OPERATING PRINCIPLE Figure 6 illustrates schematically the operating principle of this type of relay. A movable armature is shown magnetized by current flowing in an actuating coil encircling the armature, and with such polarity as to close the contacts. a reversal of the polarity of the Contacts ACtuating coil Movable armature Control Polarizing Polarizing magnet To actuating-quantity source Fig. 6. Directional relay of the electromagnetic-attraction type. actuating quantity will reverse the magnetic polarities of the ends of the armature and cause the contacts to stay open. Although a " polarizing "or field, "coil is shown for magnetizing the pol magnet, this coil may be replaced by a permanent magnet in the section between x and y. There are many physical variations possible in carrying out this principle, one of them being a construction similar to that of a d-c motor The force tending to move the armature may be expressed as follows, if we neglect saturation: F=- ks F-net force where Kl-a force-conversion constant Ip-the magnitude of the current in the polarizing coil Ia- the magnitude of the current in the armature coil K2- the restraining force (including friction) At the balance point when F-o, the relay is on the verge of operating, and the operating u constant FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS 21
FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS 21 OPERATING PRINCIPLE Figure 6 illustrates schematically the operating principle of this type of relay. A movable armature is shown magnetized by current flowing in an actuating coil encircling the armature, and with such polarity as to close the contacts. A reversal of the polarity of the actuating quantity will reverse the magnetic polarities of the ends of the armature and cause the contacts to stay open. Although a "polarizing," or "field," coil is shown for magnetizing the polarizing magnet, this coil may be replaced by a permanent magnet in the section between x and y. There are many physical variations possible in carrying out this principle, one of them being a construction similar to that of a d-c motor. The force tending to move the armature may be expressed as follows, if we neglect saturation: F = K1I pI a – K2, F = net force where K1 = a force-conversion constant. Ip = the magnitude of the current in the polarizing coil. Ia = the magnitude of the current in the armature coil. K2 = the restraining force (including friction). At the balance point when F = 0, the relay is on the verge of operating, and the operating characteristic is: K2 I pI a = — = constant K1 Fig. 6. Directional relay of the electromagnetic-attraction type
Ip and la are assumed to flow through the coils in such directions that a pickup force is produced, as in Fig. 6. It will be evident that, if the direction of either p or la(but not of both)is reversed, the direction of the force will be reversed. Therefore, this relay gets its name from its ability to distinguish between opposite directions of actuating -coil current flow, or opposite polarities. If the relative directions are correct for operation, the relay will pick up at a constant magnitude of the product of the two current If permanent-magnet polarization is used, or if the polarizing coil is connected to a source that will cause a constant magnitude of current to flow, the operating characteristic K I a kilp la still must have the correct polarity, as well as the correct magnitude, for the relay to pick up EFFICIENCY This type of relay in much more efficient than hinged-armature or plunger relays, from th standpoint of the energy required from the actuating-coil circuit. For this reason, such directional relays are used when a d-c shunt is the actuating source, whether directional action is required or not. Occasionally, such a relay may be actuated from an a-c quantity through a full-wave rectifier when a low-energy a-c relay is required RATIO OF CONTINUOUS THERMAL CAPACITY TO PICKUP As a consequence of greater efficiency, the actuating coil of this type of relay has a high ratio of continuous current or voltage capacity to the pickup value, from the thermal standpoint. TIME CHARACTERISTICS Relays of this type are instantaneous in operation, although a slug may be placed around the armature to get a short delay INDUCTION-TYPE RELAYS-GENERAL OPERATING PRINCIPLES Induction-type relays are the most widely used for protective-relaying purposes involving a- c quantities. They are not usable with d-c quantities, owing to the principle of operation. An induction-type relay is a split-phase induction motor with contacts. Actuating force is developed in a movable element, that may be a disc or other form of rotor of non-magnetic current-conducting material, by the interaction of electromagnetic fluxes with eddy urrents that are induced in the rotor by these fluxes FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS
