor approaches that of the rotating magnetic field, the rotor and the field ‘lock together’ and the rotor then rotates synchronously with the field of its own accord.

FIG 1 SYNCHRONOUS MOTOR CHARACTERISTIC The synchronous motor is a ‘single-speed’ machine, its speed of rotation being determined by the speed of the rotating magnetic field which, in turn, is decided by the frequency of the three-phase ac input to the stator windings. The synchronous motor is therefore most useful for applications requiring constant speed, eg. fans for ventilation and gyroscopes.




3.18 - 4 Rev. 00 Nov 2009

Training Centre

Equally, it is clear that the synchronous motor is most appropriate to light mechanical loads, because if the load became excessive, the ‘synchronous lock’ would be broken and the motor would stop. INDUCTION MOTORS The ac motor most commonly used on aircraft is the induction type and, dependant upon application, may be designed for operation from a three-phase, two-phase or single phase power supply It is robust, simple and cheaper than other types. The basic three-phase induction motor has no slip rings or commuter and has little to go wrong. Fig 2(a) shows the stator of the induction motor, which is almost the same as that of the synchronous motor, ie. it has three-phase windings and associated pole pieces, which as usual produce a rotating magnetic field when supplied with three-phase ac. The rotor in Fig 2(b) consists of a series of heavy copper bars connected at each end by a copper or brass ring. No insulation is required between the bars and the core on which they are mounted because of the very low voltages induced in the rotor bars. This type of rotor is a squirrel-cage and no external electrical connections are made to it. The basic principle of operation of the induction motor may be explained by Fig 3 below, where a conductor is set at right angles to a magnetic field. If the conductor is stationary and the

field moves from right to left, the change of flux through the conductor induces a voltage in it. If the conductor is part of a closed circuit, current flows in the conductor in the

FIG 2 THREE-PHASE INDUCTION MOTOR direction shown (the right hand rule for generators). This current-carrying conductor in the magnetic field then experiences a force tending to move it in the same direction as




3.18 - 5 Rev. 00 Nov 2009

Training Centre

the field’s motion (the left-hand rule for motors). The conductor therefore tends to follow the movement of the field.

Fig 3 Movement of a Conductor in a Field

Applying this principle to the squirrel-cage induction motor we see that the rotating magnetic field produced by the stator induces a voltage in the bars of the rotor. Because the bars are thick and have a low resistance, a large current flows in them which set up a magnetic field. The rotor field interacts with the stator field and, as usual when a current-carrying conductor is placed in magnetic field, causes the rotor to turn so as to line up the two magnetic fields. However, since the stator field is rotating, the rotor never quite catches up but follows a little behind. As the rotor follows the field, the relative motion between the

two is reduced, so also is the voltage induced in the rotor bars; this reduces the rotor current and the turning force acting on the rotor. The rotor speed is automatically adjusted to something less than that of the rotating field, otherwise there would be no relative motion, no current and no movement of the rotor. Thus in practice the rotor runs slightly slower than the rotating magnetic field, the amount depending upon the load. The difference in the two speeds is the slip speed and the ratio of slip speed to the speed of the rotating field, is the slip. For example, if the magnetic field is rotating at 1000 rpm, the rotor may be running to 960 rpm. The slip speed is: 9601000 rpm, and the slip is;

%41001000

40

This is a typical value of slip. As noted earlier, the slip depends upon the load; the larger the load, the greater is the slip. But in practice very little speed change occurs between a light and a heavy load and the main use of an induction motor is as a constant speed drive to a load. This motor is only started under ‘no load’ conditions. The speed varies little between ‘no load’ and ‘full load’ when running and makes the motor suitable for driving such machines as lathes, bench drills and small generators.




3.18 - 6 Rev. 00 Nov 2009

Training Centre

The starting current of all squirrel-cage motors is heavy (4-6 times the running current). This is because, if the stator windings are energised from the three-phase supply whilst the rotor is stationary, the slip is maximum and so also is the emf induced in the rotor. The low resistance of the rotor gives rise to a large rotor current which produces a magnetic field opposing and weakening the stator flux (Lenz’s Law). The back emf induced in the stator windings by the changing flux is therefore reduced so that a heavy current is taken by the stator on starting.

TWO-PHASE INDUCTION MOTOR A rotating magnetic field is also produced if two phases, 90° out of phase with each other are used instead of a three-phase supply. A two-phase induction motor is illustrated below in Fig 4.

FIG 4 TWO-PHASE INDUCTION MOTOR The production of a rotating magnetic field from a two-phase supply, 90° out of phase, is shown in Fig 5. It is a similar idea to the one previously drawn and described for a three-phase supply, and its action may be deduced in a similar manner. Two-phase induction motors are less efficient than three-phase




3.18 - 7 Rev. 00 Nov 2009

Training Centre

types and the latter are used, where possible, in preference to two-phase motors. Typically, two-phase induction motors find their greatest applications in systems requiring a servo control of synchronous devices, for example as servomotors in power follow up synchro systems. In this instance the windings are also at 90° to each other but, unlike the motors thus far described, they are connected to different voltage sources. One source is the main supply for the system and, being of constant magnitude, it serves as a reference voltage. The other source serves as a control voltage and is derived from a signal amplifier in such a way that it is variable in magnitude and its phase can either lead or lag the reference voltage, thereby controlling the speed and direction of rotation of the field and rotor.

