Electric Motors, Part Iii
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Electric motors, part III Other motors D.C. motors, single phase induction motors, synchronous motors (reluctance and hysteresis) will be covered briefly hereafter. The d.c. machine: Based on the discussion of the previous articles (part I & II), the magnetic flux and the current carrying conductors are the two indispensable elements of the electric machine. In d.c. machines the conductors producing the tangential forces are placed in the armature (inner rotating part of machine), the electromagnets (poles) are stationary and constitute one part with the frame (outer part of machine ). The flux originates in the poles which are mounted on the frame and on which fieldexciting coils are placed. The armature conductors are placed in slots punched in the armature laminations. The path of the magnetic flux consists of: a part of the frame, two poles, two air gaps between the poles and the armature, two sets of teeth on the armature and a part of the armature core. The material of the armature conductors and field coils can be copper. The slots with the armature conductors are parallel to the axis of the armature. Skewing at a slight angle to the axis can be observed to reduce the influence of the harmonic fluxes of high orders on the rotor. The armature iron must be laminated. The laminations are insulated. If insulations are not provided, the pole flux will induce an emf in the
armature iron. Currents induced in the surface of the armature iron would produce high ohmic losses I2 R without contributing much to the developed torque by the rotor. These losses are known as eddy current. The poles are laminated, the frame is solid. The main elements of a d.c. machine are: the shaft, the armature core, the commutator, the ventilating fan, the field pole, the brush assembly, the interpole (if available) and the enclosure. A d.c. motor gets its field current for excitation as well as the armature current from an external source of power. The d.c. generator can be selfexcited or separately excited. Depending on the manner the field winding is connected to the armature one, three cases can arise: series excitation, shunt and compound excitation. For the first type of excitation, the field and armature windings are connected in series.; If the residual magnetism is such that the top pole is a north pole (N) and the bottom pole is a south (S), the field winding must be connected in series with the armature so that the field current produces an N pole at the top and an S pole at the bottom. For the second, the field winding is placed in parallel with the armature winding. The resistance of a series field is made small, that for a shunt field is made high, the first will have a low voltage drop across it, while the latter will take low current levels 1% to 5% for small machines of the armature current. To produce the field flux, a definite value of mmf is necessary. In series field winding machines, the number of turns are small & the current is high. In shunt field winding machines, the number of turns are high and the current is low. For the third (compound excitations), the machine has both a series and a shunt field winding. The series winding may be connected to the armature winding so that if either produces a mmf of the same direction as that of the shunt winding, cumulative compound connection is produced or a mmf opposite to that of the shunt winding, differential compound connection is provided. In the cumulative compound wound machine, the shunt field winding may be connected either directly across the armature terminals (short shunt connection) or across the terminals which connect to the line (external circuit) and this is known as the long shunt connection. The greater part of the excitation mmf is
usually furnished by the shunt field winding. The series winding serves to make the flux of the field dependent, within narrow limits, on the load current. For the same direction of the field and the armature currents, the direction of rotation of the machine as a motor is opposite to the direction of rotation as a generator. In a motor, the induced emf and armature current and consequently the terminal voltages are in counter phase (the emf, terminal voltage and armature current are all in phase, with generators). The armature current of the d.c. motor adjusts itself to a value corresponding to the opposing torque of the load. The characteristics of the shunt, series and compound motors will be given hereafter. The flux of the shunt motor is a function of the current in the shunt field winding. The behavior of a shunt motor can be expressed in terms of starting and of running. If the terminal voltage is impressed across a shunt motor at standstill, the armature current and field flux will produce a torque. The motor will accelerate until it reaches a speed such that the load torque plus the loss torque is exactly balanced by the developed torque. The speed varies inversely with the flux. With low field current (not sufficient flux), the motor may accelerate to a very high speed and may fly apart. This means that at starting, the field must be excited first and then the voltage is impressed across the armature (or both circuits may be connected, simultaneously). An armature resistance is used to reduce the starting armature current. As the speed and counteremf rise the armature current decreases and the resistance