Synchronous motor startup lock detection circuit and method

[0001] 1. Field of the Invention

[0002] The present invention is related generally to sensorless synchronous motor drives, and more particularly to drives with startup failure detection.

[0003] 2. Description of Related Art

[0004] Permanent magnet synchronous machines (PMSMs) are widely used as electric motors because of the development of high quality power electronics and powerful signal processing microprocessors. PMSM devices are typically cheaper than DC machines and can have higher performance characteristics than DC machines. PMSM devices are also attractive of their high efficiency, low maintenance costs and high torque output for a given size dimension.

[0005] The operation of a PMSM typically calls for synchronization of the rotor angle of the motor to obtain the desired output characteristics. Prior PMSMs included position sensors, such as encoders or resolvers to contribute to synchronizing the phase excitation with the rotor position. However, the presence of an encoder or resolver in many applications and environments has several drawbacks, including cost, reliability, machine dimensions and noise immunity.

[0006] A variety of techniques to detect rotor position in PMSM motors have been developed to obtain a sensorless rotor position feedback that permit the elimination of position sensors. In some cases, the sensorless rotor position detection techniques focus on the rotor saliency and local saturation of motor coils. These types of systems have internal permanent magnets for which saliency is readily measured. Rotor saliency can also be detected by injecting a high frequency sinusoidal voltage or other disturbance that is responsive to the position of the permanent magnet in relation to the motor coils. However, signal injection produces other problems related to efficiency or audible noise.

[0007] Other types of machine systems with minimal saliency like surface mounted PM motors, use back EMF to sense rotor angle. Back EMF is also observed to extract information to estimate flux in the motor coils.

[0008] Back EMF sensorless techniques for the sensorless control of a PMSM motor present difficulties at start up and during operation at low speeds. Although rotor position may be detected using the above techniques, even when the rotor is at a standstill, controlling a PMSM motor with a surface mounted PM at startup presents problems because no saliency measurement is readily available.

[0009] During a sensorless motor drive startup, motor torque is developed to overcome drive stiction and friction to rotate and accelerate the rotor. Motor shaft stiction and friction may vary dramatically dependent upon the applied load characteristics. For example, the stiction of an outdoor pump exposed to low temperatures may increase dramatically from a nominal value. In some instances, the motor shaft may be partially jammed, which can prevent motor startup rotation. In such a case, startup failure may occur, meaning that the drive torque per ampere ratio decreases dramatically and the motor is unable to accelerate. In addition, because of the difficulties with detecting rotor position in a sensorless drive at zero or low speed, the motor speed information may not be as accurate.

[0010] In accordance with the present invention, there is provided a method for quickly determining a startup failure condition in a PMSM motor. A motor flux range is provided for determining a successful startup so that a startup failure with a flux level outside of the range can be detected. The motor flux range can be adjusted depending upon the application. During a normal start up, the motor flux builds up and stays within a certain prescribed range. The motor flux level is extracted from a sensorless PMSM drive for comparison with the range to determine whether a startup failure has occurred.

[0011] According to another feature of the present invention, an automatic restart sequence is provided when a startup failure is detected. Upon detection of a startup failure, the PMSM motor drive initializes and enters a restart cycle. In accordance with the present invention, the PMSM drive can be configured for a particular number of startup retries, with selectable restarting torque for the startup retry attempts.

[0012] In accordance with another feature of the present invention, the startup failure detection is provided as an addition to a rotor angle estimator circuit, such as that described in copending application Ser. No. 10/294,201, filed Nov. 12, 2002 entitled ROTOR ANGLE ESTIMATION FOR PERMANENT MAGNET SYNCHRONOUS MOTOR DRIVE (IR-2130). The startup failure detector obtains an input flux amplitude from an output of a vector rotator in the rotor angle estimator and compares the flux amplitude to a high and low threshold value to determine if the flux amplitude is within an appropriate range. If the flux amplitude from the rotor angle estimator is out of range, a start failure condition is indicated and the drive is set to a default sequence to turn off the motor control signals, and, if desired, restart the startup sequence with increased startup current (configurable).

[0013] For a successful startup, the motor flux level should stay within a certain band limit. If motor flux is out of the band limit, a startup failure can be detected. Upon detection of this startup failure, the drive will reinitialize and enter a restart cycle. The drive can be configured for a variable number of startup retries and variable values for the restarting torque of the startup retries. The motor flux level can be easily extracted from the sensorless control drive as described in application Ser. No. 10/294,201 (IR2130), previously mentioned.

[0014] Other features and advantages of the present invention will become apparent from the following description of the invention, which refers to the accompanying drawings.

[0019] FIG. 1A illustrates a motor drive system with sensorless position detection. FIG. 1B illustrates a control system diagram for sensorless control with inputs Ud and Uq and outputs id, iq and θ, representative of the motor currents and rotor position.

[0020] Referring now to FIG. 2, a system block diagram for a rotor angle estimator is shown as diagram 20. The rotor angle estimator shown in diagram 20 operates with a phase lock loop in which a phase lock loop error PLL_ERR is determined based on a combination of the estimated rotor angle and flux signals designated as Alpha and Beta. The operation of the rotor angle estimator is detailed in related application Ser. No. 10/294,201 (IR-2130), the contents of which are incorporated herein by reference. Vector rotator block 22 provides two outputs D and Q, of which only output Q is used for the closed loop tracking of flux. The phase lock loop operates by forcing the phase lock loop error PLL_ERR to zero in a steady state. A proportioned integrator PI compensator 24 contributes to forcing output Q, and thus the value of PLL_ERR, to zero. Although output D of vector rotator block 22 is not used for rotor angle estimation, the signal on output D includes information about the motor flux amplitude, and is used according to the present invention for detection of startup failure.