22 FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS Ip and Ia, are assumed to flow through the coils in such directions that a pickup force is produced, as in Fig. 6. It will be evident that, if the direction of either Ip or Ia (but not of both) is reversed, the direction of the force will be reversed. Therefore, this relay gets its name from its ability to distinguish between opposite directions of actuating-coil current flow, or opposite polarities. If the relative directions are correct for operation, the relay will pick up at a constant magnitude of the product of the two currents. If permanent-magnet polarization is used, or if the polarizing coil is connected to a source that will cause a constant magnitude of current to flow, the operating characteristic becomes: K2 I a = —–– = constant K1Ip Ia still must have the correct polarity, as well as the correct magnitude, for the relay to pick up. EFFICIENCY This type of relay in much more efficient than hinged-armature or plunger relays, from the standpoint of the energy required from the actuating-coil circuit. For this reason, such directional relays are used when a d-c shunt is the actuating source, whether directional action is required or not. Occasionally, such a relay may be actuated from an a-c quantity through a full-wave rectifier when a low-energy a-c relay is required. RATIO OF CONTINUOUS THERMAL CAPACITY TO PICKUP As a consequence of greater efficiency, the actuating coil of this type of relay has a high ratio of continuous current or voltage capacity to the pickup value, from the thermal standpoint. TIME CHARACTERISTICS Relays of this type are instantaneous in operation, although a slug may be placed around the armature to get a short delay. INDUCTION-TYPE RELAYS–GENERAL OPERATING PRINCIPLES Induction-type relays are the most widely used for protective-relaying purposes involving ac quantities. They are not usable with d-c quantities, owing to the principle of operation. An induction-type relay is a split-phase induction motor with contacts. Actuating force is developed in a movable element, that may be a disc or other form of rotor of non-magnetic current-conducting material, by the interaction of electromagnetic fluxes with eddy currents that are induced in the rotor by these fluxes
THE PRODUCTION OF ACTUATING FORCE Figure 7 shows how force is produced in a section of a rotor that is pierced by two adjacent a-c fluxes. Various quantities are shown at an instant when both fluxes are directed downward and are increasing in magnitude. Each flux induces voltage around itself in the rotor and currents flow in the rotor under the influence of the two voltages. The current produced by one flux reacts with the other flux, and vice versa, to produce forces that act on the rotor F FI Fig. 7. Torque production in an induction relay The quantities involved in Fig. 7 may be expressed as follows p=Φ 1 sin ot where 0 is the phase angle by which (2 leads ol. It may be assumed with negligible error that the paths in which the rotor currents flow have negligible self-inductance, and hence that the rotor currents are in phase with their voltages: dol 1 COS at ip2adtoΦgcos(ot+) We note that Fig. 7 shows the two forces in opposition, and consequently we may write the equation for the net force(F)as follows: Substituting the values of the quantities into equation 1, we get F a 1 oo sin(at +0)cos ot -sin ot cos(ot +0)1 hich reduces to FaΦΦsinθ FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS
FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS 23 THE PRODUCTION OF ACTUATING FORCE Figure 7 shows how force is produced in a section of a rotor that is pierced by two adjacent a-c fluxes. Various quantities are shown at an instant when both fluxes are directed downward and are increasing in magnitude. Each flux induces voltage around itself in the rotor, and currents flow in the rotor under the influence of the two voltages. The current produced by one flux reacts with the other flux, and vice versa, to produce forces that act on the rotor. The quantities involved in Fig. 7 may be expressed as follows: φ1 = Φ1 sin ωt φ2 = Φ2 sin (ωt + θ), where θ is the phase angle by which ø2 leads ø1. It may be assumed with negligible error that the paths in which the rotor currents flow have negligible self-inductance, and hence that the rotor currents are in phase with their voltages: d____ φ1 iφ1 α dt α Φ1 cos ωt d____ φ2 iφ2 α dt α Φ2 cos (ωt + θ) We note that Fig. 7 shows the two forces in opposition, and consequently we may write the equation for the net force (F) as follows: F = (F2 – F1) α (φ2iφ1 – φ1iφ2) (1) Substituting the values of the quantities into equation 1, we get: F α Φ1Φ2 [sin (ωt + θ) cos ωt – sin ωt cos (ωt + θ)] (2) which reduces to: F α Φ1Φ2 sin θ (3) Fig. 7. Torque production in an induction relay
