CA1297149C

CA1297149C - Control systems for an electric drive

Info

Publication number: CA1297149C
Application number: CA000598922A
Authority: CA
Inventors: Walter A. Hill; Charles L. Schwalm; Gregory S. Creelman
Original assignee: General Electric Canada Co
Current assignee: General Electric Canada Co
Priority date: 1989-05-04
Filing date: 1989-05-04
Publication date: 1992-03-10
Anticipated expiration: 2009-05-04

Abstract

CONTROL SYSTEM FOR AN ELECTRIC DRIVE

Abstract of the Disclosure A control system for an icebreaker drive has a ship's telegraph for inputting a speed reference signal representing a desired speed at which the electric motors propelling the ship are to run. The motors receive power from a controllable converter which, in turn, receives power from an AC generator or generators driven by respective prime movers. The prime movers run at a governed rated speed but slow down with overload and may stall if the load is not decreased quickly. A sensor determines the frequency of the generator and provides a frequency feedback signal. This is compared to a set frequency signal related to the speed of the prime mover and the generator frequency. The frequency feedback signal and the set frequency signal are compared and a speed reduction signal is derived related to the amount by which the frequency feedback signal is below the set frequency signal. The speed reference signal is reduced in value in accordance with the speed reduction signal. A
second speed reduction signal may be derived by comparing a signal representing the actual kilowatts of power being provided by the generators with a signal representing rated kilowatts of power for the number of generators operating at the time. Thus the speed reference signal may be reduced by either or both of the first and second reduction signals. The speed reference signal is compared to a sensed actual speed signal and the result integrated and limited to provide a torque reference signal. The torque reference signal controls the operation of the converter providing controlled current to the motor, and thus controls speed. The speed is kept constant until an overload occurs with respect to the prime mover or the generator and then the speed is reduced. This decreases the load and also indicates a change to the Captain.

Description

CONTROL SY_TEM FOR AN ELECTRIC DRIVE

Background of the Invention This invention relates to a control system for an electric drive, and in particular it relates to a control system for a generator fed synchronous motor or DC motor drive in an icebreaker.
Many types of controls are known for various drive systems which use electric motors to provide the driving power. Generally these controls are intended to maintain a selected speed or a selected torque. The requirements for a drive for an icebreaker may be somewhat different. In an icebreaker, a prime mover, such as a diesel engine, may drive a generator which provides power to a converter. The converter, in turn, provides power to an electric motor (or motors) which turns the propeller (or propellers) of the icebreaker.
The converter may provide power at a variable frequency and amplitude when the motor is an AC motor, and of course provide DC power if the motor is a DC motor. It is important that the prime mover should not be stalled by a rapidly increasing load, for example when the icebreaker encounters heavy ice. It is also important t~