[0021] Referring now to FIG. 3, a system block diagram for startup failure detection is illustrated as diagram 30. FIG. 2 illustrates all the same features provided in the rotor angle estimator of diagram 20, with the addition of startup failure detection circuitry. Vector rotator block 22 provides a flux amplitude signal from output D, which is used in the evaluation of a threshold range for startup failure detection. Initially, during drive startup, the flux amplitude signal on output D is not valid due to transients in the system upon startup. Accordingly, a start sequencer or control signal 32 is used to enable startup failure detection at AND gate 33. Once the flux amplitude signal from output D is valid, it may be used to determine whether the flux amplitude falls within the specified range to determine whether a startup failure has occurred. Accordingly, control signal 32 enables the startup detection failure indication after a specified delay to enable the flux amplitude signal to become valid and obtain a proper reading on a range of the flux amplitude level.

[0022] The flux amplitude supplied by output D from vector rotator block 22 is applied to an input of comparators 35, 36 to determine whether the flux amplitude signal falls within a given range. The flux amplitude signal is preferably compared to a high threshold FLXHITHR and a lower threshold FLXLOTHR in parallel to determine at once whether the flux amplitude is within the range specified by thresholds FLXHITHR and FLXLOTHR. The thresholds FLXHITHR and FLXLOTHR are applied to the respective comparator inputs of comparators 35, 36, along with the flux amplitude signal. A result of the comparison is provided on an output of comparators 35, 36. If the flux amplitude is below the upper threshold FLXHITHR, the output of comparator 35 is a logic zero, or low level. If the flux amplitude is greater than the lower threshold FLXLOTHR, the output of comparator 36 is a logic zero or low level as well. In a normal startup sequence, the outputs of comparators 35, 36 are both zero and no startup failure detection indication is produced. The comparison is performed after certain time interval (configurable) has elapsed. This is done to ensure sufficient time for normal flux build-up.

[0023] If the flux amplitude is above the high threshold FLXHITHR the output of comparator 35 is set to a logic “1”, or high level, to indicate that the flux amplitude is out of the bounds of the high flux threshold. If the flux amplitude is below the lower threshold FLXLOTHR, the output of comparator 36 is set to a logic “1”, or high level, to indicate the flux amplitude is out of bounds of the low flux threshold. If either of the outputs of comparators 35, 36 produces a logic “1”, or high level, a startup failure detection is indicated.

[0024] Referring now to FIGS. 4a–4c, experimental results in accordance with the present invention are illustrated in graphical plots 41–43. Graphical plot 41 in FIG. 4a shows the results of a normal startup for motor current, calculated rotor angle and calculated flux. Graphical plot 42 in FIG. 4b illustrates the detection of a startup failure. In this instance, the motor shaft is jammed and the flux amplitude falls outside the threshold boundaries, leading to a motor shutdown. In graphical plot 43 in FIG. 4c, the motor shaft is initially jammed and a startup failure detection occurs. In accordance with a feature of the present invention, an automatic restart is initiated in an attempt to determine if the jammed shaft has become free, or whether the locked condition is removed. In graphical plot 43 illustrated in FIG. 4c, three automatic restart attempts are made at points A, B and C, with the shaft of the motor being released prior to the restart attempts at point C. Accordingly, on the third attempt to restart the motor, the shaft is not jammed and the motor can start and operate as normal. In the case of each restart attempt, the flux amplitude is measured against the high and low thresholds FLXHITHR and FLXLOTHR to determine whether the motor shaft is jammed or free.

[0025] A number of variations are possible in realizing the present invention, including varying the number of automatic restart attempts, varying the delay time between the restart attempts, varying the power supply to the motor during a restart attempt, and so forth. For example, in one embodiment of the present invention, once a startup failure is detected, the next automatic restart can apply a higher startup current level to the motor in an attempt to free the blocked shaft. The amount of current and the period of time over which it is applied can be configured, depending upon application and environment, for instance, to attempt to restore motor shaft operation.

[0026] For example, the flux amplitude parameters may change over time as the motor drive system ages or deteriorates due to environmental conditions, or is subjected to high loading or harsh environments. The startup current limit thresholds may be varied to adapt to the motor drive system as it ages so that startup failure detection in the motor drive system can continue to operate consistently.

[0027] Furthermore, upon restart, the pulses, or more specifically, the retrial energy which may have any waveform, supplied to the motor can be varied in a number of different configurations. The number of restart pulses or retries may be varied, for example, so that three restarts may be attempted before a startup failure is observed. The period of time between pulses may also be varied, for example longer or shorter intervals may be used between restart attempts. The duration of the pulses or retries may be varied, so that longer or shorter pulses are applied to the motor to provide more or less current and torque.

[0028] In another variation of the present invention, the high and low threshold values can be preset in hardware, or programmable. For example, if a motor is used in different applications, or is subject to varying conditions for startup, a selectable set of threshold values can be provided to the comparators, based on manual settings or automatic sensing.

[0029] Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.