METHOD AND APPARATUS FOR CONTROLLING INDUCTION MOTORS

Abstract: The present invention provides for detecting rotor speed through the use of sense coils (240) on the stator teeth. Voltage ripples caused by the rotor slots (SC-1) is detected by combining the signals of several sense coils so that the fundamental voltage signal is cancelled or rejected and only the ripple voltages remain which is a signal proportional to speed. Further, the present invention provides for using a generator (220) to control voltage to the induction motor (222) with a PWM inverter (226) to control frequency signals to the induction motor (222).

Description: METHOD AND APPARATUS FOR CONTROLLING INDUCTION MOTORS BACKGROUND AND SUMMARY OF THE INVENTION The present invention relates generally to control methods for induction motors and, more particularly, to induction motor speed control systems for use in aircraft. Induction motors have been employed for various purposes in aircraft products. It has, for example, been proposed to use an induction motor system in an Advanced Ducted Propulsion (ADP) fan pitch control system having a dedicated generator mounted to the engine in order to provide electric power to drive the induction motor. Previously, high performance induction motor controls for similar applications have employed a field orientation technique called "vector control". The objective of vector control is to control the induction motor as a shunt wound d.c. motor by controlling the field excitation (the magnetizing current) and the torque producing current separately. In other words, the stator current of the induction motor is resolved into separately controllable torque producing and flux producing components. This can be accomplished, for example, by locking the phase of the reference system such that rotor flux is aligned entirely in the d-axis. The main issue in vector control methods is the acquisition of the fundamental flux wave of the motor rotor representing the synchronous frame of reference for field orientation. Based upon the methods of obtaining rotor flux information, the vector control technique is typically termed either "direct" or "indirect" sensing. Direct sensing methods, for example, can employ Hall effect sensors or other magnetic induction elements to measure the air gap flux vector. However, such d.c. current sensors can be relatively expensive and unreliable in the high temperature environments of aircraft engines. Indirect sensing methods are typically based on a voltage or a current (slip frequency) model of the induction motor. The current model based indirect method estimates the rotor flux vector from stator currents and rotor speed or position. In other words, slip frequency is estimated as a function of rotor time constant and torque and the flux producing components of the stator current. A disadvantage of this method is that a rotor speed/position sensor is required, such as a resolver mounted on the induction motor shaft. Thus, the motor control costs are greater while system reliability may decrease since extra control hardware and connections are involved. On the other hand, the voltage model based indirect method obtains the rotor flux vector by integrating the induced voltage detected directly via sense coils or calculated indirectly from stator currents and voltages. Sense coils are often less sensitive to variation in the motor operating parameters. However, the accuracy of this method is limited by the accuracy of integration, which is typically worse at zero and low speeds. At low speed operations an open loop slip control system can be used, but when the induction motor is already in motion (such as when it is being driven by the prime motor) the initial speed is not known. Thus, the appropriate slip command for an open loop control is not known. Another disadvantage of prior vector control systems has been in the employment of PWM inverters to produce variable voltage and variable frequency power to the induction motor stator windings. In typical aircraft engine environments, such PWM inverters must include EMI filters to decrease noise to acceptable levels. Accordingly, it is an object of the present invention to overcome these and other disadvantages in the prior art. The present invention achieves that by detecting rotor speed through the use of sense coils on the stator teeth. Voltage ripples caused by the rotor slots is detected by combining the signals of several such sense coils so that the fundamental voltage signal is canceled or rejected and only the ripple voltages remain, a signal proportional to rotor speed. Several alternative methods for implementing that technique are discussed herein. At low induction motor speeds, the present invention filters out the voltage ripples and incident third harmonics and instead estimates rotor speed from the fundamental voltage. When the induction motor is already being driven by a prime mover, the present invention selects a proper initial frequency command for initial motor start by identifying rotor speed from the PWM inverter. Further, the present invention provides for using a generator to control voltage to the induction motor, with a PWM inverter to control frequency signals to the induction motor. Other objects, advantages and novel features of the present invention will now become readily apparent to those skilled in the art from the following drawings and detailed descriptions. DESCRIPTION OF THE DRAWINGS Figure 1 shows an overall block diagram schematic of an induction motor control system according to the teachings of the present