- 2 - Case 2975 that the generators and motors shoul~ be protected against prolonged overloads. The protection of prime movers, generators and motors are of primary importance in an icebreaker. This may be complicated in large icebreakers which may have two, three or more prime movers driving respective generators. To conserve fuel the icebreaker may be running with one prime mover operating when the icebreaker is moving in an open sea. When the icebreaker encounters ice, the single prime mover and generator may be quickly overloaded.
In addition, the Captain of an icebreaker may be somewhat isolated from the conditions which exist at any time and which may change quite rapidly. A control system which attempts to maintain a selected motor speed until a critical condition, such as the stalling of the prime mover, is reached, gives no indication to the Captain of changing conditions. It is desirable that the Captain be made aware of such changing conditions before the conditions become critical.
The present invention provides a control system which protects the prime mover against stalling, which protects the generators and motors against damaging overloads while permitting limited operation at low overloads, and in addition gives some warning of changing conditions ~such as whan an icebreaker encounters heavy ice.
Summary of the Invention The control system according to the invention attempts to maintain a desired speed as set by an input signal. It reduces the speed if (a) the generator frequency starts to drop indicating the prime mover is becoming overloaded, ~b) the power output from a generator, if only one generator is on line, or from two or more generators if two or more generators are on line, has reached a predetermined level for khe number - 3 - Case 2975 of generators on line~ (c) the motor has been running in a current overload condition ~or a time inversely related to the level of the overload, and (d) the generator VAR has exceeded a predetermined level for the number of generators operating for a time inversely related to the amount o~ the excess. The reduction in speed not only serves as a protection, but the reduction in speed will also indicate that conditions are changing.
It is therefore an ob~ect of the invention to provide an improved control system suitable for use in the drive of an icebreaker.
It is another object of the invention to provids a control system having at least one prime mover driving a respective generator for providing power to a drive motor or motors, in which a decrease in generator frequency, indicating prime mover overload, results in a reduction in the speed of the drive motor, and in which a generator output above a predetermined level results in a reduction in the speed of the drive motor.
It is yet another object of the invention to provide an improved control system having at least one prime mover driving a respective generator which provides power to at least one synchronous motor connected to rotate a drive shaft.
It is a further object of the invention to provide an improved control system having at least one prime mover, each prime mover driving a respective generator, for providing DC power to at least one DC
motor connected to rotate a drive shaft.
It is also an object of the invention to provide a control system for a drive means having at least one prime mover driving a respective generator which provides powsr for at least one drive motor, in a,3 4 - case 2975 which the motor speed is reduced following a motor overload condition aftPr a time which has an inverse relationship to the degree of thR overload.
It is yet another object of the invention to provide a control system for a drive means having at least one prime mover driving a respective generator which provides power for at least one drive motor, in which the motor speed is reduced following the generator VAR exceeding a predetermined level, for the number of generators operating, after a time which has an inverse relationship to the amount of the excess.
Accordingly there is provided a control system for a drive, the drive having at least one prime mover, each prime mover driving a respective AC
generator which provides power to a controllable converter, the converter providing power to at least one electric motor for rotating a drive shaft, comprising means for providing a speed reference signal representing a desired speed, means for sensing the rotational speed of the motor and providing an actual speed signal representing the sensed speed, means for comparing the speed reference signal and the actual speed signal and providing a first difference signal representing the difference between the compared signals, means for integrating the first difference signal to provide a torque reference signal, means responsive to the torque reference signal for controlling the controllable converter to provide a - motor torque which will operate the motor at a speed approaching the speed represented by the speed reference signal, means for sensing the frequency of the output of the generator and for providing a frequency feedback signal representing the sensed frequency, the frequency feedback signal decreasing with overload on the prime mover, means for comparing - 5 - Case 2975 the frequency feedback signal with a se~ reference signal and providing a second dif~erence signal when the frequency feedback signal is less than the set referencs signal, and means responsive to the second dif*erence signal for reducing the speed reference signal and thereby reducing load on the prime mover.