placed in the armature circuit can be reduced. The field winding should never be opened suddenly as it has a high self inductance that may cause an induced emf (e=L di/dt) high enough to break the insulation. For a particular excitation, the speed as function of the armature current is parallel to the armature current axis (for example Xaxis), and the torque increases as the armature current increases, but non linearly due to the nonlinearity of the iron saturation curve. For a constant field and terminal voltage, the increased power output will cause a reduction in the counteremf, the armature current will increase more than linearly, as the torque is proportional to the counter emf times the armature current. For the series d.c. motors, the field flux is proportional to the armature current. When the armature current is very small, the flux is also very small and the speed will be very high. The series motor should never be connected to a line, if there is any possibility of losing its entire load as under such condition, the machine can destroy itself. The speed change of the series motor is very wide as function of the armature current. For low values of armature current the torque increases as the square of the current, when the machine is saturated (high armature current), the torque increases at nearly the first power of the armature current. At even higher armature current, the torque will increase at a rate less than that of the armature current. For the cumulative compound motor the characteristics usually approach the shunt or the series motor depending on the relative strengths of the shunt and the series field mmf. Speed control with d.c. machines can be accomplished through one of the following; voltage control method, resistance control of the armature and the field flux control. If Ia is the total armature current, the current in each parallel path and also in the conductors of each winding element = Ia/number of turns. The winding element changes from one armature path to another (when rotating in the magnetic field), this will cause a change in the winding element current direction. During the time of reversal, the conductors of the winding element lie in the neutral zone and the winding element is short circuited by one or two brushes of the same polarity in a lap or wave winding, respectively. If there are no further influences on the S.C. winding element, the change over of the direction of the
current is determined by the magnitude of the contact areas of the brush with the 2 adjacent commutator bars (A1 & A2), i= ia(1A1/A2); where ia is the current in the winding element, i is the current in the short circuited winding element and A is A1+A2. The current in the winding element is also given by: ia= (1t/Tc/2)i; where Tc is the time of commutation (reversal of current). The most dangerous instant of the commutation period is t=Tc (the instant at which the trailing brush edge leaves the trailing commutator bar). It is necessary, to have the current density under the trailing brush edge as low as possible. Commutation can be classified into straight line, accelerated and delayed. In order to achieve good commutation, means must be adopted to counteract the delaying action of the self induced emf and the emf induced by the armature flux in the short circuited winding element. Either the main flux can be employed or special (interpoles) poles which lie between the main poles can be used to achieve accelerating commutation. The single phase induction motors: Any three phase motor can be made to operate as a single phase induction motor by opening one of the three stator phases. The two remaining stator phases constitute a single phase winding distributed over 2/3 of the pole pitch. In general the 3phase winding is 2layer and the 1phase motor is usually single layer. The mechanical elements of the single phase induction motor are the same as those of polyphase induction motor except that a centrifugal switch used in certain types of single phase motors. The influence of the parameters (stator leakage reactance, stator resistance, main flux reactance, main flux resistance, rotor leakage reactance in stator terms and rotor resistance referred to the stator), on the performance of the single phase motor is generally, the same as that on the polyphase motor. The power factor of the single phase motor is lower than that of the polyphase one. The efficiency of the single phase motor is influenced by the increased copper losses in both the stator and rotor. Refer to part I, issue for the influence of parameters on three phase induction motors. The rotor resistance does not affect the magnitude of pullout torque in polyphase motor; it affects only the pull out slip. In single phase motors the rotor resistance influences not only the pullout slip but also the magnitude of the pullout torque. The singlephase motor has no starting torque in contrast to the polyphase motor. In order to start the single phase motor either a rotating flux (such as that in the polyphase motor) has to be produced, or a commutator has to be included with the motor. In order to produce a rotating flux at standstill a second winding (starting or auxiliary) is necessary in the stator in addition to the main winding. The axis of the starting winding has to be displaced in space with respect to the axis of the main winding. The starting winding current has to be out of time phase with the current in the main winding. A number of methods can be used to achieve the time shift between the currents in the main and starting windings: split phase, resistance startsplit phase, reactor startsplit phase, capacitor start, permanent split capacitor and two value capacitor. The starting by means of a commutator and brushes is based on the properties of the repulsion motor. Repulsion motor: It is a single phase ac commutator motor. The stator has a single phase winding and the rotor has a d.c. armature winding with commutator and brushes, the brushes are short circuited. The two types of single phase induction motors that use the principles of the repulsion motors are: repulsion start induction motor and the repulsion induction motor. For a very small output and a low starting torque,
