CONTROL SYSTEM FOR ELECTRIC CARS DRIVEN BY INDUCTION MOTORS

Abstract: The d.c. power collected from an overhead contact line (L) is converted into a three-phase alternating current by the action of a three-phase inverter (INV) of variable voltage and frequency. The three-phase alternating current is commonly supplied to a plurality of driving three-phase induction motors (IM1...IM4). The output voltage of the inverter (INV) is so constructed as to hold the motor current at a constant level. The output frequency of the inverter (INV) is deter­ mined by adding or subtracting the present value of a slip frequency (f₅) to or from the frequency signal (f₁) which correponds to the running speed of an electric car. Here, the frequency signal corresponding to the running speed of the electric car is determined by memorizing first and second pulse generators (PG1...PG4), attached to the induction motors (IM1...IM4) to indicate the maximum and the mini­ mum during the coasting run of the electric car, and by adding or subtracting the slip frequency (f₅) to and from the output of the first pulse generator (e.g. PG1) during the power run and the output of the second pulse generator (e.g. PG4) during the regeneratively braked run.

Description: TITLE OF THE INVENTION Control System for Electric Cars Driven by Induct ion Motors BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to the frequency control of a car which is driven by induction motors, and more particularly to a control system for an induction motor driven car having a large number of drive axles. Description of the Prior Art An electric car using induction motors as its traction,motors has a prospect of remarkably high enhancement of its adhesion characteristics and such motors are applied especially to electric cars. A control system for the induction motors is preferred for the -electric car in view of a comfortable ride if it performs a constant torque control. For this preference, therefore, it is general to control a motor current to a constant level while holding a slip frequency at a constant value. In order to hold the slip frequency constant, there is used a system in which the rotational frequency is detected by means of a revolving speed detector (which is frequently exemplified by a pulse generator) coupled to one of the induction motors or wheels so that the frequency obtained by adding a constant slip frequency to that rotational frequency (or by subtracting the former from the latter when in a regeneratively braked run) may be used as the operational frequency of a power transducer (e.g., an inverter). Even in case the slip frequency is controlled in a variable manner, it is necessary to detect the revolving speed of that induction motor so that the frequency may be controlled on the basis of the output of the induction motor. Now, in the case of an electric car, motors are coupled to several axles so that the drive power may be dispersed. However, the respective wheels do not always have an equal diameter but may possibly have a diameter of 860 mm, if they are new, or a diameter of 780 mm if they are worn out. Generally speaking, therefore, the electric motors have irregular revolving speeds. In this case, a variety of systems have been proposed depending upon which revolving speed detector is used to provide the basis of the control. More specifically, various systems have been proposed including one in which the outputs of all the revolving speed detectors are fed through a low preference circuit so that the minimum speed may always be referred to, another an:which:;the putputs of all the revolving speed detectors are fed through a high preference circuition the contrary, so that the maximum speed may be referred to, still another in which the average value of the outputs of all the revolving speed detectors is referred to, and so on. All of the systems thus proposed have their intrinsic advantages and disadvantages. SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to improve the tracting performance of an electric car which is driven by induction motors. According to an aspect of the present invention, of the revolving speed detecting means respectively provided for a plurality of induction motors, a first one generating the maximum speed signal and/or a second one generating the minimum speed signal are selected during the coasting run of an electric car which is driven by the induction motors. And, the revolving speed detecting means thus selected are or is memorized. During the power run of the electric car, the output frequency of a power transducer for supplying the induction motors with an electric power is controlled with reference to the output of the first revolving speed detecting means. During the regeneratively braked run of the electric car, on the other hand, the output frequency of the aforementioned power transducer is controlled with reference to the output of the second revolving speed detecting means. The induction motor corresponding to the aforementioned first revolving speed detecting means generates the minimum torque during the power run so that the corresponding wheels do not easily slip. As far as all wheels- do not ¯ slip, therefore, the output of the first revolving speed detecting means is proportional to the running speed of the electric car so that the output frequency of the power transducer is not increased even when the wheels partially slip. As a result, the constant-speed running characteristics of the induction motors can be effectively utilized so that the adhesion characteristics can be improved. During the regeneratively braked run of the electric car, the motor corresponding to'the second revolving speed detecting means generates the minimum braking torque so that the corresponding wheels do not easily skid. As far as all wheels do not.skid, therefore, the output of the second revolving speed detecting means is proportional to the running speed of the electric car so that the output frequency does not drop even if the wheels partially -skid; As a result, the constant-speed running characteristics of the induction motors can be effectively utilized so that the 'adhesion characteristics can be 'improved. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing a control system for an electric car, which is driven by induction motors, in accordance with one preferred embodiment of the present invention; Figs. 2 to 4 are diagrams illustrating the frequency-torque characteristics of the induction motors for explaining the operation of the present invention; Fig. 5 is also a block diagram showing another embodiment of the present invention, in which only the frequency control system of Fig. 1 is modified; Fig. 6 is also a block diagram showing still another embodiment of the present invention, in which only the frequency control system of Fig. 1 is modified; and Fig. 7 is a flow chart explaining the microcomputer which is shown in the embodiment of Fig. 6. DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in Fig. 1, the electric power collected from an overhead contact line L through a pantograph PA is introduced into a power transducer INV. This power transducer INV is made operative to generate a threephase alternating current of variable voltage and frequency and is desirably constructed of an inverter, if the line L is supplied with a direct current, and a combination of a converter and an inverter if the line L is supplied with an alternating current. The following description is directed to the case of the inverter. The output 3 A.C. of the. inverter INV is fed to four induction motors IM1 to Ism4. To these respective motors IM1 to Ism4, there are respectively connected wheels W1 to W4 by which an electric car is driven. The inverter INV has its output voltage VM controlled in the following manner. In a first example, a motor current 1M is detected by means of a current detector CD, and the output voltage VM of the inverter INV is so controlled that the output of the current detector CD may follow a current instruction 1MP As a result, the ratio of the output frequency f0 to the output voltage VM of the inverter INV takes a constant value. In a second example, the output voltage (i.e., the motor voltage) of the inverter INV is detected by means of a voltage detector VD, the output of which is so controlled as to become proportional to the frequency instruction fa. As a result, the motor current IM takes a substantially constant value. Therefore, it is arbitrary which control system is to be adoped or whether they are to be mixed or combined, and the construction of a voltage controller DC is determined. Now, a frequency control system constituting an essential portion of the present invention will be described in the following. The revolving speeds of the respective induction motors IM1 to IM4 are detected by means of speed detectors such as pulse generators PGl to PG4. These generators PG1 to PG4 have their outputs expressed in terms of frequency signals fl to f4. These frequency signals f; to f4 are introduced into a selective memory circuit SMC. In the shown state which is schematically expressed by means of a selecting switch SS, the one frequency signal fl is selected. On the other hand, a slip frequency setting unit Sf5 generates a slip frequency signal f and this slip frequency signal fs and the aforementioned frequency signal fl are introduced into a frequency calculating circuit F, by which the inverter output frequency signal f0 is calculated. If the electric car is in its power running state, a switch SW is held in the shown state by the action of a power run signal P so that the frequency calculating circuit F executes the calculation of f0 = fl + f . If the 5 electric car is in its regeneratively braked state, on the contrary1 the aforementioned power run signal P disappears. As a result, the switch SW is shifted to the left so that the calculating circuit F executes the calculation of fg fl - fs. Assuming now that the selective memory circuit SMC be a low preference circuit during the power run of the electric car, as in the prior art, the operation will be described with reference to Fig. 2. During the power run, the output frequencies fl to f4 of the pulse generators PGl to PG4 respectively coupled to the induction motors IM1 to IM4 are fed to a low-frequency preference circuit. If the output frequency 4, for example, of those frequencies fl to f4 is the lowest, the low-frequency preference circuit selects the output frequency f4, to which the slip frequency fs is added by the action of the frequency calculating circuit F so that the inverter INV operates at the frequency of f0 = f4 + f5. The output voltage VM of the inverter INV is so controlled, as has been described, that the motor current IM or VM/fo may -, become constant. As a result, the torques respectively generated by the induction motors are.dispersed as indicated at T1 to T4 in the frequency-torque characteris-tics of Fig. 2. It is now assumed that the wheels slip during the power run. Then, the induction motor 1M4 generating the highest torque is liable to slip so that the operating point is shifted from P4 along the arrow shown bv a thick solid line. As a result, the output frequency of the low-frequency preference circuit is shifted to f Thus, the frequency calculating circuit F executes the calculation of = f3 + fs so that the operating frequency of the inverter is slightly increased from f0 to f'0. However, this increase is not so large as that of the speed of the slipping wheels. This reduces the probability that the increase of the speed of the slipping wheels is positively fed back to invite highly idle revolutions. On the other hand, in case the slip take place in a system using a high-frequency preference circuit, the reference frequency may be shifted to the pulse generators of. the wheels which slip. This leads to large slipping. If the low-frequency preference circuit is used, as has been described hereinbefore, it is exptected that the adhesion characteristic is enhanced to an excellent level. However, the frequency-torque characteristics of all the induction motors after the slipping are