invention. Figure 2 shows a schematic of a baseline control system employing the voltage controlled generator features of the present invention. Figure 3 shows a schematic of a baseline control system employing the rotor speed estimation features of the present invention. Figure 4 shows an exemplary induction motor cross section illustrating placement of the speed estimation sense coils of the present invention. Figure 5 shows a schematic of one embodiment of speed estimation and control according to the present invention for use in the baseline control system of Figure 3. Figure 6 shows a schematic of an alternative embodiment of speed estimation and control according to the present invention for use in the baseline control system of Figure 3. Figure 7 shows a schematic of yet another alternative embodiment of speed estimation and control according to the present invention for use in the baseline control system of Figure 3. Figure 8 shows a schematic of the embodiment of Figure 7 specifically applied to the control system of Figure 3. Figure 9 shows an exemplary induction motor cross section illustrating placement of the low speed estimation sense coils of the present invention. Figure 10 shows a schematic of a low speed estimation system specifically applied to the control system of Figure 3. Figure 11 shows a schematic portion of the PWM inverter and induction motor of the present invention for use in speed estimation when the motor is already being driven by a prime mover. Figure 12 shows a schematic of a method of detecting induction motor speed for use with the arrangement of Figure 11 as applied to a baseline control system. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1, illustrating a preferred embodiment of the present invention shows schematically an induction motor control system suitable for use with a ADP fan pitch system. This control system includes an A.C. generator 20, driven, for example, by an aircraft engine or prime mover, to provide electric power to induction motor 22 through rectifier and D.C. link filter 24 and then through PWM inverter 26. Induction motor 22, for example, provides motive power to a fan blade pitch change transmission. In this embodiment, generator 20 is dedicated to this function and has its output voltage controlled by regulator 28. Speed controller 30 provides the voltage control signals to regulator 28 and the voltage and frequency control signals to inverter 26. Speed estimator 32 provides the reference frequency signals to speed controller 30 according to sensed signals from inverter 26 and/or induction motor 22, as described further below. In certain embodiments where, for example, an unregulated generator 20 is employed, speed controller 30 can also provide phase control signals to rectifier 24. The voltage control of generator 20 is a feature of the present invention which can be employed separately from the control system of Figure 1 and is best understood by individual reference to that feature used in a baseline control system such as is shown schematically in Figure 2. Briefly, instead of using the PWM inverter to control both frequency and voltage to the induction motor, the voltage is controlled by the generator and the PWM inverter controls only frequency. In this manner, a significant reduction in switching frequency noise is achieved. r - i- —I ac β More specifically, in a baseline control system 100 using indirect field orientation sensing the stator currents of inductionmotor 122 are controlled by voltage regulation of generator 120 with signals through rectifier 124 and PWM inverter 26. The D.C. link current is estimated from the generator current transformers in accordance with the following equation: The D.C. link current command is responsive to the flux and torque producing current components I d and I q as follows: The current loop includes a proportional integral (PI) regulator 140. The output of the PI regulator provides an input to voltage regulator 128, which in turn controls excitation of generator 120 via a dual switch exciter drive. That type of drive is preferred in order to obtain a rapid response (approximately 100 Hz) from the generator. The command I * , is generated from the output of the speed controller 130, shown here to include reference speed signals ω * r summed with a negative speed feedback signal from tachometer 132 and then output from a PI compensator 150. In this embodiment, frequency control is achieved by summing the rotor speed with the command slip frequency as follows: ω e =ω r +ω _ where Transient suppressor 160 is preferred to handle the regenerative power and turns itself on when the D.C. link voltage increases past a predetermined threshold level. Several speed estimation methods are available with the subject invention and are also best understood by reference individually to a baseline induction motor control system 200, such as is shown in Figure 3, having three phase A.C. generator 220, rectifier 224, PWM inverter 226 and induction motor 222. The current loop is preferably implemented in the synchronous reference frame. The I d component of the stator current produces magnetizing current for induction motor 220, while the I q component produces the electromagnetic torque. Sense coils are provided in induction motor 220 as were loops 240 on the stator teeth. As shown in Figure 4, these sense coils are preferably located 120 electrical degrees apart in a three phase motor. One sense coil 250 is, for example, located on the stator back iron. This control method is based on the fact that the air gap flux is modulated