the shaded pole motor is used. The reluctance & hysteresis motors The two types of fractional horsepower motors (synchronous) which do not need dc excitation and are self starting are the reluctance and the hysteresis ones. The reluctance motor is a synchronous motor similar in construction to an induction motor, in which the member carrying the secondary circuit has salient poles (without d.c. excitation). It starts as an induction motor but operates normally at synchronous speed. Prot.f=(mVE /Xd) sind +mv (XdXq)/2XdXq) sin 2d The cylindrical rotor machines will yield Prot.f=0 at Ef (field excitation)= 0, although the salient pole machine will produce torque and run at synchronous speed without field. Having started as an induction motor and having reached its maximum speed as an induction motor, it pulls into step and runs as a synchronous motor (by virtue of its saliency). The lower the inertia of the rotating mass (load + rotor), the easier the motor pulls into step. The stator of the reluctance motor can be polyphase or single phase. The different types of such motor are polyphase reluctance motor, split phase types reluctance motor, capacitor type reluctance motor can exist. The hysteresis motor is a synchronous motor without salient poles and without direct current excitation. It starts by virtue of the hysteresis losses induced in the hardened steel secondary member, by the revolving field of the primary, and operates at synchronous speed due to the retentivity of the secondary core. The eddycurrent losses are given by:Pe= e(afB/60.64500)2; W/lb; where e depends upon the electric resistivity of the iron and a is the thickness of the iron laminates in inches. The above equation can be written as Cef22B2
=Cef12s2B2. Thus the torque corresponding to the eddy current in the rotor= (7.04/ns)(Cesf12B2. The
hysteresis losses can be given by h (f/60)(B/64500)2 =Chf2B2 =Chsf1B2 and Th=(7.04/ns)(Chf1B2). It can be seen from the above that the torque developed as a result of the eddy current is proportional to the slip and is inversely proportional with rotor speed (it becomes 0 at synchronous speed). The rotors of such machines have a ring of special magnetic material such as cobalt or chrome steel mounted on an arbor of nonmagnetic material such as aluminum. At synchronous speed, the eddy current (starting is produced by the eddycurrent and hysteresis torque) torque is 0 and the operation of the motor is accomplished exclusively by the hysteresis torque. The stator of the hysteresis motor is usually single phase, polyphase hysteresis motors, capacitor type and shaded pole hysteresis motors are found in practice.
armature iron. Currents induced in the surface of the armature iron would produce high ohmic losses I2 R without contributing much to the developed torque by the rotor. These losses are known as eddy current. The poles are laminated, the frame is solid. The main elements of a d.c. machine are: the shaft, the armature core, the commutator, the ventilating fan, the field pole, the brush assembly, the interpole (if available) and the enclosure. A d.c. motor gets its field current for excitation as well as the armature current from an external source of power. The d.c. generator can be selfexcited or separately excited. Depending on the manner the field winding is connected to the armature one, three cases can arise: series excitation, shunt and compound excitation. For the first type of excitation, the field and armature windings are connected in series.; If the residual magnetism is such that the top pole is a north pole (N) and the bottom pole is a south (S), the field winding must be connected in series with the armature so that the field current produces an N pole at the top and an S pole at the bottom. For the second, the field winding is placed in parallel with the armature winding. The resistance of a series field is made small, that for a shunt field is made high, the first will have a low voltage drop across it, while the latter will take low current levels 1% to 5% for small machines of the armature current. To produce the field flux, a definite value of mmf is necessary. In series field winding machines, the number of turns are small & the current is high. In shunt field winding machines, the number of turns are high and the current is low. For the third (compound excitations), the machine has both a series and a shunt field winding. The series winding may be connected to the armature winding so that if either produces a mmf of the same direction as that of the shunt winding, cumulative compound connection is produced or a mmf opposite to that of the shunt winding, differential compound connection is provided. In the cumulative compound wound machine, the shunt field winding may be connected either directly across the armature terminals (short shunt connection) or across the terminals which connect to the line (external circuit) and this is known as the long shunt connection. The greater part of the excitation mmf is