illustrated by the broken line T' of Fig. 2. If the induction motor 1M3 is taken up as an example, therefore, the operating point is shifted from P3 to P3, and the generated torque is increased from T3 to T4. At this time, the wheel W3 is that which is the most liable to slip. If the wheel W3 slips too, the speed reference is shifted to the wheel W2 while leaving a high possibility that large slipping is reached like in a chain reaction. Thus, even if slip takes place, the speed reference is instantly shifted to the remaining wheels which do not slip so that the frequency is not increased to a high level. However, because the speed of the wheels generating the maximum torque so that they are the most liable to slip is referred to, the speed reference axle is so variable as to make the frequency control unstable. On the contrary, the selective memory circuit SMC used in one embodiment of the present invention is constructed in the following manner. A calculating unit CL receives the frequency signals fl to f4 during the coasting run of the electric car and selectively memorizes the frequency signal indicating the maximum and the frequency signal indicating the minimum. If the signal fl indicates the maximum and the signal f indicates the minimum, more specifically, not their values but the frequency fl or f4 itself is memorized. During the power run, moreover, the frequency signal fl at that time is used-as the frequency reference. During the regeneratively braked run, on the contrary, the frequency signal f 4 at that time is used as the frequency reference. The selecting switch SS is changed over in a manner to ensure the selective use of the frequency signals fl and f4. On the other hand, the calculating unit CL generates an adjusting signal AD, if necessary, from the relationship among the frequency signals f to f4 during the coasting run. Now, during the coasting run of the electric car in which the wheels neither slip nor skid, it is assumed that inequalities of f1 > f2 > f3 > i4 hold among the output frequencies fl to f4 of the respective pulse generators PG1 to PG4. The selective memory circuit SMC compares the output frequencies fl to f4 of the respective pulse generators PGl to PG4 during the coasting run and judges that the frequency fl is the highest and the frequency f4 is the lowest. The selectively memory circuit SMC memorizes that the selecting switch SS should be switched to the pulse generator PGl during the power run and to the pulse generator PG4 during the regeneratively braked run. With the construction thus far described, the inverter INV is run (as referred to Fig. 3(A)) by using the frequency (fl + f ), which is obtained by adding the slip frequency fs to the highest reference frequency detected, as its operating frequency fo, as illustrated in Fig. 3, so that the respective induction motors IMl to I M4 generate the torques T to T4. Here, if the idle revolutions take place, the wheel W4 generating the highest torque slips so that the induction motor IM4 has its operating point shifted, as indicted at the heavy line arrow in Fig. 3. At this time, the frequency reference is not influenced in the least even if the wheel W4 idly revolves, because it still resorts to the wheel Wl, so that the operational frequency of f0 = f1 + f of the inverter is not abruptly increased. As a result, the torque of the slipp ing wheel promptly decreases along the frequency-torque characteristic curve of Fig. 3 so that the adhesion is liable to be restored. During the regeneratively braked run, on the other hand, the frequency (f4 - f5), which is obtained by subtracting the slip frequency f5 from the lowest reference frequency f4 detected in accordance with the frequency-torque characteristic curve illustrated in Fig. 4, is used as the operational frequency of the inverter INV (as referred to Fig. 4(A)) so that the respective indudtion motors, generate the torques T1 to T4. If skidding should take place, the wheel W1 having the highest braking torque would skid so that only the induction motor IM1 would have its operating point shifted as indicated by the arrow in the same Figure. However, since the frequency reference is still provided by the wheel W4 during the regeneratively braked run, no influence is exerted even if the wheel Wl skids, so that the operational frequency f0 of the inverter will not Åabruptly drop..-As a result, the braking torque of the skidding wheel is promptly dropped so that the adhesion is liable to be restored. Here, since the wheel diameters are dispersed during both the power run and the regeneratively braked run, the torques respectively generated by the motors are dispersed among T1 to T4, , as illustrated in Figs. 3 and 4. On the other hand, the slip frequency fs should be set by the necessary total torque (T1 + T2 +T3 + T4) of the electric car. If the wheel diameters are dispersed, therefore, the slip frequency should be adjusted in a manner to take the disperse into consideration. On the other hand, it is also possible to automatically make corrections from the relationship among the levels of the frequency signals fl to f4 during the coasting run. More specifically, the adjusting signal AD shown in Fig. 1 corrects the slip frequency f from the relationship of the frequency signal f1 S or f4 selected frorr all the frequency signals fl to f4, e.g., their average value. By this correction, the power run or regeneratively braked run characteristics of the electric car can be maintained at the preset desirable ones. Fig. 5 shows the construction of only the frequency cdntrol system of another embodiment of the present invention, which is modified from that of Fig. 1. This embodiment is characterized in that a multiplier MU is disposed in the selective memory circuit SMC without correcting a slip frequency f' relative 5 to the dispersion among the wheel diameters so that the frequency signal (e.g., f1) selected may be multiplied by a constant K which is determined by the calculating circuit CL. This results in the calculation of f0 = Kfl + f' or fO = Kf4 - f1, as. illustrated in (B) of Figs. S S 3 and 4. As a result, an effect like that of the foregoing embodiment can be enjoyed. In the embodiments of Figs. 1 and 5, the selective memory circuit SMC is exemplified by the selecting switch SS, which can be replaced by stationary switching means such as a semiconductor. Fig. 6 shows still another embodiment of the frequency control system according to the present invention, in which the selective memory circuit SMC and the frequency calculating circuit F are replaced by a microcomputer MC. Fig. 7 is a chart showing the flow which is to be processed in accordance with the program of the microcomputer MC of Fig. 6. In accordance with the position of the handle of a master controller or in response to the stop signal of the inverter, there is introduced a signal indicating that the car driven by the induction motors is in its coasting run. During this coasting run, the highest frequency is selected, upon which it is determined that the pulse generator generating that signal be the one to be selected during the power run. Moreover, the minimum frequency is selected, upon which it is determined that the pulse generator generating that signal is the one to be selected during the regeneratively braked run. During the running operation, on the other hand, it is judged whether the run is of the power type or the regeneratively braked type. The output frequency of the pulse generator, which has been determined to be selected during the coasting run, is designated f-n and the calculation of f0 = f + f is executed n -O n s during the power run whereas the calculation of f0 = f - f is executed during the regeneratively braked n s run, thereby to determine the operational frequency of the inverter. By repeatedly executing a series of these processings at, a predetermined high-speed period, it is possible to exhibit a function like that of the embodiment shown in Fig. 1. Thus, if the judgements of the conditions and the processings of the signals are conducted in accordance with the software of the microcomputer, the parts such as the selecting switch can be dispensed with so that the circuit construction can be simplified with an increase in the reliability. This effect is prominent in case the control system is equipped with the microcomputer for another application. As has been described hereinbefore, according to the present invention, even if slip occurs, the operational frequency of the inverter does not abruptly fluctuate so that the adhesion can be easily restored.

Claims: WHAT IS CLAIMED IS: 1. A control system for an electric car, comprising a plurality of induction motors (IM1...IM4) corresponding to a plurality of wheels (Wi. ..W4); means (PG1...PG4) for detecting the revolving speeds of said induction motors; a common power transducer (INV) for supplying said plural induction motors with electric power; and means (SMC, F; MC) for controlling the output frequency of said power transducer on the basis of the outputs of said revolving speed detecting means, wherein said control means includes means for selecting that of said revolving speed detecting means (PGl. . .PG4) which generates the maximum during the coasting run of said electric car; means for memorizing said revolving speed detecting means selected; and means (F) for controlling the output frequency of said power transducer (INV) during the power run of said electric car with reference to the output of said revolving speed detecting means memorized during the power run. 2. A control system according to claim 1, wherein said frequency control means has means (F) for determining the output frequency of said power transducer (INV) by adding the output of slip frequency setting means (Sf5) to the output of said revolving speed detecting means (PG1...PG4) memorized during the power run. 3. A control system according to claim 1, wherein said frequency control means has means (MV) for correcting the output of said revolving speed detecting means (PG1...PG4) memorized during the power run; and means (F) for determining the output frequency of said power transducer (INV) by adding the output of said slip frequency setting means (of5) to the output of said correcting means (MV). 4. A control system for an electric car, comprising a plurality of induction motors (IM1...IM4) corresponding to a plurality of wheels (W1...W4); means (PGl. ..PG4) for detecting the revolving speeds of said induction motors; a common power transducer tINV) for supplying said plural induction motors with electric power; and means (SMC, F; MC) for controlling the output frequency of said power transducer on the basis of the outputs of said revolving speed detecting means, wherein said control means includes means for selecting that of said revolving speed detecting means (PG1...PG4) which generates the maximum during the coasting run of said electric car; means for memorizing said revolving speed detecting means selected; and means (F) for controlling the output frequency of said power transducer (INV) during the regeneratively braked run of said electric car with reference to the output of said revolving speed detecting means memorized during the regenerdtively braked run. 5. A control system according to claim 4, wherein said frequency control means has means (F) for determining the output frequency of said power transducer (INV) by subtracting the output of slip frequency setting means (Sf5) from the output of said revolving speed detecting means (PG1...PG4) memorizing during the braked run. 6. A control system according to claim 4, wherein said frequency control means has means (Mv) for correcting the output of said revolving speed detecting means (PG1...PG4) memorized during the braked run; and means (F) for determining the output frequency of said power transducer ;(INV) by subtracting the output of said slip frequency setting means (Sf5) from the output of said correcting means (MV).

Inventors: IBAMOTO, MASAHIKO, NARITA, HIROSHI, SUZUKI, KATSUAKI
Applicants: HITACHI, LTD.
Application year: 1983
Main classification: B60L3
IPC codes: B60L3, H02P5, H02P23, B60L9