Brief Description of the Drawin~s The invention will be described with reference to the accompanying drawings, in which Figures ~A and ls are simplified block schematic diagrams according to one form of the invention, and Figure 2 is a simplified block schematic diagram according to another form of the invention.
Description of the Preferred Embodiments Referring to Figures lA and lB there is shown, in simplified block form, a schematic diagram of a control system for an icebreaker drive means which has at least one synchronous motor 10 with a shaft 11 for turning a ship's propeller (not shown). It is common for an icebreaker to have a port propeller and a starboard propeller each driven by a respective motor.
However only one motor 10 is shown for simplicity. The synchronous motor 10 has a stator winding fed by three conductors 14, 15 and 16 each connected to a respective converter 17, 18 and 19. The converters 17, 18 and 19 are connected to and receive power from a bus 20 which comprises conductors 20A, 20B and 20C but is shown as a single line for ease of drawing. The bus 20 is connected to a generator 21 which is driven by a prime mover 22, such as, for example, a diesel engine 22.
There may be one, two, three or even more generators, each connected to bus 20 and each with a respective prime mover. One such additional generator is indicated in broken lines as generator 21A and it is driven by a prime mover 22A (also indicated by broken ~~ - 6 - Case 2975 lines)~ The prime movers 22 have a rated speed and they are designed to run at their rated speed, controlled by a governor. Sensors 12A, 12B and 12C
sense the current in conductors 20A, 20B and 20C and provide signals representing the sensed currents on conductors 13A, 13B and 13c. Again, for ease o~
drawing, the conductors 13A, 13B and 13C are represented by a single line 13 where convenient.
The control system requires information on how many generators are providing power to bus 20. A
signal on conductor 23 carries this informatio~. The information on the number of generators operating may, of course, be readily obtained from the switches (not shown) which connect each generator to bus 20, from sensors which detect the operation of each generator, or by any other suitable means. The control system also requires information on the frequency of the generator output (indicated on Figure 1 as Hz or Hertz). This information is detected by sensor 29A and is on conductor 29.
An exciter 24 provides power via a rectifier 25 to the rotor windings of the synchronous motor 10.
The exciter 24 receives power from a converter 26 which, in turn, receives power from the bus 20 or from a separate source (not shown). The level of current provided to exciter 24 frsm converter 26 is sensed by a sensor 2? and a signal representing the sensed current is provided on conductor 28.
Similarly, the level of current provided by converters 17, 18 and 19 on conductors 14, 15 and 16 is sensed by sensors 30, 31 and 32 respectively, and signals representing the respective currents are on conductors 33, 34 and 35.
A rotor position sensor 36 senses the position of the rotor and the speed of rotation of the ~f~-J~ 3 - 7 ~ Case 2975 rotor of the motor lO, and provides a signal on conductor 37 representing speed and on conductor 38 representing rotor position Conductors 33, 34 and 35, which carry signals representing the level of stator current, are connected to an RMS circuit 40, and the circuit ~O provides a signal on conductor 4l representing the root mean square of the stator winding currents.
A kilowatt circuit 42 or KW circuit ~2 receives signals from bus 13 and bus 20 representing generator current and voltage and provides on conductor 43 a signal representing kilowatts of power prov~ded by the connected generator or generators 2l.
A KVA feedback circuit 44 receives from bus 20 signals representing voltages VA, VB and Vc,and from bus 13 signals representing curren~s IA, IB and Ic, produced by generator 21. Circuit 44 multiplies the voltages with their respective currents and sums the product to provide a signal on conductor 45 representing the total generator KVA
loading.
Considering now the input side of the control system, an input signal representing a desired speed is introduced on conductor 58 from a ship's telegraph 60.
The input signal is applied to a ramp circuit 61 which limits the rate of change (upwards and downwards) to a maximum predetermined rate acceptable to the system and provides a speed signal on conductor 62. Conductor 62 is connected to a multiplier type circuit 63 which may reduce the signal from conductor 62 but cannot increase it. A summing point or adder 64 receives a set reference signal on conductor 65 and a Hz feedback signal or generator frequency signal on conductor 29 and it provides a difference signal on conductor 66.
Conductor 66 is connected to a regulator/limiter ~l2 s~