usually furnished by the shunt field winding. The series winding serves to make the flux of the field dependent, within narrow limits, on the load current. For the same direction of the field and the armature currents, the direction of rotation of the machine as a motor is opposite to the direction of rotation as a generator. In a motor, the induced emf and armature current and consequently the terminal voltages are in counter phase (the emf, terminal voltage and armature current are all in phase, with generators). The armature current of the d.c. motor adjusts itself to a value corresponding to the opposing torque of the load. The characteristics of the shunt, series and compound motors will be given hereafter. The flux of the shunt motor is a function of the current in the shunt field winding. The behavior of a shunt motor can be expressed in terms of starting and of running. If the terminal voltage is impressed across a shunt motor at standstill, the armature current and field flux will produce a torque. The motor will accelerate until it reaches a speed such that the load torque plus the loss torque is exactly balanced by the developed torque. The speed varies inversely with the flux. With low field current (not sufficient flux), the motor may accelerate to a very high speed and may fly apart. This means that at starting, the field must be excited first and then the voltage is impressed across the armature (or both circuits may be connected, simultaneously). An armature resistance is used to reduce the starting armature current. As the speed and counteremf rise the armature current decreases and the resistance placed in the armature circuit can be reduced. The field winding should never be opened suddenly as it has a high self inductance that may cause an induced emf (e=L di/dt) high enough to break the insulation. For a particular excitation, the speed as function of the armature current is parallel to the armature current axis (for example Xaxis), and the torque increases as the armature current increases, but non linearly due to the nonlinearity of the iron saturation curve. For a constant field and terminal voltage, the increased power output will cause a reduction in the counteremf, the armature current will increase more than linearly, as the torque is proportional to the counter emf times the armature current. For the series d.c. motors, the field flux is proportional to the armature current. When the armature current is very small, the flux is also very small and the speed will be very high. The series motor should never be connected to a line, if there is any possibility of losing its entire load as under such condition, the machine can destroy itself. The speed change of the series motor is very wide as function of the armature current. For low values of armature current the torque increases as the square of the current, when the machine is saturated (high armature current), the torque increases at nearly the first power of the armature current. At even higher armature current, the torque will increase at a rate less than that of the armature current. For the cumulative compound motor the characteristics usually approach the shunt or the series motor depending on the relative strengths of the shunt and the series field mmf. Speed control with d.c. machines can be accomplished through one of the following; voltage control method, resistance control of the armature and the field flux control. If Ia is the total armature current, the current in each parallel path and also in the conductors of each winding element = Ia/number of turns. The winding element changes from one armature path to another (when rotating in the magnetic field), this will cause a change in the winding element current direction. During the time of reversal, the conductors of the winding element lie in the neutral zone and the winding element is short circuited by one or two brushes of the same polarity in a lap or wave winding, respectively. If there are no further influences on the S.C. winding element, the change over of the direction of the
current is determined by the magnitude of the contact areas of the brush with the 2 adjacent commutator bars (A1 & A2), i= ia(1A1/A2); where ia is the current in the winding element, i is the current in the short circuited winding element and A is A1+A2. The current in the winding element is also given by: ia= (1t/Tc/2)i; where Tc is the time of commutation (reversal of current). The most dangerous instant of the commutation period is t=Tc (the instant at which the trailing brush edge leaves the trailing commutator bar). It is necessary, to have the current density under the trailing brush edge as low as possible. Commutation can be classified into straight line, accelerated and delayed. In order to achieve good commutation, means must be adopted to counteract the delaying action of the self induced emf and the emf induced by the armature flux in the short circuited winding element. Either the main flux can be employed or special (interpoles) poles which lie between the main poles can be used to achieve accelerating