WO1990006015A1

WO1990006015A1 - Vscf start system with a constant acceleration

Info

Publication number: WO1990006015A1
Application number: PCT/US1989/004444
Authority: WO
Inventors: Richard W. Reynolds; Gregory I. Rozman
Original assignee: Sundstrand Corporation
Priority date: 1988-11-21
Filing date: 1989-10-10
Publication date: 1990-05-31
Also published as: IL92286A0; JPH04501049A

Abstract

The problem of reducing the effect of inaccuracies in rotor position detection for an engine starting system is solved in an engine start control apparatus (10) operating a generator (12) as a synchronous motor which employs acceleration control. The motor (12) receives power from a main inverter and an excitation inverter (48). The inverters (46, 48) are controlled by a control unit (22) which provides for acceleration control and closed loop reactive power control. The control unit (22) includes a pulse width modulation generator (100) which is responsive to a voltage command and a commutation command to develop switching signals for controlling the switches (S1-S6) in the main inverter (46). The voltage command is used to vary the duty cycle of the PWM signals. The commutation angle command is used to control the timing of the PWM signals. The commutation angle command is determined in accordance with an acceleration reference (112) and rotor position (70) feedback in a closed loop manner to maintain generally constant acceleration.

Description

V5CF START SYSTEM WITH A CONSTANT ACCELERATION

Field of the Invention

This invention relates to electrical power systems and more particularly to a dual mode control system therefor including a generate mode of operation and a start mode of operation.

Background of the Invention

Conventional electrical power systems utilize a synchronous electrical generator for generating AC power. Particularly, such a generator may include a rotor and a stator having a stator coil. In applications such as an aircraft, the rotor is driven by an engine so that electrical power is developed in the stator coil. owing to the variation in engine speed, the frequency of the power developed in the generator windings is similarly variable. This variable frequency power is converted to constant frequency power in a variable speed constant frequency (VSCF) system including a power converter which may develop, for example, 115/200 V_ac power at 400 Hz. Such known converters are controlled by a generator/converter control unit (GCCU) .

In order to provide aircraft engine starting, such known power systems have operated the generator as a motor. Specifically, an external power source is coupled through a start control_^to the generator to energize the stator coil and thus develop motive power to start the engine. The -.components required in such a start control increase the weight of the aircraft and take up valuable space. To minimize the size and weight of such start controls, certain known aircraft VSCF power systems have utilized the existing converter and GCCU for the start control.

In the start mode of operation, the converter may be supplied power from any 400 Hz power source, such as, for example, an auxiliary unit generator or an external power source. However, each such power source might have a different available capacity for use in engine starting. Therefore, the GCCU must be configured to provide engine starting from any such available power sources and to limit the amount of power drawn.

. Rozman et al. co-pending application entitled VSCF Start System with Selectable Input Power Limiting, serial no. , filed , and owned by the assignee of the present invention (Docket no. B02951-AT1-USA) , which is hereby incorporated by reference herein, discloses a start control which provides input power limitations in accordance with input power requirements. Specifically, the start control described therein uses closed loop control of the commutation angle at speeds above a preselected minimum to control current and to limit input power. The commutation angle represents the phase advance, which is the angle between the applied voltage and the rotor position.

To insure smooth engine starting, it is desirable that closed loop control be used at all speeds.

Developed motor torque is proportional to field excitation and the torque angle, the torque angle being the angle between applied voltage and back EMF. It is desirable to operate the motor at unity power factor to avoid excessive reactive power flow, which creates heating losses in the motor and the converter. The power factor is controlled by the magnitude of the field excitation. At low speeds the torque angle is small. Since the commutation angle is controlled utilizing information from a rotor position sensor, any error from the position sensor undesirably affects the torque developed in the motor.

Additionally, losses in the motor and converter are a function of motor current. It is therefore desirable to maintain the motor current at a low level and to increase it only when it is necessary to overcome engine drag torque.

The present invention is intended to overcome one or more of the problems as set forth above.

Summary of the Invention

In accordance with the present invention, a start control system for a brushless DC machine is operable to control rotor acceleration.