FIG 5 ROTATING MAGNETIC FIELD FROM A TWO-PHASE SUPPLY




3.18 - 8 Rev. 00 Nov 2009

Training Centre

SINGLE-PHASE INDUCTION MOTORS Single-phase induction motors are used extensively in low-power applications such as blowers and switch motors used in communication equipment. A single-phase induction motor has only one stator winding so it is not capable of producing a rotating magnetic field of the type described earlier. The field produced by the single-phase winding alternates according to the frequency of the supply, and can be said to alternate along the axis of the single winding, rather than to rotate. As the field changes polarity every half cycle, it induces currents in the rotor which tries to turn it through 180°, but as the force is exerted through the axis shown, there is no turning force on the rotor. This type of motor cannot, therefore be self-starting. If the rotor is given a start however, it will be given a push every half cycle that will keep it rotating. Since the field is pulsating, rather than rotating, single-phase induction motors produce a pulsating torque and are not as smooth running as two or three-phase motors.

FIG 6 SINGLE PHASE INDUCTION MOTOR It is impracticable to start a motor by turning it over by hand, so an electric device must be incorporated into the stator circuit such that it will cause a rotating field to be generated on starting. Once the motor has started, this device can be switched out of the stator, since the rotor and stator together will generate their own rotating field to keep the motor turning. The starting device takes the form of an auxiliary stator winding spaced 90° from the main winding, and connected in series with an impedance to the main supply. This impedance is chosen to produce as great a phase displacement as possible between




3.18 - 9 Rev. 00 Nov 2009

Training Centre

the currents in the main and auxiliary windings so that the machine starts up virtually as a two-phase motor. A switch, usually operated by centrifugal action, cuts out the auxiliary winding when approximately 75% of synchronous speed has been attained and the machine continues to run on the main stator winding. Alternatively, contacts in the auxiliary winding circuit may be closed by the high stator current which flows through a relay coil when the supply is switched on; the contacts opening as the motor current falls during acceleration from rest.

FIG 7 ELECTRIC STARTER USING AN AUXILIARY WINDING

The impedance device used can be inductive or capacitive, or a combination of both. Consider Fig 8 below, which shows a simplified schematic of a typical capacitor start motor. The stator consists of the main winding, and a starting winding which is connected in parallel with the main winding and spaced at right angles to it. The 90° electrical phase difference between the two windings is obtained by connecting the auxiliary winding in series with a capacitor and starting switch.

FIG 8 SCHEMATIC OF A CAPACITOR STARTER MOTOR




3.18 - 10 Rev. 00 Nov 2009

Training Centre

On starting, the switch is closed, placing the capacitor in series with the auxiliary winding. The capacitor is of such a value that the auxiliary winding is effectively a resistive-capacitive circuit in which the current leads the line voltage by approximately 45°. The main winding has enough inductance to cause the current to lag the line voltage by approximately 45°. The two currents are therefore 90° out of phase, and so are the magnetic fields which they generate. The effect is that the two windings act like a two-phase stator and produce the revolving field required to start the motor. When nearly full speed has been attained, a device cuts out the starting winding and the motor runs as a plain single-phase induction motor. Since the special starting winding is only a light winding, the motor does not develop sufficient torque to start heavy loads. Because a two-phase induction motor is more efficient than a single-phase motor, it is often desirable to keep the auxiliary winding permanently in the circuit so that the motor will run as a two-phase induction motor. The starting capacitor is usually made quite large, in order to allow a large current to flow through the auxiliary winding. The motor can thus build up a large starting torque. When the motor comes up to speed, it is not necessary that the auxiliary winding shall continue to draw the full starting current, and the capacitor can be reduced, therefore two capacitors are used in parallel for starting and one is cut out when the motor comes up to speed. Such a motor is called ‘capacitor-start, capacitor-run induction motor’

A disadvantage of this type of split-phase motor is the high starting current (nearly four times the full load current). The direction of rotation can be change by reversing the connections to either of the stator windings.




3.18 - 11 Rev. 00 Nov 2009

Training Centre

SHADED-POLE INDUCTION MOTOR The single-phase motors considered in the preceding sections all employed stators having uniform air gaps with respect to their rotor and stator windings, which are uniformly distributed around the periphery of the stator. The starting methods employed thus far were generally based on a split-phase principle of producing a rotating magnetic field to initiate rotor rotation. The great virtue of this motor lies in its utter simplicity; a single-phase rotor winding, a cast squirrel-cage rotor, and special pole pieces. No centrifugal switches, capacitors, special starting windings, or commutators are used. With but a single-phase winding it is inherently self-starting. There must be some auxiliary means of producing the effect of a rotating magnetic field, therefore, with a single-phase supply and only one stator winding. Figure 9 shows the general construction of a two-pole shaded-pole motor. The special pole pieces are made up of laminations, and a short-circuited shading coil (or a single-turn solid copper ring) is wound around the smaller segment of the pole piece. The shading coil, separated from the main ac field winding serves to provide a phase-splitting of the main field flux by delaying the change of flux in the smaller segment.