- 8 - Case 2975 circuit 67 which includes a proportional plus integral controller and a limiterO Circuit 67 integrates and limits the signal received from conductor 66 and provides an output signal on conductor 68 to circuit 63. When the signal representing generator ~requency or Hz on conductor 29 is at rated or normal level, it represents a prime mover operating within iks limits.
The signal ~n conductor 29 then equals or may be more than the set reference signal on conductor 65 and the regulator/limiter 67 provides a signal on conductor 68 that does not affect or alter in multiplier circuit 63 the signal from conductor 62. The speed signal is passed unchanged. However, if the load increases to a level that begins to overload the prime mover 22, the generator frequency will begin to fall and this is indicated by the signal on conductor 29. There will then be a signal on conductor 66 which after integration and limiting will provide a signal on conductor 68 that reduces the speed signal in multiplier circuit 6~. This will result in a reduction of the load on the generator and the prime mover.
The signal from multiplier circuit 63 is on conductor 70. Conductor 70 is connected to a multiplier type circuit 71. A circuit 72 is connected to conductor 23 and receives a signal representing the number of generators operating. Circuit 72, which may be referred to as "the number of generators" circuit, provides predetermined reference signals for various parameters in response to the signal on conductor 23 representing the number of generators operating. One reference signal provided by circuit 72 represents the kilowatt limit, that is, it provides a signal representing the maximum number of kilowatts that can be provided by the number of generators in operation.
This signal is on conductor 73 which is connected to ~7~
- g - Case 2975 summing point or adder 74. Al~o connected to ad~er 74 is conductor 43 which carries the kilowatt feedback signal from circuit 42.
The adder 74 provides a difference signal on conductor 75 ~o a regulator/limiter circuit 76. The circuit 76 integrates and limits the signal from conductor 75 and provides on conductor 77 the inte~rated and limited signal. The circuit 71 is responsive to this signal so that it proportionally reduces the speed signal from conductor 70 when the kilowatt feedback signal (conductor 43) exceeds the kilowatt reference signal (conductor 73), and allows the speed signal to pass unchanged when the kilowatt feedback signal is less than the kilowatt reference signal. The resulting signal i6 on conductor 78 which is connected to summing point or adder 80.
Adder 80 also receives a speed feedback signal from conductor 81. This signal comes from a constant multiplier circuit 82, connected to conductor 37. A difference or error signal from the adder 80 is on conductor 83 which is connected to a speed regulator circuit 84. The speed regulator circuit 84 comprises a speed error limiting circuit 85, a proportional plus integral circuit 86, and a limiter circuit 87 that has changeable limits. The speed regulator circuit 84 has an input 88 for setting a limiting error level suitable for a particular installation. The signal passed by limiter 85 is integrated in proportional plus integral controller 8~ and the resulting signal is limited by a variable limiter 87 which receives a signal from conductor 90 for controlling the torque limits. The output from speed regulator circuit 84 is on conductor 91 which is connected to vector rotator 92. The signal on conductor 91 represents a torque reference.
It will be recalled that a KVA signal is on lo - case 2975 conductor 45. Conductor 45 is connected to a summing point or adder 93. Also connected to adder 93 is a conductor 94 which carries a KV~ reference signal from circuit 72 representing a KVA reference for the number of generators in operation. A di~ference signal is on conductor 95 and is applied as input Y to circuit 96.
Conductor 94 is also connected to circuit 96 and -provides an input signal X. Circuit 96 divides signal Y by signal X and provides the result on conductor 97 and this signal represents generator KVA overload.
Conductor 97 is connected to an inverse time delay circuit 98 which provides an output signal on conductor 100 after an overload signal (a generator KVA overload signal) persists for a predetermined variable time.
The time delay is inversely proportional to the degree of the overload, that is, a small overload results in a relatively long time delay and a larger overload results in a shorter time delay. The signal on conductor 100 is a triggering signal and is applied to circuit 99 which may be considered equivalent to a relay. The circuit 99 performs a switching function between a high level and a low level. It is normally at the high level and a signal on conductor 100 switches it to a low level. Thus circuit 99 provides a low signal on conductor 107 in response to a generator overload that has persisted for a time that is inversely proportional to the degree of overload.
Similarly, circuit 98 once again provides a signal on conductor 100 that switches the circuit 99 to its normal high level in response to an underload persisting for a time inversely proportional to the degree of generator underload.