Broadly, there is disclosed herein a start control system for a brushless DC machine having a rotor and a stator having a stator coil which is controllably energized from a source of DC power defining a positive and a negative DC voltage for imparting rotation to the rotor. The control system includes means for sensing the rotational position of the rotor, and switching means coupled between the source of DC power and the stator coil for alternately applying the positive and negative voltage to the coil according to the rotational position of the rotor. Means are included for developing an acceleration reference representing a desired rotor acceleration level, and means for generating a feedback value representing actual acceleration. Control means are coupled to the developing means, the generating means and the switching means for modifying the rotational position at which the positive and negative voltages are applied to the coil according to a difference between the actual acceleration and the desired acceleration. Specifically, the disclosed start system is used for starting an engine using a brushless synchronous generator operating as a motor. The motor receives power from a main inverter and an excitation inverter. These inverters are controlled by a control unit which provides for constant acceleration.

The control unit includes a pulse width modulation (PWM) generator which is responsive to a voltage command and a commutation command to develop switching signals for controlling the switches in the main inverter. The voltage command is a level corresponding to rotor speed plus a boost voltage to offset the IR drop of the machine at low speeds. The voltage command is used to vary the duty cycle of the PWM signals.

In order to develop the commutation angle command an acceleration command is integrated to provide a speed command which is compared to actual speed to develop a speed error. The speed command is further integrated to provide a position command which is compared to actual rotor position to develop a position error. The position error and the speed error are summed to provide the commutation angle command which is limited prior to being transferred to the PWM generator. The commutation angle command is used to phase advance the main inverter output.

Thus, in the start mode of operation the motor is controlled by the main and the excitation inverters to provide a substantially- constant acceleration over the starting speed range of the engine utilizing closed loop control of the commutation angle. It is another object of the invention to provide an engine start control system which controls a converter to start a synchronous motor while reducing the effect of the accuracy of a rotor position detector on system performance. It is a further object of the invention to provide an engine start control system which controls a converter to start a synchronous motor while permitting output power to be controlled in accordance with the motor load, i.e., the engine drag torque.

Further features and advantages of this invention will readily be apparent from the specification and from the drawings.

Brief Description of the Drawings Figure 1 is a combined diagrammatic illustration-block diagram of an electrical system incorporating the start system of the present invention;

Figure 2 is a generalized block diagram of the electrical power system including a control system for the generate mode of operation and the start mode of operation;

Figure 3 is a block diagram of the control system specifically illustrating the start mode of operation;

Figure 4 is a schematic diagram illustrating the main inverter of figure 3 ; Figure 5 is a more detailed block diagram of the generator/convertor control unit of figure 3; and

Figure 6 is a more detailed block diagram of a generator/converter control unit, similar to that of figure 5, according to an alternative embodiment of the invention.

Description of the Invention

Referring first to figure 1, an electrical power system 10 includes a main generator 12, an AC exciter 14 for providing main field current to the generator 12 and a permanent magnet generator (PMG) 16. Each of the main generator 12, exciter 14 and PMG 16 are driven by an engine IS through a common shaft 20.

A generator/convertor control unit (GCCU) 22 receives the power developed by the PMG and delivers a controlled current to a field winding 26 of the exciter 14. As is conventional in brushless power systems, rotation of the shaft 20 by the engine 18 results in generation of a polyphase voltage in armature windings 28 of the exciter 14. This polyphase voltage is rectified by a rectifier bridge, illustrated generally at 30, and the rectified power is coupled to a field winding 32 of the main generator 12. The current in the field winding 32 and the rotation of the shaft 20 sets up a rotating magnetic field in space occupied by a set of main generator stator windings 34. The stator windings 34 develop polyphase output power which is delivered to a converter 36 over a bus 38 comprising at least three conductors 38a, 38b, and 38c.

In a typical application, the engine 18 is the main engine in an aircraft, and the converter 36 is part of a variable speed constant frequency (VSCF) system for. delivering constant frequency power to an AC bus 40 for powering aircraft loads (not shown) , as controlled by the GCCU 22.

During engine start, the engine 18 is started using the main generator 12 operating as a motor. Particularly, the main generator 12 receives power from the converter 36 which is controlled by the GCCU 22. For ease of explanation herein, the main generator 12 is referred to as a motor when operated as such in the start mode of operation.