FIG 9 As shown in Fig 10b, when the flux in the field poles tend to increase, a short-circuit current is induced in the shading coil, which by Lenz’s law opposes the force and the flux producing it. Thus, as the flux increases in each field pole, there is a concentration of the flux in the main segment of each pole, while the shaded segment opposes the main field flux.




3.18 - 12 Rev. 00 Nov 2009

Training Centre

FIG 10 At point c shown in Fig 10a, the rate of change of flux and of current is zero, and there is no voltage induced in the shaded coil. Consequently, the flux is uniformly distributed across the poles. When the flux decreases, the current reverses in the shaded coil to maintain the flux in the same direction, as in Fig 10d. The result is that the flux crowds in the shaded segment of the pole. An examination of Figs 10b, c and d will reveal that at intervals b, c and d, the net effect of the flux distribution in the pole has

been to produce a sweeping motion of flux across the pole face representing a clockwise rotation. The flux in the shaded segment is always lagging the flux in the main segment in time as well as in physical space (although a true 90º relation does not exist between them). The result is that a rotating magnetic field is produced and the rotor always turns in the direction of the rotating field. For the type of shaded-pole motor shown in Fig 9, the rotation is clockwise since the flux in the shaded segment lags the main flux. In order to reverse the direction of rotation, it would be necessary to unbolt the pole structure and reverse it physically. The shaded-pole motor is rugged, inexpensive, and small in size, and it requires little maintenance. Unfortunately, it has a very low starting torque, low efficiency, and a low power factor. The last two considerations are not serious in a small motor.




3.18 - 13 Rev. 00 Nov 2009

Training Centre

HYSTERESIS MOTORS A Hysteresis motor works on the principle that in a material with a large Hysteresis loop, the magnetic flux lags behind the current which produced it by almost 90°, while in a material with a small Hysteresis loop the two are almost in phase. A stator of small Hysteresis loop material is supplied with a polyphase input, as is the rotor which is made of large Hysteresis loop material (usually cobalt steel). The result is that the flux in the rotor lags that in the stator by almost 90°. The rotor will then move in an attempt to line up its field with that of the stator. Thus, as the stator field rotates, the rotor follows it. The effect on the rotor of the rotating stator field is that if the rotor is stationary, or turning at a speed less than the synchronous speed, every point on the rotor is subjected to successive magnetising cycles. As the stator field reduces to zero during each cycle, a certain amount of flux remains in the rotor material, and since it lags on the stator field it produces a torque at the rotor shaft which remains constant as the rotor accelerates up to the synchronous speed of the stator field. This latter feature is one of the principle advantages of Hysteresis motors and for this reason they are chosen for such applications as autopilot servomotors, which produce mechanical movements of an aircraft’s flight control surfaces. When the rotor reaches synchronous speed, it is no longer subjected to successive magnetising cycles and in this condition it behaves as a permanent magnet.

SINGLE-PHASE COMMUTATOR MOTOR The synchronous and induction types of ac motor all have one thing in common - they are essentially single-speed or constant-speed motors, their running speed being determined by the frequency of the supply. Constant speed motors have many uses, but where a variable speed is required some other type of motor must be used. The commonest single-phase variable-speed motors are series or commutator motors. They are used as blower motors in communication equipment. In an ordinary dc motor the direction of rotation depends upon both the direction of the current in the armature windings and the direction of current in the field coils. If one changes direction, the direction of rotation is reversed, if both change direction together, the direction of rotation is not altered. When alternating current is applied to a series motor, the current through the armature and field change simultaneously and, therefore, the motor will rotate in one direction. The number of field turns in the ac series motor is less than in the dc series motor, in order to decrease the reactance of the field so that the required amount of current will flow. Cutting down the size of the field reduces the motor torque. The characteristics of the ac series motor are similar to those of the dc series motor. It is a varying-speed machine, with low speed for large loads and high speeds for light loads. The starting torque is also very high.




3.18 - 14 Rev. 00 Nov 2009

Training Centre

SINCE THE AC SERIES MOTOR HAS THE SAME GENERAL CHARACTERISTICS AS THE DC SERIES MOTOR, A SERIES MOTOR HAS BEEN DESIGNED WHICH CAN OPERATE BOTH ON AC AND DC. THIS AC/DC MOTOR IS CALLED A ‘UNIVERSAL MOTOR’ AND FINDS WIDE APPLICATION IN SMALL ELECTRIC APPLIANCES. UNIVERSAL MOTORS OPERATE AT A LOWER EFFICIENCY THAN EITHER THE AC OR DC SERIES MOTOR AND ARE BUILT IN SMALL SIZES ONLY.

PIA Module 3 (Electrical Fundamentals) Sub Module 3.18 (AC

Documents

Transcript of PIA Module 3 (Electrical Fundamentals) Sub Module 3.18 (AC