It will be recalled that conductor 41 carries a signal representing the root mean square of the motor stator currents. Conductor 41 is connected to a - 11 - Case 2975 summing point or adder 101. Also connected to adder 101 is a conductor 102 which has a set reference value representing the rated motor current. Adder 101 provides an error signal or difference signal on conductor 103 which is connected to a circuit 104 as input signal Y. Conductor 102 is also connected to circuit 104 and provides an input signal X. Circuit 104 provides on conductor 105 a signal representing Y/X
which is a motor current overload signal. Conductor 105 is connected to an inverse time d~lay circuit 106 which provides a time delay that is inversely proportional to the level of the motor current overload signal on conductor 105. Circuit 106 then provides a triggering signal on conductor 108. Conductor 108 is connected ~o circuit 110 (circuit 110 is similar to circuit 99). Circuit 110 normally provides a high signal on conductor 111 and is switched or triggered to a low level by the signal on conductor 10~. As before, the inverse time delay circuit 106 terminates the triggering signal on conductor 108 when the motor stator current signal on conductor 105 represents an underload condition that has persisted for a time inversely proportional to the degree and duration of the underload.
A selector circuit 112 receives signals from conductors 107 and 111, and it selects the lower of the two signals and provides the selected signal on conductor 114. Conductor 114 is connected to a ramp circuit 115 which limits the rate of change of the signal on conductor 114. That is, the ramp circuit provides a controlled rate of change from a low level signal to a high level signal and vice versa. Thus the ramp circuit provides on conductor 90 a high level signal representing high limits for limiter 87 in normal operation, and in response to either a - 12 - Case 2975 persisting generator KVA overload or a mokor stakor current overload, provides a low level signal representiny low limits ~or limiter 87. A low limit for limiter 87 will tend to reduce the torgue reference signal on conductor 91.
A flux sensor 116 is connected to conductors 14, 15, 16, 38 and 91 for receiving respectively signals representing the three motor stator voltages, the rotor position signal ~, and the torque re~erence signal. From these signals it determines two values, namely the voltage angle ~ and the flux level. It provides a signal representing the voltag~ angle ~ on conductor 117 and a signal representing flux level on conductor 118.
Vector rotator 92 receives from conductor 91 a torque re~erence signal and from conductor 117 a signal representing the voltage angle ~. Vector rotator 92 provides on conductors 120, 121, and 122 stator current reference signals for producing the 20 desired torque. Conductors 120, 121 and 122 are connected to respective summing points or adders 123, 124 and 125. Also connected to adders 123, 124 and 125 are conductors 33, 34 and 35 respectively. Each adder thus receives current reference signals and signals representing actual current for the respective phase.
The adders 123, 124 and 125 provide on conductors 126, 127 and 128 difference signals which are applied to current regulator/limiters 130, 131 and 132 respectively. Each of the current regulator/limiters 30 130, 131 and 132 includes a proportional plus integral controller which integrates the incoming signal and a limiter which limits the controller output signal. The outputs on respective conductors 136, 137 and 138 are applied to respective converter controls 133, 134 and 35 135. Converter controls 133, 134 and 135 are connected - 13 ~ Case 2975 to conver~ers 17, 18 and 19 respectively, and provide control signals for the converters so that the converters 17, 18 and 19 provide the required currents to the respective motor phases.
A function generator 140 is connected to conductor 37 and receives from conductor 37 a signal representing actual motor speed. Function generator 140 limits the speed signal and provides on conductor 141 a flux reference signal. The function generator 140 reduces the possible effect of flux saturation and the flux reference is generally a function of motor speed. Conductor 141 is connected to summing point or adder 142 and applies thereto the flux reference signal. Also connected to adder 142 is conductor 118 which has a signal representing actual flux level.
Adder 142 provides on conductor 143 an error or difference signal, and conductor 143 is connected to a flux regulator/limiter circuit 144. Circuit 144 includes a proportional plus integral controller and a limiter. The output signal is on conductor 145 and represents a desired exciter current. Conductor 145 is connected to a summing point or adder 146. Also connected to adder 146 is conductor 28 which has a signal representing actual exciter current. Adder 146 provides on conductor 147 a difference or error signal. Conductor 147 is connected to exciter control circuit 148 which provides a control signal to converter 26 for controlling the current provided by exciter 24.
It is believed that the operation of the control system of Figure 1 will be apparent from the preceding description. However a brief summary of the operation may be useful. A signal representing desired speed is input via the ship's telegraph on conductor 58. If the prime mover or prime movers are being .3~ ~