Referring now to figure 2, the electrical power system 10 is illustrated in greater detail in block diagram form. The converter 26 includes an AC/DC converter 42 connected by a DC link 44 to a DC/AC converter 46. Particularly, according to the illustrative embodiment of the invention, the AC/DC converter 42 comprises a full wave bridge rectifier circuit of conventional construction which is operable to convert three phase AC power to DC power, the DC link 44 includes a conventional filter, and the DC/AC converter 46 comprises a main inverter circuit, described more specifically below relative to figure 4. The converter 36 also includes an excitation inverter 48 connected to the DC link -44 for developing AC power for the motor field during the start mode of operation.

The AC side of the rectifier 42 is connected to a movable contact 50 of a converter input relay (CIR) . The relay CIR also includes respective first and second fixed contacts 51 and 52. The second fixed contact 52 is connected through a filter circuit 54 and generator bus relay (GBR) 56 to the AC bus 40. The first fixed contact 51 is connected to a first fixed contact 57 of a generator relay (GR) . The GR relay also includes a movable contact 58 and a second fixed contact 59. The movable contact 58 is connected to the main generator 12, i.e., to the windings 34 shown in Figure 1. The second fixed contact 59 is connected to a first fixed contact 60 of a converter output relay (COR) . The COR relay also includes a movable contact 61 and a second fixed contact 62. The movable contact 61 is connected to the output of^~the main inverter 46. The second fixed contact 62 is connected through an output filter 64 to the filter circuit 54. The COR relay also includes respective first and second field control switches 65 and 66. The first switch 65 connects the exciter field winding 26 to the GCCU 22. The second switch 66 connects the excitation inverter 48 to an AC start field winding 67 of the exciter 14. Specifically, the excitation for the wound field main generator/motor 12 cannot be supplied at zero speed by the exciter 14. Accordingly, the excitation inverter 48 and the start field winding 67 are included functioning as a rotary transformer. Specifically, AC power delivered to the exciter AC field winding 67 develops corresponding AC power in the armature windings 28 for powering the motor field winding 32.

During engine start, the relays GR, CIR and COR are operated as shown in solid line in figure 2. Conversely, in the generate mode, these relays GR, CIR and COR are operated as shown in dashed line in figure 2.

Although the relays GR, CIR and COR are shown as providing a single line connection, each of the relays is provided with suitable switches to switch three phase power, as is well known.

The GCCU 22 includes a speed converter 68 which receives a rotor position signal on a line 70 from a rotor position sensor 72 associated with the main generator 12. The position sensor 72 may be, for example, a conventional resolver. The rotor position signal 70 is also transferred to a main inverter control 74. The speed converter 68 may perform a derivative operation for converting rotor position to speed, as is well known. The main inverter control also receives the speed signal on a line 76 from the speed converter 68. The main inverter control 74 develops base drive commands on a line 88 for controlling the inverter 46. An exciter inverter control 78 also receives the speed signal on the line 76 from the speed converter 68 and an excitation inverter current signal on a line 80 from an excitation inverter current sensor 79. The excitation ^""inverter control 78 develops base drive commands on a line 90 for driving the switches of the excitation inverter 48. In the generate mode of operation, with the relay contacts GR, CIR and COR as illustrated in dashed lines, three phase power developed by the main generator 12 is delivered through the GR relay movable contact 58, its first fixed contact 57, through the CIR relay first fixed contact

51 and its movable contact 50 to the rectifier 42. The rectifier 42 converts the three phase AC power to DC power which is transferred over the DC link 44 to the inverter 46 which converts the power to AC power of constant frequency. The constant frequency AC power from the inverter 46 is delivered through the CIR relay movable contact 61 to the second fixed contact 62, through the output filter 64, and the filter 54 to the AC bus 40. Field power is developed by the exciter generator 14 utilizing the DC field winding 26 powered from the GCCU 22 through the first field control switch 65.