Since sinusoidal flux waves were assumed, we may substitute the rms values of the fluxes for the crest values in equation 3 Apart from the fundamental relation expressed by equation 3, it is most significant that the net force is the same at every instant. This fact does not depend on the simplifying assumptions that were made in arriving at equation 3. The action of a rel influence of such a force is positive and free from vibration. Also, although it may not be immediately apparent, the net force is directed from the point where the leading flux pierces the rotor toward the point where the lagging flux pierces the rotor. It is as thoug the flux moved across the rotor, dragging the rotor along In other words, actuating force is produced in the presence of out-of-phase fluxes. One flux alone would produce no net force. There must be at least two out-of-phase fluxes to produce any net force, and the maximum force is produced when the two fluxes are 90 out of phase. Also, the direction of the force-and hence the direction of motion of the relay's movable member-depends on which flux is leading the other a better insight into the production of actuating force in the induction relay can b obtained by plotting the two components of the expression inside the brackets of equation 2, which we may call the"per-unit net force. " Figure 8 shows such a plot when 8 is assumed to be 90. It will be observed that each expression is a double-frequency sinusoidal wave completely offset from the zero-force axis Total force 9, sin(at +acos at sin wt cos(wt +o) Fig. 8. Per-unit net force The two waves are displaced from one another by g0 in terms of fundamental frequency or by 180 in terms of double frequency. The sum of the instantaneous values of the two waves is 1.0 at every instant. If 0 were assumed to be less than 90, the effect on Fig. 8 would to raise the zero-force axis, and a smaller per-unit net force would result. When 0 is zero, the two waves are symmetrical about the zero-force axis, and no net force is produced. If we let 8 be negative, which is to say that 2 is lagging l, the zero-force axis is raised still higher and net force in the opposite direction is produced. However, for a given value of 0. the net force is the same at each instant. FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS
24 FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS Since sinusoidal flux waves were assumed, we may substitute the rms values of the fluxes for the crest values in equation 3. Apart from the fundamental relation expressed by equation 3, it is most significant that the net force is the same at every instant. This fact does not depend on the simplifying assumptions that were made in arriving at equation 3. The action of a relay under the influence of such a force is positive and free from vibration. Also, although it may not be immediately apparent, the net force is directed from the point where the leading flux pierces the rotor toward the point where the lagging flux pierces the rotor. It is as though the flux moved across the rotor, dragging the rotor along. In other words, actuating force is produced in the presence of out-of-phase fluxes. One flux alone would produce no net force. There must be at least two out-of-phase fluxes to produce any net force, and the maximum force is produced when the two fluxes are 90° out of phase. Also, the direction of the force-and hence the direction of motion of the relay’s movable member-depends on which flux is leading the other. A better insight into the production of actuating force in the induction relay can be obtained by plotting the two components of the expression inside the brackets of equation 2, which we may call the "per-unit net force." Figure 8 shows such a plot when θ is assumed to be 90°. It will be observed that each expression is a double-frequency sinusoidal wave completely offset from the zero-force axis. The two waves are displaced from one another by 90° in terms of fundamental frequency, or by 180° in terms of double frequency. The sum of the instantaneous values of the two waves is 1.0 at every instant. If θ were assumed to be less than 90°, the effect on Fig. 8 would be to raise the zero-force axis, and a smaller per-unit net force would result. When θ is zero, the two waves are symmetrical about the zero-force axis, and no net force is produced. If we let θ be negative, which is to say that φ2 is lagging φ1, the zero-force axis is raised still higher and net force in the opposite direction is produced. However, for a given value of θ, the net force is the same at each instant. Fig. 8. Per-unit net force