- 14 - Case 2975 overloaded, causing the generator frequency to fall, the spaed signal is reduced by circuit 63. The resulting reduction in the speed of the driving motors not only prevents the prime mover from stalling but indicates to the Captain the changing conditions. The speed signal may also be reduced by circuit 71 if the kilowatts being generated exceed the maximum rating for the number of generators in use. The desired speed signal is, of course, not reduced by circuits 63 or 71 if the prime mover(s3 is operating within the normal operating range and if the kilowatts being generated are within the acceptable range of values.
The desired speed signal is compared with an actual speed signal in adder 80 and the difference is limited and integrated to provide a torque reference signal. ~he torque reference signal passes through a limiter 87 that has changeable limits. The normal limit is a high limit in response to a "high" signal on conductor 114 and has little effect on the torque reference signal except for limiting extremes.
However, the limiter 87 can be changed (at a rate controlled by ramp circuit 115) to a low limit in response to a "low" signal on conductor 11~. The low limit will restrict the torque reference signal temporarily until operation returns to acceptable levels as will be apparent from the preceding description. A low signal on conductor 114 will occur if either the maximum rated motor current is exceeded or the maximum rated generator KVA is exceeded, and it will occur after a time delay that is inversely proportional to the amount of the excess. Thus, the inverse time delay permits a low level of either motor current overload or a low level of generator KVA
overload to persist for a predetermined time (motors and generators are normally designed to run under a low - 15 - Case 2975 overload for a predetermined time before overheating occurs) before the torque signal is limited, whereas a higher level of overload is permitted to persist for a much shorter time. The "low" signal is removed after the motor or generator has been run below its rated level for a time su~ficient to permit recovery (i.e. to permit return to an acceptable temperature).
The tor~le reference signal on conductor 91, along with a signal representing the voltage angle ~, is used by vector rotator 92 to provide current reference signals, for producing a desired torque, to a converter control and converter for each motor phase.
A flux re~erence signal on conductor 141 is derived from the speed signal on conductor 37, suitably limited. The flux reference signal is compared to a flux signal derived from a flux sensor 116. The difference signal, on conductor 143, is integrated and limited to provide a desired exciter current reference signal on conductor 145. This is compared to a signal representing actual exciter current from conductor 28 and the difference is used by exciter control circuit 148 to control converter 26 which provides power to exciter 24. The output fxom exciter 24 is rectified and provides a rotor field current (producing the required flux) for motor 10.
Referring now to Figure 2, there is shown in block schematic form, a control system for an icebreaker having DC motors for rotating the ship's propellers. Parts or circuits in Figure 2 which are similar to those in Figure 1 generally bear like designation numbers. As before, it is customary to have two propellers on an icebreaker, each driven by a respective motor, although a ship may be driven by a single motor. For convenience only one motor is shown in Figure 2. A DC motor 150 receives power from a - 16 - Cass 2975 reversing converter 152 over conductor 151 and through a switch 153. A field winding 154 receives power over conductor 155 from a converter 156. A shaft from motor 150 turns a propeller (not shown). A sensor 158, such as, for example, a tachometer, provides on conductor 81 a signal representing motor speed. A sensor ~0 senses current provided to the main winding of motor 150 and provides a signal representing current on conductor 161. Similarly, a sensor 163 senses the field current provided over conductor 155 to field winding 154, and provides a signal representing field current on conductor 164.
As before, generator 21 (and 21A, etc.) pro~ide power ~or the system; in Figure 2 the power is supplied to converters 152 and 156.
The control for converter 156 comprises a summing point or adder 165 which receives a set input at 162 representing a desired terminal voltage.
Conductor 151 is also connected to adder 165 to provide a signal representing actual voltage. A difference signal or error signal is on conductor 166 which is connected to a regulator/limiter circuit 167. Circuit 167 includes a proportional plus integral controller and a limiter which integrates and limits the input signal and provides an output on conductor 163. This signal represents a desired current for providing the desired terminal voltage. Conductor 168 is connected to a summing point or adder 170. Also connected to adder 170 is conductor 164 which has a signal representing field current. A di~ference or error signal on conductor 171 is applied to a regulator/limiter circuit 172. Circuit 172 includes a proportional plus integral controller and a limiter ~or integrating and limiting the signal applied to it and providing a signal representing the result on conductor - 17 Case 2975 173. Conductor 173 is connected to a converter control 174 which provides control signals to converter 156 for providing the required field current.
In Figure 2, it is conductor 161 which carri~s a signal representing motor current for adder 101 ~rather than the conductors 133, 134 and 135 of Figure 1). Otherwise the portion of the control circuitry which results in the torque reference signal on conductor 91 is substantially the same as that of Figuxe 1. The torque reference signal on conductor 91 is applied to a summing point or adder 176. Also connected to adder 176 is conductor 161 with the signal representing actual motor current. The output from adder 176 is a difference signal on conductor 177 and is applied to a regulator/limiter circuit 178. The circuit 178 includes a proportional plus integral controller and a limiter and it provides an integrated and limited signal on conductor 1~0. Conductor 180 is connected to a converter control 181 which provides control signals via conductor 182 for controlling converter 152.
The operation of the control system of Figure 2 is very similar to that of Figure 1 except that a DC
motor provides the driving power in Figure 2. It is believed that the operation of the control system of Figure 2 will be clearly understood. Very briefly, an input signal representing a speed reference is input at conductor 58. The speed reference signal may be reduced by circuit 63 if the generator frequency drops indicating a prime mover overload, or by circuit 71 if a generator kilowatt limit (for the number of generators on circuit) is exceeded. The speed reference signal (after passing through circuits 63 and 71~ is compared to a signal representing actual speed and the difference is limited and integrated, and then - 18 - Case 2975 is passed through a limiter 87 with changeable limits.
If there is a generator KVA overload, a time delay circuit provides a time delay that is inversely proportional to the amount of the overload and then causes the limits of limiter 87 to be reduced, If there is a motor current overload, a time delay circuit provides a time delay that is inversely proportional to the amount of the overload and then causes the limits of limiter 87 to be reduced. The output from limiter 87 is a torque reference signal on conductor 91. The torque reference signal may be considered as a signal representing a current which will provide a desired torque. ~his signal is compared to a signal representing actual motor current and the resulting signal is integrated and limited and applied to a converter control circuit 181. Converter control circuit 181 controls converter 152 which provides current to the main winding of motor 150. In the field control, the actual terminal voltage is compared to a desired voltage signal and the resulting signal is integrated and limited to provide a signal representing a field current which will produce the desired voltage, and this is compared to a signal representing actual field current. The resulting difference signal is integrated, limited and applied to a converter control 174 for controlling converter 156 which provides field current.
Whila the foregoing detailed description of the preferred embodiments has been with respect to an icebreaker, it should be understood that the electric drive system of the present invention may find other applications such as for example in ships, electric powered vehicles and possibly locomotives.