by the slot ripple. Several methods are proposed to detect the rotor speed from the voltage ripple on the fundamental voltage signal due to the rotor slots. Figure 5 shows schematically one speed estimator 232 for calculating of rotor speed where only the back iron sense coil 250 is employed. Low pass filter 252 rejects the PWM frequency. A synchronous notch filter 254, including four multipliers 256, two low pass filters 258 and a summer 260, rejects the fundamental (speed) voltage signal component of the sense coil signal and passes the rotor slot components of the signal to a speed detector 262. This detector includes, for example, a zero cross detector 264, a pulse converter 266, a counter 268, a latch 270 and a block 272 which produces 1/T (period between rotor slots) and scales the output signal to the rotor speed. That output signal is now proportional to the rotor speed and is applied to a speed controller 230 having PI 232 and a slip calculator 234. Slip calculator 234 computes ω * s from the stator current component references , d and rotor time constant (1/T r = R- L r ), where R- is rotor resistance and L r is rotor self inductance. The commanded electrical frequency ω e is the sum of the rotor speed ω r and commanded slip frequency ω * s . The electrical angular position is obtained at the output of integrator 274 (which is also scaled by the poles pair) and applied to sine and cosine functional blocks 276 and 278. The output of these functional blocks is in turn supplied to notch filter 254. Figure 6 shows, alternative, a diagram of schematic a single phase compensation technique for speed estimation for use when the induction motor is driven by a PWM inverter and a single sense coil 240 and back iron sense coil 250 are employed. In general, back iron sense coil 250 is used to compensate for the fundamental voltage and detect the rotor slot voltage on the output of summer 280 after voltage signals of each sense coil are subject to low pass filters 282. The reminder of the control system is analogous to that of Figure 5. Similarly, Figure 7 shows, alternatively, a schematic diagram of a three phase compensation technique for speed estimation. The three sense coils 240, located 120 electrical degrees apart on the stator, provide signals to low pass filters 290 to reject PWM switching frequency. The resulting signals are then added by summer 292 to cancel out the fundamental voltage signal. As overall speed estimator and speed controller system using this three phase alternative is shown schematically in Figure 8. Flux analyzer 294 produces sine and cosine signals for the coordinate transformations while the speed detector signals calculated form the rotor slot ripples are used for speed control. At low induction motor speeds, it has been found that magnetic staturation exists and a third harmonic frequency signal is introduced into the sense coil signals which is relatively difficult to filter out. In such situations, an alternative embodiment of the present employs three sense coils 340 wired around the pole pitch to include a plurality of stator teeth, as shown in Figure 9. The outputs of these sense coils are voltage produced directly by the stator flux and are used estimate stator and rotor flux linkage in the control circuit of Figure 10 according to the following equations: θ=tan .' λ β- The estimated flux linkages and voltages from these sense coils are used to estimate synchronous frequency according to the following equation: ω.<λ - λp λ y«λ ^(λp ) 2 ) The rotor speed is estimated by the following equations: ω r =ω e -ω 5 Further, the present invention provides an arrangement for identifying rotor speed when the induction rotor is already being driven by a prime mover. Specifically, during initial start, two phase legs 450 and 452 of PWM inverter 426 are supplied with D.C. current to the induction motor 422, while the third phase leg 454 is kept open, as shown in Figure 11. The voltage speed induced in the third phase leg includes a signal having a frequency equal to a product of the number of and the rotor speed. That signal is applied to low pass filter 456 to filter out the PWM switching frequency. The analog signal detected thereby is passed through a zero cross detector 458, to produce a pulse train output signal having a frequency which is proportional to the rotor speed. Various methods can be used to detect this frequency and use it to select a proper frequency command for initial start of induction motor 422, as shown in Figure 12. Although the present invention has been described above in detail with respect to certain preferred embodiments, the same are by way of illustration and example only, and not to be taken as limitations since those of ordinary skill in this act will now recognize that several variations are contemplated by the present invention. Accordingly, the spirit and scope of the present invention are only limited by the attached claims.

Claims: We claim: 1. An induction motor control system having an A.C. generator providing power to an induction motor through a D.C. rectifier and a PWM inverter, including at least one sense coil mounted about a stator tooth of said induction motor for estimating rotor speed by detection of ripple voltages on the fundamental frequency signal detected by said sense coil.

Inventors: BANSAL MADAN, HEGLUND WILLIAM S, ROZMAN GREGORY I, TSAI SHAN-CHIN, BANSAL, MADAN, HEGLUND, WILLIAM, S., ROZMAN, GREGORY, I., TSAI, SHAN-CHIN
Applicants: SUNDSTRAND CORP, SUNDSTRAND CORPORATION
Application year: 1995
Main classification: H02P7
IPC codes: G01P3, H02P21