In some induction-type relays one of the two fluxes does not react with rotor currents produced by the other flux. The force expression for such a relay has only one of the components inside the brackets of equation 2. The average force of such a relay may still be expressed by equation 3, but the instantaneous force is variable, as shown by omitting one of the waves of Fig 8. Except when 8 is 90 lead or lag, the instantaneous force will actually reverse during parts of the cycle; and, when 8-0, the average negative force equals the average positive force. Such a relay has a tendency to vibrate, particularly at values of 8 close to zero Reference 2 of the bibliography at the end of this chapter gives more detailed treatment of induction-motor theory that applies also to induction relays TYPES OF ACTUATING STRUCTURE The different types of structure that have been used are commonly called: (1)the"shaded- pole"structure;(2)the"watthour-meter"structure; (3)the "induction-cup"and the double-induction-loop"structures; (4)the"single-induction-loop"structure Shaded-Pole Structure. The shaded-pole structure, illustrated in Fig. 9, is generally actuated by current flowing in a single coil on a magnetic structure containing an air gap. The air- gap flux produced by this current is split into two out-of-phase components by a so-called shading ring "generally of copper, that encircles part of the pole face of each pole at the Rotor 中 Shading ring actuating-quantity Direction 中 Shading ring source of force Fig 9. Shaded-pole structure. air gap. The rotor, shown edgewise in Fig. 9, is a copper or aluminum disc, pivoted so as to rotate in the air gap between the poles. The phase angle between the fluxes piercing the Hisc is fixed by design, and consequently it does not enter into application considerations The shading rings may be replaced by coils if control of the operation of a shaded-pole relay is desired. If the shading coils are short-circuited by a contact of some other relay torque will be produced; but, if the coils are open-circuited, no torque will be produced because there will be no phase splitting of the flux. Such torque control is employed where directional control"is desired. which will be described later Watthour-Meter Structure. This structure gets its name from the fact that it is used for watthour meters. As shown in Fig. 10, this structure contains two separate coils on two different magnetic circuits, each of which produces one of the two necessary fluxes for driving the rotor, which is also a disc. FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS 25
FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS 25 In some induction-type relays one of the two fluxes does not react with rotor currents produced by the other flux. The force expression for such a relay has only one of the components inside the brackets of equation 2. The average force of such a relay may still be expressed by equation 3, but the instantaneous force is variable, as shown by omitting one of the waves of Fig. 8. Except when θ is 90° lead or lag, the instantaneous force will actually reverse during parts of the cycle; and, when θ = 0, the average negative force equals the average positive force. Such a relay has a tendency to vibrate, particularly at values of θ close to zero. Reference 2 of the bibliography at the end of this chapter gives more detailed treatment of induction-motor theory that applies also to induction relays. TYPES OF ACTUATING STRUCTURE The different types of structure that have been used are commonly called: (1) the "shadedpole" structure; (2) the "watthour-meter" structure; (3) the "induction-cup" and the "double-induction-loop" structures; (4) the "single-induction-loop" structure. Shaded-Pole Structure. The shaded-pole structure, illustrated in Fig. 9, is generally actuated by current flowing in a single coil on a magnetic structure containing an air gap. The airgap flux produced by this current is split into two out-of-phase components by a so-called "shading ring," generally of copper, that encircles part of the pole face of each pole at the air gap. The rotor, shown edgewise in Fig. 9, is a copper or aluminum disc, pivoted so as to rotate in the air gap between the poles. The phase angle between the fluxes piercing the disc is fixed by design, and consequently it does not enter into application considerations. The shading rings may be replaced by coils if control of the operation of a shaded-pole relay is desired. If the shading coils are short-circuited by a contact of some other relay, torque will be produced; but, if the coils are open-circuited, no torque will be produced because there will be no phase splitting of the flux. Such torque control is employed where "directional control" is desired, which will be described later. Watthour-Meter Structure. This structure gets its name from the fact that it is used for watthour meters. As shown in Fig. 10, this structure contains two separate coils on two different magnetic circuits, each of which produces one of the two necessary fluxes for driving the rotor, which is also a disc. Fig. 9. Shaded-pole structure