In the start mode of operation, the relays GR, CIR and COR are controlled so that their contacts are positioned as shown solid lines. Particularly, the AC bus 40 is connected to any available power source. The AC power is delivered through the filter 54, to the second fixed contact

52 and movable contact 50 of the CIR relay to the rectifier 42. The AC voltage is then rectified and transferred through the DC link 44 to the main inverter 46 where it is converted to AC power. The AC power from the main inverter 46 is delivered through the movable contact 61 and the first fixed contact 60 of the COR relay, and subsequently through the second fixed contact 59 and movable contact 58 of the GR relay to the armature windings of the main generator/motor 12. Field power to the main generator 12 is provided from the excitation inverter 48 through the second COR field control switch 66 to the exciter generator AC field winding 67, as discussed above. Referring now to figure 3, a block diagram representation more specifically illustrates the operation of the electrical power system 10 according to the invention in the start mode of operation, as discussed immediately above. A power source 82 is coupled to the rectifier 42 which is coupled through the DC link and filter 44 to both the main inverter 46 and the excitation inverter 48. The GCCU 22 receives a reactive power feedback signal on a line 85 from a conventional reactive power detector circuit 86 which senses reactive power from the main inverter 46 to the motor 12. The GCCU 22 also receive the position signal on the line 70 from the rotor position sensor 72 and the excitation current signal on the line 80 from the current detector 79. As discussed above, the GCCU 22 develops the base drive commands for the main inverter 46 on the line 88 and the base drive commands for the excitation inverter 48 on the line 90.

Referring to figure 4, a schematic diagram illustrates one alternative circuit for the main inverter 46. Particularly, the inverter 46 is a voltage source inverter having six power switch circuits S1-S6. The six power switch circuits S1-S6 are connected in a 3-phase bridge configuration. Each of the power switch circuits S1-S6 is driven by an associated respective base drive circuit B1-B6. The base drive circuits B1-B6 are driven by the signals on the line 88 from the GCCU 22 in a conventional manner. The switch circuits S1-S6 are connected between the plus voltage DC rail and the minus voltage DC rail of the DC link filter 44. The 3-phase armature windings 34 of the main generator 12 are connected by the lines 38a-38c, respectively, to junctions 92a-92c between pairs of series-connected switch circuits S1-S6. A neutral line 94 to the main generator 12 is connected at a junction between filter capacitors CI and C2 across the DC link filter 44.

Although not shown, the excitation inverter 48 may be of generally similar construction to the main inverter 46 illustrated in figure 4. Alternatively, other circuits may be utilized for either or both of the main inverter 46 and the excitation inverter 48, as is well known.

With reference to figure 5, a block diagram illustrates the implementation of the GCCU 22, see figure 3, according to the invention, including the main inverter control 74 and the excitation inverter control 78.

The main inverter control 74 includes a PWM generator 100. The PWM generator 100 receives the position signal on the line 70, a voltage command on a line 102, and a commutation angle command on a line 104. The PWM generator 100 derives the base drive commands which are transferred on the line 88 to the base drive circuits B1-B6 of the main inverter 46, see Figure 4. The PWM generator 100. may be of any conventional construction. Particularly, the PWM generator 100 develops base drive signals to control the output voltage of the main inverter 46, by varying the duty cycle of the PWM signals. The duty cycle is proportional to the voltage command received on the line 102. The fundamental frequency of the inverter output is determined by motor speed. The output waveforms are synchronized to the input of the rotor position as determined by the sensor 72, see Figure 3. The phase difference between rotor position and inverter output is adjusted in accordance with the commutation angle command on the line 104?

The voltage command on the line 102 is formed by converting the rotor position signal on the line 70 to a speed signal on the line 76 via the speed signal converter 68. A multiplier 106 multiplies the speed signal by a constant from a block 108. Particularly, the constant represents a desired volt/hertz ratio. A summer 110 receives the output from the multiplier 106 and a constant Vo, which is proportional to "boost" voltage, from a block 111. The boost voltage is required to offset the IR drop of the machine at low speed. The output of the summer 110 is the voltage command on the line 102.

The commutation angle command on the line 104 is developed responsive to an acceleration reference generated at a block 112. This reference may be generated by any known means as is well known. The acceleration reference

112 is applied to an integrator _114 which develops a speed command on a line 116. The speed command on the line 116 is applied to a further integrator 118 which develops a position command on a line 120. The speed command on the line 120 is applied to a summer 122 which also receives the speed feedback signal on the line 76 from the speed signal converter 68. The output of the summer 122 is a speed error on a line 124. The position command on the line 12 is applied to an additional summer 126 which also receives the position feedback signal on the line 70. The output of the summer 126 is a position error on a line 128.