Claims

1. A control system for a drive, said drive having at least one prime mover driving a respective AC
generator which provides power to a controllable converter, said converter providing power to at least one electric motor for rotating a drive shaft, comprising:
means for providing a speed reference signal representing a desired speed, means for sensing the rotational speed of said motor and providing an actual speed signal representing the sensed speed, means for comparing said speed reference signal and said actual speed signal and providing a first difference signal representing the difference between the compared signals, means for integrating said first difference signal to provide a torque reference signal, means responsive to said torque reference signal for controlling said controllable converter to provide a motor torque which will operate said motor at a speed approaching the speed represented by said speed reference signal, means for sensing the frequency of the output of said generator and providing a frequency feedback signal representing the sensed frequency, said frequency feedback signal decreasing with overload on said prime mover, means for comparing said frequency feedback signal with a set reference signal and providing a second difference signal when said frequency feedback signal is less than said set reference signal, and means responsive to said second difference - 20 - Case 2975 signal for reducing said speed reference signal and thereby reducing load on said prime mover.

2. A control system as defined in claim 1 and further comprising means for determining the number of said generators operating and providing a kilowatt reference signal representing the maximum rated kilowatt output for the number of generators operating, means for determining the actual kilowatts being output by said generators and providing an actual kilowatt signal representing this, means for comparing said actual kilowatt signal with said kilowatt reference signal and providing a third difference signal representing the amount by which said actual kilowatt signal is greater than said kilowatt reference signal, and means responsive to said third difference signal for reducing said speed reference signal and thereby reducing load.

3. A control system as defined in claims 1 or 2 and further comprising limiter means having changeable limits for receiving the integrated first difference signal and providing said torque reference signal, said limiter means having a high limit for limiting extremes in said integrated first difference signal and a low limit for considerably limiting said integrated first difference signal, means for determining generator KVA and providing a KVA feedback signal representing this, means for comparing said KVA feedback signal with a reference KVA signal representing a maximum KVA
for the number of generators operating and deriving therefrom a generator KVA overload signal, and time delay means for receiving said generator - 21 - Case 2975 KVA overload signal, providing a time delay inversely proportional to the degree of overload represented by said KVA overload signal, and at the end of the time delay providing a triggering signal to said limiter means for causing said limiter means to operate at said low limit.

4. A control system as defined in claim 1 or 2 and further comprising limiter means having changeable limits for receiving the integrated first difference signal and providing said torque reference signal, said limiter means having a high limit for limiting extremes in said integrated first difference signal and a low limit for considerably limiting said integrated first difference signal, means for determining motor current and providing a motor current signal representing the determined current, means for comparing said motor current signal with a maximum motor current reference signal and deriving therefrom a motor current overload signal, and time delay means for receiving said motor current overload signal, providing a time delay inversely proportional to the degree of overload as represented by said motor current overload signal, and at the end of the time delay providing a triggering signal to said limiter means for causing said limiter means to operate at said low limit.

5. A control for a drive as defined in claim 2 in which said at least one electric motor is at least one synchronous motor each having a stator winding and a field winding, and in which said at least one controllable converter is three controllable converters each providing current for a respective phase of said synchronous motor, and further comprising:

- 22 - Case 2975 exciter means including an exciter for providing current to said field winding and a field winding converter for providing power to said exciter, means for determining the position of the rotor of said synchronous motor and providing a rotor position signal representing the position, flux sensor means for receiving signals representing the voltages of the three phases of said synchronous motor, said torque reference signal and said rotor position signal, and providing a signal representing the voltage angle of the flux and a signal representing flux level, vector rotator means for receiving said torque reference signal and said signal representing the voltage angle of the flux, and providing a desired current signal for each phase of said motor, a respective adder means for each phase for receiving a respective desired current signal and a respective sensed actual current signal, and providing respective fourth difference signals, and a respective regulator means for each phase for receiving a respective fourth difference signal, integrating and limiting said signal, and providing a respective converter control signal, said converter control signal being applied to a respective one of said controllable converters for controlling said converters to provide current at a desired amplitude and time for a respective motor phase to rotate said motor according to said speed reference signal with limitations.