commutation. The single phase induction motors: Any three phase motor can be made to operate as a single phase induction motor by opening one of the three stator phases. The two remaining stator phases constitute a single phase winding distributed over 2/3 of the pole pitch. In general the 3phase winding is 2layer and the 1phase motor is usually single layer. The mechanical elements of the single phase induction motor are the same as those of polyphase induction motor except that a centrifugal switch used in certain types of single phase motors. The influence of the parameters (stator leakage reactance, stator resistance, main flux reactance, main flux resistance, rotor leakage reactance in stator terms and rotor resistance referred to the stator), on the performance of the single phase motor is generally, the same as that on the polyphase motor. The power factor of the single phase motor is lower than that of the polyphase one. The efficiency of the single phase motor is influenced by the increased copper losses in both the stator and rotor. Refer to part I, issue for the influence of parameters on three phase induction motors. The rotor resistance does not affect the magnitude of pullout torque in polyphase motor; it affects only the pull out slip. In single phase motors the rotor resistance influences not only the pullout slip but also the magnitude of the pullout torque. The singlephase motor has no starting torque in contrast to the polyphase motor. In order to start the single phase motor either a rotating flux (such as that in the polyphase motor) has to be produced, or a commutator has to be included with the motor. In order to produce a rotating flux at standstill a second winding (starting or auxiliary) is necessary in the stator in addition to the main winding. The axis of the starting winding has to be displaced in space with respect to the axis of the main winding. The starting winding current has to be out of time phase with the current in the main winding. A number of methods can be used to achieve the time shift between the currents in the main and starting windings: split phase, resistance startsplit phase, reactor startsplit phase, capacitor start, permanent split capacitor and two value capacitor. The starting by means of a commutator and brushes is based on the properties of the repulsion motor. Repulsion motor: It is a single phase ac commutator motor. The stator has a single phase winding and the rotor has a d.c. armature winding with commutator and brushes, the brushes are short circuited. The two types of single phase induction motors that use the principles of the repulsion motors are: repulsion start induction motor and the repulsion induction motor. For a very small output and a low starting torque,
the shaded pole motor is used. The reluctance & hysteresis motors The two types of fractional horsepower motors (synchronous) which do not need dc excitation and are self starting are the reluctance and the hysteresis ones. The reluctance motor is a synchronous motor similar in construction to an induction motor, in which the member carrying the secondary circuit has salient poles (without d.c. excitation). It starts as an induction motor but operates normally at synchronous speed. Prot.f=(mVE /Xd) sind +mv (XdXq)/2XdXq) sin 2d The cylindrical rotor machines will yield Prot.f=0 at Ef (field excitation)= 0, although the salient pole machine will produce torque and run at synchronous speed without field. Having started as an induction motor and having reached its maximum speed as an induction motor, it pulls into step and runs as a synchronous motor (by virtue of its saliency). The lower the inertia of the rotating mass (load + rotor), the easier the motor pulls into step. The stator of the reluctance motor can be polyphase or single phase. The different types of such motor are polyphase reluctance motor, split phase types reluctance motor, capacitor type reluctance motor can exist. The hysteresis motor is a synchronous motor without salient poles and without direct current excitation. It starts by virtue of the hysteresis losses induced in the hardened steel secondary member, by the revolving field of the primary, and operates at synchronous speed due to the retentivity of the secondary core. The eddycurrent losses are given by:Pe= e(afB/60.64500)2; W/lb; where e depends upon the electric resistivity of the iron and a is the thickness of the iron laminates in inches. The above equation can be written as Cef22B2
=Cef12s2B2. Thus the torque corresponding to the eddy current in the rotor= (7.04/ns)(Cesf12B2. The
hysteresis losses can be given by h (f/60)(B/64500)2 =Chf2B2 =Chsf1B2 and Th=(7.04/ns)(Chf1B2). It can be seen from the above that the torque developed as a result of the eddy current is proportional to the slip and is inversely proportional with rotor speed (it becomes 0 at synchronous speed). The rotors of such machines have a ring of special magnetic material such as cobalt or chrome steel mounted on an arbor of nonmagnetic material such as aluminum. At synchronous speed, the eddy current (starting is produced by the eddycurrent and hysteresis torque) torque is 0 and the operation of the motor is accomplished exclusively by the hysteresis torque. The stator of the hysteresis motor is usually single phase, polyphase hysteresis motors, capacitor type and shaded pole hysteresis motors are found in practice.
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