The errors on the lines 124 and 128 are applied to respective multipliers 130 and 132, the outputs of which are applied to a summer 134. The multipliers 130 and 132 are utilized to scale the errors relative to one another. The output of the summer 134 is applied to a limit function block 136 which develops the commutation angle command on the line 104. The purpose of the limit function 136 is to prevent the control system from operating in an unstable region which can occur if commutation angle command exceeds maximum voltage, which is a function of speed and motor parameters.

The excitation inverter control 78 controls field current in the motor 12 and is controlled by varying the duty cycle of a PWM signal from a PWM generator 140. Specifically, the PWM generator 140 develops the base drive command signals on the line 90 for controlling the excitation inverter 48, see Figures 2 and 3. The duty cycle is proportional to a voltage reference which is applied to the PWM generator 140 on a line 142. The voltage reference on the line 142 is formed by a compensation unit 144 which receives the power feedback on the line 85 and develops a current reference, applying it to a summer 146. The reactive power is controlled to be equal to zero, which corresponds to unity power factor operation. The other input of the summer 146 is the current feedback on the line 80. The summer 146 develops an error on a line 148 which is applied to a compensation unit 150 which utilizes, for example, proportional and integral control to develop the voltage reference on the line 142 which is applied to the PWM generator 140.

The operation of the GCCU illustrated in figure 5 is now discussed. At the beginning of the start motoring mode of operation, i.e., the speed is zero, the commutation angle command on the line 104 is zero. Specifically, when start operation is initiated, the integrators 114 and 118 are reset to zero utilizing a conventional reset function (not shown), the position feedback on the line 70 is zero, and the speed feedback on the line 76 is zero. Accordingly, the speed error on the line 24 and the position error on the line 128 are both zero, resulting in the commutation angle command on the line 104 being zero. Alternatively, a minimum commutation angle command could be provided for, as described in the co-pending application incorporated by reference herein. Also, the voltage command on the line 102 is equal to the boost voltage ø. The excitation inverter current is controlled to maintain unity power factor.

The PWM generator 100 begins to develop base drive commands to the main inverter 46 according to the initial rotor position. The interaction between the magnetic field established in the rotor and stator current causes movement of the main generator rotor and thus the shaft 20. As speed increases, the voltage command on the line 102 is increased proportionally according to the volt/hertz ratio set at the block 108 to increase the duty cycle and speed up the motor. Simultaneously, the commutation angle command on the line 104 is controlled according to the position error on the line 128 and the speed error on the line 124. Specifically, since the acceleration reference 112 is a constant, the speed command on the line 116 increases linearly over time owing to the integration function performed by the integrator 114. The position command on the line 120 increases according to the integration of the speed command by the integrator 118. As long as the rotor position operates according to the position command, and the speed increases linearly according to the speed command, the commutation angle command on the line 104 remains constant and smooth starting is provided. Any inaccuracies in the position feedback on the line 70 causes corresponding inaccuracies in the speed feedback on the line 76, and thus also the speed error and the position error. Resultantly, the commutation angle command on the line 104 changes responsive^thereto. Also, errors could result from changes in engine drag torque. Specifically, if drag torque changes, then rotation of the shaft 20 is impeded causing errors in both speed and position, resulting in corresponding change in the commutation angle command.

To compensate for any such errors, the commutation angle is increased or decreased according to the particular error. Particularly, if engine drag torque increases, it is necessary to develop additional motor torque which is done by creating a difference between rotor position and developed torque. By firing the inverter switches sooner, known as phase advance, a field is created which leads the rotor. A change in phase advance is accomplished by changing the commutation angle, resulting in a similar change in torque angle and also motor current. Simultaneously, field current is controlled utilizing the excitation inverter control 78 to maintain unity power factor operation.

With reference to figure 6, a detailed block diagram of a generator/converter control unit 22' according to an alternative embodiment of the invention is illustrated. Specifically, the GCCU 22• is generally similar to the GCCU 22 described above relative to figure 5, but further, includes additional control elements. More specifically, the main inverter control 74' includes further control elements for modifying the boost voltage Vo to be selected in accordance with the synchronous machine condition and motor load. The excitation inverter control 78' provides constant dynamic characteristics to the reactive power closed loop over the full range of motor speed.