6. A control system for a drive as defined in claim 5, and further comprising flux limiter means for receiving said actual speed signal and providing a related flux reference signal, - 23 - Case 2975 adder means for comparing said flux reference signal and said signal representing flux level, and providing a fifth difference signal, regulator means for receiving said fifth difference signal, integrating and limiting said fifth difference signal, and providing as a result a desired exciter current signal, means for sensing the actual current provided by said field winding converter to said exciter, and providing an actual exciter current signal representing the sensed signal, and adder means for receiving said desired exciter current signal and said actual exciter current signal and providing a sixth difference signal representing an exciter control signal, said exciter control signal being applied to said field winding converter for controlling said field winding converter to provide a field current for producing a desired flux.

7. A control system for a drive as defined in claim 2 in which said at least one electric motor is at least one DC motor having a main winding and a field winding, and in which said at least one controllable converter is at least one controllable reversible converter, and further comprising field converter means for providing field current to said field winding, adder means for receiving said torque reference signal and a signal representing actual sensed current, and providing a seventh difference signal representing the difference, and regulator means for receiving said seventh difference signal, integrating and limiting said seventh difference signal to provide a converter control signal for controlling said controllable - 24 - Case 2975 reversible converter according to said torque reference signal.

8. A control system for a drive as defined in claim 7, and further comprising sensor means for sensing the voltage at said main winding and providing a terminal voltage signal representing the sensed voltage, adder means for receiving said terminal voltage signal and comparing it with a set terminal voltage reference, and producing an eighth difference representing the difference, regulator means for receiving said eighth difference signal, integrating and limiting said eighth difference signal, and providing a desired field current signal, sensor means for sensing the current provided by said field converter to said field winding and providing an actual field current signal representing this, adder means for receiving said desired field current signal and said actual field current signal and providing a ninth difference signal representing the difference, and regulator means for receiving said ninth signal, integrating and limiting said ninth difference signal and providing the integrated and limited signal as a field control signal, said field control signal being applied to said field converter for controlling said field converter to provide a desired field current.

9. In a control system for a ship drive, said ship drive having a prime mover driving a respective AC generator, said generator providing power to at least one controllable converter, said controllable converter providing power to an electric - 25 - case 2975 Claim 9 continued:
motor having a main winding and a field winding for rotating a propeller for moving the icebreaker, a speed control means comprising input means for introducing a speed reference representing a desired motor speed, first sensing means for sensing the actual motor speed and providing an actual speed signal representing the sensed speed, second sensing means for sensing the frequency of the output of said generator and providing a frequency feedback signal representing the sensed frequency, the sensed frequency decreasing with overload on the prime mover, first adder means for receiving said frequency feedback signal and a set frequency signal representing a frequency related to normal operation of said prime mover, and providing a first difference signal representing the amount by which said frequency feedback signal drops below said set frequency reference signal, first regulator means for receiving said first difference signal, integrating and limiting said first difference signal to provide a first reduction signal, first reduction means for receiving said first reduction signal and said speed reference signal, and reducing said speed reference signal in accordance with said first reduction signal, second adder means for receiving the reduced speed reference signal from said first reduction means and said actual speed signal from said first sensing means, and providing a second difference signal representing the difference between said reduced speed reference signal and said actual speed signal, - 26 - case 2975 second regulator means having at least an integrator and a limiter with changeable limits, for receiving said second difference signal, integrating and limiting said second difference signal to provide a torque reference signal, and means responsive to said torque reference signal for controlling said motor, the torque reference signal being decreased in response to a drop in said frequency feedback signal thereby decreasing load on the prime mover.

10. In a control system as defined in claim 9, and further comprising reference means for providing a kilowatt reference signal representing the rated maximum power output obtainable from the number of generators that are in operation, sensor means for determining the actual kilowatts being generated by said generators and providing an actual kilowatt signal representing the determined kilowatts, third adder means for receiving said actual kilowatt signal and said kilowatt reference signal, and providing a third difference signal related to the amount by which the actual kilowatt signal is greater than said kilowatt reference signal, third regulator means for receiving said third difference signal, integrating and limiting said third difference signal to provide a second reduction signal, and means for receiving said second reduction signal and said speed reference signal and reducing said speed reference signal in accordance with said second reduction signal.