For simplicity, elements similar to those shown above in figure 5 are referenced with like, primed reference numerals. For example, the PWM generator 100' of figure 6 is generally similar to the PWM generator 100 of figure 5. Accordingly, such elements referenced with primed numerals are not described in detail herein below.

The torque and current of the motor 12 are primarily a function of the boost voltage at or near zero speed. The boost voltage is equal to the stator current of the machine multiplied by the resistance of the stator winding. The stator winding resistance is a function of temperature and may vary according to the ambient temperature. Such variations in resistance can sufficiently affect the engine start operation. If the boost voltage is configured for operation under cold temperature conditions, then hot engine restart can cause severe overcurrent conditions. Conversely, if the boost voltage is set up for a hot engine, then it may significantly slow down start during a cold temperature condition.

The principal difference in the main inverter control 74' is that the output from the summer 134^« is coupled to an input 200 of a mode selection switch 202. A first output 204 from the switch 202 is coupled as the input of the limit function 136'. A second output 206 is coupled to a memory unit 208 which is coupled to the summer 110'. Both the memory unit 208 and the mode selection switch 202 are operated in accordance with a speed comparator 210 which receives the speed signal on the line 76^f. Specifically, the speed comparator 210 determines the position of the switch 202 and the operation of the memory unit 208.

At low speeds, for example below speeds in the range of 20 to 50 rpm, the speed comparator 210 causes the switch 202 to connect the input 200 with the second output 206. Accordingly, the boost voltage applied to the summer 110' represents the output from the summer 134'. At such speeds, the commutation angle command to the limit function 136' is equal to zero since the second switch output 204 is disconnected. Resultantly, the boost voltage is initially controlled to maintain acceleration so that it changes with rotor resistance changes.

Once rotor speed exceeds the preselected minimum speed set at the speed comparator 210, then the mode switch 202 couples the input 200 with the first output 204 so that the commutation angle is controlled to maintain acceleration, and the memory unit 208 is operable to hold the last value of the boost voltage, which last value is used thereafter and applied to the summer 110* for the remainder of the engine start operation.

Thus, the main inverter control 74' is operable at engine start to select the boost voltage in accordance with the engine drag torque to counteract any variations in stator winding resistance.

An AC exciter, such as the exciter 14, is an induction machine which operates at a slip of greater than one. At zero speed, the slip is equal to one. At maximum operating speed the slip is equal to two. Such variations in frequency cause an increase in the output voltage of the AC exciter 14. As a result, for a given constant AC exciter current, the main field current is increasing as speed increases. Therefore, it is desirable that the gain of the closed loop reactive power control be a function of speed.

The excitation inverter control 78' differs principally from the excitation inverter control 78 of figure 5 in including a gain scheduler 212 coupled between the compensation unit 144' and the summer 146'. The gain scheduler 212 receives the speed signal on the line 76' from the speed, signal convertor 68'. The gain scheduler 212 applies a gain to the output of the compensation speed 144' according to the actual rotor speed. Thus, adaptive control of the excitation inverter control loop is provided which provides constant dynamic characteristics of the reactive power closed loop over the full range of motor speed.

According to the present invention, the effective inaccuracies of the position detector on system performance is reduced_ by employing the closed loop acceleration control.

Moreover, the acceleration control according to the invention permits the synchronous motor output power to be changed in accordance with motor load, i.e. engine drag torque. Specifically, since the losses in the synchronous motor and converter are a function of motor current, it is desired to maintain low motor current and increase it, by increasing the commutation angle command, only when it is necessary to overcome the drag torque.

The GCCU 22 described herein can be implemented with suitable electrical or electronic circuits, or with a software programmed control unit, as is obvious to those skilled in the art.

Thus, the invention broadly comprehends a start control system for a brushless DC machine utilizing constant acceleration control.

Claims

We Claim:

1. A start control system for a brushless machine having a rotor and a stator having a stator coil which is controllably energized from a source of DC power defining a positive and a negative DC voltage for imparting rotation to the rotor, comprising: means for sensing the rotational position of said rotor; switching means coupled between the source of DC power and said stator coil for alternately applying the positive and negative voltage to said coil according to the rotational position of said rotor; means for developing an acceleration reference signal representing a desired rotor acceleration level; means for generating a feedback signal representing actual rotor acceleration; and control means coupled to said developing means, said generating means and said switching means for modifying the rotational position at which the positive and negative voltage is applied to said coil responsive to said desired rotor acceleration and said actual rotor acceleration to provide constant acceleration of said rotor.

2. The start control system of claim 1 further comprising means for sensing the speed of rotational movement of said rotor, and wherein said switching means varies the length of time for alternately applying the positive and negative voltage to the coil according to rotor speed.

3. The start control system of claim 1 wherein said control means includes means for comparing said acceleration reference signal to said feedback signal.

4. The start control system of claim 1 wherein said DC machine includes a field coil and said start control system further includes means for controllably energizing the field coil to maintain unity power factor operation.

5. The start control system of claim 4 further comprising means for sensing the speed of rotational movement of said rotor, and means for modifying the energization of the field coil according to the rotor speed.

6. A constant acceleration start control system for a brushless DC machine having a rotor and a stator having a stator coil which is controllably energized from a source of DC power defining a positive and a negative DC voltage for imparting rotation to the rotor, comprising: means for sensing the actual rotational position of the rotor; switching means coupled between the source of DC power and the stator coil for alternately applying the positive and negative voltage to the coil according to the rotational position of said rotor; means for developing an acceleration reference signal representing a desired rotor acceleration; means coupled to said developing means for determining a rotor speed reference representing a desired rotor speed and a position reference representing a desired rotor position; means for generating a rotor speed signal representing rotational speed of said rotor; and control means coupled to said generating means, said determining means, said sensing means and said switching means for modifying the rotational position at which the positive and negative voltages are applied to the coil according to a first difference between said desired and actual rotor speed and a second difference between said desired and actual rotor position to provide generally constant acceleration starting.

7. The start control system of claim 6 further comprising means coupled to said generating means for and said switching means for varying the length of time for alternately applying the positive and negative voltage to the coil according to rotor speed.

8. The start control system of claim 6 wherein said control means includes means for summing said first difference and said second difference.

9. The start control system of claim 6 wherein , 2 said determining means includes first means for integrating said acceleration reference to obtain said speed reference 4 and second means for integrating said speed reference to obtain said position reference.

-10. The start control system of claim 6 wherein 2 the DC machine includes a field coil and said start control system further includes means for controllably energizing 4 the field coil to maintain unity power factor operation.

11. The start control system of claim 6 wherein 2 the DC machine includes a field coil and said start control system further includes means for sensing reactive power in 4 the stator coil and means coupled to said sensing means for controllably energizing the field coil in accordance with 6 said reactive power to maintain unity power factor operation.

12. The start control system of claim 6 further 2 comprising means for modifying the energization of the field coil according to the rotor speed.

13. The start control system of claim 7 further 2 comprising means coupling said varying means with said control means and said generating means for varying the 4 length of time for alternately applying the positive and negative voltage to the coil according to the first ₆ difference between said desired and actual rotor speed and said second difference between said desired and actual rotor position at speeds below a preselected minimum rotor speed.

14. A start control system for a brushless DC machine having a rotor and a stator having a stator coil which is controllably energized from a source of DC power defining a positive and a negative DC voltage for imparting rotation to the rotor, comprising: means for sensing the actual rotational position of said rotor; switching means coupled between the source of DC power and said stator coil for alternately applying the positive and negative voltage to said coil according to the rotational position of said rotor; means for developing a rotor position reference signal representing a desired rotational position of said rotor; and control means coupled to said developing means, said sensing means and said switching means for modifying the rotational position at which the positive and negative voltage is applied to said coil responsive to said desired rotational position and said actual rotational position to minimize the effect of inaccuracies of said sensing means.

15. A start control system for a brushless DC 2 machine having a rotor and a stator having a stator coil which is controllably energized from a source of DC power 4 defining a positive and a negative DC voltage for imparting rotation to the rotor, comprising: , means for sensing the actual rotational position of said rotor; 8 switching means coupled between the source of DC power and said stator coil for alternately applying the 10 positive and negative voltage to said coil according to the rotational position of said rotor; ¹² means coupled to said sensing means for developing a reference signal representing changes in torque applied to 14 the rotor; and control means coupled to said developing means, 16 and said switching means for modifying the rotational position at which the positive and negative voltage is 18 applied to said coil to control current through said coil responsive to changes in the torque applied to the rotor.